Phosphorus transport and retention in a channel draining an urban, tropical catchment with informal settlements

Abstract

Abstract. Urban catchments in sub-Saharan Africa (SSA) are increasingly becoming a major source of phosphorus (P) to downstream ecosystems. This is primarily due to large inputs of untreated wastewater to urban drainage channels, especially in informal settlements (or slums). However, the processes governing the fate of P in these catchments are largely unknown. In this study, these processes are investigated. During high runoff events and a period of base flow, we collected hourly water samples (over 24 h) from a primary channel draining a 28 km 2 slum-dominated catchment in Kampala, Uganda, and from a tertiary channel draining one of the contributing slum areas (0.54 km 2 ). The samples were analysed for orthophosphate (PO 4 -P), particulate P (PP), total P (TP), suspended solids (SS) and hydrochemistry. We also collected channel bed and suspended sediments to determine their geo-available metals, sorption characteristics and the dominant phosphorus forms. Our results showed that the catchment exported high fluxes of P (0.3 kg km 2 d −1 for PO 4 -P and 0.95 for TP), which were several orders of magnitude higher than values normally reported in literature. A large proportion of P exported was particulate (56 % of TP) and we inferred that most of it was retained along the channel bed. The retained sediment P was predominantly inorganic (> 63 % of total sediment P) and consisted of mostly Ca and Fe-bound P, which were present in almost equal proportions. Ca-bound sediment P was attributed to the adsorption of P to calcite because surface water was near saturation with respect to calcite in all the events sampled. Fe-bound sediment P was attributed to the adsorption of P to iron oxides in suspended sediment during runoff events given that surface water was undersaturated with respect to iron phosphates. We also found that the bed sediments were P-saturated and showed a tendency to release P by mineralisation and desorption. During rain events, there was a flushing of PP which we attributed to the resuspension of P-rich bed sediment that accumulated in the channel during low flows. However, firstflush effects were not observed. Our findings provide useful insights into the processes governing the fate and transport of P in urban slum catchments in SSA.

Phosphorus dynamics in a highly polluted urban drainage channel-shallow reservoir system in the Brazilian semiarid

“…Eutrophication of urban waters is a worldwide problem which is associated with high phosphorus (P) loadings (ruley and rusch 2004, Bedore et al 2008, Nyenje et al 2014, Hobbie et al 2017. In Brazil, despite the use of separate systems for sanitary sewer and stormwater runoff, only 50% of residences are connected to a sanitary sewer system and only 43% of the sewage is treated (SNIS -National Information System on Sanitation 2015).…”

Anaerobic digestion in Uganda: risks and opportunities for integration of waste management and agricultural systems

“…1). For example, reported values of TP sampled from local drainage canals ranged from 1.6 to 13 mg L −1 , whereas the digestate concentrations reported in this study averaged 214 mg L −1 (Kanyiginya et al ., 2010; Nyenje et al ., 2014; Fuhrimann et al ., 2015; Katukiza et al ., 2015). This study reports digestate physiochemical characteristics prior to discharge, whereas published values of water quality from drainage canals estimate storm water diluted discharges.…”