Civil Engineering Reference
In-Depth Information
/L, H
2
S
0.3 mg / L, free chlorine and turbidity
2 NTU.
30
In every case, of course,
careful examination of the prospective water would be necessary to determine suita-
bility and pretreatment. Normally at least a 5-micron cartridge filter should precede
the ED / EDR stack.
The precipitation of slightly soluble salts in the standard ED process may be min-
imized by ion-exchange softening and / or reducing the pH of the brine through acid
addition and / or the addition of a precipitation-inhibiting agent. However, a certain
degree of fouling is unavoidable. Membranes should be regularly washed or cleaned
in place with dilute acid and alkali solutions to restore performance when required.
Case History
Due to the characteristics of Lake Granbury, located in north central Texas, treatment
beyond conventional surface water treatment is required to produce a palatable drink-
ing water supply. Lake Granbury is a brackish surface water, which has a total dis-
solved solids and chlorides concentration approaching 2,200 mg / L and 900 mg / L,
respectively. Based on these concentrations, the water tastes salty and does not meet
the Minimum Drinking Water Standards for the state of Texas. Therefore, the water
must be demineralized before it can be deemed an acceptable water supply.
The first phase of the ultimate 21-mgd facility consists of an intake structure in
Lake Granbury, 4,000 ft of raw water line along with a 38-cm (15-in) concentrate
return line, a conventional surface water treatment plant with EDR having a capacity
of 13.2 ML / d (3.5 mgd), and a transmission system consisting of 38 km (24 miles)
of 61-cm (24-in) and 46-cm (18-in) pipeline along with a 11.4-ML (3-mg) standpipe
to deliver treated water to the Johnson County participants. Since that time, the EDR
plant has effectively reduced the total dissolved solids of the brackish water found in
Lake Granbury to produce a potable water in conformance with the requirements of
the state of Texas and the Safe Drinking Water Act.
The modular components of the EDR facility makes it easy to expand. In 1998,
the capacity was expanded by 1.5 mgd, bringing the total capacity to 5.0 mgd. The
improvements included adding raw water and demineralizer feed pump capacity as
well as increasing the capacity of the EDR system.
DISPOSAL OF MEMBRANE WASTE STREAMS
Introduction
Disposal of membrane waste streams is a primary consideration in determining the
feasibility of a membrane process. The issues involved in disposal range from purely
practical to regulatory to political. Many of these issues are common to residual dis-
posal from any water treatment plant. A thorough review of this subject is presented
in a handbook published by ASCE, AWWA, and the U.S. EPA
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and is also discussed
in Chapter 23, Residuals Management. Because the application of UF and MF systems
for potable water treatment is relatively new, there is not extensive experience with
disposal of large volumes of waste from these processes. Most engineering and reg-
ulatory work in membrane waste stream disposal has been with RO and NF processes.
