Civil Engineering Reference
In-Depth Information
TABLE 15-12. Method of Concentrate Disposal by U.S. Membrane Plants
Percentage of
Plant in
Planning
Stages
Percentage of
Operating
Plants
Type of Disposal
Surface water discharge
48
60
Sewer discharge
23
21
Land application
12
0
Deep well injection
10
13
Evaporation pond
6
2
(Adapted from
Membrane Concentrate Disposal,
by permission. Copyright
1993, American Water Works
Association and AWWA Research Foundation.)
centrate quality and quantity, characteristics of the receiving water, and regulatory
environment, determine the feasibility of the surface discharge. Surface discharge re-
quires an NPDES permit. Water quality and quantity limits, pretreatment conditions,
planning requirements, outfall location and design, and monitoring plans must all be
considered in applying for a permit. Otherwise, given a nearby receiving water, surface
water discharge is relatively simple.
Discharge to Sewer System
Discharging concentrates to the sewer is usually lim-
ited to smaller membrane plants because of capacity limitations within the collection
system and wastewater treatment plant. The concentrate stream must also meet pre-
treatment requirements to ensure no deleterious effects on the sewer system and waste-
water plant. For some concentrates, the cost of pretreatment for sewer discharge can
be prohibitive. Some membrane waste streams may be of sufficient quality to bypass
a part of the wastewater treatment plant. For example, some concentrates are dis-
charged such that only disinfection is provided. A fee for discharge to the sewer may
be required, depending on who controls the water and wastewater agencies.
Ocean Discharge
Where a treatment plant is located near the sea, ocean discharge
can be an attractive alternative. Ocean discharge is considered separately from surface
water discharge due to the general differences in quality, quantity, and uses of these
receiving waters. However, as with surface water discharge, an NPDES permit is re-
quired. In general, an outfall should be located to maximize mixing and dilution.
Contaminants in the concentrate stream, such as heavy metals or high salt concentra-
tions, or conditions such as extremes in pH or low oxygen concentrations, may be
harmful to the local marine environment.
Land Application
Two land application methods are currently used for membrane
concentrate disposal: irrigation systems and rapid infiltration systems. Spray irrigation
applies the concentrate to parks, golf courses, and crops and other vegetation. The
concentrate waste stream provides a benefit by replacing the use of more valuable
water resources. However, spray irrigation is limited by several constraints. The quality
of the concentrate must not harm the vegetation, so dilution may be necessary. For
crops tolerant of salts, such as golf course grasses and citrus trees, the irrigation water
can have a chloride ion concentration no higher than 1,000 mg / L.
31
Adequate land
