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The Encina Wastewater Authority (EWA) optimized the beneficial use of bioenergy at their water pollution control facility in Carlsbad, California through a comprehensive operations management planning approach. Bioenergy refers to the energy associated with biosolids processing, such as biogas from anaerobic digestion and the energy demand for biosolids drying. The Encina Water Pollution Control Facility (EWPCF) has a design capacity of 36 million gallons per day (mgd) liquid and 38 mgd solids. Major operating costs at the Encina WPCF are labor, chemical, biosolids, and energy. Keeping labor costs constant, EWA has lowered their current and projected operating costs by increasing the use of chemicals, reducing aeration air and electrical demands, producing more biogas, and decreasing the amount of biosolids requiring dewatering and drying.

Implementing Chemically Enhanced Primary Treatment (CEPT) has resulted in increased anaerobic treatment and decreased aerobic treatment. Chemical addition of ferric chloride and polymer upstream of the primary sedimentation tanks flocculates the settleable solids to improve removal efficiency in the primary sedimentation tanks. The enhanced primary sedimentation performance has increased the total suspended solids (TSS) and biochemical oxygen demand (BOD) removal rates. Biogas production has increased because a greater fraction of the sludge being fed to the anaerobic digesters is primary sludge, and volatile solids destruction of primary sludge is greater than secondary sludge. Greater BOD removal in primary treatment has resulted in lower aeration air demand in secondary treatment, resulting in energy savings. CEPT reduced overall plant energy demand by approximately 45 percent due to reduced aeration demands and shut down of associated auxiliary system that support the secondary treatment process.

A biosolids heat drying system with high solids centrifuges and three-pass rotary drum dryers have been conceptually designed. A key issue with heat drying is the energy requirement. Biogas from anaerobic digestion will be the primary source of fuel. The gas consumption of the dryer has a direct impact on EWPCF's cogeneration operation. The amount of energy required for biosolids drying depends on the efficiency of dewatering prior to drying, the amount of biosolids requiring drying, and the efficiency of the drying system itself. With more biogas being produced because of CEPT, less biosolids requiring drying, and the plan to use a highly efficient three-pass dryer, it was projected that more biogas will be produced than will be consumed by the dryer. The excess biogas will be beneficially used in the cogeneration system. Usually, supplemental fuel such as natural gas is required in addition to the biogas to fuel the heat drying process.

An energy management strategic plan was prepared to integrate the existing cogeneration and the future biosolids heat drying operations. The objectives were to meet the existing and future plant energy demands with a high efficiency energy system that beneficially uses the biogas, is cost-effective, and does not significantly increase air emissions. EWA decided that this system should have operational flexibility to adapt to varying operating conditions and be robust to varying energy rates. It would also provide additional power reliability by having onsite generation. The energy plan took a holistic approach that considered wastewater treatment, CEPT, aeration air demand, anaerobic digestion heating requirements and biogas production, and energy requirements of biosolids heat drying. Using exhaust gas from lean burn engines to supply some of the heat demand of the dryers was evaluated. In addition, heat recovered from condensate produced by the sludge drying system was evaluated and will be used for sludge heating for anaerobic digestion. Engine-driven, multi-stage aeration blowers were compared to motor-driven, multi-stage blowers and single-stage blowers. The continued use of internal combustion engine technology as well as emerging technologies such as microturbines and fuel cells were evaluated for the cogeneration system.

The energy management strategic plan considered the cogeneration and heat drying system at the EWA facilities as part of a comprehensive operations management plan that optimized the beneficial use of the available biogas.
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Document Type: Research Article

Publication date: 2004-01-01

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