This is the initial public offering of our common stock. We are selling 8,078,566 shares of common stock, and the selling stockholders named in this prospectus are selling 5,921,434 shares of common stock. We will not receive any proceeds from the shares of common stock sold by the selling stockholders. Prior to this offering, there has been no public market for our common stock. The initial public offering price of our common stock is expected to be between $7.00 and $9.00 per share. We have applied to list our common stock on the NASDAQ Global Market under the symbol “ERII.”

We have granted the underwriters an option to purchase a maximum of 2,100,000 additional shares to cover over-allotments of shares.

The underwriters expect to deliver the shares to purchasers on or about          , 2008.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

You should rely only on the information contained in this document or to which we have referred you. We have not authorized anyone to provide you with information that is different. This document may only be used where it is legal to sell these securities. The information in this document may only be accurate on the date of this document.

We incorporated in Virginia in April 1992 and reincorporated in Delaware in March 2001. Our principal executive offices are located at 1908 Doolittle Drive, San Leandro, California 94577. Our telephone number is (510) 483-7370. Our website address is www.energyrecovery.com. Information contained on our website is not incorporated by reference into this prospectus, and you should not consider information contained on our website to be part of this prospectus.

“ERI,” the ERI logo, “Making Desalination Affordable,” “PX Pressure Exchanger,” “PX” and other trademarks or service marks of ERI appearing in this prospectus are the property of ERI. This prospectus contains additional trade names, trademarks and service marks of other companies. We do not intend our use or display of other companies’ trade names, trademarks or service marks to imply a relationship with, or endorsement or sponsorship of us by, these other companies.

This prospectus includes market and industry data and forecasts that we obtained from internal research, publicly available information and industry publications and surveys. Industry publications and surveys generally state that the information contained therein has been obtained from sources believed to be reliable. Unless otherwise noted, statements as to our market position relative to our competitors are approximated and based on the above-mentioned third-party data and internal analysis and estimates as of the date of this prospectus. Although we believe the industry and market data and statements as to market position to be reliable as of the date of this prospectus, we have not independently verified this information and it could prove inaccurate. Industry and market data could be wrong because of the method by which sources obtained their data and because information cannot always be verified with certainty due to the limits on the availability and reliability of raw data, the voluntary nature of the data gathering process and other limitations and uncertainties. In addition, we do not know all of the assumptions regarding general economic conditions or growth that were used in preparing the forecasts from sources cited herein.

Until          , 2008 (25 days after the date of this prospectus) all dealers that effect transactions in these securities, whether or not participating in this offering, may be required to deliver a prospectus. This is in addition to the dealer’s obligation to deliver a prospectus when acting as an underwriter and with respect to unsold allotments or subscriptions.

Investing in our common stock involves a high degree of risk. You should carefully consider the risks and uncertainties described below before making a decision to buy our common stock. If any of the following risks materializes, our business, financial condition and results of operations could be harmed. Consequently, the trading price of our common stock could decline, and you may lose all or part of your investment. Before deciding to purchase any shares of our common stock, you should also refer to the other information contained in this prospectus, including “Forward-Looking Statements” and our consolidated financial statements and the related notes.

Our primary product is the PX device, and sales of our PX device historically have accounted for almost 100% of our revenue. While we sell a variety of models of the PX device depending on the design of the desalination plant and its desired output, all of our models rely on the same basic technology we have developed over the past 11 years. We expect that the revenue from our PX devices will continue to account for most of our revenue for the foreseeable future. Any factors adversely affecting the demand for the PX device, including competition, customer spending and industry regulations, would cause a significant decline in our revenue. Some of the factors that may affect sales of our PX device may be out of our control.

The demand for our products may decrease if the construction of desalination plants declines. We derive substantially all of our revenue from the sale of products and services, directly or indirectly, to the municipal water supply, hotel and resort, and agricultural industries. Construction of desalination plants and subsequent installation of our products may be deferred or cancelled as a result of many factors, including changing governmental regulations, energy costs and reduced energy conservation capital spending. For instance, desalination projects on islands are often delayed due to unpredictable weather patterns. In addition, a significant amount of revenue generated by our original equipment manufacturer, or OEM, customers is dependent on long-term relationships, which are not always supported by long-term contracts. This revenue is particularly susceptible to variability based on changes in the spending patterns of such OEM customers. We have experienced and may in the future experience significant variability in our revenue, on both an annual and a quarterly basis, as a result of these factors. Pronounced variability or an extended period of reduction in spending by our customers and construction of desalination plants could negatively impact our business and make it difficult for us to accurately forecast our future sales, which could lead to increased spending by us that is not matched with equivalent or higher revenue.

Due to delays in, or failure to obtain the approval of or permitting for, plant construction because of political factors, adverse financing conditions or other factors, especially in countries with political unrest, planned SWRO projects can be cancelled or delayed. Even though we may have a signed contract to produce a certain number of PX devices by a certain date, if a customer requests a delay of shipment and we accordingly delay shipment of our PX devices, our results of operations and revenue will be negatively impacted.

A limited number of our EPC customers accounts for a substantial portion of our net revenue. One EPC customer accounted for approximately 49% of our net revenue and two EPC customers accounted for approximately 48% of our net revenue for the three months ended March 31, 2008 and March 31, 2007, respectively. Specifically, Geida and its affiliated entities, accounted for approximately 49% of our net revenue for the three months ended March 31, 2008 and Inima Servicios and Geida and its affiliated entities accounted for approximately 26% and 22% of our net revenue, respectively, for the three months ended March 31, 2007. In 2007, three EPC customers, including their affiliated entities, accounted for 56%

Our operating results may fluctuate due to a variety of factors, many of which are outside of our control. Due to the fact that a single order for our PX devices for a particular desalination plant may represent significant revenue, we have experienced significant fluctuations in revenue from quarter to quarter, and we expect such fluctuations to continue. As a result, comparing our operating results on a period-to-period basis may not be meaningful. You should not rely on our past results as an indication of our future performance. If our revenue or operating results fall below the expectations of investors or securities analysts or below any guidance we may provide to the market, the price of our common stock would likely decline substantially.

In addition, factors that may affect our operating results include, among others:

Our customer contracts generally contain holdback provisions pursuant to which the final installments to be paid under such sales contracts are due up to 24 months after the product has been shipped to the customer and revenue has been recognized. Typically, between 10 and 20 percent, and in some instances up to 30 percent, of the revenue we receive pursuant to our customer contracts are subject to such holdback provisions and are accounted for as unbilled receivables until we deliver invoices for payment. As of March 31, 2008, we had approximately $4.7 million of current unbilled receivables

Given the specialized nature of our business, we must hire highly skilled professionals with industry-specific experience. Our ability to successfully grow depends on recruiting skilled and experienced employees. We often compete with larger, better known companies for talented employees. Also, retention of key employees, such as our chief executive officer, who has over 30 years of experience in the water treatment industry, is vital to the successful execution of our growth strategies. Our failure to retain existing or attract future key personnel could harm our business.

Since 2004, we have invested over $3 million in research and development costs associated with our PX products. From time to time, our customers have expressed a need for greater processing efficiency. In response, and as part of our strategy to enhance our energy recovery solutions and grow our business, we plan to continue to make substantial investments in the research and development of new technologies. For instance, we are in the process of developing the PX-1200 Titan as a product for use in increasingly larger desalination plants. While this product has the potential to provide greater capacity, it will be priced higher and may not perform as well as our other PX devices. It is possible that potential customers may not accept the new pricing structure. It is also possible that the release of this product may be delayed if testing reveals unexpected flaws. Our future success will depend in part on our ability to continue to design and manufacture new products, to enhance our existing products and to provide new value-added services. We may experience unforeseen problems in the performance of our existing and new technologies or products. Furthermore, we may not achieve market acceptance of our new products and solutions. If we are unable to develop competitive new products, or if the market does not accept such products, our business and results of operations will be adversely affected.

If there is a downturn in the SWRO industry, our sales would be directly and adversely impacted. In addition, changes in SWRO technology could reduce the demand for our devices. For example, a reduction in the operating pressure used in SWRO plants could reduce the need for and viability of our energy recovery devices. Membrane manufacturers are actively working on lower pressure membranes for SWRO that could potentially be used on a large scale to desalinate sea water at a much lower pressure than is currently necessary. Similarly, an increase in the recovery rate would reduce the number of energy recovery devices required and would reduce the demand for our product. Any of these changes would adversely impact our revenue and growth.

Our PX devices utilize ceramic components that have to date demonstrated high durability, high corrosion resistance and long life in SWRO applications. Because most of our PX devices have only been installed for several years, it is difficult to accurately predict their performance or endurance over a longer period of time. Accordingly, our value proposition to customers may not be fulfilled and our opportunity to sell replacement components or units may be limited.

Our sales efforts involve substantial education of our current and prospective customers about the use and benefits of our PX products. This education process can be extremely time consuming and typically involves a significant product evaluation process. While the sales cycle for our OEM customers, who are involved with smaller desalination plants, averages one to three months, the average sales cycle for our international EPC customers, who are involved with larger desalination plants, ranges from six to 16 months and has, in some cases, extended up to 24 months. Most of our EPC customers are located internationally or are themselves governmental entities. In addition, these customers generally must make a significant commitment of resources to test and evaluate our technologies. As a result, our sales process involving these customers is often subject to delays associated with lengthy approval processes that typically accompany the design,

A significant portion of our annual sales typically occurs during the fourth quarter, which we believe generally reflects EPC customer buying patterns. Any delays or cancellation of expected sales during the fourth quarter would reduce our quarterly and annual revenue from what we anticipated. Such a reduction might cause our quarterly and annual revenue or quarterly and annual operating results to fall below the expectations of investors or securities analysts or below any guidance we may provide to the market, causing the price of our common stock to decline.

We rely on a limited number of vendors to produce the ceramics used in our products. For the three months ended March 31, 2008, three ceramics suppliers represented approximately 60% of our purchases from all of our suppliers, and for the three months ended March 31, 2007, two ceramics suppliers represented approximately 56% of purchases from all of our suppliers. For the years ended December 31, 2007, 2006 and 2005, two ceramics suppliers represented approximately 52%, 59% and 47%, respectively, of our purchases from all of our suppliers. From time to time our demand has grown faster than the supply capabilities of these vendors. If any of our suppliers were to cancel or materially change its commitment with us or fail to meet the quality or delivery requirements needed to satisfy customer orders for our products, we could lose customer orders, be unable to develop or sell our products cost-effectively or on a timely basis, if at all, and have significantly decreased revenue, which would harm our business, operating results and financial condition. We are currently in the process of qualifying a fourth supplier of ceramics. However, our qualification process is rigorous and there is no assurance that such additional supplier will be approved as a qualifying supplier. If we are unable to qualify this additional ceramics supplier, we may be exposed to increased risk of supply chain disruption and capacity shortages.

We rely on a single foundry to produce all of our stainless steel castings for use in our PX products. Our reliance on a single manufacturer of stainless steel castings involves a number of significant risks, including reduced control over delivery schedules, quality assurance, manufacturing yields, production costs and lack of guaranteed production capacity or product supply. We do not have a long term supply agreement with our supplier and instead secure manufacturing availability on a purchase order basis. Our supplier has no obligation to supply products to us for any specific period, in any specific quantity or at any specific price, except as set forth in a particular purchase order. Our requirements represent a small portion of the total production capacities of our supplier and our supplier may reallocate capacity to other customers, even during periods of high demand for our products. We have in the past experienced and may in the future experience quality control issues and delivery delays with our supplier due to factors such as high industry demand or the inability of our vendor to consistently meet our quality or delivery requirements. If our supplier were to cancel or materially change its commitment with us or fail to meet the quality or delivery requirements needed to satisfy customer orders for our products, we could lose time-sensitive customer orders, be unable to develop or sell our products cost-effectively or on a timely basis, if at all, and have significantly decreased revenue, which would harm our business, operating results and financial condition. We may qualify additional suppliers in the future which would require time and resources. If we do not qualify additional suppliers, we may be exposed to increased risk of capacity shortages due to our complete dependence on our current supplier.

The market for energy recovery devices for desalination plants is competitive and continually evolving. The PX device competes with slow cycle isobarics, Pelton wheels and hydraulic turbochargers. Our three primary competitors are Calder AG, Fluid Equipment Development Company and Pump Engineering Incorporated. We expect competition to persist and intensify as the desalination market opportunity grows. Many of our current and potential competitors may have significantly greater financial, technical, marketing and other resources than we do and may be able to devote greater resources to the development, promotion, sale and support of their products. Also, our competitors may have more extensive customer bases and broader customer relationships than we do, including long-standing relationships or exclusive contracts

We warrant our products for up to five years. We test our products in our manufacturing facilities through a variety of means. However, there can be no assurance that our testing will reveal latent defects in our products, which may not become apparent until after the products have been sold into the market. Accordingly, there is a risk that warranty claims may be filed due to product defects. We may incur additional operating expenses if our warranty provisions do not reflect the actual cost of resolving issues related to defects in our products. If these additional expenses are significant, they could adversely affect our business, financial condition and results of operations. While the number of warranty claims has not been significant to date, we are in the initial stages of offering such warranties to our customers. Accordingly, we cannot quantify the error rate of our products and cannot assure that a large number of warranty claims will not be filed in a given year. As a result, our operating expenses may increase if a large number of warranty claims are filed in any specific year, particularly towards the end of any given warranty period.

We depend on our ability to protect our proprietary technology. We rely on trade secrets, patent, copyright and trademark laws and confidentiality agreements with employees and third parties, all of which offer only limited protection. We hold five United States patents and nine counterpart international patents relating to specific proprietary design features of our PX technology. The terms of these patents will begin to expire in 2011, at which time we could become more vulnerable to increased competition. In addition, we have applied for two new United States patents and 14 international counterpart patents covering our current and anticipated future PX designs. We do not hold patents in many of the countries into which we sell our PX devices, including Saudi Arabia, Algeria and China, and accordingly, the protection of our intellectual property in those countries may be limited. We also do not know whether any of our pending patent applications will result in the issuance of patents or whether the examination process will require us to narrow our claims, and even if patents are issued, they may be contested, circumvented or invalidated. Moreover, while we believe our remaining issued patents are essential to the protection of the PX technology, the rights granted under any of our issued patents or patents that may be issued in the future may not provide us with proprietary protection or competitive advantages, and, as with any technology, competitors may be able to develop similar or superior technologies to our own now or in the future. In addition, our granted patents may not prevent misappropriation of our technology, particularly in foreign countries where intellectual property laws may not protect our proprietary rights as fully as those in the United States. This may render our patents impaired or useless and ultimately expose us to currently unanticipated competition. Protecting against the unauthorized use of our products, trademarks and other proprietary rights is expensive, difficult and, in some cases, impossible. Litigation may be necessary in the future to enforce or defend our intellectual property rights or to determine the validity and scope of the proprietary rights of others. This litigation could result in substantial costs and diversion of management resources, either of which could harm our business.

Third parties could claim that our technology infringes their proprietary rights. In addition, we may be contacted by third parties suggesting that we obtain a license to certain of their intellectual property rights they may believe we are infringing. We expect that infringement claims against us may increase as the number of products and competitors in our market increases and overlaps occur. In addition, to the extent that we gain greater visibility, we believe that we will face a higher risk of being the subject of intellectual property infringement claims. Any claim of infringement by a third party, even those without merit, could cause us to incur substantial costs defending against the claim, and could distract our management from our business. Furthermore, a party making such a claim, if successful, could secure a judgment that requires us to pay substantial damages. A judgment against us could also include an injunction or other court order that could prevent us from offering our products. In addition, we might be required to seek a license for the use of such intellectual property, which may not be available on commercially reasonable terms, or at all. Alternatively, we may be required to develop non-infringing technology, which could require significant effort and expense and may ultimately not be successful. Any of these events

Our existing manufacturing facilities are capable of meeting current demand and demand for the foreseeable future. However, the future growth of our business depends on our ability to successfully expand our manufacturing, research and development and technical testing facilities. Larger products currently under development will require the design and construction of new manufacturing capacity. We intend to add new facilities or expand existing facilities as the demand for our devices increases. However, we cannot ensure that suitable additional or substitute space will be available to accommodate any such expansion of our operations.

We have historically relied on outside financing to fund our operations, capital expenditures and expansion. We may require additional capital from equity or debt financing in the future to fund our operations, or respond to competitive pressures or strategic opportunities. We may not be able to secure such additional financing on favorable terms, or at all. The terms of additional financing may place limits on our financial and operating flexibility. If we raise additional funds through further issuances of equity, convertible debt securities or other securities convertible into equity, our existing stockholders could suffer significant dilution in their percentage ownership of our company, and any new securities we issue could have rights, preferences or privileges senior to those of existing or future holders of our common stock, including shares of common stock sold in this offering. If we are unable to obtain necessary financing on terms satisfactory to us, if and when we require it, our ability to grow or support our business and to respond to business challenges could be significantly limited.

Our products are used in SWRO desalination plants which are often times constructed and maintained through government subsidies. The rate of construction of desalination plants depends on each government’s willingness and ability to allocate funds for such projects. For instance, some desalination projects in the Middle East and North Africa are funded by budget surpluses driven by high crude oil and natural gas prices. If governments divert funds allocated for such projects to other projects or do not have budget surpluses, the demand for our products could decline and negatively affect our revenue base, which could harm the overall profitability of our business.

In addition, various water management agencies could alter demand for fresh water by investing in water reuse initiatives or limiting the use of water for certain agricultural purposes. Certain uses of water considered to be wasteful could be curtailed, resulting in more available water and less demand for alternative solutions such as desalination.

The manufacture of our products is highly technical, and our products may contain latent defects or flaws. We test our products prior to commercial release and during such testing have discovered and may in the future discover flaws and defects that need to be resolved prior to release. Resolving these flaws and defects can take a significant amount of time and prevent our technical personnel from working on other important tasks. In addition, our products have contained and may in the future contain one or more flaws that were not detected prior to commercial release to our customers. Some flaws in our products may only be discovered after a product has been installed and used by customers. Any flaws or defects discovered in our products after commercial release could result in loss of revenue or delay in revenue recognition, loss of customers and increased service and warranty cost, any of which could adversely affect our business, operating results and financial condition. In addition, we could face claims for product liability, tort or breach of warranty. Our contracts with our customers contain provisions relating to warranty disclaimers and liability limitations, which may not be upheld. Defending a lawsuit, regardless of its merit, is costly and may divert management’s attention and adversely affect the market’s perception of us and our products. In addition, if our business liability insurance coverage proves inadequate or future coverage is unavailable on acceptable terms or at all, our business, operating results and financial condition could be harmed.

Historically, we have derived a significant portion of our revenue from customers whose SWRO facilities utilizing the PX device are outside the United States. Many of such customers’ projects are in emerging growth countries with relatively young and unstable market economies and volatile political environments. We also have sales and technical support personnel stationed in Africa, Asia and the Middle East, among other regions, and we expect to continue to add personnel in additional countries. As a result, any governmental changes or reforms or disruptions in the business, regulatory or political environment in the countries in which we operate or sell our products could have a material adverse effect on our business, financial condition and results of operations.

Sales of our products have to date been denominated principally in U.S. dollars. Over the last several years, the U.S. dollar has weakened against most other currencies. Future increases in the value of the U.S. dollar, if any, would increase the price of our products in the currency of the countries in which our customers are located. This may result in our customers seeking lower-priced suppliers, which could adversely impact our operating results. A larger portion of our international revenue may be denominated in foreign currencies in the future, which would subject us to increased risks associated with fluctuations in foreign exchange rates.

Our international contracts and operations subject us to a variety of additional risks, including:

As we continue to expand our business globally, our success will depend, in large part, on our ability to anticipate and effectively manage these and other risks associated with our international operations. Our failure to manage any of these risks successfully could harm our international operations and reduce our international sales, which in turn could adversely affect our business, operating results and financial condition.

Future growth in our business, if it occurs, will place significant demands on our management, infrastructure and other resources. To manage any future growth, we will need to hire, integrate and retain highly skilled and motivated employees. We will also need to continue to improve our financial and management controls, reporting and operational systems and procedures. If we do not effectively manage our growth, our business, operating results and financial condition would be adversely affected.

As a public company, SEC rules require that we maintain internal control over financial reporting that provides reasonable assurance regarding the reliability of financial reporting and preparation of published financial statements in accordance with generally accepted accounting principles. Accordingly, we will be required to document and test our internal controls and procedures to assess the effectiveness of our internal control over financial reporting. In addition, our independent registered public accounting firm will be required to report on the effectiveness of our internal control over financial reporting. In the future, we may identify material weaknesses and deficiencies which we may not be able to remediate in a timely manner. Material weaknesses may exist when we report on the effectiveness of our internal control over financial reporting for purposes of our attestation required by reporting requirements under the Securities Exchange Act of 1934 after this offering, with our first reporting obligation being in our Annual Report on Form 10-K for the year ending December 31, 2009. If we fail to achieve or maintain effective internal control over financial reporting, we will not be able to conclude that we have maintained effective internal control over financial reporting or our independent registered public accounting firm may not be able to issue an unqualified report on the effectiveness of our internal control over financial

We prepare our financial statements to conform with generally accepted accounting principles, or GAAP, in the United States. These accounting principles are subject to interpretation by the SEC and various other bodies. A change in those policies can have a significant effect on our reported results and may affect our reporting of transactions completed before a change is announced. Changes to those rules or the interpretation of our current practices may adversely affect our reported financial results or the way we conduct our business.

In the future, we may acquire companies or assets that we believe may enhance our market position. We may not be able to find suitable acquisition candidates and we may not be able to complete acquisitions on favorable terms, if at all. If we do complete acquisitions, we cannot assure you that they will ultimately strengthen our competitive position or that they will not be viewed negatively by customers, financial markets or investors. In addition, any acquisitions that we make could lead to difficulties in integrating personnel and operations from the acquired businesses and in retaining and motivating key personnel from these businesses. Acquisitions may disrupt our ongoing operations, divert management from day-to-day responsibilities, increase our expenses and harm our operating results or financial condition. Future acquisitions may reduce our cash available for operations and other uses and could result in an increase in amortization expense related to identifiable assets acquired, potentially dilutive issuances of equity securities or the incurrence of debt, any of which could harm our business, operating results and financial condition.

As a public company, we will incur significant legal, accounting and other expenses that we did not incur as a private company. In addition, the Sarbanes-Oxley Act of 2002, or Sarbanes-Oxley, as well as rules subsequently implemented by the SEC and the NASDAQ Global Market, or NASDAQ, have imposed various requirements on public companies, including requiring changes in corporate governance practices. Our management and other personnel will need to devote a substantial amount of time to these compliance requirements. Moreover, these rules and regulations will increase our legal and financial compliance costs and will make some activities more time-consuming and costly. For example, we expect these rules and regulations to make it more difficult and more expensive for us to obtain director and officer liability insurance, and we may be required to accept reduced policy limits and coverage or incur substantial costs to maintain the same or similar coverage. These rules and regulations could also make it more difficult for us to attract and retain qualified persons to serve on our board of directors, our board committees or as executive officers.

There are no directly comparable U.S. companies known to us whose securities are currently being publicly traded in the U.S. stock market. Additionally, our common stock has no prior trading history. Factors affecting the trading price of our common stock will include:

In addition, if the market for stocks of companies in industries related or similar to ours, or the stock market in general, experiences loss of investor confidence, the trading price of our common stock could decline for reasons unrelated to our business. The trading price of our common stock might also decline as a result of events that affect other companies in our industry even if these events do not directly affect us. Some companies that have had volatile market prices for their securities have had securities class actions filed against them. If a suit were filed against us, regardless of its merits or outcome, it could result in substantial costs and divert management’s attention and resources. This could harm our business, operating results and financial condition.

Prior to this offering, there has been no public market for shares of our common stock. Although we expect to apply to list our common stock on NASDAQ, an active public trading market for our common stock may not develop or, if it develops, may not be maintained after this offering. Our company and the representatives of the underwriters will negotiate to determine the initial public offering price. The initial public offering price may be higher than the trading price of our common stock following this offering. As a result, you could lose all or part of your investment.

If our existing stockholders sell, or indicate an intention to sell, substantial amounts of our common stock in the public market after the lock-up agreements and other legal restrictions on resale discussed in this prospectus lapse, the trading price of our common stock could decline. See “Shares Eligible for Future Sale” below. Based upon shares outstanding as of March 31, 2008, we will have outstanding a total of 47,917,474 shares of common stock upon completion of this offering, an increase of approximately 20% from the number of shares outstanding prior to this offering. Of these shares, only the 14,000,000 shares of common stock sold in this offering and 839,279 shares of common stock not subject to lock-up agreements will be freely tradeable, without restriction, in the public market immediately following this offering.

The lock-up agreements entered into by the underwriters with our officers and directors and other current holders of our common stock will expire 180 days from the date of this prospectus, although those lock-up agreements may be extended under certain circumstances. The underwriters, however, may, in their sole discretion, release these parties from the restrictions of the lock-up agreements. After the lock-up agreements expire, based upon shares outstanding as of March 31, 2008, up to an additional 33,078,195 shares of common stock will be eligible for sale in the public market, 22,718,694 of which are held by our directors, executive officers and other affiliates and will be subject to volume limitations under Rule 144 under the Securities Act and various vesting agreements. In addition, as of March 31, 2008, the 3,407,430 shares of common stock that are either subject to outstanding warrants or options or reserved for future issuance under our employee benefit plans will become eligible for sale in the public market to the extent permitted by the provisions of various vesting agreements, the lock-up agreements and Rules 144 and 701 under the Securities Act. If these additional shares are sold, or if it is perceived that they will be sold, in the public market, the trading price of our common stock could decline.

The trading market for our common stock may be affected by the research and reports that securities or industry analysts publish about us or our business. We do not currently have, and may never obtain, research coverage by securities and industry analysts. If no securities or industry analysts commence coverage of our company, the trading price for our stock and the trading volume could decline. In the event we obtain securities or industry analyst coverage, if one or more of the analysts who covers us downgrades our stock or publishes inaccurate or unfavorable research about our business, our stock price could decline. In addition, if we obtain analyst coverage, the analysts’ projections may have little or no relationship to the results we actually achieve and could cause our stock price to decline if we fail to meet their projections. If one or more of these analysts ceases coverage of our company or fails to publish reports on us regularly our stock price or trading volume could decline.

Upon completion of this offering, our directors and executive officers and their affiliates will beneficially own, in the aggregate, approximately 50% of our outstanding common stock, assuming no exercise of the underwriters’ option to purchase additional shares, compared to approximately 17% of our outstanding common stock represented by the shares sold in this offering, assuming no exercise of the underwriters’ option to purchase additional shares. As a result, these stockholders will be able to exercise significant influence over all matters requiring stockholder approval, including the election of directors and approval of significant corporate transactions, such as a merger or other sale of our company or its assets. This concentration of ownership will limit your ability to influence corporate matters and may have the effect of delaying or preventing a third party from acquiring control over us. For more information regarding the ownership of our outstanding stock by our executive officers and directors and their affiliates, please see the section titled “Security Ownership of Certain Beneficial Owners and Management” below.

The initial public offering price per share will be substantially higher than the net tangible book value per share of our common stock outstanding prior to this offering. As a result, investors purchasing common stock in this offering will experience immediate dilution of $6.37 per share assuming an initial public offering price of $8.00 per share. In addition, we have issued options and warrants to acquire common stock at prices significantly below the initial public offering price. To the extent outstanding options are ultimately exercised, there will be further dilution to investors in this offering. This dilution is due in large part to the fact that our earlier investors paid substantially less than the initial public offering price when they purchased their shares of our stock. In addition, if the underwriters exercise their option to purchase additional shares, if outstanding warrants to purchase our common stock are exercised or if we issue additional equity securities, you will experience additional dilution.

We could spend the proceeds from this offering in ways our stockholders may not agree with or that do not yield a favorable return. We intend to use the net proceeds from this offering for general corporate purposes, which may include expansion of our sales and marketing and research and development efforts, capital expenditures, and potential acquisitions of, or investments in, complementary businesses, products and technologies. However, we do not have more specific plans for the net proceeds from this offering and will have broad discretion in how we use the net proceeds of this offering. If we do not invest or apply the proceeds of this offering in ways that improve our operating results, we may fail to achieve expected financial results, which could cause our stock price to decline.

After the completion of this offering, we do not anticipate declaring any cash dividends to holders of our common stock in the foreseeable future. Consequently, investors must rely on sales of their common stock after price appreciation, which may never occur, as the only way to realize any future gains on their investment. Investors seeking cash dividends should not purchase our common stock.

Provisions in our certificate of incorporation and bylaws, as amended and restated upon the closing of this offering, may have the effect of delaying or preventing a change of control or changes in our management. Our amended and restated certificate of incorporation and amended and restated bylaws to become effective upon completion of this offering include provisions that:

In addition, we are subject to the provisions of Section 203 of the Delaware General Corporation Law regulating corporate takeovers. Section 203 generally prohibits us from engaging in a business combination with an interested stockholder subject to certain exceptions.

For information regarding these and other provisions, please see the section titled “Description of Capital Stock” below.

This prospectus includes forward-looking statements that relate to future events or our future financial performance and involve known and unknown risks, uncertainties and other factors that may cause our actual results, levels of activity, performance or achievements to differ materially from any future results, levels of activity, performance or achievements expressed or implied by these forward-looking statements. Words such as “believe,” “expect,” “anticipate,” “estimate,” “intend,” “plan,” “likely,” “will,” “would,” “could” and similar expressions or phrases identify these forward-looking statements.

All forward-looking statements involve risks and uncertainties. The occurrence of the events described, and the achievement of the expected results, depend on many events, some or all of which are not predictable or within our control. Actual results may differ materially from expected results.

Factors that may cause actual results to differ from expected results include:

See the section above titled “Risk Factors” for a more complete discussion of these risks and uncertainties and for other risks and uncertainties. These factors and the other risk factors described in this prospectus are not necessarily all of the important factors that could cause our actual results to differ materially from those expressed in any of our forward-looking statements. Other unknown or unpredictable factors also could harm our results. Consequently, actual results or developments anticipated by us may not be realized or, even if substantially realized, may not have the expected consequences to, or effects on, us. Given these uncertainties, we caution you not to place undue reliance on such forward-looking statements.

All future written and verbal forward-looking statements attributable to us or any person acting on our behalf are expressly qualified in their entirety by the cautionary statements contained or referred to in this section. We undertake no obligation to update publicly or revise any forward-looking statements, whether as a result of new information, future events or otherwise. In light of these risks, uncertainties and assumptions, the forward-looking events discussed in this prospectus might not occur.

We estimate that our net proceeds from this offering will be approximately $56.7 million, assuming an initial public offering price of $8.00 per share, which is the midpoint of the range set forth on the cover page of this prospectus, and after deducting underwriting discounts and commissions and estimated offering expenses. Each $1.00 increase or decrease in the assumed initial public offering price of $8.00 per share would increase or decrease, as applicable, the net proceeds to us by approximately $7.5 million, assuming the number of shares offered by us, as set forth on the cover page of this prospectus, remains the same and after deducting the underwriting discounts and commissions payable to us. If the underwriters’ option to purchase additional shares in this offering is exercised in full, we estimate that our net proceeds will be approximately $72.4 million. We will not receive any proceeds from the sale of shares of our common stock by the selling stockholders.

We intend to use the net proceeds to us from this offering for working capital and other general corporate purposes, including to finance our growth, develop new products and fund capital expenditures. Additionally, we may expand our current business through acquisitions of other businesses, products or technologies. However, we do not have agreements or commitments for any specific acquisitions at this time.

Pending our use of the net proceeds from this offering, we intend to invest the proceeds in short-term, investment-grade interest-bearing instruments.

We have never declared nor paid cash dividends on our common stock. We currently intend to retain all available funds and any future earnings for use in the operation of our business and do not anticipate paying any dividends on our common stock in the foreseeable future. Any future determination to declare dividends will be made at the discretion of our board of directors and will depend on our financial condition, operating results, capital requirements, general business conditions and other factors that our board of directors may deem relevant.

The following table sets forth our capitalization as of March 31, 2008:

The information set forth in the table should be read together with the information set forth under “Management’s Discussion and Analysis of Financial Condition and Results of Operations,” and our consolidated financial statements and accompanying notes, each appearing elsewhere in this prospectus.

Our net tangible book value as of March 31, 2008 was $21.4 million, or approximately $.54 per share. Net tangible book value per share represents the amount of total tangible assets, less our total liabilities, divided by 39,838,908 shares of common stock outstanding.

Net tangible book value dilution per share to new investors represents the difference between the amount per share paid by purchasers of shares of common stock in this offering and the as adjusted net tangible book value per share of common stock immediately after completion of this offering. After giving effect to our sale of 8,078,566 shares of common stock in this offering at an assumed initial public offering price of $8.00 per share, which is the midpoint of the range listed on the cover page of this prospectus, and after deducting the underwriting discounts and commissions and estimated offering expenses, our net tangible book value as of March 31, 2008 would have been $78.2 million, or $1.63 per share. This represents an immediate increase in net tangible book value of $1.09 per share to existing stockholders and an immediate decrease in net tangible book value of $6.37 per share to purchasers of common stock in this offering, as illustrated in the following table:

A $1.00 increase or decrease in the assumed initial public offering price of $8.00 would increase or decrease, as applicable, our as adjusted net tangible book value per share after this offering by $0.16 per share and increase or decrease, as applicable, dilution per share to new investors in this offering by $0.84 per share, assuming the number of shares offered by us, as set forth on the cover of this prospectus, remains the same and after deducting the estimated underwriting discounts and commissions and offering expenses payable by us.

If the underwriters exercise their option to purchase additional shares of our common stock in full in this offering, the net tangible book value per share after this offering would be $1.88 per share, the increase in net tangible book value per share to existing stockholders would be $1.34 per share and the decrease in net tangible book value per share to new investors purchasing shares in this offering would be $6.12 per share.

The following table presents as of March 31, 2008 the differences between the existing stockholders and the purchasers of shares in this offering with respect to the number of shares purchased from us, the total consideration paid and the average price paid per share:

The above discussion and tables assume no exercise of 1,333,308 shares of common stock issuable upon the exercise of stock options outstanding as of March 31, 2008 with a weighted average exercise price of $2.54 per share and 2,074,122 shares of common stock issuable upon the exercise of warrants outstanding as of March 31, 2008 with a weighted average exercise price of $0.52 per share. If all of these options and warrants were exercised, new investors ownership would be diluted by approximately 1% and total consideration would increase by approximately $4.5 million. In addition, if all these options and warrants were exercised, then as adjusted net tangible book value per share would decrease from $1.63 to $1.61, resulting in an increase in dilution per share to new investors in this offering to $6.39 per share.

You should read the following selected consolidated historical financial data below in conjunction with “Management’s Discussion and Analysis of Financial Condition and Results of Operations” and the consolidated financial statements, related notes and other financial information included in this prospectus. The selected financial data in this section is not intended to replace the consolidated financial statements and is qualified in its entirety by the consolidated financial statements and related notes included in this prospectus.

The selected consolidated statements of operations data for each of the three years in the periods ended December 31, 2007, 2006 and 2005 and the consolidated balance sheet data as of December 31, 2007 and 2006 are derived from our audited consolidated financial statements and related notes included elsewhere in this prospectus, and the selected consolidated statements of operations data for each of the two years ended December 31, 2004 and 2003 and the consolidated balance sheet data as of December 31, 2005, 2004 and 2003 are derived from our audited consolidated financial statements and related notes not included in this prospectus. The consolidated statement of operations data for the three months ended March 31, 2008 and 2007 and the consolidated balance sheet data at March 31, 2008 are derived from our unaudited consolidated financial statements included in this prospectus. The unaudited consolidated financial statements include, in the opinion of management, all adjustments that management considers necessary for the fair presentation of the financial information set forth in those statements. Our historical results are not necessarily indicative of the results that should be expected in the future and results for the three months ended March 31, 2008 are not necessarily indicative of results to be expected for the full year. The amounts below are in thousands, except per share data.

The following discussion and analysis of our financial condition and results of operations should be read in conjunction with our consolidated financial statements and related notes included elsewhere in this prospectus. This discussion contains forward-looking statements that involve risks and uncertainties. Our actual results could differ materially from those discussed below. Factors that could cause or contribute to such differences include, but are not limited to, those identified below, and those discussed in the section titled “Risk Factors” included elsewhere in this prospectus.

We were founded in 1992 and are in the business of designing, developing and manufacturing energy recovery devices for sea water reverse osmosis, or SWRO, desalination plants. In early 1997, we introduced the initial version of our energy recovery device, the PX. In November 1997, we introduced and marketed our first ceramic-based PX device. As of March 31, 2008, we had shipped over 4,000 PX devices to desalination plants worldwide, including in China, Europe, India, Australia, Africa, the Middle East, North America and the Caribbean.

A majority of our net revenue has been generated by sales to large engineering, procurement and construction firms, or EPCs, who are involved with the design and construction of larger desalination plants. Sales to EPCs often involve a long sales cycle, or the time between the initial project tender and the time the PX device is shipped to the client, which can range from six to 16 months. A single EPC desalination project can generate an order for numerous PX devices and generally represents an opportunity for significant revenue. We also sell PX devices to original equipment manufacturers, or OEMs, which commission smaller desalination plants, order fewer PX devices per plant and have shorter sales cycles.

Due to the fact that a single order for PX devices by an EPC for a particular plant may represent significant revenue, we often experience significant fluctuations in net revenue from quarter to quarter. In addition, our EPC customers tend to order a significant amount of equipment for delivery in the fourth quarter and, as a consequence, a significant portion of our annual sales typically occurs during that quarter.

A limited number of our EPC customers accounts for a substantial portion of our net revenue. One EPC customer accounted for approximately 49% of our net revenue and two EPC customers accounted for approximately 48% of our net revenue for the three months ended March 31, 2008 and March 31, 2007, respectively. Specifically, Geida and its affiliated entities accounted for approximately 49% of our net revenue for the three months ended March 31, 2008 and Inima Servicios and Geida and its affiliated entities accounted for approximately 26% and 22% of our net revenue, respectively, for the three months ended March 31, 2007. In 2007, three EPC customers, including their affiliated entities, accounted for 56% of our net revenue, and in 2006, two EPC customers, including their affiliated entities, accounted for 29% of our net revenue. Specifically, Acciona Water, Geida and its affiliated entities and Doosan Heavy Industries represented approximately 20%, 23% and 13% of our net revenue in 2007, respectively, and GE Ionics and Geida and its affiliated entities accounted for approximately 18% and 11% of our net revenue in 2006, respectively. In 2005, GE Ionics and Multiplex Degremont JV accounted for 19% and 17% of our net revenue, respectively. We do not have long-term contracts with our EPC customers and instead sell to them on a purchase order basis or under individual stand-alone contracts. Orders may be postponed or delayed by our customers on short or no notice.

In the three months ended March 31, 2008 and the years ended 2007 and 2006 most of our revenue was attributable to sales outside of the United States. We expect sales outside of the United States to remain a significant portion of our revenue for the foreseeable future.

Our revenue is principally derived from the sales of our PX devices. We receive a small amount of revenue from the sale of booster pumps, which we manufacture and sell in connection with PX devices to smaller desalination plants. We also receive incidental revenue from services, such as product support, that we provide to our PX customers.

Our consolidated financial statements are prepared in accordance with generally accepted accounting principles in the United States, or GAAP. These accounting principles require us to make estimates and judgments that can affect the reported amounts of assets and liabilities as of the date of the consolidated financial statements as well as the reported amounts of revenue and expense during the periods presented. We believe that the estimates and judgments upon which we rely are reasonable based upon information available to us at the time that we make these estimates and judgments. To the extent there are material differences between these estimates and actual results, our consolidated financial results will be affected. The accounting policies that reflect our more significant estimates and judgments and which we believe are the

We recognize revenue in accordance with SEC Staff Accounting Bulletin No. 104, Revenue Recognition. Revenue is recognized when the earnings process is complete, as evidenced by an agreement with the customer, transfer of title occurs, fixed pricing is determinable and collection is probable. Transfer of title typically occurs upon shipment of the equipment pursuant to a written purchase order or contract. Emerging Issues Task Force No. 00-21, Revenue Arrangements with Multiple Deliverables requires us to allocate the purchase price between the device and the value of the undelivered services by applying the residual value method. Under this method, revenue allocated to undelivered elements is based on vendor-specific objective evidence of fair value of such undelivered elements, and the residual revenue is allocated to the delivered elements. Vendor specific objective evidence of fair value for such undelivered elements is based upon the price we charge for such product or service when it is sold separately. We may modify our pricing practices in the future, which could result in changes to our vendor specific objective evidence of fair value for such undelivered elements. Our purchase agreements typically provide for the provision by us of field services and training for commissioning of a desalination plant. Recognition of the revenue in respect of those services is deferred until provision of those services is complete. The services element of our contracts represent an incidental portion of the total contract price.

Under our revenue recognition policy, evidence of an arrangement has been met when we have an executed purchase order or a standalone contract. Typically, our smaller projects utilize purchase orders that conform to our standard terms and conditions that require the customer to remit payment generally within 30 to 90 days from product delivery. In some cases, if credit worthiness cannot be determined, prepayment is required from the smaller customers.

For our large projects, stand-alone contracts are utilized. For these contracts, consistent with industry practice, the customers typically require their suppliers, including our company, to accept contractual holdback provisions whereby the final amounts due under the sales contract are remitted over extended periods of time. These retention payments typically range between 10% and 20%, and in some instances up to 30%, of the total contract amount and are due and payable when the customer is satisfied that certain specified product performance criteria have been met upon commissioning of the desalination plant, which in the case of our PX device may be 12 months to 24 months from the date of product delivery as described further below.

The specified product performance criteria for our PX device generally pertains to the ability of our products to meet our published performance specifications and warranty provisions, which our products have demonstrated on a consistent basis. This factor, combined with our historical performance metrics measured over the past 10 years, provides us with a reasonable basis to conclude that the PX device will perform satisfactorily upon commissioning of the plant. To help ensure this successful product performance, we provide service, consisting principally of supervision of customer personnel, and training to the customers during the commissioning of the plant. The installation of the PX device is relatively simple, requires no customization and is performed by the customer under the supervision of our personnel. We defer the fair value of the service and training component of the contract and recognize such revenue as services are rendered. Based on these factors, we have concluded that delivery and performance have been completed when the product has been delivered (title transfers) to the customer.

We perform an evaluation of credit worthiness on an individual contract basis to assess whether collectibility is reasonably assured. As part of this evaluation, we consider many factors about the individual customer, including the underlying financial strength of the customer and/or partnership consortium and our prior history or industry specific knowledge about the customer and its supplier relationships. To date, we have been able to conclude that collectibility was reasonably assured on our sales contracts at the time the product was delivered and title has transferred; however, to the extent that we conclude that we are unable to determine that collectibility is reasonably assured at the time of product delivery, we will defer all or a portion of the contract amount based on the specific facts and circumstances of the contract and the customer.

Under the stand-alone contracts, the usual payment arrangements are summarized as follows:

Under the terms of the retention payment component, we are generally required to issue to the customer a product performance guarantee in the form of a collateralized letter of credit, which is issued to the customer approximately 12 to 24 months after the product delivery date. The letter of credit is collateralized by restricted cash on deposit with our financial institution (see Restricted Cash under “Summary of Significant Accounting Policies”). The letter of credit remains in place for the performance period as specified in the contract, which is generally 24 months and which runs concurrent with our standard product warranty period. Once the letter of credit has been put in place, we invoice the customer for this final retention payment under the sales contract. During the time between the product delivery and the issuance of the letter of credit, the amount of the final retention is classified on the balance sheet as unbilled receivable, of which a portion may be classified as long term to the extent that the billable period extends beyond one year. Once the letter of credit is issued, we invoice the customer and reclassify the retention amount from unbilled receivable to accounts receivable where it remains until payment, typically 120 to 150 days after invoicing (see Note 3—Balance Sheet Information: Unbilled Receivables).

Shipping and handling charges billed to customers are included in sales. The cost of shipping to customers is included in cost of revenue.

We do not provide our customers with a right to return our products. However, we accept returns of products that are deemed to be damaged or defective when delivered, subject to the provisions of the product warranty. Historically, product returns have not been significant.

We sell our products to EPC companies that are not subject to sales tax. Accordingly, the adoption of EITF Issue No. 06-3, How Taxes Collected from Customers and Remitted to Governmental Authorities Should Be Presented in the Income Statement (That is, Gross versus Net Presentation), does not have an impact on our consolidated financial statements.

We sell products with a limited warranty for a period of one to two years. In August 2007, we modified the warranty to offer a five-year term on the ceramic components for new sales agreements executed after August 7, 2007. We accrue for warranty costs based on estimated product failure rates, historical activity and expectations of future costs. We periodically evaluate and adjust the warranty costs to the extent actual warranty costs vary from the original estimates.

We may offer extended warranties on an exception basis and these are accounted for in accordance with Financial Accounting Standards Board Technical Bulletin 90-1, Accounting for Separately Priced Extended Warranty and Product Maintenance Contracts for Sales of Extended Warranties.

Prior to January 1, 2006, we accounted for stock-based employee compensation arrangements in accordance with the provisions of Accounting Principles Board Opinion No. 25, Accounting for Stock Issued to Employees, or APB 25, and FASB Interpretation No. 44, Accounting for Certain Transactions Involving Stock Compensation, an Interpretation of APB Opinion No. 25, or FIN 44, and had adopted the disclosure provisions of Statement of Financial Accounting Standards No. 123, Accounting for Stock-Based Compensation, or SFAS 123, and SFAS No. 148, Accounting for Share-Based Compensation—Transition and Disclosure, or SFAS 148.

In February 2005, we offered to each of our employees the option to borrow from us an amount equal to the aggregate exercise price for all of their outstanding options pursuant to full recourse promissory notes at 3.76% interest, which are due in February 2010. The interest rate on the notes was deemed to be below market rate, resulting in a change in the deemed exercise price for the options. As a result, we are accounting for these options as variable option awards. For the three months ended March 31, 2008 and March 31, 2007, we recorded $135,000 and $195,000, respectively, of stock-based compensation related to the options exercised with promissory notes. For 2007, 2006 and 2005, we recorded $783,000, $1.1 million and $1.0 million, respectively, of stock-based compensation related to the options exercised with promissory notes. All of our executive officers and directors have subsequently repaid their notes.

Effective January 1, 2006, we adopted the fair value recognition provisions of SFAS No. 123(R), Share-Based Payment, using the prospective transition method, which requires us to apply the provisions of SFAS 123(R) only to awards granted, modified, repurchased or cancelled after the adoption date. Upon adoption of SFAS 123(R), we selected the Black-Scholes option pricing model as the most appropriate method for determining the estimated fair value for stock-based awards. The Black-Scholes model requires the use of highly subjective and complex assumptions to determine the fair value of stock-based awards, including the option’s expected term and the price volatility of the underlying stock. The value of the portion of the award that is ultimately expected to vest is recognized as expense over the requisite vesting period on a straight-line basis in our consolidated statements of operations and the expense is reduced for estimated forfeitures. SFAS 123(R) requires forfeitures to be estimated at the time of grant and revised, if necessary, in subsequent periods if

To determine the inputs for the Black-Scholes option pricing model, we are required to develop several assumptions, which are highly subjective. These assumptions include:

We use comparable public company data to determine volatility, as our common stock has not yet been publicly traded. We use a weighted average calculation to estimate the time our options will be outstanding as prescribed by Staff Accounting Bulletin No. 107, Share-Based Payment. We estimate the number of options that are expected to be forfeited based on our historical experience and expected future forfeiture patterns. The risk-free rate is based on the U.S. Treasury yield curve in effect at the time of grant for the estimated life of the option. We use our judgment and expectations in setting future dividend rates, which is currently expected to be zero.

The absence of an active market for our common stock also requires our management and board of directors to estimate the fair value of our common stock for purposes of granting options and for determining stock-based compensation expense. In response to these requirements, our management and board of directors estimate the fair market value of common stock on an annual basis, based on factors such as the price of the most recent common stock sales to investors, the valuations of comparable companies, the status of our development and sales efforts, our cash and working capital amounts, revenue growth and additional objective and subjective factors relating to our business.

The following table shows the stock option grants during 2007 and the three months ended March 31, 2008:

In 2007, our board of directors determined that the fair market value of common stock for options granted that year was $5.00 per share. The fair value of the common stock for options granted was estimated by our board of directors with input from management and by reference to our stock price in conjunction with the sale of 1,000,000 shares of our common stock at $5.00 per share in a private placement to third parties in May 2007. In March 2008, we retained Finance Scholars Group, or FSG, an independent valuation firm, to prepare independent analyses of the value of our common stock for 2007, 2006 and 2005 related to the grants of options on those shares. These valuations were prepared in conformity with Uniform Standards of Professional Appraisal Practice using standard methodologies for valuing options. FSG’s analysis used the discounted cash flow methodology as well as trading multiples of companies in related industries based on the comparability of revenue and cash generation to estimate the fair value of the options as of each valuation date. For the trading multiples, five publicly-traded companies in related industries were selected based on FSG’s own research as well as information provided by our investment bankers. Because EBITDA multiples were more variable and less reliable than revenue multiples due to negative cash flow in some periods for several of the selected comparable companies, FSG relied on revenue multiples as a basis of comparison. For the discounted cash flow valuation, the projected cash flows were discounted at a rate that reflected the trading variability of similar companies, risk-free bond returns, equity risk and specific risks related to our company and industry as of each valuation date. The discounted cash flow methodology was used as confirming evidence of the reasonableness of the trading multiple estimates. For 2005, FSG relied on only trading multiples for the selected comparable companies as there were no available contemporaneous cash flow projections. For 2006, FSG used both discounted cash flow and the trading multiples for the selected comparable companies to determine values for the options. For 2007, FSG used pricing from our private placement of common stock in May 2007, the cash flow projections contained in the related private placement memorandum and trading multiples for the selected comparable companies. The concluded estimate of market value of shares in each year was adjusted for the lack of marketability by using discounts to reflect their lack of liquidity. FSG’s conclusion was that as of June 30, 2007, 2006 and 2005, the fair market value of our common stock

Based on the estimated initial public offering price of $8.00 per share, which is the mid-point of the price range listed on the cover page of this prospectus, the aggregate intrinsic value of options outstanding as of March 31, 2008 was $7.3 million, of which $3.0 million related to vested options and $4.3 million related to unvested options.

For options granted during 2007 and the three months ended March 31, 2008, we determined the fair value at date of grant using the Black-Scholes option pricing model. The following table summarizes the assumptions used in determining the fair value of stock options granted.

We account for equity instruments issued in exchange for the receipt of goods or services from non-employees in accordance with the consensus reached by the Emerging Issues Task Force, or EITF, in Issue No. 96-18, Accounting for Equity Instruments That Are Issued to Other Than Employees for Acquiring, or in Conjunction with Selling, Goods or Services. Costs are measured at the fair market value of the consideration received or the fair value of the equity instruments issued, whichever is more reliably measurable. The value of equity instruments issued for consideration other than employee services is determined on the earlier of the date on which there first exists a firm commitment for performance by the provider of goods or services or on the date performance is complete, using the Black-Scholes pricing model.

Inventories are stated at the lower of cost (using the weighted average cost method) or market. We calculate inventory reserve for excess and obsolete inventories based on estimated future demand of the products and spare parts. Cost of inventory is determined in accordance with Statement of Financial Accounting Standards No. 151, Inventory Costs, an amendment of ARB No. 43, Chapter 4, or SFAS 151.

We record a provision for doubtful accounts based on our historical experience and a detailed assessment of the collectability of our accounts receivable. In estimating the allowance for doubtful accounts, our management considers, among other factors, (1) the aging of the accounts receivable, (2) our historical write-offs, (3) the credit worthiness of each customer and (4) general economic conditions. Our allowance for doubtful accounts was $107,000, $121,000, $230,000 and $150,000 at March 31, 2008 and December 31, 2007, 2006 and 2005, respectively. If we were to experience unanticipated collections issues, it could have an adverse affect on our operating results in future periods.

We account for income taxes in accordance with SFAS No. 109, Accounting for Income Taxes, or SFAS 109, issued by the Financial Accounting Standards Board, or FASB. SFAS 109 requires an entity to recognize deferred tax liabilities and assets. Deferred tax assets and liabilities are recognized for the future tax consequence attributable to the difference between the tax bases of assets and liabilities and their reported amounts in the financial statements. Deferred tax assets and liabilities are measured using the enacted tax rate expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that included the enactment date. Valuation allowances are provided if, based upon the available evidence, management believes it is more likely than not that some or all of the deferred assets will not be realized or the use of prior years’ net operating losses may be limited.

On July 13, 2006, the FASB issued Interpretation No. 48, Accounting for Uncertainty in Income Taxes – An Interpretation of FASB Statement No. 109, or FIN 48. FIN 48 clarifies the accounting for uncertainty in income taxes recognized in any entity’s financial statements in accordance with SFAS 109 and prescribes a recognition threshold and measurement attributes for financial statement disclosure of tax positions taken or expected to be taken on a tax return. Under FIN 48, the impact of an uncertain income tax position on the income tax return must be recognized at the largest amount that is more likely than not to be sustained upon audit by the relevant taxing authority. An uncertain income tax position will not be recognized if it has less than a 50% likelihood of being sustained. Additionally, FIN 48 provides

We adopted the accounting policy that interest recognized in accordance with Paragraph 15 of FIN 48 and penalty recognized in accordance with Paragraph 16 of FIN 48 are classified as part of income taxes. The amounts of interest and penalty recognized in the statement of operations and statement of financial position for 2007 were insignificant.

Our operations are subject to income and transaction taxes in the United States and in foreign jurisdictions. Significant estimates and judgments are required in determining our worldwide provision for income taxes. Some of these estimates are based on interpretations of existing tax laws or regulations. The ultimate amount of tax liability may be uncertain as a result.

We are subject to taxation in the U.S. and various states and foreign jurisdictions. There are no ongoing examinations by taxing authorities at this time. Our various tax years from 1997 through 2007 remain open in various taxing jurisdictions.

The following table sets forth certain data from our historical operating results as a percentage of revenue for the years indicated:

Net revenue is reported net of volume discounts. We derive our revenue principally from sales of our PX devices. Our net revenue increased by $2.0 million, or 28%, to $9.1 million in the three months ended March 31, 2008 from $7.1 million in the three months ended March 31, 2007. These increases were principally due to higher sales of our PX-220 device, which resulted primarily from increased market acceptance of the device and the overall growth of the desalination market. Prices were relatively constant for our PX devices in the three months ended March 31, 2008, 2007 and 2006. In the three months ended March 31, 2008, the sales of PX devices accounted for approximately 91% of our revenue increase, with pump sales accounting for approximately 6% of the increase. In the three months ended March 31, 2007, the sales of PX devices accounted for approximately 93% of the increase, with pump sales accounting for approximately 3% of the increase and spare parts and services accounting for the remainder of the increase.

Gross profit represents our net revenue less our cost of revenue. Our cost of revenue consists primarily of raw materials, personnel costs (including stock-based compensation), manufacturing overhead, warranty costs, capital costs, excess and obsolete inventory expense, and manufactured components. The largest component of our cost of revenue is raw materials, principally ceramic materials, which we obtain from several suppliers. Gross profit, as a percentage of net revenue, remained relatively constant at 60% in the three months ended March 31, 2008 from the three months ended March 31, 2007. Stock compensation expense included in cost of revenue was $24,000 in the three months ended March 31, 2008 and $25,000 in the three months ended March 31, 2007.

Sales and marketing expense consists primarily of personnel costs (including stock-based compensation), sales commissions, marketing programs and facilities cost associated with sales and marketing activities. Sales and marketing expense increased by $152,000, or 13%, to $1.3 million in the three months ended March 31, 2008 from $1.2 million in the three months ended March 31, 2007. This increase was primarily related to growth in our sales that resulted in higher headcount with sales and marketing employees increasing to 16 at March 31, 2008 from 11 at March 31, 2007. Of the $152,000 increase in sales and marketing expenses in the three months ended March 31, 2008, $31,000 of such increase related to compensation and employee related benefits, $56,000 related to consultant fees, $28,000 related to travel and office expenses and $63,000 related to sales and marketing efforts costs, offset by a $16,000 decrease to occupancy. In addition, our sales team is compensated in part by commissions, resulting in increased sales expense as our sales levels increase. Stock-based compensation expense included in sales and marketing expense was $74,000 in the three months ended March 31, 2008 and $71,000 in the three months ended March 31, 2007.

As a percentage of our net revenue, sales and marketing expense decreased to 15% in the three months ended March 31, 2008 from 17% in the three months ended March 31, 2007. The decrease in the three months ended March 31, 2008 was attributable principally to the increase in our net revenue that quarter, which grew at a higher rate than our sales and marketing expenses.

We plan to continue to invest heavily in sales and marketing by increasing the number of our sales personnel and we expect sales and marketing expenses in absolute dollars to increase in future periods. Our sales personnel are not immediately productive and therefore the increase in sales expense that we incur when we add new sales personnel is not immediately offset by increased revenue and may never result in increased revenue. The timing of our hiring of new sales personnel and the rate at which they generate incremental revenue could therefore affect our future period-to-period financial performance.

General and administrative expense consists primarily of personnel (including stock-based compensation) and facilities costs related to our executive, finance and human resources organizations, as well as fees for professional services. Professional services consist of fees for outside legal and audit services and preparation for operating as a public company.

General and administrative expense increased by $1.9 million, or 244%, to $2.7 million in the three months ended March 31, 2008 from $773,000 in the three months ended March 31, 2007. This increase reflected in part the increase in general and administrative employees to 17 at March 31, 2008 from 11 at March 31, 2007.

As a percentage of our net revenue, general and administrative expense was 29% in the three months ended March 31, 2008 and 11% in the three months ended March 31, 2007. The primary reason for the increase in general and

We expect to incur significant additional accounting and legal costs after this offering related to compliance with rules and regulations implemented by the SEC and NASDAQ, as well as additional insurance, investor relations and other costs associated with being a public company. Consequently, we expect general and administrative expenses in absolute dollars to increase in future periods.

Research and development expenses include costs associated with the design, development, testing and enhancement of our products. Research and development expenses include employee compensation (including stock-based compensation), supplies and materials, consulting expenses, travel and facilities overhead. All research and development expenses are expensed as incurred.

Research and development expense increased by $120,000, or 31%, to $509,000 in the three months ended March 31, 2008 from $389,000 in the three months ended March 31, 2007. As a percentage of our net revenue, research and development expense increased to 6% in the three months ended March 31, 2008 from 5% in the three months ended March 31, 2007.

Compensation and employee-related benefits accounted for $88,000 of the increase, while consulting and legal services and research and development accounted for another $67,000 of the $120,000 increase from the three months ended March 31, 2007 to the three months ended March 31, 2008. Headcount in our research and development department increased to eight at March 31, 2008 from six at March 31, 2007. The foregoing increases were offset by net expense decreases totaling $35,000 in travel related expenses. Stock-based compensation expense included in research and development expense was $33,000 for the three months ended March 31, 2008 and $35,000 for the three months ended March 31, 2007.

We believe that continued spending on research and development to develop new PX devices and other products is critical to our success and, consequently, we expect to increase research and development expenses in absolute dollars in future periods.

Other income (expense), net includes interest income on cash balances and losses or gains on conversion of non-United States dollar transactions into United States dollars. Our losses or gains on currency conversions have not been material to date because our international sales have been denominated principally in United States dollars, and our foreign currency exposure risk has been limited to expense incurred in our overseas operations. If we are successful in increasing our international sales we may be subject to currency conversion risks because some of the international sales could be denominated in foreign currencies. We have historically invested our available cash balances in money market funds, short-term United States Treasury obligations and commercial paper.

Other income (expense), net increased by $629,000 to $626,000 in the three months ended March 31, 2008 from $(3,000) in the three months ended March 31, 2007. The increase in net interest and other income from the three months ended March 31, 2007 to the three months ended March 31, 2008 was primarily attributable to gains on foreign currency transactions of $619,000 in the three months ended March 31, 2008 and higher average cash balances, which resulted in higher interest income in the three months ended March 31, 2008 of $29,000, versus $15,000 in the three months ended March 31, 2007.

Our net revenue increased by $15.4 million, or 77%, to $35.4 million in 2007 from $20.1 million in 2006, and by $9.4 million in 2006, or 88%, from $10.7 million in 2005. These increases were principally due to higher sales of our PX-

The following geographic information includes net revenue to our domestic and international customers based on the customers’ requested delivery locations, except for certain cases in which the customer directed us to deliver our products to a location that differs from the known ultimate location of use. In such cases, the ultimate location of use is reflected in the table below instead of the delivery location. The amounts below are in thousands, except percentage data.

Gross profit represents our net revenue less our cost of revenue. Our cost of revenue consists primarily of raw materials, personnel costs (including stock-based compensation), manufacturing overhead, warranty costs, capital costs, excess and obsolete inventory expense, and manufactured components. The largest component of our cost of revenue is raw materials, principally ceramic materials, which we obtain from several suppliers. Gross profit, as a percentage of net revenue, remained relatively constant at 58% in 2007 as compared to 59% in 2006 and 56% in 2005. Stock compensation expense included in cost of revenue was $117,000 in 2007, $143,000 in 2006 and $88,000 in 2005.

Sales and marketing expense increased by $1.6 million, or 43%, to $5.2 million in 2007 from $3.6 million in 2006, and by $1.9 million in 2006, or 105%, from $1.8 million in 2005. These increases were primarily related to growth in our sales that resulted in higher headcount with sales and marketing employees increasing to seven at December 31, 2007 from six at December 31, 2006 and four at December 31, 2005. In addition, our sales team is compensated in part by commissions, resulting in increased sales expense as our sales levels increase.

As a percentage of our net revenue, sales and marketing expense decreased to 15% in 2007 from 18% in 2006 and 17% in 2005. The decrease in 2007 was attributable principally to the significant increase in our net revenue that year, which grew at a greater rate than our sales and marketing expenses.

With respect to the $1.6 million increase in sales and marketing expenses in 2007, $734,000 of such increase related to compensation and employee related benefits, $259,000 related to consultant fees, $249,000 related to travel and related expenses, $151,000 related to increased occupancy costs and $125,000 related to sales and marketing efforts. From 2005 to

General and administrative expense increased by $927,000, or 28%, to $4.3 million in 2007 from $3.4 million in 2006, and by $915,000 in 2006, or 37%, from $2.5 million in 2005. These increases reflected in part the increase in general and administrative employees to 13 at December 31, 2007 from eight at December 31, 2006 and from six at December 31, 2005.

As a percentage of our net revenue, general and administrative expense was 12% in 2007, 17% in 2006 and 23% in 2005. The decrease of general and administrative expense as a percentage of net revenue was attributable principally to the significant increases in our net revenue.

The primary reason for the increase in general and administrative expenses was the growth in our operations that resulted in higher headcount including the recruitment of an officer, renting of additional facility space, increased travel and increased bank fees. With respect to the $927,000 increase in such expenses in 2007, $513,000 related to compensation, employee-related benefits and professional services fees, $139,000 related to bank charges, $46,000 related to office supplies and equipment, $89,000 related to occupancy costs, and $349,000 related to other expenses (general recruiting, patent amortization and travel), offset by $184,000 related to bad debt. With respect to the $915,000 increase in 2006, $870,000 related to compensation, employee-related benefits and professional service fees. Stock based compensation expense included in general and administrative expense was $388,000 in 2007, $428,000 in 2006 and $731,000 in 2005.

Research and development expense increased by $438,000, or 35%, to $1.7 million in 2007 from $1.3 million in 2006, and by $637,000 in 2006, or 101%, from $630,000 in 2005. As a percentage of our net revenue, research and development expense decreased to 5% in 2007, from 6% in 2006 and in 2005.

Compensation, employee-related benefits, consulting services and depreciation of development equipment accounted for $151,000 of the $438,000 increase from 2006 to 2007. The remainder of the increase in 2007 was primarily attributable to $173,000 in product development costs and $98,000 in travel expense. Compensation, employee-related benefits, consulting services and depreciation of development equipment accounted for $413,000 of the $637,000 increase from 2005 to 2006. Stock-based compensation expense included in research and development expense was $159,000 in 2007, $183,000 in 2006 and $98,000 in 2005.

Other income (expense), net increased by $432,000 to $413,000 in 2007 from $(19,000) in 2006, and decreased by $162,000 to $(19,000) in 2006 from $(182,000) in 2005. The increase in net interest and other income from 2006 to 2007 was primarily attributable to gains on foreign currency transactions of $355,000 in 2007 and higher average cash balances, which resulted in higher interest income in 2007. The decrease in net interest expense from 2005 to 2006 was primarily attributable to a reduction in the use of the line of credit and associated interest expense due to increased profitability.

The following table sets forth our unaudited quarterly consolidated statement of operations data for each of our eight fiscal quarters in the period ended March 31, 2008. The quarterly data have been prepared on the same basis as the audited consolidated financial statements included elsewhere in this prospectus, and reflect all adjustments, consisting only of normal recurring adjustments, necessary for a fair presentation of this information. Our results for these quarterly periods are not necessarily indicative of the operating results for a full year or any future period.