For plants larger than 450,000 GPD (1,704 m3/d), ISI builds custom-designed desalination systems.
The Production Capacity of a desalination plant is a key parameter and refers to the amount of water that the plant can supply in one day, or 24 hour period. The standard units for Production Capacity include gallons per day (USgpd) and cubic meters per day (m3/d). Other Production Capacity unit examples are Imperial gallons per day and tons per day. The Proposal Generator requires that this field be completed by choosing the correct radio button. Note that the Production Capacity is not guaranteed because membranes are influenced by site specific parameters such as salinityThe amount of salt in your water., temperature, and conversion. If the client requires a guaranteed production rate, it is necessary to submit to our engineering staff a water analysis that details the maximum and minimum conditions for the incoming feed water. If this is not available, we would recommend that the next larger size system be specified. ISI provides systems in many different production capacities, with standard systems ranging from 60,000gpd (227 m3/d) through 450, 000gpd (1514 m3/d). For example, a 200,000gpd standard salinityThe amount of salt in your water. system would be designated as a 200S. See specification data sheets for detailed information on all plant sizes.
To achieve capacities greater than 450,000 gpd (1514 m3/d), multiple systems can be selected in Step 3. For example, to achieve 600,000gpd (2270 m3/d), a 300,000 gpd system could be selected, then a quantity of (2) could be entered in step 3.
While the costs are typically higher, there are advantages to using multiple smaller systems rather than a single large system. Having multiple small systems allows for easier maintenance, since one system can be running while the other is being maintained. Similarly, in cases where the system demand is variable, or where storage capacity is marginal, one large system would experience a lot of starts and stops as it tried to match demand, while if multiple smaller systems are used, they could more closely match demand and minimize the number of starts and stops. Finally, having multiple smaller systems can provide better reliability, since if one system experiences an unforeseen failure that forces it to shutdown, there would still be other systems available to provide water for critical needs. If one large system were to experience a similar shutdown, the customer would be completely without water.
The proposal generator allows the customer to look at multiple different scenarios to provide the same capacity and compare the cost implications.
Membrane-based desalination systems are very sensitive to feed water chemistry. In order to pick the correct system, it is necessary to choose the correct maximum salinity range that may be encountered. The standard units for salinity are Total Dissolved Solids (TDS) which is measured as mg/l or parts per million (ppm). As the TDS increases, the operating pressure of the membrane desalination system increases which requires a matching high pressure pump. In order to supply standard equipment, we have designed our systems around three TDS ranges: 30-35,000 mg/l, 35,001-40,000 mg/l and 40,001-45,000 mg/l. These are offered as Low, Standard and High, respectively because seawater varies worldwide and a single system cannot properly treat all these ranges. It is important to choose the system that includes the highest TDS for your site. If a guaranteed performance threshold is needed, it is necessary to have a water analysis of the raw incoming feed water. With a water analysis, our engineers are able to provide guaranteed performance with respect to production and final water quality.
A water analysis should include the following:
ISI offers two options for energy recovery:
1) Turbine Type ("Standard")- This option offers good efficiency, low capital cost, and high reliability. The turbine acts like a turbocharger. One side of the turbine extracts energy form the high pressure brine stream. The other side takes in the full output flow from the high pressure pump and boosts it to the pressure required by the membranes.
2) Pressure Exchanger Type ("Enhanced Energy Recovery") - This option provides the highest energy efficiency possible. It has a higher initial capital cost, but quickly pays for itself, especially in regions where local power costs are high. This option utilizes the ERI (R) pressure exchanger to exchange pressure from the brine stream directly to an equal volume flow of feedwater. A small boost pump then increases the pressure to that required by the membranes. In this option the high pressure pump flow is roughly equal to the product flow. Boost pump flow and high pressure pump flow are mixed at the membrane feed header.