The importance of sewage effluent discharge in the export of dissolved organic carbon from United Kingdom rivers

Abstract

The flux of fluvial carbon from the terrestrial biosphere to the world's oceans is known to be an important component of the global carbon cycle but within this pathway the flux and return of carbon to the river network via sewage effluent has not been quantified. In this study monitoring data from 2000 to 2016 for the dissolved organic carbon (DOC) concentration, biochemical oxygen demand (BOD) and chemical oxygen demand (COD) of the final effluent of sewage treatment works from across England were examined to assess the amount of DOC contributing to national‐scale fluvial fluxes of carbon. The study shows that the median concentration of DOC in final effluent was 9.4 mg C/l compared to 4.8 mg C/l for all surface waters for the UK over the study period, and the DOC in final effluent significantly declined over the study period from 11.0 to 6.4 mg C/l. Rivers receiving sewage effluent showed a significant, on average 19%, increase in DOC concentration downstream of sewage discharges. At the scale of the UK, the flux of DOC in final effluent was 31 ktonnes C/yr with a per capita export of 0.55 kg C/yr and compared to an average annual flux of DOC from the UK of 859 ktonnes C/yr, i.e. only 3.6% of national‐scale flux. The lability of this DOC was limited, with only 7.4% loss of final effluent DOC concentration over in‐stream residence times of up to 5 days. The direct decline in DOC concentration from sewage treatment works was not large enough on its own to explain the declines observed in DOC concentration in UK rivers at their tidal limit.