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Secondary Treatment at the JWPCP
SECONDARY INFLUENT PUMPING STATION
The Secondary Influent Pumping Station pumps primary effluent to the secondary treatment facilities through a 14-foot diameter pipe. Secondary treatment is located at a higher elevation than primary treatment. Consequently, primary effluent must be pumped and conveyed under pressure to the secondary treatment biological reactors. Five natural gas fired engine/pump assemblies capable of pumping 175 million-gallons-per-day (MGD) are used to convey the primary effluent.
Certain organisms will remove carbonaceous material by breaking the material down into simpler compounds which are then either oxidized to carbon dioxide and water vapor or assimilated into new cells. The restructuring of the carbonaceous material into new organisms requires energy, which is supplied by the oxidation reaction. The net results of the reactions are the consumption of oxygen and carbonaceous material, the production of more organisms (activated sludge) and oxidation products, and the liberation of energy.
The biological reactors at JWPCP convert finely divided and dissolved organic matter that passes through primary treatment into settleable solids that can be removed by final clarification. Primary effluent comes in contact with biological floc in the reactors and the mixture (mixed liquor) is mixed by surface aerators. High purity oxygen supplied by cryogenic oxygen plants is dissolved into the mixed liquor by these surface aerators.
Treatment is accomplished in eight reactors. Each reactor has an average design capacity of 50 MGD. The reactors are subdivided into four stages, each outfitted with three aerators/mixers to facilitate oxygen dissolution and mixing. The reactors are covered to retain the high purity oxygen gas introduced to the system and permit a high degree of oxygen utilization by the activated sludge. Primary effluent and activated sludge enter the first stage, flowing through the four stages in a plug-flow manner.
The first stage of the reactors is operated as an anaerobic selector, limiting the exposure to oxygen to suppress the growth of certain undesirable organisms in the activated sludge. In the following three stages, the activated sludge consumes organic matter in the mixed liquor and produces more organisms. The fourth stage of some of the reactors also functions as a pH adjustment stage, stripping carbon dioxide from the mixed liquor to achieve a neutral pH effluent. After passing through the reactor, the mixed liquor from the fourth stage flows over a weir into the clarifier inlet channels and into the clarifiers.
The purpose of the final clarifiers is to separate the activated sludge solids from the biological reactor's mixed liquor. This constitutes the final step in the production of a stable effluent that is low in BOD and suspended solids.
Each reactor has a bank of 26 sedimentation tanks. Floatable material is skimmed off the top, collected, and directed to the J.O.-C sewer line. As the mixed liquor flows through the sedimentation tank, the solids settle to the bottom of the tank and are scraped to two hoppers where the sludge is collected and drawn off to each respective return sludge pumping station. There is one pumping station for each reactor. Each station consists of three pumps that pump activated sludge to the inlet of the reactors to keep an effective concentration of microorganisms in the reactors. A portion of the activated sludge is removed from the reactor/clarifier system to maintain a desired population of microorganisms in the reactors.
DISSOLVED AIR FLOTATION (DAF)
The flotation thickening system is designed to concentrate, or thicken, the waste activated sludge produced in secondary treatment. Activated sludge is thickened using a mixture of pressurized air and water. Air is mixed with water under pressure in a pressure vessel causing the air to dissolve in the water. The pressurized air-water mixture is fed to covered air flotation tanks. At the inlet of each flotation tank, the pressurized air-water mixture combines with the waste activated sludge. Polymer is also added to the tanks to aid in the flocculation of solids. As the mixture enters the flotation tank, air comes out of solution creating bubbles that attach to the sludge, causing these solid particles to rise to the surface forming a thickened, blackened mat. The solids on the surface of the flotation tank are collected using skimmers. The collected solids are then pumped to the anaerobic digestion system at Primary Treatment. The underflow, or clarified effluent, is returned to the secondary influent force main.
CRYOGENIC OXYGEN PLANT
The Cryogenic Oxygen Plant utilizes a process that includes the filtration and compression of air prior to separation into its elemental components, namely oxygen and nitrogen. Oxygen and nitrogen have different boiling points, permitting the use of the distillation process to achieve separation. Oxygen boils at -297 degrees Fahrenheit and nitrogen boils at -320 degrees Fahrenheit. Pure oxygen greater than 98%, by volume, is used rather than air to reduce the size of the reactors, increase biological activity, and reduce the power demand.
The Cryogenic Oxygen Plant includes three air separation trains. The three trains run independently or together to provide the necessary oxygen for the reactors. Incoming air is filtered, compressed, and pre-cooled prior to distillation. In the distillation process, liquid air is distilled by boiling off the liquid nitrogen in the liquid air mixture, leaving behind pure liquid oxygen for storage/use. Three liquid oxygen storage tanks are used to supply pure oxygen to the reactors when the Cryogenic Oxygen Plant is not operational.
SECONDARY EFFLUENT PUMPING STATION
Secondary effluent is either pumped or gravity fed to the ocean. Gravity feed is utilized when tidal conditions permit and total plant flow will not result in substantial headloss. When gravity feed cannot be utilized, five pumps with a capacity of 170 MGD, each, are available for use. Disinfection of secondary effluent is achieved upstream of the pumping station using a bleach solution. Bleach is injected into the effluent to achieve a residual of approximately 1-2 mg/l.