Municipal gravity sewers: An unrecognised source of nitrous oxide

Abstract

Nitrous oxide (N2O) is a primary ozone-depleting substance and powerful greenhouse gas. N2O emissions from secondary-level wastewater treatment processes are relatively well understood as a result of intensive international research effort in recent times, yet little information exists to date on the role of sewers in wastewater management chain N2O dynamics. Here we provide the first detailed assessment of N2O levels in the untreated influent (i.e. sewer network effluent) of three large Australian metropolitan wastewater treatment plants. Contrary to current international (IPCC) guidance, results show gravity sewers to be a likely source of N2O. Results from the monitoring program revealed hydraulic flow rate as a strong driver for N2O generation in gravity sewers, with microbial processes (nitrification and possibly denitrification) implicated as the main processes responsible for its production. Results were also used to develop a presumptive emission factor for N2O in the context of municipal gravity sewers. Considering the discrepancy with current IPCC Guidelines, further work is warranted to assess the scale and dynamics of N2O production in sewers elsewhere.

Influence of infrastructure on water quality and greenhouse gas dynamics in urban streams

“…While sanitary sewer lines are known to leak dissolved N, N 2 O losses are not accounted for in greenhouse gas budgets of the large WWTPs that these pipes feed into. Short et al (2014) measured intake lines from three municipal WWTPs and estimated that N 2 O emissions from sewer lines alone are on the same order of magnitude (1.7 g N 2 O person yr −1 ) as current IPCC estimates for per capita emissions from secondary WWTPs. Their study demonstrates the importance of constraining biogenic gas emissions from streams, which flow alongside and may receive gaseous inputs from aging sanitary sewer lines.…”

“…Septic systems, primarily used in low-density residential areas, are designed to settle out waste solids and leach N-rich liquid waste into subsurface soils and groundwater. Sanitary sewer infrastructure may influence GHG abundance and emission from streams directly via diffusion of gases out of gravity sewer lines (Short et al, 2014) or indirectly by microbial processing along surface and subsurface flow paths (Yu et al, 2013;Beaulieu et al, 2011). While the present study focuses mainly on first-to third-order streams influenced by sanitary sewer lines or septic systems, it is also worth mentioning that WWTPs are known to be a source of CH 4 and N 2 O in urban areas and contribute point-source GHG loading to larger rivers and coastal areas Strokal and Kroeze, 2014;Alshboul et al, 2016).…”

“…Sewage leaks are likely the primary source of N 2 O emissions from small urban streams (Short et al, 2014). Several studies have documented that wastewater leakage from municipal sewers often accounts for more than 50 % of dissolved N in urban streams (Kaushal et al, 2011;Pennino et al, 2016;Divers et al, 2013).…”

“…N 2 O emissions from uncollected human waste (i.e., leaky sanitary sewer lines, septic system effluent, dug pits) are largely unmeasured globally (Strokal and Kroeze, 2014;UNEP, 2013) and warrant further study in the context of watershed management as well as local GHG accounting. Direct emissions from wastewater treatment plants are well documented (Foley et al, 2010;Townsend-Small et al, 2011;Strokal and Kroeze, 2014;UNEP, 2013); however, the upstream losses of N 2 O from delivery pipes into streams and rivers are not well documented (Short et al, 2014). Short et al (2014) measured N 2 O concentrations in WWTP influent in Australia and determined that sanitary sewers are consistently supersaturated with N 2 O, with concentrations in excess of equilibrium by as much as 3.5 µM.…”

Comparing the Life Cycle Energy Consumption, Global Warming and Eutrophication Potentials of Several Water and Waste Service Options

“…The volatilization of N 2 O in sewer systems was calculated based on the equations derived from Short et al [67]. The nitrogen and phosphorus outputs for the septic elements came from the pilot testing datasets from the Barnstable County Department of Health and the Environment in Cape Cod.…”