Energy and Greenhouse Gas Implications for Removal of Micro-Constituents in Wastewater

Enforcement of increasingly stringent effluent limits, and application of more sophisticated treatment processes for reduction of micro-constituents (MCs), increases the energy consumed in municipal wastewater treatment processes. Some MCs 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 MCs, 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 MCs. 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 nearly triple the plant energy consumption.The increased electricity needed to reduce the micro-constituents in wastewater effluents results in higher emissions of greenhouse gases (principally carbon dioxide) from electricity generating stations. The emission rate of GHGs from electricity generation depends on the mix of generation types, including fossil-fuel thermal, nuclear, hydro-electric, wind, geothermal and other types, employed in different geographical area in North America. It is therefore possible to calculate the increase in electricity-related GHG emissions due to use of more advanced energyintensive processes for reduction of MC concentrations in municipal effluents. In environmental stewardship, efforts should be focused on minimizing overall environmental impact. A rush to invest in the high costs of capital improvements, resulting from a decision to impose ultrastringent levels of treatment on the municipal effluents, may not be the wisest path until the impact on the total environment and human health is understood.