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Wastewater to Energy: Exploiting Opportunities for Energy Recovery in Municipal and Industrial Wastewater

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Designed in the 1960's and constructed in the mid-1970's, the Milton Regional Sewer Authority's (MRSA's) 3.42 MGD WWTP is in need of a complete facility upgrade. With rising energy costs and more strict regulations the upgrade will take on a much different feel than the original plant design. The upgrade will need to not only account for new nitrogen and phosphorus effluent requirements, but also seek out opportunities to minimize energy dependence through biogas production, electrical generation, biosolids drying and sale of electricity to the Grid.

The end goal for this project is to create a wastewater treatment facility where net energy consumption is zero. The objectives are to maximize energy recovery by exploiting all reasonable sources; to minimize energy use throughout the facility; and to identify the most feasible methods of energy recovery. Anaerobic treatment will be used were applicable to meet these objectives through lower connected horsepower and biogas production. The project will incorporate the anaerobic treatment of industrial wastewater along with improvements in operational efficiencies. It is anticipated that the following cost savings will be realized:

• Electricity Cost: Net savings of between $205,000 and $382,700, annually

• Sludge Disposal Cost: Net savings of $231,000, annually

• Total O&M Cost: Net Savings of between $501,300 and $679,000, annually

The MRSA WWTP is currently rated for 3.42 MGD, roughly half of that flow comes from a food processing facility. It is projected that the ConAgra facility will increase flow to 2.5 MGD, while the municipal flow will increase to 2.0 MGD by expansion of the service area. The resultant 4.5 MGD flow will overload the current plant both hydraulically and organically.

Aging facilities combined with projected overload conditions make an upgrade of the current facilities necessary. The following goals have been identified for the facility upgrade:

• New tankage on the current treatment plant site must be kept at a minimum

• New technology will take energy usage into account

• Anaerobic technology will be used to generate biogas for energy recovery

• Waste heat from the power generator will be used for sludge drying

• Effluent nutrient limits will be 6 mg/l total Nitrogen and 0.8 mg/l total Phosphorus

The food processing wastewater has certain properties that lend itself to anaerobic treatment. These properties include wastewater strength, temperature and bio-degradability. A pilot scale anaerobic treatment study was conducted to determine the ability to anaerobically treat the food processing wastewater, the quantity and quality of biogas generated from the process, as well as the downstream treatability of the anaerobic effluent.

Results of the study were favorable. The ultimate effluent goal of 10 mg/l BOD and 10 mg/l TSS was achieved by following an ADI Bulk Volume Fermentor (ADI-BVF) with a sequencing batch reactor (SBR). Biogas was generated in the expected quantity and better than expected quality. By harvesting the biogas and using it to produce power, the MRSA WWTP will be self-sustaining. Waste heat from the power production will also be captured and used in a sludge drying application.

A demonstration project is planned to anaerobically treat 200,000 gpd of raw influent wastewater. The project will raise the temperature of the municipal wastewater (cold side) via a heat exchanger using non-contact cooling water (NCCW, @ 110 F) as the hot side. The resultant warm municipal wastewater will enter an anaerobic treatment tank and be discharge back into the head of the WWTP. It is expected that 60% BOD and TSS removal will occur in this reactor while creating additional biogas.

This project demonstrates how it is possible to convert an energy intensive wastewater treatment process into a "Green" wastewater treatment process by efficient anaerobic treatment and wastewater to energy processes.
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Keywords: ANAEROBIC TREATMENT; BIOSOLIDS DRYING; RENEWABLE ENERGY; WASTEWATER TO ENERGY

Document Type: Research Article

Publication date: 2007-10-01

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