Achieving Stringent Effluent Limits Takes a Lot of Energy!
Enforcement of increasingly stringent effluent limits, and application of more sophisticated treatment processes for reduction of chemicals of potential concern, increases the energy consumed in municipal wastewater treatment processes. Energy consumption factors (ECFs) have been developed for wastewater treatment plant configurations based on the quantity of electrical energy used per cubic meter of wastewater treated (kWh/m3) using published technical data, actual plant data and from operating costs algorithms provided in design and costing software. Several secondary or equivalent treatment processes, including facultative ponds, lagoons, rotating biological contactors and trickling filters have similar energy consumption factors, in the range of 100-400 kWh/1000 m3 treated. Activated sludge processes, use more energy, typically in the range of 350 – 650 kWh/1000 m3 treated. Oxidation ditches and
extended aeration facilities have higher energy consumption factors, ranging from 450 – 1050 kWh/ 1000 m3 treated. Some compounds of potential concern (CPCs) can be reduced or eliminated effectively when secondary processes are operated to fully nitrify ammonia.
Upgrading non-nitrifying treatment facilities to full nitrification would result in an increase in energy consumption of approximately 300 to 500 kWh/1000 m3 treated. Other CPCs, such as pesticides, and some pharmaceuticals are relatively resistant to biological oxidation. Membrane processes such as reverse osmosis (RO) or nanofiltration (NF) and advanced oxidation processes (AOP), such as UV/ozone or UV/hydrogen peroxide, have also been shown as effective for many CPCs. The combined energy consumption for an AOP consisting of UV/ozone would total between 220 and 420 kWh/1000 m3 treated. Inclusion of RO may triple or quadruple the plant energy consumption. Wastewater facilities have a number of options to reduce the impact of increased energy consumption, by use of efficient equipment, optimized operating procedures, automation and full recovery of the energy value of biogas from anaerobic sludge digestion.