Regional Water System Plant Design and Upgrades
Grand Strand Water and Sewer Authority, South Carolina
Grand Strand Water and Sewer Authority (GSW&SA) is a public utility company
serving coastal areas in South Carolina, specifically Horry County and
Georgetown County. GSW&SA was the lead agency in the construction of the Bull
Creek Water System mandated by DHEC to improve the water quality provided to
its customers. GSW&SA sought to develop a surface water source to serve the
present and future needs of the area.
HUA was contracted to complete a Master Plan for the overall system. The
evaluation included hydraulic analysis of the raw water and finished water
pumping systems and the water treatment plant. The study included investigation
of intake screens, capacity, and the possibility of installing a second raw
water intake and pump station.
Key Elements
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Design of original 21 MGD water treatment plant.
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Optimization study for operation of water treatment plant.
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Study and design of the high- rate modifications - 21 MGD to 30 MGD.
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Design of new 15 MGD treatment train.
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Present plant capacity 45 MGD.
Treatment Techniques/Optimization
The water quality in Bull Creek is very seasonal and is characterized by
relatively high levels of color and Total Organic Carbon (TOC) which are
precursors of trihalomethane (THM). During periods of high precipitation in the
late summer, color values over 400 PCU and TOC of 65 PPM may occur, yielding
THM formation potential in excess of 300 PPB.
Many treatment techniques and chemical combinations were evaluated in addition
to chlorine and standard coagulants. These included volatile organic stripping
by aeration, potassium permanganate, chlorine dioxide, sodium aluminate, and
powdered activated carbon (PAC). The basic objective was to produce high
quality potable water in simultaneous compliance with the CT criteria of the
Surface Water Treatment Rule and the THM disinfection by-products limitations.
The optimization work was very successful and yielded a plant mechanical
configuration and operating protocol whereby frequent measurements of color and
TOC are accomplished in order to anticipate impending treatment difficulties.
Changes in these parameters cause the operators to modify procedures and to
energize standby treatment chemical pumps according to established practices.
The delivered water quality, while using chlorine dioxide on top of the filters
and chloramines in the distribution system, is typically below 2 PPM TOC, 15
PPB THM and 3 color units.
Water Treatment Plant Design
The design for the original water treatment facility consisted of a 21
MGD surface water treatment plant with full chemical feed equipment, sludge
handling facilities, finished water pumping (low service) and instrumentation
and control equipment. The project also included remote pumping stations
consisting of ground storage, pumping equipment, and chlorination and
ammoniation equipment. Due to the isolated location of the water treatment
facility, it was determined that the pumping should consist of low service
pumping at the plant transferring water at a low pressure to the remote
stations where the disinfectant is adjusted and the water is pumped into the
distribution system.
Alum Sludge Treatment and Disposasal
The use of alum in the treatment process also required the design of an
alum sludge handling facility. The design included sludge production
determinations, testing of similar sludge, and calculation of loading
parameters. Based on these evaluations it was determined that the liquid alum
sludge could be suitable for use on a nearby turf grass farm constructed on
water treatment plant property. To accommodate this, soil testing was completed
at the turf grass farm to predict specific loading rates in order to acquire
satisfactory acreage. All of this ultimately led to the design and construction
of a liquid sludge irrigation system. This project component has allowed GSW &
SA to minimize alum sludge disposal costs.
Water Transmission and Storage/ Pump Stations
The transmission and water storage components of the project included
the design and construction of two prestressed concrete ground storage
reservoirs, and the construction of one 500,000 gallon elevated storage tank.
This design effort involved the hydraulic analysis and the sizing for future
expansion as well as the construction management services. HUA also provided
design of approximately 47 miles of transmission main to water system
participants. The project included the design of three major pump stations, the
raw water pump station, the finished water pump station, the South Pump
station, and the Conway Pump station.
Plant Start-Up and Treatment
Following the completion of the construction phase of the project,
Hobbs, Upchurch & Associates participated in the start-up and testing of the
facility. This start-up included conducting a corrosion analysis to assist in
the selection of a corrosion inhibitor as well as the performance of a CT study
to determine the contact time throughout the process at different flow rates.
Water Treatment Plant Upgrades
HUA has designed numerous upgrades to the Bull Creek Water Treatment
Plant. The plant was also high rated from 21 MGD to 30 MGD. This increased the
filter flow rate up to 6 gpm/sf. This upgrade was made possible by a
modification to the effluent control weir on the filter. The modification was
facilitated by the innovative effluent pipe design by HUA on the original
Greenleaf filters. The modification also had the beneficial effect of extending
filter run time by 50% or greater.
The chlorine system was also redesigned to utilize sodium hypochlorite in lieu
of gas chlorine for both safety and treatment reasons.
In 2000, Hobbs, Upchurch & Associates designed and constructed a 15 MGD train
that will parallel the existing 31 MGD water treatment plant at Bull Creek. The
project consisted of upgrade of the existing raw water pump station,
modifications to the raw water main, installation of flash mix, clarifier, and
filters, construction of a new administration building which houses additional
chemical feed systems for the entire plant, upgrade of the existing sludge
handling facilities, and the addition of a 2 MG clearwell. The project also
consisted of modification to the finished water pump station and the
installation of analytical and control equipment for operation of the facility.
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