Water Sensitive Urban Design in the Sydney Region PRACTICE NOTE 9 Wastewater reuse On site waste water treatment options include septic tanks aerated wastewater treatment systems and composting toilets Water sensitive development involves simple design and management practices that take advantage of natural site features and minimise impacts on the water cycle It is part of the contemporary trend towards more sustainable solutions that protect the environment T h i s Wa t e r S e n s i t i v e P r a c t i c e N o t e g i v e s a g e n e r a l introduction to the options available for on site waste water treatment and re use Septic tanks Aerated wastewater systems Greywater reuse systems Wastewater reuse Introduction Septic tanks The majority of water used for indoor domestic purposes is discharged after use as wastewater Wastewater can be collected by a reticulated sewage system and treated at a conventional wastewater treatment plant Alternatively it can be collected treated and re used on site thereby promoting more efficient water use This has many significant economic and environmental benefits for the community However on site reuse of domestic wastewater is subject to various restrictions due to concerns about effluent quality maintenance and health issues Septic tanks are widely used throughout Australia in areas without reticulated sewerage About 12 of all households nationally rely on septic tanks The conventional system involves the underground installation of a concrete tank and an absorption trench see Figure 1 Wastewater from house Inspection port Effluent discharge Slotted pipe Scum zone Detention zone Types of wastewater There are two main types of domestic wastewater Septic tank Absorption trench filled with gravel Sludge zone blackwater wastewater from the toilet greywater all other domestic wastewater including wastewater from bathrooms kitchens and laundries A typical household discharges approximately 35 litres of blackwater and 105 litres of greywater per person per day The potential for on site treatment and reuse will depend on its quality Greywater contributes about 65 of the volume of domestic wastewater 70 of the phosphorus and 63 of the BOD biological oxygen demand whilst blackwater contributes about 35 of the volume of wastewater 61 of suspended solids 82 of nitrogen and 37 of BOD The potential presence of pathogens in greywater is substantially lower than in blackwater However several authors have shown that greywater may contain pathogens Thus both greywater and blackwater require adequate treatment before onsite reuse On site treatment and reuse options include septic tanks aerated systems and greywater reuse systems These options are mainly applicable to rural and rural residential locations 2 Figure 1 Septic tank absorption trench Wastewater is partially treated in the septic tank by anaerobic processes These remove about 30 of phosphorus 20 of nitrogen 60 of suspended solids 50 of BOD and reduce the concentration of biological contaminants Final treatment occurs via an absorption trench The effluent then percolates to the soil where it is subject to further contaminant removal processes by soil organisms before reaching surface or ground waters Guidance for the design of septic tanks and the disposal of effluent from on site wastewater treatment systems is provided in Australian Standards AS1546 and AS1547 respectively Installation of a septic tank requires approval from the local council Ongoing operation also requires council approval and regular inspection About 40 of septic systems have been found to be not operating correctly thereby contributing nutrients to waterways and causing significant water management problems Common reasons for failure of septic tank and absorption trench systems are Water Sensitive Urban Design in the Sydney Region PRACTICE NOTE 9 Wastewater reuse the volume of wastewater discharged to the septic tank is greater than its design volume failure to periodically remove sludge from the septic tank insufficient area of absorption trench to accept effluent from the septic tank Greywater reuse systems There are two main types of greywater reuse systems primary and secondary systems In a primary system greywater is collected and distributed by gravity or a pump for underground lawn and garden watering see Figure 3 inappropriate soil type for absorption of effluent Shower bath basin Aerated systems Effluent discharge Air Aeration chamber Laundry Toilet There are a number of different aerated wastewater treatment systems available for on site management and reuse of wastewater These systems rely on mechanical devices to mix aerate and pump the effluent subjecting it to accelerated aerobic and anaerobic decomposition using one or two tanks see Figure 2 Wastewater from house Storage tank greywater treatment Underground drip irrigation Reticulated sewage system Overflow to sewage system Fig 3 Primary greywater reuse system Careful selection of detergents and washing products is required to minimise possible harmful Settling impacts on plants or soil due to accumulation of chlorination salts nutrients and trace metals A guide to suitable chamber detergents is provided by Mobbs 1998 As untreated greywater may contain harmful bacteria Outlet it should not be applied directly to vegetables Sludge Septic tank see Fig 1 Pump Sludge return Fig 2 Aerated wastewater treatment system Provided that the required management and maintenance regimes are adhered to including periodic sludge removal the effluent should be clear and odourless and meet NSW Department of Health guidelines Effluent quality should be better than 30 mg l suspended solids concentration 20 mg l BOD 5 0 5 mg l free residual chlorine and 10 organisms per 100 ml for faecal coliforms It can then be disposed of by surface or underground irrigation A minimum irrigation area of 200 m 2 is usually required Secondary systems incorporate a storage tank for greywater treatment This supplies greywater for toilet flushing and garden irrigation via a pump see Figure 4 The system can also supply underground drip irrigation of garden areas Shower bath basin Storage tank greywater treatment Sprinkler Laundry Toilet Pump Underground drip irrigation Reticulated sewage system Overflow to sewage system Fig 4 Secondary greywater reuse system 3 Wastewater reuse Costs Construction costs for wastewater systems can vary considerably Palmer et al 2001 found that the average cost to install a
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