Assessment of a novel development policy for the control of phosphorus losses from private sewage systems to the Loch Leven catchment, Scotland, UK

Legislation to control nutrient enrichment of inland waters has been developed and implemented across local, regional and international scales. In the EU, measures must be identified to ensure that all inland water bodies meet ecological guidelines as set by the Water Framework Directive (WFD) by 2015 or 2027. However increasing demand for rural development, associated with projected population increase, confound existing nutrient management approaches. Here we assess the efficacy of a rural development policy that was designed to ensure that the private sewage systems (PSS) of new developments do not increase the phosphorus (P) load to the environment within a lake catchment. In outline this policy involves mitigating 125% of the calculated P output of a development by modifying an existing, third party PSS. The assumption that PSS discharge a hierarchal reduction in P output with increasing treatment level (i.e. primary treatment (10 mg l−1) > secondary treatment (5 mg l−1) > tertiary treatment (2 mg l−1)) lies at the core of this policy. This study assesses the effectiveness of the policy instrument in achieving a reduction in nutrient discharge from PSS to the catchment. To do this, seven PSS (four with primary, one with secondary and two with tertiary treatment) were monitored over a four-month period to provide a range of P discharge concentrations across treatment types. These data were used to assess the potential impact of future rural development on P losses to the catchment using the expected, and the hypothetical, population increase rate of 1.3% yr−1 over a 90 year projection. No significant differences in TP discharge concentration were observed among PSS or treatment levels of PSS sampled. To ensure this policy meets its aim, improvement in technology and management of PSS along with alternative mitigation measures are required.