i n c u n rv v D c r t v cts. Photo by Riziero Vertolli Benny Seminerio the superv isor of the Mid Halton and South-East Wastewater Treatment Plants. Recycling Oakville's used water (C ontinued from page 1) the QEW and east of Trafalgar Road. When fully expanded to accommo date growth, including that occurring in Milton, it will be the largest such facility in town. The Southeast plant at Winston Churchill Boulevard and Lakeshore Road services Oakville south of the QEW and east of Navy Street. The Southwest facility at Lakeshore Road near Coronation Park services Oakville south of the QEW west of Navy Street. The average flow of both the Mid Halton and Southeast plants is around 80,000 cubic metres of sewage a day, about 35,000 cubic metres for the Southwest facility. While the process is a complicat ed one, the following is a basic thumbnail sketch of how sewage and sludge (the heavier biosolids separat ed from the main stream) are treated at the Mid Halton Wastewater Treatment Plant: Raw sewage is first collected and transported through the network of gravity-fed sevVers and pumping sta tions to the treatment plants where it is first automatically sifted to ensure that only suitably sized particles pass through the remainder of the system. The removed debris, is disposed as landfill. At this point, sewage enters a tank where grit and sand is separated and removed to landfill, after which the remaining liquid is returned to the main flow. In the primary settling phase, where the average "detention period" is three hours, larger organic particles settle to the bottom and create a heavy sludge, which is then collected by an underwater mechanism. This stage removes 45% to 50% of pollu tants. What remains is then pumped into the "primary digester" for more treatment. During the complex five-hour aeration component of the system, a carefully controlled bacteriological balance is maintained through the use of large quantities of oxygen and return sludge from the final settling tanks. "The bacteria is our inventory, it does the work for us," says Seminerio. "Basically it uses the sewage as food." These "bugs" are so small that other "representative organisms" are constantly monitored to determine the volumes of bacteria necessary to digest the sewage to ensure that they remain in proper equilibrium with their food. "We're simulating what Mother Nature does, but we've made it more efficient," said Seminerio. Effluent from the aeration tanks then enters the final settling tanks, which again employ underwater col lector mechanisms to remove the majority of sludge for pumping back to the aeration tanks to maintain the bacteriological balance. Prior to final settling for phospho rus removal, effluent continues on and comes in contact with a concen trated sodium hypochlorite solution in the chlorine mixing chamber. This disinfects the wastewater to. kill any remaining harmful bacteria. Effluent then flows by gravity to Lake Ontario through the outfall sewer from the South-West plant at the end of a pipe one kilometer out in the lake and 35 feet down. From start to finish, the entire process takes 12 hours. "It's almost as clean as drinking water," said Seminerio. All the sludge that's been accumu lating is pumped through a heat exchanger and pre-heated prior to the primary anaerobic digester, which is fitted with a steel casing and a gas mixing system. Here, the necessary environment is created for the sludge's decomposition. After approximately 30 days at a temperature of 35°C, the digested sludge flows to the unheated sec ondary digester where it settles and its more liquid component is returned to the aeration tanks for more treat ment. One byproduct of this process is methane gas, which is collected then used to power an on-site generator that provides electricity as well as heat for digestion and the buildings. Meanwhile, the thickened sludge is drawn off the bottom of the sec ondary digester and pumped to a tanker truck station. Unlike Toronto, which bums its sludge, Halton Region transports it free to farmers for use on their fields as fertilizer. "The Ministry encourages recy cling that way," says Mohammed. terrace ford lincoln E N D Photo by Riziero Vertolli Bacteriological agents mL\ with wastewater as part of the process ongoing at the Mid Halton Wastewater Treatment Plant. sa les O F M O D E L C LEA R O U T! 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