Practical measures for reducing phosphorus and faecal microbial loads from onsite wastewater treatment system discharges to the environment: a review

Onsite wastewater treatment systems (OWTS), the majority of which are septic tanks, are a contributing factor to phosphorus and faecal microbial loads. OWTS contribute to waterbodies failing to meet Water Framework Directive (WFD) objectives and as such, measures to improve the quality of OWTS discharges are required. Literature has been reviewed for a range of measures designed to reduce phosphorus and pathogen concentrations in effluent from OWTS. A feasibility assessment focussed on their application, effectiveness, efficiency, cost and ease of adaptation. A wide range of measures have been identified that could potentially improve water quality. Results show no one solution could be applied to reduce phosphorus and pathogen loadings to the water environment. The literature suggests that OWTS need to be designed to the local flow and load characteristics of the effluents streams,as well as site specific conditions. With that in mind, measures such as awareness raising, site planning, and maintenance are likely to contribute to reduction of impact of OWTS on the environment. The level of load reduction possible from measures such as awareness raising is difficult to quantify, but it is low-cost and relatively easy to implement. Those most effective for phosphorus and pathogen removal are post-tank measures that maximise physical removal, through adsorption and filtering, and maintain good conditions for biological breakdown of solids and predation of pathogens.