Fluid handling – Processes
Reexamination Certificate
2002-06-13
2003-12-02
Lee, Kevin (Department: 3753)
Fluid handling
Processes
C137S236100, C137S526000
Reexamination Certificate
active
06655402
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to water quality management and more particularly to a system and method for substantially preventing sewer solid accumulation in an urban drainage system.
BACKGROUND OF THE INVENTION
Most urban drainage systems have evolved into a complex network that includes combined sewer systems (including interceptor sewers), separated sanitary sewer systems, stormwater sewer systems, channels, and culverts. This network conveys domestic and industrial wastewater to wastewater treatment plants during dry weather (referred to as “dry weather flow”) with the addition of stormwater runoff during periods of wet weather (collectively referred to as “wet weather flow”). Domestic wastewater includes sewage from a household. Industrial wastewater includes industrial processing waste including solids and liquids.
A “combined sewer system” collects domestic and industrial wastewater, and stormwater runoff. This mixture is called combined sewage. A “separated sanitary sewer” collects domestic and industrial wastewater. A “stormwater sewer system” collects stormwater. During dry weather or small rainstorms, combined sewage from combined sewer systems and wastewater from sanitary sewer systems receive full treatment before discharge to receiving waters. During larger rainstorms, inflows can exceed the capacity of these sewer systems or the wastewater treatment plant itself. The excess flows are known as combined sewer overflows (CSOs) and sanitary sewer overflows (SSOs) “Wet weather flow” discharges include combined sewer overflows, sanitary sewer overflows, and stormwater runoff. Dry and wet weather flow include both sewer solids and liquid. Combined sewer overflows, stormwater runoff, and sanitary sewer overflows are major contributors to the degradation of many urban lakes, streams and rivers.
CSOs and SSOs may be diverted respectively to CSO and SSO storage tanks to substantially reduce or eliminate the frequency and volume of CSOs/SSOs to receiving waters. These storage tanks are located in order to intercept the CSOs/SSOs before they enter the receiving waters. They store the excess wet weather flow during rainstorms. Stormwater storage tanks similarly store excess stormwater during rainstorms. During this period, sewer solids in the wet weather flow settle to the bottom of the tank. When flows subside after a rainstorm, their liquid contents are drained or pumped back into the appropriate sewer systems and conveyed to the wastewater treatment plant where they are treated. After the liquid contents of the tank are emptied, the settled solids remain on the floor of the tank. Sewer solids deposited in combined sewer and sanitary sewer systems during low flow dry weather periods are major contributors to the CSO/SSO-pollution load, causing serious water quality and health problems.
One of the underlying reasons for considerable sewer solids deposition is the combined sewer hydraulic design. Combined sewers are sized to convey many times the anticipated peak dry weather flow. Combined sewers can carry up to 1000 times the expected background sewage flow. Ratios of peak to average dry weather flow usually range from 2 to 10 for interceptor sewers. The oversized combined sewer segments possess substantial sedimentation potential during dry-weather periods. Dry weather flow velocities are typically inadequate to maintain settleable solids in suspension, and a substantial amount of sewer solids tends to accumulate in the pipes. During rain storms, the accumulated solids may resuspend and, because of the limited hydraulic capacity of the interceptor sewers, overflow to receiving waters. Suspended solids concentrations of several thousand parts per million are not uncommon for CSOs. This can produce shock loadings detrimental to receiving waters. Accumulation of sewer solids in sewer pipes also results in a loss of flow carrying capacity that may restrict/block flow and cause an upstream surcharge or local flooding.
Sewer solid accumulation in urban drainage systems also creates septic conditions that pose odor, health hazards, and corrosion problems for these systems. “Sewer solids” as used herein may include sediment, sludge, debris or the like. “Combined sewer system” as used herein includes both combined sewers and CSO storage tanks. “Sanitary sewer system” as used herein includes both sanitary sewers and SSO storage tanks. “Urban drainage system” as used herein includes combined sewers, sanitary sewers, stormwater sewers, and CSO/SSO/stormwater storage tanks.
A variety of flushing systems have been used to purge the sewer solids deposited in combined sewers, stormwater conveyance systems and CSO storage tanks. By creating high-speed flushing waves to resuspend deposited solids, the resuspended solids are washed to strategic locations such as to a point where the wastewater stream is flowing with sufficient velocity, to another point where flushing will be initiated, to a storage sump which will allow later removal of the stored contents, or the wastewater treatment plant. Flushing reduces the amount of solids resuspended during storm events, lessens the need for CSO treatment and sludge removal at downstream storage facilities and allows the conveyance of more flow to the wastewater treatment plant or to the drainage outlet.
One such system is the Hydrass® flushing system comprised of a balanced hinged gate. The gate is weighted to close during low flows allowing the flow to be retained behind the gate. Once the force created by the retained water becomes sufficient, the gate tilts. This releases the surcharged water and flushes the sediment from the sewer. Once the force of the surcharged water is relieved, the gate returns to the closed position to repeat the line surcharging.
Another system is the Hydroself® flushing system which uses a storage impoundment to retain water. Periodically this water is released creating a hydraulic surge which flushes deposited sediment from the storage tank floor and along sewer lines. The release can be triggered manually or automatically with a preset water level monitor and controller.
The gate flushing system also requires a storage impoundment for the flush water. This is created by erecting two walls in the sewer pipe. A heavy gate is placed in the sewer or storage tank perpendicular to the flow and water is held behind the gate. When the water level behind the gate reaches a predetermined level, the heavy gate is opened and water is released to flush sediment downstream of the gate. The impoundment floor must have a slope of 5 to 20% to prevent debris accumulation. When the water reaches a predetermined level, it is released causing a hydraulic surge that flushes the storage tank and sewer line.
The tipping flushers system uses a cylindrical stainless steel vessel suspended above the maximum water level on the back wall of the storage tank. The system requires a water filling system. As the vessel is filled with water, the center of gravity shifts and causes the vessel to rotate and discharge its contents down the back wall of the tank. A curved fillet at the intersection of the wall and tank floor redirects the flush water horizontally across the floor of the storage tank. The flushing force removes the sediment and debris from the tank floor and transports it to a collection sump located at the opposite end of the tank. These flushing systems all require an extramural source of water and/or complex control instrumentation.
Accordingly, there has been a need for a novel system and method that substantially removes sewer solids from urban drainage systems between storms. There is also a need for a novel system and method that may be used in urban drainage systems for substantially reducing sewer solids and associated pollutants from reaching receiving waters. There is a still further need for a novel system and method that operate under atmospheric pressure and hydrostatic head build-up. There is an additional need for a novel system and method that do not require an extramural source of water for flus
Lee Kevin
Novatt Janine R.
U.S. Environmental Protection Agency
LandOfFree
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