The Road to Energy Autarky in Municipal Water Resource Recovery Facilities

Abstract

Wastewater treatment has traditionally been one of the largest power users in a municipality's energy budget. New technologies and operating methods have allowed some water resource recovery facilities (WWRF) to actually achieve energy autarky. General treatment strategies for wastewater treatment around the world involve the growth of bacteria to consume carbon and generate biomass or methane. This treatment is typically divided into physical treatment processes, primary clarification, and biological treatment processes, aeration and secondary clarification. Typically, 35% of the carbon in raw sewage can be separated with primary clarification. The remaining carbon is treated though aerobic biological processes which converts carbon to biosolids and CO2, and ammonia to nitrate and nitrogen. The resulting solids are separated from the treated wastewater through the secondary clarification process. The primary and secondary solids can be combined and digested anaerobically, generating a biogas, which is contains approximately 60% to 70% methane. The generated biogas can heat anaerobic digesters, dry the dewatered biosolids reducing transportation costs, and power engines to generate electricity. Large quantities of air must be supplied to allow the secondary process to work correctly. Typically, it takes 1.2 pounds of oxygen for every pound of biochemical oxygen demand (BOD) and 4.3 pounds of oxygen for every pound of ammonia, including organic ammonia that is present in the wastewater entering the secondary treatment process. Inefficiencies of mass transfer require more oxygen to be supplied. Because air is only 21% oxygen, large quantities of air must be compressed and bubbled through the wastewater to provide the needed oxygen. Because of the inefficiencies of the oxygen transfer, energy consumption estimates for secondary wastewater treatment can range from 0.4 to 2.6 kWh per lb BOD removed. Improvements in gas transfer methods continue to be made. A shift has slowly been taking place from the use of course bubble aeration systems to fine bubble and micro-bubble aeration systems. The increase in surface area due to smaller bubbles increases the efficiency of gas transfer. The use of deeper tanks for aeration also improves transfer efficiency since the pressure at the deeper depths allows more oxygen to be dissolved into the liquid phase. There is, however, a tradeoff to deeper tank depths. Increased depths equate to higher discharge pressures for air compression equipment, which means more power must be used to provide this increased compression. Typical depths of aeration have increased from about 12 feet to between 20 and 30 feet. Even with these improvements, a typical wastewater plant remains a net consumer of electrical power. Several new developments in wastewater treatment have emerged in the recent decades that have converged to allow the power required for wastewater treatment to be substantially reduced, and the capacity for biogas generation to increase greatly. One of these developments was the discovery of anaerobic bacteria that consumes ammonia as its food source, and uses nitrite as an electron acceptor. These bacteria, called anammox, were first used WWRFs in side-stream processes, to reduce the total ammonia returning to the plant from sludge handling processes. Research into the mechanism of the nitrification/denitrification process led to a discovery that the traditional process path of nitrification and denitrification had a shortcut path, referred to as nitrite shunt. The nitrite shunt pathway reduces the total amount of oxygen needed to convert ammonia to nitrogen. Concerns with energy use for wastewater treatment combined with goals for nutrient limits, led to the realization that these anammox bacteria and the nitrite shunt pathway could be utilized in the mainstream treatment process, and an ideal configuration could be established whereby: Additional carbon can be removed in the primary treatment process though either chemical co-precipitation or through biological adsorption in a very High Rate Activated Sludge (HRAS) process, providing additional feed to the anaerobic digesters The additional feed to the anaerobic digesters allows greater biogas production Improvements in the digestion process, along with co-digestion or highly digestible organic feed stock (i.e. Fats, Oils and Greases or FOG) allows yet further increases in biogas production The decrease in carbon to the secondary treatment process (through improved primary treatment) reduced the amount of oxygen that had to be supplied for secondary treatment The environment that allows the nitrite shunt pathway and also favors anammox bacteria further reduces the amount of air required for secondary treatment The convergence of these changes and discoveries has allowed several plants to achieve energy autarky. Even if not fully implemented to achieve energy independence, the partial implementation of some of these changes could allow significant energy savings.