Demonstration of Zero Discharge Desalination Technology for Concentrate Management

Zero Discharge Desalination (ZDD) technology utilizes electrodialysis metathesis (EDM) to remove troublesome ions from membrane desalination concentrate so that the solution can be further processed to increase the yield of fresh water from brackish groundwater that is difficult to treat by RO or NF alone. ZDD technology is particularly applicable to treatment of groundwater containing appreciable concentrations of scaling salts such as gypsum (CaSO₄), calcium phosphate, barium sulfate, etc. EDM, a variant of conventional electrodialysis, utilizes four ion-exchange membranes, two depleting compartments, and two concentrating compartments. The RO concentrate flows through one of the depleting compartments, and the diluate can be returned to the system as RO feed or blended with RO permeate depending on project-specific water quality requirements. A solution of NaCl flows through the second depleting compartment. Application of an electric potential to the electrodes of the EDM stack causes the anions from the RO concentrate to migrate through anion-exchange membranes where they meet Na⁺ ions from the NaCl compartment to form a concentrated salt stream rich in Na₂SO₄. Similarly, the cations from the RO concentrate meet Cl^- ions from the NaCl stream to form another concentrated salt stream rich in CaCl₂. Because Na₂SO₄ and CaCl₂ are highly soluble salts, the concentrations of these two streams can achieve very high levels with minimal scale potential. In fact, when CaSO₄ is the dominant salt in the groundwater, all of the salts removed from the groundwater can be packed into two EDM concentrate streams, each containing only about 1% of the water that was in the RO feed. Thus the ZDD process has the potential to recover 98% of the water from groundwater compared to typical yields of about 75% when the same groundwater is treated by RO alone.

The high recovery obtainable by the ZDD process is particularly beneficial in arid regions where groundwater is scarce and concentrate disposal is problematic. Typical disposal methods – dumping in a sewer, deep-well injection, evaporation ponds, or hauling by truck – are impractical, too expensive, or damaging to the environment in many locations. At the very least, the ZDD technology can reduce the costs and or size of conventional disposal methods leading to lower operational or capital costs. The ZDD technology has the advantage that the salts are in a highly concentrated solution. In this concentrated state, the salts can be separated and recovered as marketable products. In one example, the two EDM concentrate streams are combined to precipitate CaSO₄, which has potential markets for soil augmentation or the production of gypsum wallboard. The supernatant from the precipitation contains NaCl that can be recovered and reused in the EDM stack. Continuing laboratory research is directed toward recovery of useful products from the EDM concentrate streams so that even more water can be recovered to further approach Zero Discharge Desalination.

The ZDD technology has been evaluated in progressively larger pilot studies at the Brackish Groundwater National Desalination Research Facility (BGNDRF) in Alamogordo, New Mexico. From the first pilot with 1 gpm capacity to the latest with 20 gpm capacity, refinements to the process have been incorporated. Major efforts have been directed toward dealing with silica that occurs in the groundwater of that region. In the 1-gpm pilot, precipitation of silica was observed in the stream that circulated through the EDM diluate compartments and the RO concentrate. Ions were removed from that stream by the EDM, and water was removed by the RO. However, because silica is not ionized appreciably at neutral pH, the silica accumulated in that circulating stream until it became supersaturated and precipitated. Precipitation of the silica could be prevented by purging some of that stream, but the purge stream represented a loss of water from the process. The water loss was reduced by treating the purge stream to remove silica and then returning the liquid to the process. Although the silica removal was successful, it added complexity and chemical cost to the ZDD process. The ultimate solution was to replace the RO with nanofiltration (NF). Certain NF membranes are highly permeable to silica, and the presence of silica is not harmful to drinking water. The 20-gpm pilot contained the selected NF membrane, and the silica problem was completely solved. Elimination of the silica purge allowed recovery of 98% of the brackish water to be produced as fresh water.