Three types of ZDD systems were evaluated during the ZDD Demonstration Project: greenfield, bolt-on, and EDM-only. The greenfield configuration was tested in Alamogordo, New Mexico (BGNDRF), and La Junta, Colorado; the bolt-on configuration was tested in Brighton, Colorado; and, the EDM-only configuration was briefly tested in La Junta, Colorado, and piloted at Beverly Hills, California. The choice of configuration is dependent upon the water being treated by ZDD and the product water quality requirements.
The first ZDD system evaluated simulates a greenfield site (i.e. no existing desalination equipment) and involves a brackish water that is fed to a NF or RO to produce a permeate quality that meets primary drinking water standards. The concentrate from this NF is fed to an EDM system where offending dissolved constituents such as calcium sulfate are removed. This is shown in Figure 1 below. This configuration also shows how an existing plant could install different membranes in their existing system and install a new EDM to achieve high recovery. RO membranes have high silica rejection, so a silica purge, silica treatment system, or use of silica-permeable membranes are needed where moderate to high levels of silica are present in the groundwater.
The second type of ZDD system tested simulates a bolt-on approach and involves ZDD desalinating the concentrate from an existing desalination plant.This is shown in Figure 2 below. In this case, the EDM is placed at the front of the treatment scheme, because electrodialysis operates more efficiently at higher concentration. Additionally, the EDM removes calcium sulfate and other dissolved constituents to a level that allows the NF system to operate efficiently and with a lesser chance for scale formation. A silica purge stream is necessary at sites with high silica in their RO concentrate. If NF270 membranes are utilized to desalinate EDM diluate, the rate of silica purge is expected to be 10% of the waste, or less than 0.2% of the initial feed to the existing plant RO. If RO is used, the waste volume could be 5-10% of the initial feed.
Some brackish water and plant RO concentrate has a low enough concentration that RO/NF systems are not required in the ZDD process. In this case, the EDM desalinates the feed water or RO/NF concentrate directly and the diluate can either be mixed with the existing plant's feed or permeate. This process is shown in Figure 3 below.
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