Wastewater reusePRACTICENOTE 9Water Sensitive Urban Designin the Sydney Region• Septic tanks• Aerated wastewater systems• Greywater reuse systemsWater sensitive development involves simple design andmanagement practices that take advantage of natural sitefeatures and minimise impacts on the water cycle. It is partof the contemporary trend towards more ‘sustainable’solutions that protect the environment.This Water Sensitive Practice Note gives a generalintroduction to the options available for on-site waste watertreatment and re-use.On-site waste water treatment options includeseptic tanks, aerated wastewater treatmentsystems and composting toilets.Wastewater reuse2IntroductionThe majority of water used for indoor domesticpurposes is discharged after use as ‘wastewater’.Wastewater can be collected by a reticulatedsewage system and treated at a conventionalwastewater treatment plant. Alternatively, it can becollected, treated and re-used on-site, therebypromoting more efficient water use. This has manysignificant economic and environmental benefits forthe community. However, on-site reuse of domesticwastewater is subject to various restrictions due toconcerns about effluent quality, maintenance andhealth issues.Types of wastewaterThere are two main types of domestic wastewater:• blackwater - wastewater from the toilet• greywater - all other domestic wastewater,including wastewater from bathrooms, kitchensand laundries.A typical household discharges approximately 35litres of blackwater, and 105 litres of greywater, perperson per day. The potential for on-site treatmentand reuse will depend on its quality. Greywatercontributes about 65% of the volume of domesticwastewater, 70% of the phosphorus, and 63% ofthe BOD (biological oxygen demand), whilstblackwater contributes about 35% of the volume ofwastewater, 61% of suspended solids, 82% ofnitrogen and 37% of BOD.The potential presence of pathogens in greywater issubstantially lower than in blackwater. However,several authors have shown that greywater maycontain pathogens. Thus, both greywater andblackwater require adequate treatment before on-site reuse.On-site treatment and reuse options include septictanks, aerated systems, and greywater reusesystems. These options are mainly applicable torural and rural-residential locations.Septic tanksSeptic tanks are widely used throughout Australia inareas without reticulated sewerage. About 12% ofall households nationally rely on septic tanks. Theconventional system involves the undergroundinstallation of a concrete tank and an absorptiontrench (see Figure 1).Figure 1: Septic tank & absorption trenchWastewater is partially treated in the septic tank byanaerobic processes. These remove about 30% ofphosphorus, 20% of nitrogen, 60% of suspendedsolids, 50% of BOD, and reduce the concentrationof biological contaminants. Final treatment occursvia an absorption trench. The effluent thenpercolates to the soil where it is subject to furthercontaminant removal processes by soil organismsbefore reaching surface or ground waters.Guidance for the design of septic tanks and thedisposal of effluent from on-site wastewatertreatment systems is provided in AustralianStandards AS1546 and AS1547 respectively.Installation of a septic tank requires approval fromthe local council. Ongoing operation also requirescouncil approval and regular inspection.About 40% of septic systems have been found tobe not operating correctly, thereby contributingnutrients to waterways and causing significantwater management problems. Common reasons forfailure of septic tank and absorption trench systemsare:Wastewater from houseSludge zoneDetention zoneScum zoneEffluent dischargeAbsorption trench filled with gravelSlotted pipeSeptic tankInspection portWastewater reusePRACTICENOTE 9Water Sensitive Urban Designin the Sydney Region3• the volume of wastewater discharged to theseptic tank is greater than its design volume• failure to periodically remove sludge from theseptic tank• insufficient area of absorption trench to accepteffluent from the septic tank• inappropriate soil type for absorption of effluent.Aerated systemsThere are a number of different aerated wastewatertreatment systems available for on-site managementand reuse of wastewater. These systems rely onmechanical devices to mix, aerate and pump theeffluent, subjecting it to accelerated aerobic andanaerobic decomposition using one or two tanks(see Figure 2).Fig 2: Aerated wastewater treatment systemProvided that the required management andmaintenance regimes are adhered to, includingperiodic sludge removal, the effluent should beclear and odourless, and meet NSW Department ofHealth guidelines. Effluent quality should be betterthan 30 mg/l suspended solids concentration, 20mg/l BOD5, 0.5 mg/l free residual chlorine and 10organisms per 100 ml for faecal coliforms. It canthen be disposed of by surface or undergroundirrigation. A minimum irrigation area of 200 m2 isusually required.Greywater reuse systemsThere are two main types of greywater reusesystems: primary and secondary systems In aprimary system, greywater is collected anddistributed by gravity or a pump for undergroundlawn and garden watering (see Figure 3).Fig 3: Primary greywater reuse systemCareful selection of detergents and washingproducts is required to minimise possible harmfulimpacts on plants or soil due to accumulation ofsalts, nutrients and trace metals. A guide to suitabledetergents is provided by Mobbs (1998). Asuntreated greywater may contain harmful bacteria,it should not be applied directly to vegetables.Secondary systems incorporate a storage tank forgreywater treatment. This supplies greywater fortoilet flushing and garden irrigation via a pump (seeFigure 4). The system can also supply undergrounddrip irrigation of garden areas.Fig 4: Secondary greywater reuse systemWastewater from houseSeptic tank (see Fig 1)Effluent dischargePumpSludgeAeration chamberSettling & chlorination chamberOutletSludge returnAirShower, bath & basinLaundryToiletReticulated sewage systemOverflow to sewage systemStorage tank & greywater treatmentUnderground drip irrigationShower, bath & basinLaundryToiletReticulated sewage systemOverflow to sewage systemStorage tank & greywater treatmentUnderground drip irrigationPumpSprinklerWastewater reuse4CostsConstruction costs for wastewater systems can varyconsiderably. Palmer et al (2001) found that theaverage cost to install a septic system is $4,300,and the
View Full Document
Unlocking...