Hydrated Oil Shale Ash Mitigates Greenhouse Gas Emissions from Horizontal Subsurface Flow Filters for Wastewater Treatment

Previous pilot-scale studies have shown outstanding levels of efficiency in phosphorus removal by using hydrated oil shale ash (HOSA) sediments in horizontal subsurface flow (HSSF) filters with low greenhouse gas emissions. However, no long-term full-scale experiment has been conducted using this material. From September 2013 to December 2015, two HSSF filters with different hydraulic loading regimes (NH1 with a stable loading regime and NH2 with a fluctuating regime), used to treat municipal wastewater, were analysed to estimate greenhouse gas (GHG) fluxes and to develop a treatment system with minimised GHG emissions. The fluxes of CO2, CH4 and N2O, as well as their emission factors were significantly lower when compared with studies where regular filter materials (sand, gravel, etc.) are in use. The fluctuating loading regime significantly increased CO2 and N2O fluxes (median values of -3.3 and 2.6 mg CO2-C m(-2) h(-1), and 5.7 and 8.6 mu g N2O-N m(-2) h(-1) for NH1 and NH2 regimes, respectively), whereas no impact could be seen on CH4 emissions (median 93.3 and 95.6 mu g CH4-C m(-2) h(-1), for NH1 and NH2, respectively). All GHG emissions were strongly affected by the chemical composition of the water entering into the system. The water purification efficiency of the system was satisfactory for most water quality parameters and excellent for phosphorus. Thus, the HOSA-filled filters have a good potential for municipal wastewater treatment with low GHG emission.