SYSTEMS APPROACH TO INTEGRATING BIOSOLIDS AND ENERGY MANAGEMENT

Biogas is a valuable resource for wastewater treatment plants where anaerobic digestion is used for solids stabilization because it offers the opportunity to substantially reduce plant energy costs. Optimizing the production and beneficial use of biogas is a critical component of energy management and should be evaluated by taking a systems approach which considers gas and energy management in conjunction with other plant process and operations decisions. This is especially the case when there are more demands for biogas at wastewater treatment plants (e.g., boilers, cogeneration, dryers). This paper presents a systems model to describe the interrelationships between key wastewater treatment plant variables in an effort to assist plant owners with their decision about how to create optimal and integrated solids and energy management plans.Wastewater agencies are faced with certain independent variables which are beyond their control and other variables which are adjustable or dependant on process or operations decisions. Independent variables include utility rates, emissions restrictions, influent flows and loads, existing plant tank sizes and layout, labor rates, availability of natural gas, and geography. Adjustable or dependant variables include process choices, operations choices (e.g., batch vs. continuous), total plant electrical and heat demands, and the decision to accept additional optional waste streams (e.g., organic waste from agriculture, food processing, etc.). Energy management strategies consider the optimal use of biogas verses purchased utilities in response to these independent and dependent variables. Energy management strategies include demand side management, resource management of biogas, and rate analysis and utility interaction. Selection of the appropriate operating scenarios and technologies for biogas use, for example: cogeneration and biosolids heat drying, are the resulting outcome from the interaction of many variables. The ability of wastewater agencies to identify which variables are key and the interactions between these variables for their specific plant is crucial for good strategic planning. This paper synthesizes the variables that influence biogas and energy management decisions into a systems model to assist decision makers.Case studies from three wastewater facilities are presented to illustrate the systems model. The three facilities described are: the Encina Water Pollution Control Facility (EWPCF, located in Carlsbad, CA), Lethbridge Wastewater Treatment Plant (LWWTP, Lethbridge, Alberta) and Chambers Creek Wastewater Treatment Plant (CCWWTP, located in Pierce County, WA). Two have anaerobic digesters and either already have biosolids heat dryers or plan to install dryers in the near future. The unique features of each of these plants relevant to the energy, biogas, and biosolids management decisions are presented and applied to the systems model. EWPCF has a cogeneration system and has decided to add heat drying. CCWWTP has biogas fuel boilers and is evaluating whether to add cogeneration. In making energy, biogas, and solids processing decisions, each agency evaluated numerous technologies (e.g., internal combustion engines, gas turbines, Stirling engines, fuel cells, gas storage), operating scenarios (e.g., varied energy and dryer operations), and the associated cost, risk, and flexibility implications. Results of the technological and cost-benefit evaluations were evaluated and compared with respect to biogas and energy management goals. Although the outcomes are different at each facility, the decision making process was similar and consistent with the systems model approach.