ISI: Desalination Consulting, Design Services and Equipment

Desalination consulting, design services and equipment

Capacity:  How much water is required?
- The capacity of a system is usually defined in US gallons per day (USgpd) or cubic meters per day.
- The determination of capacity should take into consideration future water use, losses, and unexpected downtime.
- The more critical the water supply, the more units that should be considered. For instance, if 1000 m3/d are required, it would be prudent to design the water supply with (4) 250 m3/d systems, so that any downtime experienced by a single unit will have less effect on the overall production. In general, fewer units = less cost, more units = more reliability.
- When determining the capacity of the system, but sure to allow for future expansion of the system and/or infrastructure. Future expansion can be easily accomplished if it is accommodated in the initial design with oversized electrical supply and oversized process piping. If these are not taken into account, future expansion can be costly.

Product Quality:   What final quality of the water is required?
- Water quality is usually defined as Total Dissolved Solids (TDS)
- A typical potable water requirement requires that the plant provide water with less than 500 ppm TDS. For example, if a lower TDS is required, the system may have to be designed with a second pass. A second pass can reduce the water quality to <50 ppm or lower, however the additional equipment costs more and requires additional power and operating costs. Other water quality parameters used to measure quality include pH, boron, conductivity, chlorination, alkalinity and other parameters. These quality requirements are sometimes guaranteed by the equipment manufacturer.

Feed Water Quality Testing:   What is the quality of the water source?
Testing the quality of your water source is a critical step in the design of your desalination system, as it determines the type and size of necessary pre-treatment measures. The following tests should be performed:

- Feed Water Analysis
A Feed Water Analysis measures the water chemistry of the raw unfiltered water you wish to treat. Determining this information is very important to the success of your system, as desalination pumps and membranes react differently to different chemical constituents. The following parameters should be included in a Feed Water Analysis, measured in mg/l as ion:

NH4	Ammonia	NO3	Nitrate
K	Potassium	Cl	Chloride
Na	Sodium	F	Fluoride
Mg	Magnesium	SO4	Sulfate
Ca	Calcium	Boron	Boron
Sr	Strontium	SiO2	Silicon
Ba	Barium	CO2	Carbon Dioxide
CO3	Carbonate	TDS	Total Dissolved Solids
HCO3	Bicarbonate	pH and water temperature are also measured

- Silt Density Index (SDI)
SDI is a measurement of the fouling potential of suspended solids and materials in feed water. With this test, a filter is exposed to the feed water under pressure and filtration rates are calculated and recorded. Long term operation of membrane systems require an SDI of less than 3.0. RO systems can operate at SDI values up to 5.0, but reduced membrane life can be expected due to fouling.

- Turbidity
Turbidity is a measurement of the amount of suspended solids in water, sometimes described as "cloudiness". Feed water is exposed to a light-measuring instrument called a nephelometer, with particle density measured in NTU's. In general, an NTU reading of < 1.0 is the allowable standard for drinking water.

Pretreatment System:  How will you get the water?
The Pretreatment System is a critical plant component that conditions the raw water so that it is suitable for the reverse osmosis system. Pre-treatment systems are designed based on feed water quality test results, to ensure that the SDIs and other measurements meet the membrane manufacturer's requirements.

- Intakes There are 2 different intake arrangements available for RO systems: open intakes and closed intakes. An open intakes system uses a channel or pipe that extends into an open body of water. These systems are employed where flows are large or where drilling for water is not feasible. Open intakes require a robust pretreatment system due to the fact that open water sources are subject to silt, debris, oil, turbulence, temperature variability and other biological effects.

Closed intakes refer in general to drilled boreholes or wells. Drilled boreholes can source seawater or brackish water. Water quality is generally better from a closed intake system and there are fewer variable influences.

Fouling:   What steps can be taken to prevent problems?
Fouling occurs when debris accumulates on the membrane surface, leading to loss of production and costly membrane replacement. Inorganic fouling is the result of process parameters that did not accommodate various chemical characteristics of the intake water. Organic fouling is the result of high nutrient water that provides nourishment for bacteria slimes to grow in the system. Potential types of fouling are very specific to each site and must be accommodated on a case by case basis. The effects of fouling can be minimized by thoroughly understanding the raw intake water chemistry and designing the system to avoid problems in the first place.

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