Methane emission during municipal wastewater treatment

Abstract

Municipal wastewater treatment plants emit methane. Since methane is a potent greenhouse gas that contributes to climate change, the abatement of the emission is necessary to achieve a more sustainable urban water management. This requires thorough knowledge of the amount of methane that is emitted from a plant, but also of the possible sources and sinks of methane on the plant. In this study, the methane emission from a full-scale municipal wastewater facility with sludge digestion was evaluated during one year. At this plant the contribution of methane emissions to the greenhouse gas footprint were slightly higher than the CO₂ emissions related to direct and indirect fossil fuel consumption for energy requirements. By setting up mass balances over the different unit processes, it could be established that three quarters of the total methane emission originated from the anaerobic digestion of primary and secondary sludge. This amount exceeded the carbon dioxide emission that was avoided by utilizing the biogas. About 80% of the methane entering the activated sludge reactor was biologically oxidized. This knowledge led to the identification of possible measures for the abatement of the methane emission.

Introduction

During wastewater treatment, the greenhouse gases carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) can be emitted to the atmosphere (Hofman et al., 2011). Carbon dioxide is produced indirectly as a result of fossil fuel combustion to generate the energy required for the operation of the wastewater treatment plant, or it is produced directly during the respiration of organic matter. In the latter case it concerns short-cycle carbon dioxide that does not contribute to increased atmospheric carbon dioxide concentrations. Nitrous oxide is expected to be emitted during biological nitrogen removal from wastewater, through nitrification and subsequent denitrification (Kampschreur et al., 2009). Since nitrous oxide has a global warming potential of 300 CO₂-equivalents over a 100 year time horizon (IPCC, 2007), even a low emission contributes significantly to a WWTP's greenhouse gas footprint. Not in the least due to its high impact, nitrous oxide emission from wastewater treatment processes recently received a lot of attention. Methane, having a global warming potential of 25 CO₂-equivalents over a 100 year time horizon, is expected to be formed in the sewer system (Guisasola et al., 2008) and in those parts of the WWTP where anaerobic conditions prevail. Hitherto, the emission of methane from wastewater treatment received far less attention than the nitrous oxide emission.

Only two peer reviewed studies investigated the methane emissions of municipal wastewater treatment plants with activated sludge. Both studies used grab sampling to monitor the emissions. Czepiel et al. (1993) studied a small (12,500 PE) WWTP in Durnham, New Hampshire, USA; while Wang et al. (2011) investigated the methane emission from a large plant (1,500,000 PE) in Jinan, China. These WWTPs had no anaerobic digestion facility. The Dutch Foundation for Applied Water Research monitored methane emissions on three wastewater plants: Papendrecht (40,000 PE), Kortenoord (100,000 PE) and Kralingseveer (360,000 PE) (STOWA, 2010). The latter plant – which is also the subject of the present study – had an anaerobic sludge digestion facility, while the former two did not. The reported results from these studies are presented in Table 1. The share of methane in the total greenhouse gas emission from wastewater handling can mount up to 75% expressed as CO₂-equivalents (Foley and Lant, 2007).

Methane is emitted from a WWTP after it enters the plant via stripping from the incoming wastewater, or after it is formed at the plant itself. The influent of a WWTP contains dissolved methane that is formed in the sewer system. Recent studies indicate that methane formation in sewer systems can be substantial (Foley et al., 2009; Guisasola et al., 2008), but actual quantities of methane entering a WWTP have as yet not been reported. As far as methane production at the plant itself is concerned, it can be expected that in anaerobic zones, sludge thickeners and buffer and storage tanks methane is formed. In plants equipped with a digester for the anaerobic digestion of the surplus sludge, this can be expected to be a major source of methane. The biogas from the digester is usually combusted in a gas engine or flared, resulting in methane emissions with the off-gas (Liebetrau et al., 2010; Woess-Gallasch et al., 2010). Dissolved methane that leaves the digester, sludge thickeners and storage tanks will either be stripped during downstream processing (e.g. dewatering of the digested sludge) or will remain dissolved in the reject water and as such it will end up in the aeration tanks of the WWTP, as does the methane in the influent stream. There it can be stripped during aeration, or it can be biologically oxidized by the microorganisms in the activated sludge. The potential methane oxidation of activated sludge systems has, to the authors' best knowledge, not been reported in the literature.

The objective of this study was to determine the contribution of methane to the greenhouse gas footprint of a wastewater treatment plant and to suggest measures to curb methane emissions. To obtain these goals, a one-year measurement campaign was carried out. The WWTP under study is fully covered, except for the secondary settling tanks, and all ventilation air is discharged through one stack. Gas measurements allowed estimation of total methane emissions, except for the exhaust of the biogas-based combined heat and power facility. Mass balances were used to reveal the sinks and sources of methane in this WWTP. Therefore, additional gaseous and liquid sampling was performed. To achieve reliable liquid sampling particular attention was paid to the sampling method.