Wastewater Treatment Plant
 In
1988 Hobbs, Upchurch & Associates began planning and design of an upgrade of
the 1.8 MGD wastewater treatment plant at Siler City. The plant was
experiencing severe problems caused by large peak stormwater inflow, a large
constituent of industrial water including discharge from poultry processing
plants and the relatively low flow (approaching zero at times) of the small
receiving stream.
The Town chose to seek funding through the State Revolving Loan Program
administered by the Construction Grants and Loan Section of DEM. The option
required an update of the 201 Facilities Plan, which proved to be an extremely
difficult and time consuming task because of environmental groups trying to
protect the Rocky River which received the plant discharge via Love's Creek.
A federally endangered species, the Cape Fear Shiner, raised particular
concern, because of its habitat several miles downstream of the plant
discharge. Environmental planning included time of travel stream studies,
stream macro-invertebrate populations, fish and mollusk population
investigations, and consideration of the waters' nutrient sensitivity.Effluent
Limits
The plant was subject to bypassing of mixed liquor from the first stage
aeration basins during high stormwater flow and regularly experienced foaming
at the discharge point. Because of low receiving stream flows and the emphasis
on improving the condition of the Rocky River, Siler City was given the
strictest discharge limits presently in effect in North Carolina. Effluent
limits of 5 mg/l BOD, 1 mg/l ammonia and .5 mg/l phosphorus were imposed.Design
The design of the plant posed several problems, including the need for
the addition of a large volume of stormwater flow equalization; the need for a
large volume of aerobic sludge digestion; sludge thickening and sludge holding;
and the need to incorporate the oddly configured plant into the final design
while maintaining compliance during the construction period.
Extensive research was performed on nutrient removal technologies, both
proprietary and non-proprietary. Each alternative was analyzed financially to
balance the cost of capital expenditures against the operation and maintenance
costs and the expected reliability of the processes.
After much study, engineers proposed and designed an extended aeration
treatment plant with capability of some biological phosphorus removal augmented
by chemical nutrient precipitation capabilities and followed by tertiary
filtration. The design used zoned jet aeration capable of creating zones of low
and high dissolved oxygen content within the aeration basin to facilitate
biological nutrient removal. By careful analysis of the plant influent, we were
able to provide a non-proprietary process at considerable cost savings which,
when augmented by chemical nutrient removal, provides a reliable and
controllable method of nutrient removal. Controllable dissolved oxygen levels
and extended contact time in the aeration basin along with reactor
clarification and tertiary filtration guarantees the plant can meet discharge
limits on BOD and ammonia.
All existing plant basins, including clarifiers, were used in the design for
conversion into flow equalization and sludge handling facilities. This resulted
in the design of a completely new 4 MGD wastewater treatment plant including
influent pumping, scum, grit and grease removal facilities, jet aerated
oxidation ditch, chemical addition and flocculation, reactor clarification,
tertiary sand filtration, chlorine disinfection, dechlorination and
non-mechanical cascade aeration prior to discharge. The design included
extensive process control and monitoring systems.
Extensive pre-qualification procedures were required for all equipment
suppliers. Second rate suppliers and equipment were eliminated and the best
available equipment was chosen for the plant. The plant operator and town staff
were extensively involved in selection of available equipment as well as
choosing configuration of plant processes.
This project received a 1994 Engineering Excellence "Grand Conceptor Award"
from the American Council of Engineering Companies of North Carolina (ACEC/NC).
Projects that receive this award undergo a through third part review and are
rated on requirements such as exceeding client's expectation.
|
