AWWA ACE63129

Cost-effective disposal of the concentrated residual stream from a reverse osmosis (RO)system is one of the primary factors affecting the feasibility of RO projects either near theocean or inland areas. This paper reviews the typical regulatory concerns related to awide range of traditional and innovative concentrate disposal alternatives. Anunderstanding of these concerns is then used to describe pilot testing programs that willsupport the utilities efforts to obtain discharge permits or verify performance objectives.A typical low pressure brackish RO facility achieves an overall total dissolved solids (TDS) rejection of 97-98%,which means most of the dissolved minerals in the feedwater are concentrated 5-6 fold asthe feedwater pass through the RO unit. A typically high quality brackish feedwatermeets most of the drinking water standards with the possible exception of a few tracecontaminants, hardness and total dissolved solids. As a result the brackish feedwater andthe resulting residual stream should not be inherently hazardous to humans or theenvironment. By virtue of its elevated TDS, however, it is not similar to freshwater andtherefore raises concerns regarding impacts on the environment. In the past, theregulatory concerns associated with RO concentrates have been focused on the potentialtoxic effects of trace elements in the concentrate. As the relatively benign nature of theconcentrate has been demonstrated, the concerns have shifted to the mass of dissolvedsolids associated with the concentrate from an RO facility.The study compares the mass of dissolved solids from a typical 10 mgd brackish water ROfacility to several sources of dissolved solids. Assuming the brackish water has aconcentration of 2000 mg/l TDS, a 10 mgd facility operating at 80% recovery discharges80 tons of dissolved solids per day. If this is a previously unused brackish water sourceof water, this represents a new salt load on the receiving water. By comparison thecomposite increase in dissolved solids in municipal wastewater associated with typicalurban activities such as commercial cooling tower blowdown, soaps and detergents,water softeners, evaporation, washing, cooking and wastewater is 100-200 mg/l. Theincrease in salt in the receiving water associated with the RO facility is equivalent to thatfrom a 100 mgd wastewater treatment facility. Another typical source of salt in theenvironment is road deicing, but it has decreased over the years to the point where forexample, the State of Colorado only used approximately 35,000 tons of salt last yearfor deicing, which is similar to the amount of salt produced by one 10 mgd desalter. Thevolume of concentrate can be decreased by increasing the RO recovery ratio, but themass of solids in the concentrate remains the same. Includes 10 references, figure.

Product Details

Edition:

Vol. - No.

Published:

06/01/2006

Number of Pages:

11

File Size:

1 file , 260 KB