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Treatment Process

marsh.jpg Primary Treatment

Once materials that can clog or damage the pumps have been removed from the wastewater, primary treatment begins.  Organic and inorganic solids are removed in the primary clarifier through sedimentation and flotation.  The flow is dispersed, and its velocity is reduced to encourage denser materials to settle out and light materials to float to the surface.  This removes approximately 40 to 60 percent of suspended solids.

Secondary Treatment

After primary treatment, the water flows to the aeration basins.  Here, a population of microorganisms is maintained, fed by organics in the wastewater.  Tiny air bubbles are forced through the water encouraging even and thorough distribution of chermicals to remove phosphorus.  Water then travels to a secondary clarifier to further separate solids from the flow.

Phosphorus Removal

Federal and state reulations require that phosphorus be removed from the wastewater plant effluent before discharge in Seneca Lake.  Since hosphorus acts as a nutrient that promotes algae growth in lakes and streams, all wastewater plants discharging to bodies of water tributary to the Great Lakes must remove phosphorus to less than 1 mg/1.  At the Marsh Creek Plant, phosphorus is removed by adding clairfier iron salts and polymer at several points in the treatment process.  The chemical reaction with soluble phosphorus forms an insoluble precipitate which settles to the bottom of the clarifiers along with the sludge.

Final Clarification

Sewage leaving the aeration train is directed via a 26 in pipe to a flow splitter box between two final clarifiers. These circular clarifiers are 60 ft in diameter. During the side-by-side testing period of May 1993 to September 1993, the secondary treatment system removed approximately 85 percent of the primary effluent BOD. Approximately 89 percent of the BOD was removed during the full-scale testing period of October 1993 to January 1994.

Solid Handling

Sludge from the primary treatment clarifiers and final settling tanks is pumped to the sludge thickeners. The sludge thickeners are 25 ft by 25 ft in size and were outfitted with an odor control system built in 1985.

In the original design, a plate-and-frame filter press provided mechanical sludge de-watering. This unit was quite efficient and produced dry solids in the 30 to 40 percent range, but it was costly to operate. Solids from the plate-and-frame press were transported to the landfill. The high energy costs and slow production, along with chemical and maintenance costs, forced digester an evaluation of alternate de-watering methods. In addition, raw sludge from the primary settling tanks was being sent through the plate-and-frame press, which created both volume and odor problems.

In 1985, two anaerobic digesters were constructed for sludge stabilization. These units are each 50 ft in diameter. The digesters are providing a much needed storage capacity for the sludge generated at the plant. When the operators began phosphorous removal, the quantity of sludge almost doubled. Phosphorous precipitate sludge does not respond well to settling in the sludge thickening process or to de-watering. With the addition of the two digesters and the odor control system for the thickeners, the volume of sludge to be treated by the plate-and-frame filter press was reduced, and the increased sludge volume due to phosphorous removal could easily be handled. The digester system currently provides a volatile solids reduction of around 50 to 60 percent.
In 1990, a Komline Sanderson belt filter press was added for improved solids handling. The big advantage of this belt filter press is its ability to handle greater volumes than the older plate-and-frame filter press. Though the de-watering capability was somewhat reduced (21 percent solids cake) with the belt filter press, the increased volume that could be put through the unit more than made reacter up for this fact. The ease of operating this unit also led to the virtual retiring of the older plane-and-frame press. At this time, the plate-and-frame press serves only a back-up role.

During the summer months, a portion of the sludge stream is diverted to 15,000 sq ft of drying beds that were provided in the 985 upgrade. This enables plant operators to shut down the mechanical de-watering units for approximately two months to perform yearly maintenance on the equipment.

Sludge is removed from the plant and trucked to a sanitary landfill. Approximately 3,500 cu yds of sludge is taken to the landfill each year. Tests on the sludge were done to determine if all requirements of the NYSDEC Part 360 solids regulations could be met for land spreading. After receiving positive results, sludge has been tilled into farmer fields since 1992. In 1994, the City is building a composting system to reduce its sludge disposal costs.

Composting

In an effort to reduce sludge disposal costs, increase available disposal options, and provide for a more environmentally sound means of sludge disposal, the design and construction of an in-vessel composting operation is planned for the Marsh Creek plant. The facility will be constructed beginning in the spring of 1994 with expected completion within 12 months. With the exception compost of the final product storage area, the system is to be constructed within the limits of the existing covered sludge drying bed facility.

De-watered sludge, carbon amendment (sawdust), and recycled compost are carried daily in an enclosed conveyor to the top of the totally enclosed bioreactor vessel to begin the composting process. Air is distributed underneath the compost material, while moist exhaust air is removed from the top of the reactor vessel. The daily plug flow movement of the compost combines with the upward flow of the air to ensure that the entire process remains aerobic. Temperatures in excess of 55 degrees C (131 degrees F) for a minimum of three days ensure pathogen reduction and weed-seed destruction. The totally enclosed conveying and reactor systems capture and hold odors until final discharge of the composting system air to aeration basins. Total in-vessel retention time is between 14 and 21 days, depending on product quality characteristics. The compost is then put on paved cure pads to stabilize further. This process usually takes another 14 to 21 days, during which additional organic stabilization occurs and moisture and compost temperature reduction are achieved.

The final compost is to be marketed as a soil amendment. Several area farmers and nurseries have shown interest in the product.