Pennsylvania Filtration Plant Helps Meet NPDES Discharge Requirements

Dec. 28, 2000

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In the early 1990s, the Milford-Trumbauersville Area Sewer Authority in Bucks County, Pennsylvania, recognized the need to upgrade the existing sewage treatment plant. This was necessary to increase capacity and to meet future more stringent requirements set by the Pennsylvania Department of Environmental Protection for Unami Creek — among the few U.S. creeks with low discharge requirements set by the National Pollutant Discharge System (NPDES).

The existing treatment plant provides secondary biological treatment via the extended aeration activated sludge process and tertiary filtration with sand filters.

The Authority retained Pennoni Associates to perform engineering design and construction management for the construction of a one million-gallon per day plant upgrade. With the assistance of plant manager Lee Myers, they investigated various tertiary filters used successfully at other locations.

To expand the existing discharge flow, the Authority added two additional USFilter Davco Products’ circular clarifiers. USFilter Microfloc products’ Trident Process was selected for the advanced wastewater treatment. In addition to reliability and optimal performance, the Trident technology provided a small footprint, and required no additional air supply system or associated electrical and control equipment.

The Trident process utilizes an upflow adsorption clarification combining flocculation and clarification in a single unit occupying less than 20 percent of the floor space traditionally needed by such plants. The adsorption clarifier uses special buoyant media held in place by a screen at the top of the clarifier tank.

From the adsorption clarifier, the flow passes through the mixed media filtration chamber, after which it is collected by a Triton direct retention underdrain.

The wastewater generated by both the filter backwash cycles and the flush cycles of the clarifier is recycled back to a settling basin. Both the backwash water and the flush water are disposed into a special holding tank, de-watered, and then returned (approximately 90 percent) back to the head of the Trident units for re-treatment.

The primary purpose of the Trident units is to remove soluble phosphorous. High levels of phosphorous encourage plant growth within the stream, and that would have an adverse effect on the aquatic life in the river.

The treatment plant has proven very successful. The discharge into the trout laden Unami Creek is of such high quality that it meets or exceeds the minimum public drinking water standards. Most importantly, the facility far exceeds the contaminate removal expectations and requirements as defined in the NPDES regulatory permit. Over 99.5 percent of all contaminants are removed from the waste stream prior to discharge. Phosphorous influent levels are reduced from an average 6.85ppm to below 0.3ppm.

"The plant performs very well," stated Lee Myers. "In fact, the plant effluent remains unchanged even during the occasional upsets in the concentration of suspended solids in the secondary effluent."

The plant is the first application on the East Coast using upflow clarification and mixed media filter technology to remove phosphorous and suspended solids from wastewater.