Scotland’s centre of expertise for waters Practical measures for reducing phosphorus and faecal microbial loads from onsite wastewater treatment system discharges to the environment A review i Scotland’s centre of expertise for waters This document was produced by: Juliette O’Keeffe and Joseph Akunna Abertay University Bell Street, Dundee DD1 1HG and Justyna Olszewska, Alannah Bruce and Linda May Centre for Ecology & Hydrology Bush Estate Penicuik Midlothian EH26 0QB and Richard Allan Centre of Expertise for Waters (CREW) The James Hutton Institute Invergowrie Dundee DD2 5DA Research Summary Key Findings Onsite wastewater treatment systems (OWTS), the majority of local flow and load characteristics of the effluents streams, which are septic tanks, are a contributing factor to phosphorus as well as site specific conditions. With that in mind, measures and faecal microbial loads. OWTS contribute to waterbodies such as awareness raising, site planning, and maintenance failing to meet Water Framework Directive (WFD) objectives are likely to contribute to reduction of impact of OWTS on and as such, measures to improve the quality of OWTS the environment. The level of load reduction possible from discharges are required. Literature has been reviewed for measures such as awareness raising is difficult to quantify, a range of measures designed to reduce phosphorus and but it is low-cost and relatively easy to implement. Those most pathogen concentrations in effluent from OWTS. A feasibility effective for phosphorus and pathogen removal are post-tank assessment focussed on their application, effectiveness, measures that maximise physical removal, through adsorption efficiency, cost and ease of adaptation. A wide range of and filtering, and maintain good conditions for biological measures have been identified that could potentially improve breakdown of solids and predation of pathogens. water quality. A full summary of the measures reviewed is presented in Results show no one solution could be applied to reduce Section 7 of the report. The following table presents a selection phosphorus and pathogen loadings to the water environment. of the most practical measures to reduce P or pathogen releases The literature suggests that OWTS need to be designed to the from OWTS. Measure Removal of P Removal of Practicality Site requirements Cost Likely uptake possible pathogens possible Using P-free Yes - up to 50% No Legislation will ensure None Low Guaranteed detergents this is implemented Reducing food Yes - quantity Unknown Awareness raising could None Low - awareness Possible with waste flushed unknown assist. May be more raising awareness raising to drains practical for hotels, restaurants Appropriate site and Likely Likely May require change Increased distance to Related to increased Possible setback distances in building regulations water body land take and pipe (linked to risk based distances approach) Risk based measures Unknown Unknown Targeting measures None Cost of Currently being to most at risk sites consultation, applied in England deregulation Awareness raising Unknown Unknown Practical if providing None Low if electronic; Likely advice on operations, costs associated inspection and with leaflets or maintenance public events Replacing old tanks Yes Yes Practical where current Access for machinery High Possible with new tanks: system is poorly and adequate space Tank design functioning. Baffles may for new system (baffles and shape) be more practical for pathogen reduction than P reduction Increased Hydraulic Yes Yes Practical as an inspection Access to pipe Low Likely Retention Time measure for site owners/ connections and (HRT) – correcting occupiers to improve knowledge of OWTS misconnections function Increased HRT Unknown Yes Practical as a Access to desludging Relatively low Likely - desludging maintenance measure; equipment; may have unintended consideration of end impact on P releases use of sludge Measure Removal of Removal of Practicality Site requirements Cost Likely uptake P possible pathogens possible Introducing chemical Yes Yes Depends on site, scale of Access for dosing, Medium (depending Possibly as a additives improvement required may be more suited to on additive and polishing step and dosing mechanism. multi-chamber system. dosing frequency) In tank chemical use may May require electricity destabilise microbes Soak away, Yes Yes Could provide additional Land requirement, High, depending Likely drainfield, or treatment at sites with suitable soil conditions on level of site work mound system direct discharges to water and slope. Electricity required body need if pumps used Lagoons/WSP Yes Yes Depends on site and Land requirement, and Installation and Possible polishing requirement. protections against maintenance costs Can allow for UV exposure to pathogens may be high treatment or chlorination Removing P from Yes Unknown Depends on site and Land requirement for High Possible for discharged effluent polishing requirement, treatment area, or additional polishing using ochre could dose in WSP or dosing mechanism use as filter medium Constructed wetland Yes Yes Practical where space Land requirement, Installation and Likely available, allows for protection against maintenance increased retention time, exposure to pathogens, costs may be high facilitates increased substrates and depending on absorption of both P and vegetation harvesting system pathogens over time. Electricity need if pumps used Sand filter Yes Yes Practical where adequate None Installation and Likely space allows maintenance costs Peat filter Yes Yes may be high Alternative filter Yes Yes Practical where space Land requirement, Installation and Possible with media available, and proven electricity need maintenance costs further evidence to be safe (no additional if pumps used; may be high pollutant releases) consideration of filter material disposal Combination systems Yes Yes Practical where adequate Land requirement May be high Possible for sites space on site allows higher for site with depending on in sensitive areas mixed treatments; system electricity need if pumps used Package treatment Yes Possible May allow for treatment Similar to septic tanks, Range of costs, Possible plants where limited space requires electricity can be cheaper available onsite than septic tanks to install, but maintenance costs may be higher than septic tanks Introduction Research Undertaken The 2013 WFD classification identified 220 WFD baseline The project, in seeking to identify measures to improve OWTS rivers and 71 baseline lochs in Scotland as being impacted discharges, considered: by phosphorus in their chemistry and/or ecology. Faecal microbial loads are also recognised as a contributing factor 1. The available measures for reducing phosphorus and faecal to downgraded protected areas. In particular, pathogen microbial loads from septic tanks and other OWTS. pollution can result in contamination of bathing waters and 2. An assessment of the feasibility of applying such measures shellfish waters, increasing the risk of human exposure to to domestic households or larger private/ communal septic pathogens and associated impacts on industries such as tanks, and the practicality of retrofitting any additional shellfish growing. treatment. 3. Measures to deliver sustainable waste management solutions Large numbers of properties in rural Scotland (estimated including energy generation and/or nutrient recovery that to be circa 160,000) are not connected to mains sewerage may reduce pressures on waterbodies and, at the same time, systems and instead rely on OWTS to process their domestic deliver value. wastewater. These systems, mainly septic tanks, private 4. The load reductions which could potentially and realistically sewage treatment works, and package treatment plants, are be achieved through each measure, individually and thought to contribute to the phosphorus and faecal microbial collectively. loads that impact on the status of WFD waterbodies and protected areas. Contents 1 Introduction 5 2 Aims 8 3 Sources of data and information 8 4 About Septic Tanks 9 5 Mitigation measures for reducing pollutant loading to the environment from septic tanks 10 5.1 Measures for reducing phosphorus discharges to the environment 10 5.1.1 Pre-tank measures for reducing phosphorus discharges to the environment 11 5.1.2 In-tank measures for reducing phosphorus discharges to the environment 12 5.1.3 Post-tank measures for reducing phosphorus discharges to the environment 17 5.2 Measures for reducing pathogen discharges to the environment 22 5.2.1 Pre-tank measures for reducing pathogen discharges to the environment 23 5.2.2 In-tank measures for reducing pathogen discharges to the environment 24 5.2.3 Post-tank measures for reducing pathogen discharges to the environment 25 5.2.4 Pre-fabricated package treatment plants 28 5.3 Estimated load reductions of practical measures 30 5.3.1 P-reducing measures 30 5.3.2 Pathogen reducing measures 30 5.4 Comparison of system costs 32 6 Sustainable waste management solutions 33 6.1 Nutrient recovery 33 6.2 Energy generation potential 33 7 Conclusions 34 7.1 Summary of measures 34 7.2 Further work 37 8 References 38 Annex 1: Development and workings of septic tanks 44 Sources and concentrations of P in effluent 45 Sources and concentrations of pathogens in effluent 46 Annex 2: Factors affecting pathogen loading and survival 47 Pathogen loading 47 Pathogen survival 47 Abbreviations AD Anaerobic Digestion BOD Biological Oxygen Demand cfu Colony