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Aeration Structures

Summary. The aeration structures (basins) are designed to promote the growth of helpful bacteria which consume the nutrients in the sewage. This is called the "activated sludge" process. By injecting large amounts oxygenated air, these bacteria consume the urea, ammonia, carbohydrates, fats, etc. in the sewage and convert it into bacterial mass or "sludge."

History, Design and Operation. The original aeration structures, built in the early 1970's, consist of four square basins, a return sludge pump room, a control room and appurtenant channels and inlet and outlet chambers. Each basin is 50 feet square with a nominal water depth of 14 feet. Each has a volume of approximately 260,000 gallons. The Northerly basins (# 1 and #2) have been converted to primary influent flow equalization basins. These hold higher flows during peak times of the day. The stored primary influent is then slowly added back into the treatment process during low flows, typically after midnight. The Southerly basins (#3 and #4) are currently operated in series and a single large basin with an anoxic zone at the head of the tank.

In 1994 two addtional aeration basins (#5 and #6) were added. These concrete rectangular basins each measure 110 feet long by 25 wide by 14.3 feet deep for a total volume of 308,000 gallons each, Both have anoxic zones at the head and end of the basins to promote the bacterial conversion of nitrates into inert nitrogen gas.

All aeration tanks were modified with the anoxic zones in 2008 and part of the MF/RO Project.

Aeration in each basin is accomplished by air blowers feeding fine bubble diffusers at the bottom of each basin. Air flow is regulated by automatic dissolved oxygen (DO) control systems.

Activated sludge is returned from the bottom of the secondary sedimentation tanks to the front of the aeration basins. When the sludge levels increase sludge is removed ("wasted") at a steady rate by diverting it into either the Sludge Thickener or to the Headworks. The activated sludge process can be operated in several modes including complete mix or variations approaching conventional, step aeration or contact stabilization.

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