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 the
ocean or inland areas. This paper reviews the typical regulatory concerns related to a
wide range of traditional and innovative concentrate disposal alternatives. An
understanding of these concerns is then used to describe pilot testing programs that will
support 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 as
the feedwater pass through the RO unit. A typically high quality brackish feedwater
meets most of the drinking water standards with the possible exception of a few trace
contaminants, hardness and total dissolved solids. As a result the brackish feedwater and
the resulting residual stream should not be inherently hazardous to humans or the
environment. By virtue of its elevated TDS, however, it is not similar to freshwater and
therefore raises concerns regarding impacts on the environment. In the past, the
regulatory concerns associated with RO concentrates have been focused on the potential
toxic effects of trace elements in the concentrate. As the relatively benign nature of the
concentrate has been demonstrated, the concerns have shifted to the mass of dissolved
solids associated with the concentrate from an RO facility.
The study compares the mass of dissolved solids from a typical 10 mgd brackish water RO
facility to several sources of dissolved solids. Assuming the brackish water has a
concentration of 2000 mg/l TDS, a 10 mgd facility operating at 80% recovery discharges
80 tons of dissolved solids per day. If this is a previously unused brackish water source
of water, this represents a new salt load on the receiving water. By comparison the
composite increase in dissolved solids in municipal wastewater associated with typical
urban activities such as commercial cooling tower blowdown, soaps and detergents,
water softeners, evaporation, washing, cooking and wastewater is 100-200 mg/l. The
increase in salt in the receiving water associated with the RO facility is equivalent to that
from a 100 mgd wastewater treatment facility. Another typical source of salt in the
environment is road deicing, but it has decreased over the years to the point where for
example, the State of Colorado only used approximately 35,000 tons of salt last year
for deicing, which is similar to the amount of salt produced by one 10 mgd desalter. The
volume of concentrate can be decreased by increasing the RO recovery ratio, but the
mass of solids in the concentrate remains the same. Includes 10 references, figure.
| Edition : | Vol. - No. |
| File Size : | 1
file
, 260 KB |
| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 11 |
| Published : | 06/01/2006 |
