Options to reduce greenhouse gas emissions during wastewater treatment for agricultural use

Abstract

Treatment of primarily-domestic sewage wastewater involves on-site greenhouse gas (GHG) emissions due to energy inputs, organic matter degradation and biological nutrient removal (BNR). BNR causes both direct emissions and loss of fertilizer value, thus eliminating possible reduction of emissions caused by fertilizer manufacture. In this study, we estimated on-site GHG emissions under different treatment scenarios, and present options for emission reduction by changing treatment methods, avoiding BNR and by recovering energy from biogas. Given a typical Israeli wastewater strength (1050mg CODl−1), the direct on-site GHG emissions due to energy use were estimated at 1618 and 2102g CO2-eqm−3, respectively, at intermediate and tertiary treatment levels. A potential reduction of approximately 23–55% in GHG emissions could be achieved by fertilizer preservation and VS conversion to biogas. Wastewater fertilizers constituted a GHG abatement potential of 342g CO2-eqm−3. The residual component that remained in the wastewater effluent following intermediate (oxidation ponds) and enhanced (mechanical–biological) treatments was 304–254g CO2-eqm−3 and 65–34g CO2-eqm−3, respectively. Raw sludge constituted approximately 47% of the overall wastewater fertilizers load with an abatement potential of 150g CO2-eqm−3 (385kg CO2-eq dry tonne−1). Inasmuch as anaerobic digestion reduced it to 63g CO2-eqm−3 (261kg CO2-eq dry tonne−1), the GHG abatement gained through renewable biogas energy (approx. 428g CO2-eqm−3) favored digestion. However, sludge composting reduced the fertilizer value to 17g CO2-eqm−3 (121kg CO2-eq dry tonne−1) or less (if emissions, off-site inputs and actual phytoavailability were considered). Taking Israel as an example, fully exploiting the wastewater derived GHG abatement potential could reduce the State overall GHG emissions by almost 1%. This demonstrates the possibility of optional carbon credits which might be exploited in the construction of new wastewater treatment facilities, especially in developing countries.