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The goal of wastewater or sewage treatment systems is to reduce the concentrations of organic pollutants in raw sewage such that the clarified waste can be discharged into natural bodies of water without disrupting the ecosystem. While liquid sewage can also result from agricultural and industrial operations, Oxford's is primarily domestic, resulting from everyday human activities such as showers, sinks, and toilets. This sewage is treated to remove organic nutrients (pollutants) so that it can be safely discharged into Four Mile Creek. This water eventually joins the Ohio River which is eventually tapped as a water supply for cities downstream (e.g., Louisville). The City of Oxford Sewage Treatment Plant treats Oxford's three million gallons of sewage per day using a combination of two microbial processes known as aerobic digestion with activated sludge followed by anaerobic digestion.
Raw sewage enters the plant via large underground pipes. It is initially filtered to remove large, non-biodegradable solids by passage through bar screens. The resultant debris is deposited in dumpsters and hauled away to the landfill. The liquid component of the sewage is piped to a large tank for grit removal where large particles of sand and grit settle out. Resultant effluent then goes to sedimentation basins where more sedimentable solids and grease are removed via settling and microbial degradation.
The clarified sewage then enters the activated sludge component of the treatment plant. This process is based on maximizing (activating) the growth of aerobic heterotrophic bacteria, fungi and protozoa such that they break down the dissolved organics and/or incorporate them into new microbial cells (biomass) which then settle out of suspension, together with undigested material, as "sludge." Since the desired microbial populations are aerobic, it is crucial that the system be highly aerated. During the first phase of secondary sewage treatment, the sewage is transferred to aerobic digesters in which large pumps keep dissolved oxygen levels high. The sewage is continually circulated through trickle filters housed in "biotowers" or "activated growth units" where redwood slats are stacked and act as substrates for the attachment and growth of layers of aerobic, heterotrophic microorganisms. The liquid sewage is sprayed over the slats to enhance aerobic conditions, and the organic compounds are broken down as the sewage trickles past the microbes growing on the slats. Clarified effluent (water portion of the sewage) runs through the tower, reenters the aeration tank, and is recirculated back through the towers for further clarification. As the microbial populations in the aeration tank increase, as seen by the appearance of foam on the surface, cells begin to clump together to form (activated) sludge, a proportion of which settles out of the suspension. Some of the activated sludge is then used to inoculate incoming effluent from the "primary tanks" to keep degradative microbial population levels high. The rest of the sludge is further broken down in later steps.
Following treatment in the biotowers, the further-clarified effluent (water) flows into post-secondary sedimentation tanks where it is allowed to settle once again. The importance of settling is obvious as a twelve-foot core sample shows a drastic gradation in clarity (displayed by our guide Randy Goble). The surface of the tank is slowly swept by a skimmer to remove surface growth on residual organics.
As an adjunct to the aerobic digestion phase of secondary sewage treatment, the sludge generated in the aeration and sedimentation tanks, material removed by the skimmer, and sedimented solids, and sludge generated at each treatment step are continuously collected from each of the sedimentation basins, combined and treated by anaerobic digestion processes. Anaerobic digesters are heated to 35-38C to maintain a diverse community of bacteria at densities of 109 to1010 cells per ml. These microorganisms break down complex organic compounds in a series of steps to form methane or "natural" gas. The methane (CH4) is then collected in a large inflatable structure, combined with commercial methane from Oxford Natural Gas Company, and used to heat the digesters.
Even after it has been through the anaerobic digesters, about 1% of the sewage ends up as undigestable sludge. Each day 25,000 to 30,000 gallons of this undigestable sludge is dried and pressed, then loaded into dump trucks to be stored for pickup on a routine basis. This dried sludge is eventually converted into fertilizer and spread on farmers' fields.
The now nearly purified water flows from the post-secondary sedimentation tanks into a large basin, where it is chlorinated to kill any remaining potential pathogens. This tertiary effluent is then dechlorinated by aeration as it flows down a series of steps prior to discharge from the sewage treatment plant.
The combination of these three processes allows about 99% of Oxford's daily three million gallons of raw sewage to be recycled ... purified and safely discharged into Four Mile Creek.
