April 2009, Vol. 21, No.4

Plant Profile

Washington County (N.Y.) Wastewater Treatment Plant

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Location: Fort Edward, N.Y.
Startup date: 1988
Service population: 15,000
Number of employees: 11
Design flow: 2.28 mgd (8630 m3/d)
Average daily flow: 2.3 mgd (8710 m3/d)
Peak flow: 5 mgd (18,900 m3/d)
Annual operating cost: $680,000
 

As at most treatment plants, the staff at the Washington County (N.Y.) Wastewater Treatment Plant seeks to keep the energy bills low, get the best performance possible, and be good stewards of the environment. The staff operates a 2.28-mgd (8630-m3/d) conventional activated sludge treatment process to serve its 15,000 residents. To keep utility costs down, the staff members have carried out several projects to make the plant’s operation more energy-conscious.

For example, the plant switched its waste activated sludge thickening process from dissolved-air flotation to a rotating sludge thickener. This change reduced power consumption for thickening by 90%.

After the waste activated sludge is thickened to 4% solids, it is mixed with primary sludge and digested in two 45-ft-diameter (14-m-diameter) anaerobic digesters. The biogas from the digesters is captured and used to generate electricity. Biogas-derived electricity provides between 10% and 25% of the plant’s power. The heat from burning the biogas also is used to heat some areas of the plant.

The liquid processes at the plant begin as influent enters the headworks via three 5-mgd (18,900-m3/d) screw pumps. The flow passes through a mechanical screen and splits into two grit chambers on its way to two rectangular primary clarifiers. After primary treatment, the wastewater passes through the activated sludge tanks and two secondary clarifiers.

A new disinfection system is scheduled to begin operating this month. The plant’s National Pollutant Discharge Elimination System permit requires disinfection between May 1 and Oct. 31. After disinfection, the effluent is discharged to the Hudson River.

However, before the effluent reaches the river, it’s used as the basis of the plant’s most innovative energy reduction project — a geothermal heat pump to heat its new 2000-ft2 (186-m2) garage. Rather than using a conventional geothermal heat source, such as wells or buried pipe loops, the plant uses its effluent. The heat pump is supplied with 10 gal/min (38 L/min) of effluent by the plant water system to produce 64,389 Btu/h.

The system uses no external heat exchanger. A 30-mesh and a 20-mesh strainer, in series, filter effluent before it enters the heat pump, and effluent is pumped directly through the machine. Gravity carries used effluent back to the head of the plant. Plant staff installed the machine under direction from the heat-pump supplier.

The effluent temperature drops between 7°F and 9°F (4°C and 5°C) as the heat pump extracts this heat to warm the air in the garage. The heat pump produces approximately 3.68 Btu for each Btu put into the system. This makes its operation 30% cheaper to run than a natural gas system (at current market prices). The heat pump also produces 70% less greenhouse gases than using natural gas while doing the same work. The heat pump draws 5.12 KWh of electric power.

The heat pump costs about $7000. A natural gas furnace with a similar heating capacity would have cost only $1400; however, with the energy savings, the plant expects a simple payback on the equipment in 3 to 4 years.

The system uses no external heat exchanger. A 30-mesh and a 20-mesh strainer, in series, filter effluent before it enters the heat pump, and effluent is pumped directly through the machine. Gravity carries used effluent back to the head of the plant. Plant staff installed the machine under direction from the heat-pump supplier.The effluent temperature drops between 7°F and 9°F (4°C and 5°C) as the heat pump extracts this heat to warm the air in the garage. The heat pump produces approximately 3.68 Btu for each Btu put into the system. This makes its operation 30% cheaper to run than a natural gas system (at current market prices). The heat pump also produces 70% less greenhouse gases than using natural gas while doing the same work. The heat pump draws 5.12 KWh of electric power.The heat pump costs about $7000. A natural gas furnace with a similar heating capacity would have cost only $1400; however, with the energy savings, the plant expects a simple payback on the equipment in 3 to 4 years.