Estimating the phosphorus load to waterbodies from septic tanks

Septic tanks are widely used across the UK for the disposal of household waste in rural areas. Sewage is a rich source of phosphorus (P) and one of the functions of these systems is to remove much of this P from waste water before it reaches groundwater or surface waters. This is necessary because increased P inputs to waterbodies encourages algal growth and degrades water quality. In recent years, much effort has been invested in reducing the output of P from large point sources, such as waste water treatment works (WWTWs). This has led to the assumption that agriculture is now the main source of P entering waterbodies in rural catchments. However, there is mounting evidence that this is not the case. Small point sources of P in these areas, such as septics tanks, may also be important sources of P. Very little is known about how effective septic tank systems are at removing P from sewage effluent, and how much P they release to surface waters. It is important that this is now quantified so that it can be placed into context in relation to other sources of P in rural areas, such as agriculture. This information is essential to help us manage our water resources better and to enable us to meet the requirements of the Water Framework Directive. In theory, septic tanks systems can effectively remove the majority of P from household waste if they are sited, maintained and used properly. The extent of P removal is dependant upon many inter-related factors. These include soil grain size and chemical composition, proximity to the water table, proximity to surface water, capacity of the system in relation to the number of people using it, chemical composition of the sewage that is received by the system, and the frequency at which the tank is emptied of sludge. Also, the effective functioning of the tank itself is dependent upon several interconnected components that are responsible for capturing solids and breaking down organic materials. Failure of any one of these components will reduce the extent to which P is retained by the system. To date, most estimates of the contribution of P from septic tanks to water bodies have used a simple export coefficient approach applied at the catchment, regional or national scale. This is a very general method that takes no account of local variation in P removal due to the influence of factors such as soil type, hydrology, and the location, age and level of maintenance of the system itself, all of which may have a significant impact on P transfer from septic tanks to waterbodies. It also tends to assume that all tanks are properly maintained and functioning correctly, which is often not the case. In order to assess the contribution of septic tank systems to P loads to water more accurately, more information is needed on these factors and their impact on P mobility. The availability of the additional information necessary for improving these calculations was investigated through a case study of the Loch Leven catchment. This revealed that little is known about the age, size, location, method of discharge or level of maintenance of septic tanks in this area. A study by SNH strongly suggests that about 750 properties within the catchment are served by private sewage treatment facilities such as septic tanks. Of these, less than 10 per cent are registered with the SEPA and the data that exist on these are insufficient for estimating their P load to the loch. A similar situation is believed to exist across the UK. Literature studies suggest that the P load to waterbodies from properly located and efficiently functioning septic tanks should be very small. However, there is strong anecdotal evidence that a large number of septic tanks across the Loch Leven catchment are not working effectively. This is because many are not de-sludged regularly, some are being used beyond their original design capacity and others discharge directly to a watercourse. A review of SEPA monitoring data for this catchment supported this view. The data showed that the P concentration of many of the septic tank effluents monitored was very high at the location of the outflow. No data exist on the P content of the effluent beyond this point. It is recommended that a detailed study of P losses from septic tanks across the catchment should be undertaken. This study should include: o Determination of the number, location and type of septic tanks o Estimation of the P load to surface waterbodies from these systems o Assessment of P losses to groundwater In conclusion, this review suggests that septic tank systems are probably a significant and underestimated source of P inputs to waterbodies in rural catchments across the UK. It is, therefore, highly recommended that research is undertaken to quantify the parameters needed to estimate these P inputs more accurately. In particular, effort should be focused on identifying the factors that most influence P losses to waterbodies from these systems to enable effective management of the problem