City of Greenfield, IN, upgrades wastewater treatment facility

The City of Greenfield owns and operates a Class III activated sludge wastewater treatment facility...

Challenge
The City of Greenfield owns and operates a Class III activated sludge wastewater treatment facility. In October 2002 design improvements to increase daily average flow capacity to 4.0 million gallons per day (mgd) and peak capacity to 10.0 mgd. were needed. The plant's main problem was the washing out of solids in their secondary clarifiers during high flow conditions.

Solution
The solution was a Trans-Flo® secondary clarifier upgrade. Trans-Flo® Clarifier systems were installed in three existing 20 ft x 80 ft x 11 ft side water depth (SWD) concrete tanks that previously contained siphoning bridge clarifiers sludge removal equipment thereby increasing the three tanks' overflow rate. Secondary capacity was increased further by operating the Trans-Flo® Clarifier in parallel with a new Envirex single 80 ft. diameter x 14 ft SWD Rim-Flo® Tow-Bro® Clarifier. The additional upgrades to the facility included the construction of new screening, grit removal and raw sewage pumping. The exisiting primary clarifiers were expanded from two to four tanks. The exisiting aerobic digester was converted to a fourth aeration tank.

The Trans-Flo® secondary clarifier incorporates a major design breakthrough. It is a combination of three proven wastewater technologies, with the process benefits of each. In a single basin, the Trans-Flo clarifier combines a Rim-Flo® peripheral side feed and takeoff clarifier, Tow-Bro® hydraulic sludge removal and a chain and scraper collector mechanism. This new design produces a secondary effluent that is lower in suspended solids, and fresher more concentrated sludge. As a result, Trans-Flo® clarifiers can occupy a smaller space with lower construction and equipment costs than conventional clarifiers.

The operation of the Trans-Flo® Clarifier begins when influent is introduced into a channel through inlet orifices and extends the entire length of the tank greatly reducing the velocity. Flow moves downward then transversely across the tank, up and back to the effluent channel. The full volume of the tank is used with short circuiting eliminated and solids settling uniformly.

Settled solids are moved a short distance by the chain and flight mechanism to multiple hydraulic sludge removal headers. The flights move at one foot per minute (0.3 m per minute) preventing disturbance to the sludge blanket. The sludge removal headers are equipped with a series of orifices to continuously remove settled sludge uniformly across the width of the tank. The thickest layer of the sludge blanket is always removed first so that the solids required to maintain the activated sludge process are returned quickly without unnecessary dilution.

Because solids deposition is uniform along the tank's length, sludge removal headers are evenly spaced to keep the average detention time of the settled solids in the range of 20 minutes. This assures a fresher and more concentrated sludge to be returned to the aeration basin.

Results
Besides the excellent process performance, the plant personnel said that they have not experienced any mechanical problems since their primary and Trans-Flo® secondary clarifiers were placed on line more than four years ago. The Trans-Flo® Clarifier is also producing a TSS effluent discharge of less than 10 parts per million. According to the plant operator, "The installation of the Trans-Flo® Clarifier performed 100% better than the previous rectangular siphoning clarifiers."

December 2008

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