Package System Helps Handle Variable Floc

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The Village of Dexter, N.Y., is using a package treatment system that features a submerged, fixed-film aeration system to comply with the state of New York’s effluent discharge requirements. Like many eastern cities, the town on the eastern shores of Lake Ontario experiences infiltration and inflow problems as a result of an old sewer system.

The village had faced compliance problems because of wet-weather flows, which can be six times the daily average flow and last up to two days. Dexter has spent hundreds of thousands of dollars refurbishing the system, but infiltration and inflow is still a problem throughout the system.

The city and its engineers had the choice of continuing massive rehabilitation of the sewer system or redesigning the treatment plant. Dexter decided to purchase a factory-built FAST® (Fixed Activated Sludge Treatment) system from Smith & Loveless, Inc.

The FAST treatment system combines attached bacterial growth and suspended growth simultaneously, thus ensuring a constant bacterial population even during high inflow and infiltration events. A submerged media with a high surface area rests within the aeration zone. The aeration system transfers oxygen to the wastewater, and at the same time pumps the wastewater through the submerged media. Some of the bacterial growth attaches to the surface of the media while suspended growth occurs within the aeration system.

The biological solids that slough off the media are transported into a separate clarification system. These solids are generally excess, and the sludge from the clarifier is wasted to sludge holding. Because the system adjusts itself to load conditions, the plant operator does not have to adjust the system in terms of return activated sludge to maintain the bacterial population. The only required operation is occasional air scouring of the submerged media to dislodge excess bacteria from the media.

The Dexter plant was designed to provide an average monthly effluent quality of 30 mg/L BOD and 30 mg/L TSS. Established design criteria indicated the total BOD load to the system would not exceed 418 lbs/BOD/day while flow to the plant could range from the basic domestic flow of about 250,000 gpd to 1.5 mgd during high infiltration periods. The infiltration flows offer little or no additional BOD contribution above the domestic flow.

The state required duplicity of aeration zones in the system, so the plant was divided into two separate trains, each capable of handling 125,000 gpd under normal conditions. One train was required to hydraulically handle up to 1 mgd in case the other was shut down for maintenance.

The influent is pumped into a mechanically cleaned bar screen, and the flow is then divided equally between the two treatment systems. After initial treatment, the flow is directed into two 30 ft. diameter peripheral feed Smith & Loveless Kraus-Fall clarifiers.

The piping arrangement allows flow from the package plants to be diverted into either of the two clarifiers. Return activated sludge from the clarifiers can flow back into the splitter box at the bar screen headworks, or be wasted to an aerobic digester.

The FAST process has minimized washout of bacteria during high flows and lowered the average BOD and TSS levels to 15 mg/L, maintaining consistent effluent quality throughout peak flow periods. Since installing the system, Dexter has not had a wastewater discharge violation.

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