Septic tanks as point source of pollutants and their impact on water quality
Phosphorus (P) plays an important role in water quality as it is considered a major pollutant when it enters fresh water systems from agricultural runoff or as a point source discharge from urban wastewater treatment plants (WWTP) or from onsite wastewater treatment systems (OSWTS) in rural areas such as septic tanks (ST). Septic tank systems (STS) are the most widely used systems for treatment and disposal of domestic wastewater (Figure 1) around the world (Table 1) where connection to the mains sewerage network system is inaccessible, impractical and costly.
The preference for septic tank systems use is partly because of their relative simplicity, low costs and treatment capabilities, however, their performances is highly variable and they often produce effluent with varied quality.Septic tank systems often fail due to aging, user neglect, poor management and lack of maintenance posing risks to surface and ground waters quality.Although most modern STS discharge their effluent to soil systems for final contaminants removal in the soakaway/drainage field, soil treatment is also variable and in the case of failing systems, contaminants often reach surface and ground waters. Moreover, historically some tanks were designed to discharge their effluent directly to watercourses without secondary soil treatment, posing ecological impacts on water quality.
Septic tank effluent contains a wide variety of pollutants including pathogens, faecal bacteria, phosphorus (P), nitrogen (N), organic matter (OM), suspended solids (SS), pharmaceutical compounds and household detergents and chemicals that pose risks to fresh water resources. Septic tanks in the UK are neither regulated nor monitored for performance and often fail, with the consequence that they discharge their effluents to the environment without treatment.
Despite some views that STs’ impacts on water quality are proportionally negligible relative to total pollutant loads, other studies have argued that wastewater discharges including ST pose greater risks to water quality than agricultural diffuse sources.It is important to evaluate STS discharges and to understand their loading impacts to place them in perspective with other rural diffuse pollution sources in order to prioritise and target effective mitigation measures.
Septic tank design function and size
Figure 2:Typical septic tank unit.
Septic tanks are designed to store wastewater and maximise the removal of solids and pollutants by physical settlement and microbial hydrolysis of organic material into inorganic soluble simple molecules (primary treatment). A typical STS comprises of a ST unit (single or multi chambers) (Figure 2), a drainage field (soakaway) with a network of perforated pipes for effluent distribution (Figure 1) for further physical chemical and biological processing by soil particles (secondary treatment). Gravity and displacement play an important part in septic tank operation: if 10 litres wastewater were run from the kitchen sink and dropped into the tank, 10 litres of partially treated sewage effluent will exit the tank to the soakaway soil system. Septic tanks should be pumped out (desludged) to remove the accumulated sludge and scum layers regularly (2-5 years). The tank volume must be big enough to allow for a minimum of 24 hours residence time for the average daily waste of 150-180 litres per person in the tank (Table 2). An over-sized tank is not cost effective, while an under-sized tank reduces effluent residence time and discharges insufficiently treated effluent to the soakaway system causing soil blockage and flooding.
Table 2: Estimated minimum ST size depending on the number bedrooms and number of people in a household based on water use of 180 L/person/day.
When ST influent becomes effluent
Figure 3: Transformation of domestic wastewater from septic tank influent (STI) to septic tank effluent (STE).
Domestic waste material that enter the tank is termed septic tank influent (STI) and is generally comprised of kitchen wastes, toilet flushing, shower and bathtub washings, washing machine and dishwasher wastes. During residence time of waste material inside the tank, the influent undergoes physical, chemical and microbiological processes under anaerobic condition inside the tank (primary treatment) to become partially treated septic tank effluent (STE), (Figure 3). Septic tank unit is not effective in removing N and P or bacteria, and both the anaerobic condition and biochemical processes inside the tank convert most organic N and P to ammonium-N (NH4-N) and inorganic soluble phosphates (PO4), while, total nitrogen (TN) and total phosphorus (TP) remain unchanged.
Effluent secondary treatment in the soil system
Despite the primary treatment inside the tank, the resulting effluent still contain large concentration of contaminants such as N, P, SS, OM, bacteria and pathogens as well as pharmaceutical organic compounds and household detergents and chemicals. Therefore, STE requires further and essential treatment and purification through the soakaway soil system before it is released to the environment.
Soil is an excellent medium for the treatment and the removal of STE contaminants. As effluent enters the soil system, it filters and infiltrates the soil system by surface filtration and straining processes. When the effluent seeps through the soil, a biological mate is formed at the base of the soakaway area in which, much of the decomposition of the suspended material and OM of the effluent take place through biological processing. Other chemical and physical processes such as sorption, cation exchange, nitrification, denitrification, precipitation and complexation of effluent pollutants with soil particles occur to reduce effluent pollutant concentration further in the soil system. Bacterial removal occurs by filtration and straining through soil pores, which blocks the initial physical movement of bacteria through the soil. These surface retained bacteria are then subjected to ultraviolet light (UV), desiccation and are less likely to survive.
How important the quality of soakaway soil
The soakaway’s soil quality is a crucial factor in the failure/success of any STS and the level of effluent treatment it provides. In poorly structured soils (heavy clay soils), effluent ponding may occur reducing the soil efficiency in treating and retaining STE. Equally, in coarse textured soils (coarse sandy soils), effluent movement becomes rapid and soil-effluent contact time is greatly reduced resulting in insufficient biological and chemical processes to occur. Fine textured soils (clay and silty soils) have greater surface area which is ideal for dissolved pollutants removal by chemical processes such as sorption. However, the presence of discontinuities such as fissure and cracks in the subsoil can provide preferential flow paths for percolating liquids, which reduce treatment contact time between effluent and soil particles. In cases where site and soakaway soil characteristics are not suitable for ST conventional wastewater disposal, an alternative system such as septic tank-mound system and reed bed treatment system is used.
Septic tank-mound system
Figure 4:septic tank-mound system.
Mound system is suitable for shallow sites that do not meet setback distance between STS and water table or in sites that have low or high soil permeability rates. A ST-mound system is an elevated soil system which comprises: a ST unit, a pumping chamber and the mound itself (Figure 4). The mound itself is comprised of a layer of sand as a filling material on top of the natural soil, followed by gravel layer which engulfs and supports the distribution pipes. A layer of geotextile fabric is placed to cover the gravel followed by top soil over the entire mound. The effluent is elevated by a pump from the tank to flow through the fill material where it is processed before entering the natural soil. The total depth of the natural soil and the sand together should equal the required setback distance from percolation pipes to groundwater of 1.2 m.
Reed bed treatment systems
Figure 5:Reed bed treatment system.
Reed bed systems (Figure 5) are considered to be effective and of low operational cost as alternative treatment for secondary or tertiary treatments. They require large area for effective effluent treatment and they are not recommended as standalone secondary treatment systems. The principle of reed treatment is the ability of reed plants to survive in waterlogged conditions and to transfer oxygen from leaves through theirroot systems to a gravel bed, promoting the growth of bacteria and microorganisms. Septic tank effluent is allowed to seep through the gravel bed for pollutant removal by physical filtration, chemical precipitation and aerobic and anaerobic bacterial digestion. Reed beds are designed to detain the wastewater for 5 to 7 days allowing sufficient time for the settling and filtering of suspended solids, nitrification/denitrification to occur, breakdown of organic matter and nutrient removal by micro-organisms and plant uptake. Reed bed systems are more effective at nitrifying effluents, converting effluent ammonia into nitrates, nitrites and nitrogen gas than most package sewage treatment plants. They are effective in the removal of SS, BOD, TN, faecal coliforms and TP. However, they are often criticized for their low performance particularly in winter months and the unreliable long term performance. Reed beds that receive effluent with a high level of suspended solids are susceptible to block up more rapidly, diminishing their ability and their effectiveness of contaminants removal with time.
