Plano, S. and Berrisford, M., Plano Engineering Ltd, UK
Abstract Local authorities and utility companies responsible for operating Wastewater Treatment Plants (WWTPs) are confronted with a growing range of challenges such as more stringent environmental laws, increasing hydraulic capacity, energy and sludge disposal costs. The tightening of discharge limits under the water frame directive (WFD), will affect negatively energy consumption, GHG emission and construction costs unless a new design approach based on Energy and Carbon footprint analysis coupled with more effective operational efficiency strategies is used. Most biological wastewater treatment plants (WWTPs) employ aerobic processes to ensure effluent quality standards fixed by the law. Aeration is the highest energy footprint for municipal and industrial wastewater treatment plants, ranging from 50 to 75% of total energy consumption.
In this paper, a novel method is proposed for optimising the set-up of existing plants and monitoring oxygen transfer efficiency at field conditions from wastewater aeration basins with using submerged air-diffusers.
Keywords Aeration, Oxygen Transfer Efficiency, Submerged air diffusers, Energy Consumption,
Introduction Since 1914, when the activated sludge process was developed by Ardern and Lockett in the laboratory of the Manchester Sewage Treatment Works, thousands of full-scale wastewater biological plants are operated in Europe and in the rest of the world. The activated sludge process and its many variants are now the main process for sewage treatment and have had a huge impact on environmental improvement in the past century.
As the European Union has expanded there have been a series of directives aimed at the prevention of water pollution and protection of cross-border water resources. In the 21st century operators and managers of wastewater plants are facing new challenges such as more stringent standards for nitrogen and phosphorus levels, increasing the energy and sludge treatment costs are involved. Moreover, the U.S. Environmental Protection Agency has estimated that 4% of the equivalent anthropogenic greenhouse gas emissions in the world result from methane and nitrous oxide produced from wastewater and solid wastes systems.
Most biological wastewater treatment plants employ aerobic processes to ensure effluent quality standards. Aeration is the highest energy footprint for municipal and industrial wastewater treatment plants, ranging from 50 to 75% of total energy consumption.
In the last two decades, with the increasing energy costs, more efficient fine pore diffusers have replaced coarse pore diffusers and surface aerators. Although fine pore diffusers can improve the energy efficiency of aeration basins in the short term, they are subject to fouling and scaling, resulting over time in increased head loss and reduced Oxygen Transfer Efficiency, both contributing ultimately to increased plant energy costs and costs of regular cleaning and downtime.
When evaluating a given aeration system (blowers + air pipes + air diffusers + diffuser valves + blower valves), a number of factors intrinsic to the aeration devices will affect the performance of diffusers including the process layout, the mode of operation of process, the control methodology used and the maintenance of the equipment. For submerged air diffusers these factors include: diffuser type, diffuser placement, diffuser density, gas flow rate per diffuser, basin geometry, wastewater composition, process type and flow regime, process loading, DO control, deterioration of diffusers, mechanical integrity of diffusers.
