Riverbed sediments buffer phosphorus concentrations downstream of sewage treatment works across the River Wensum catchment, UK

Abstract

Purpose Wastewater effluent discharged into rivers from sewage treatment works (STWs) represents one of the most important point sources of soluble reactive phosphorus (SRP) pollution and is a major driver of freshwater eutrophication. In this study, we assess the ability of riverbed sediments to act as a self-regulating buffering system to reduce SRP dissolved in the water column downstream of STW outflows. Materials and methods River water and riverbed sediment samples were collected from ten tributary outlets across the River Wensum catchment, Norfolk, UK, at monthly intervals between July and October 2016, such that 40 sediment and 40 water samples were collected in total. Of these locations, five were located downstream of STWs and five were on tributaries without STWs. Dissolved SRP concentrations were analysed, and the equilibrium phosphorus concentration (EPC 0 ) of each sediment sample was measured to determine whether riverbed sediments were acting as net sources or sinks of SRP. Results and discussion The mean SRP concentration downstream of STWs (382 μg P L −1 ) was double that of sites without a STW (185 μg P L −1 ), whilst the mean EPC 0 for effluent impacted sites (105 μg P L −1 ) was 70% higher than that recorded at unaffected sites (62 μg P L −1 ). Regardless of STW influence, riverbed sediments across all ten sites almost always acted as net sinks for SRP from the overlying water column. This was particularly true at sites downstream of STWs which displayed enhanced potential to buffer the river against increases in SRP released in sewage effluent. Conclusions Despite EPC 0 values revealing riverbed sediments were consistently acting as sinks for SRP, elevated SRP concentrations downstream of STWs clearly demonstrate the sediments have insufficient SRP sorption capacity to completely buffer the river against effluent discharge. Consequently, SRP concentrations across the catchment continue to exceed recommended standards for good chemical status, thus emphasising the need for enhanced mitigation efforts at STWs to minimise riverine phosphorus loading.

Streambank Legacy Sediments in Surface Waters: Phosphorus Sources or Sinks?

“…Studies have shown that there is a strong correlation between the EPC 0 , soluble reactive phosphorus (SRP), and dissolved reactive phosphorus (DRP) in stream water [43,49]. The EPC 0 in sediments may change depending upon the SRP concentration in the sediment and stream waters [49] as, potentially, in the case of our urban, P-rich, Brandywine zoo site (Table 3). One study [49] looked at the influence of sewage treatment works (STWs) on riverbed sediment to determine if sediments could act as a buffer for the increase in stream water P concentrations.…”

“…A study in Maryland (Kimages creek) that investigated legacy sediments reported an EPC 0 value of 0.010 mg L −1 [47], while that from till bank material near Lake Pepin in Minnesota had an EPC 0 value of less than 0.1 mg L −1 [48]. Studies have shown that there is a strong correlation between the EPC 0 , soluble reactive phosphorus (SRP), and dissolved reactive phosphorus (DRP) in stream water [43,49]. The EPC 0 in sediments may change depending upon the SRP concentration in the sediment and stream waters [49] as, potentially, in the case of our urban, P-rich, Brandywine zoo site (Table 3).…”

“…The EPC 0 in sediments may change depending upon the SRP concentration in the sediment and stream waters [49] as, potentially, in the case of our urban, P-rich, Brandywine zoo site (Table 3). One study [49] looked at the influence of sewage treatment works (STWs) on riverbed sediment to determine if sediments could act as a buffer for the increase in stream water P concentrations. They found that regardless of STW influence, the riverbed sediments always acted as sinks in the water column.…”

Can an Integrated Constructed Wetland in Norfolk Reduce Nutrient Concentrations and Promote In Situ Bird Species Richness?

“…Despite improvements in the treatment of domestic and industrial wastewaters, the current levels of nutrient fluxes in sewage effluent remain substantially higher than at the beginning of the twentieth century, and are frequently far higher than would be required to elicit ecological recovery of surface waters (Sayer et al 2010;Bowes et al 2012Bowes et al , 2016Naden et al 2016;McCall et al 2017). Small rural sewage treatment works (STWs) have been implicated in the eutrophication of surface waters in the United Kingdom (Mainstone and Parr 2002;Neal et al 2005;Jarvie et al 2006;Roberts and Cooper 2018). Elevated nutrient concentrations in receiving waters downstream of STWs have been shown to result in excessive phytoplankton and periphyton growth which can compromise both the ecology and the ecosystem services provided by waterbodies (Jeppesen et al 2000;Flynn et al 2002;Hutchins et al 2010;Bowes et al 2012).…”

Assessing the environmental and economic efficacy of two integrated constructed wetlands at mitigating eutrophication risk from sewage effluent

“…Nutrient‐enriched sewage effluent discharged from wastewater treatment plants (WWTPs) is a major global driver of freshwater eutrophication (Neal et al ., 2005; Jarvie et al ., 2006; Bowes et al ., 2012; Roberts and Cooper, 2018). In order to reduce the environmental toxicity of sewage effluent, wastewater undergoes numerous stages of processing at WWTPs, including screening through filters to remove coarse material (pre‐treatment), holding in settling tanks to encourage sedimentation of suspended fines (primary treatment) and promoting the degradation of organics through biological oxidation (secondary treatment) (Spellman, 2013).…”