The goal of 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 Rumpke 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. The sewage is transferred to an aerobic digester in which large pumps keep dissolved oxygen levels high. The sewage is then 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 organics are broken down as the sewage trickles past the microbes growing on the slats. Clarified effluent 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 clarified effluent flows to large 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. Finally, clarified liquid effluent is chlorinated in large tanks to kill any remaining potential pathogens and then dechlorinated by aeration as it flows down a series of steps prior to discharge into Four Mile Creek.
The sludge generated in the aeration tanks, material removed by the skimmer, and sedimented solids generated at each treatment step are 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.
The combination of these processes allow about 99% of Oxford's daily three million gallons of raw sewage to be safely discharged into Four Mile Creek. Even after all of these steps, however, each day 25,000 to 30,000 gallons of undigestable sludge is dried and pressed, then loaded into dump trucks to be spread as fertilizer.
