Pilot Study of Calcium Removal Pretreatment and Desalination of Irrigation Drainage
* Is use of preferential precipitation (seeding) and nanofiltration as pretreatment for low-pressure reverse osmosis a practical, cost-effective means of achieving high recovery rates when desalinating agricultural drainage and other high hardness waters?
The pilot tests will determine whether:
* Preferential precipitation can prevent calcium sulfate fouling at high feedwater recovery rates
* Steady operations can be maintained for extended periods
* The expected life of the first-pass tubular nanofiltration membranes exceeds three years
Need and Benefit
Reclamation has a legal obligation to provide drainage service to Federal water users in the 700, 000-acre San Luis Unit of the Central Valley Project in California. Based on recent studies, Reclamation has projected that by year 2050 254, 000 acres in this area will require drainage service and the gross drainage volume will reach about 140, 000 acre-feet (AF) per year. This volume can be reduced to about 110, 000 AF per year through certain on-farm measures, including recycling about 16 percent and blending the water (which has an average total dissolved solids [TDS] of 7, 000 milligrams per liter [mg/L]) in a 1:40 ratio with low salinity (300 mg/L) Central Valley Project (CVP) water.
However, the relatively high salinity of the drainwater precludes greater on-farm recycling without desalination since the high value crops grown in this area require irrigation water with <500 mg/L TDS. Thus, to achieve a total on-farm recycling solution to the drainage problem in the San Luis Unit, approximately 110, 000 tons per year of salt will have to be removed from 110, 000 AF per year of drainwater.
Desalination at high water recovery is highly desirable for two reasons:
* Every gallon of water that is recovered is a gallon of water that does not have to be taken from some competing user
* Reject disposal is a major issue because San Luis Unit drainwater contains an average of 150 mg/L of selenium (with some locations exceeding 600 mg/L). Since selenium concentrations above 5 mg/L are considered potentially harmful for waterfowl, disposing of brine in evaporation ponds is highly problematic. Recognizing this problem, a new state law (SB 1372) in California exempts solar evaporators (i.e., evaporation basins designed and operated to avoid standing water) used as part of a zero-discharge integrated farm drainage management (IFDM) system from toxic waste regulations. However, to qualify for this exemption, the solar evaporator must be less than 2 percent of the farmland served by the IFDM system. Thus, to achieve a total zero-discharge on-farm recycling solution for San Luis Unit drainage, a reverse osmosis (RO) desalination process with a recovery rate greater than 80 percent is required.
The technical problem in achieving a recovery above 80 percent when desalting San Luis Unit drainwater with RO is the water's extreme hardness (typical values range from 1, 000 to 2, 500 mg/L expressed as CaCO[v3]). Indeed, most as collected subsurface drainwater in this area is already nearly saturated with of calcium sulfate (CaSO[v4]). As water permeates the RO membranes, CaSO[v4] deposits on the feed side as mineral scale. These deposits foul the membrane, blocking the passage of water. The two conventional approaches for overcoming mineral scale fouling of RO membranes are:
* Removing the sparingly soluble compounds ahead of the RO unit using lime/soda softening
* Adding sequestering agents to the feed stream to act as scale inhibitors. The extreme hardness of the drainwater makes lime/soda softening too costly. Past and current pilot tests have shown that water recoveries of 50 percent to 60 percent can be achieved with scale inhibitors and careful pH control, but 80 percent or more is out of reach.
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