A new treatment plant in Orange County, Calif., is one of about 15 in the country to recycle wastewater back into the groundwater drinking water supply -- a plan called indirect potable reuse. Two experts answer your questions on water recycling.
Why use "dirty" water when there's plenty of ocean water?
Shivaji Deshmukh responds:
"Dirty" is subjective. Wastewater in Orange County has approximately 1,000 milligrams per liter (mg/L) of salinity while ocean water has 40,000 mg/L. All coastal communities discharge treated wastewater into the ocean. With the treatment technologies present today, it is easier and more cost-effective to treat wastewater than ocean water.
Desalination of seawater costs more than indirect potable reuse projects. In the case of the Groundwater Replenishment System and most desalination plants currently in design, similar treatment schemes with pretreatment, reverse osmosis and disinfection are used. However, ocean water contains approximately 40 times more salt than wastewater. Reverse osmosis is a high-pressure process that separates dissolved constituents from the feed stream. The pressure requirements for reverse osmosis are dependent on the salt concentration of the feed water. Therefore, more energy is required for saltier feed streams. Estimates for indirect potable reuse projects using advanced treatment range from $600 to $800 per acre-foot. Desalination of ocean water is estimated at over $1,000 per acre-foot. An acre-foot is 325,850 gallons or approximately enough water for 2 families for one year.
Cheryl McGovern responds:
It should be considerably more efficient to process filtered wastewater effluent by reverse osmosis than to process sea water due to the much lower level of total dissolved solids in the filtered wastewater effluent. This is going to vary depending upon a number of variables, such as the quality of water being desalinated and the available options for disposal of the waste brine stream generated by desalination.
Dr. Bahman Sheik, an expert on water recycling, has stated that treating wastewater is about six times more efficient than reverse osmosis of ocean water because it requires less energy. He has pointed out that not all ocean water is clean. A lot of wastewater is discharged into the ocean from coastal cities, ocean-going vessels such as large cruise ships, cargo ships and smaller pleasure boats, as well as industries, etc.
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