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Concentrate Management

All desalination processes have two outgoing process streams – the product water which is lower in salt than the feed water, and a concentrated stream that contains the salts removed from the product water.  Even distillation has a “bottoms” solution that contains salt from the vaporized water.  The higher concentrated stream is called the “concentrate.”  The nature of the concentrate stream depends on the salinity of the feed water, the amount of product water recovered, and the purity of the product water.

To determine the volume and concentration of the two outgoing streams, a mass balance is constructed.  The recovery rate of water, the rejection rate of salt, and the input flow and concentration are needed to solve equations for the flow and concentration of the product and concentrate. 

Figure 1 illustrates the relationships between these parameters. 

Parameters highlighted in purple are unknown, Q is flow rate, C is concentration, subscript “f” is for feed, “p” is product, “c” is concentrate, Rec is the recovery and Rej is rejection, both represented as fractions. 

Depending on the recovery and rejection, the concentrate stream can range from 2 to 4.5 times higher than the feed concentration.  A mildly brackish groundwater desalination system with average rejection of 90% and water recovery of 80% would produce the higher multiplier while a seawater system with 50% recovery and 98% rejection would produce the lower multiplier. Table 1 is an example of a calculation of concentrations and flows.  Use any consistent units.  You can download the excel spreadsheet with active formulas to make your own calculations.

Table 1.  Calculation of product and concentrate flow and concentration

What to do with Concentrate?

Seawater desalination plants discharge this concentrate back to the sea.  However, the distribution system must be carefully designed to encourage mixing (see reports #98 and 132 below).  High salinity plumes can be avoided by blending the seawater concentrate with tertiary treated municipal wastewater.  If the wastewater was originally product from the desalination plant, the blended effluent should closely match seawater composition.

Managing the concentrate from inland desalination plants treating brackish surface or groundwater is a costly proposition.  Options for low salinity concentrate are to discharge to surface water through the municipal wastewater plant and landscape irrigation.  For facilities with high salinity concentrate, further treatment is needed to recover as much water as possible prior to discharging to lined evaporation ponds.  If the area is underlain be a confined deep saline aquifer, deep well injection may be an option.  Reports on all of the alternatives are listed by category below.

Reclamation Issues

1. Reclamation managed water supplies with high natural salt load: the Colorado River Basin Project, Salton Sea, San Joaquin Valley, and the San Luis Valley Project.
   
   
2. Trust lands with declining water quality

See our desalination and concentrate management research with the WateReuse Research Foundation.

Science and Technology Program Projects

Desalination and Water Purification Research Program Reports

• Multiple Management Method Evaluation
• Saline Wetlands and Landscape Irrigation
• Solar Ponds
• Secondary Precipitation and Zero Discharge
• Modeling Discharge Plumes
• Deep Well Injection
• Thermal Processes