Macrophyte trenches for septic tank effluen

Percival R. Thomas (7219154)

T. Kalaroopan (7219157)

English

Loughborough University Institutional Repository

E   SANITATION AND WASTEWATER: THOMAS, P.R. and KALAROOPAN16120th WEDC Conference Colombo, Sri Lanka, 1994AFFORDABLE WATER SUPPLY AND SANITATIONMacrophyte trenches for septic tank effluentP.R. Thomas, La Trobe University and T. Kalaroopan, Rural City of Wodonga, Australia.IN LOW- AND MIDDLE-INCOME areas of developing coun-tries and in rural areas of developed countries, it is usualto provide septic tanks and associated drainfields orsoakage pits as excreta disposal systems for individualhouseholds as well as for small communities up to about300 people.  Although such treatment systems are effi-cient and economical, septic tanks often have a badreputation because of either inadequacy in the design ofthe unit or more often because of the failure of thedrainfield which follows it.  Many situations exist wherethe odorous septic tank effluent either ponds on theground or seeps into the nearby stormwater drain due topoor performance of the drainfield causing health risks topeople.  This can be attributed to one of the following:• inappropriate soil condition;• excessive organic/hydraulic loading; or• faulty design.To avoid pollution problems the design and construc-tion of the effluent disposal system is as important as themain part of the septic tank system.  Proper functioningof the drainfield depends on percolation of the effluentinto the soil profile and adequate aeration of the bed.Several alternatives to the standard drainfield systemhave been developed because of the pollution problemsencountered with the standard drainfields (AWRC, 1988)and one such alternative is the use of macrophyte trenchesor constructed wetlands.  It has been shown that con-structed wetlands have the potential to provide a rela-tively low cost, technologically simple method ofwastewater treatment (Finlayson, 1983; Scholes et al.,1986; Davies, 1988).Macrophyte trenchesA “macrophyte trench” can be defined as a wetlandspecifically constructed for the purpose of pollution con-trol and waste management, at a location other thanexisting natural wetlands.  There are two basic types ofconstructed wetlands, the free water surface wetland andthe subsurface flow wetland, the latter being consideredto have some advantages over the other.  In the subsur-face flow wetland the flow is maintained below themedia surface and there is little risk of odours, insectvectors or public health problems.  A subsurface flowwetland system can be a discharge type or a non-dis-charge type, however, for an average household septictank a non-discharge type can be reliably designed andconstructed because of the low effluent flows.  For higherflows, a discharge system can be used with potential foreffluent reuse.Aquatic plant species for use in macrophyte trenchestreating wastewater should generally be selected usingthe following criteria (Mitchell, 1978):• rapid and relatively constant growth rate;• ease of propagation;• capacity for absorption of pollutants;• tolerance of hyper-eutrophic conditions; and• ease of harvesting and potential usefulness of har-vested material.Also, it is preferable to select from native plant specieswhich grow locally in the area.  Examples of aquaticmacrophytes that have been used in the artificial wetlandsystems are:Floating plants:• Eichhornia crassipes (Water Hyacinth)• Spirodela (Duckweed)• Salvinia molesta (Salvinia)Emergent plants:• Schoenoplectus validus (Great Bulrush)• Juncus ingens (Giant Rush)• Phragmites (Common Reed)• Typha spp. (Cumbungi or Cattail).These plants are able to transfer oxygen into the bed,creating aerobic microzones around the plant roots andanaerobic zones away from them.  As a result, aerobic andanaerobic bacteria will both carry out the breakdown ofthe organic matter and removal of nitrogen throughnitrification and denitrification processes.  Wetlands cansignificantly reduce biochemical oxygen demand (BOD5),suspended solids (SS), nitrogen, pathogens and metalsthrough their complex chemical/biological processes aswell as some uptake by the vegetation.  Phosphorusremoval in many constructed wetland systems is noteffective because the gravel media offer limited contactopportunities between the wastewater and the soil, anddue to the short hydraulic retention times.  Removal ofBOD5 in the wetlands can be approximated by the first-order plug flow kinetics but they do not have a strongrelationship with the hydraulic retention times as well aswith aspect ratio (length:width) (Reed and Brown, 1992).E   SANITATION AND WASTEWATER: THOMAS, P.R. and KALAROOPAN162Home septic tank systemsFor individual houses, macrophyte trenches to treat sep-tic tank effluent can be designed and constructed tosatisfy owner’s landscape requirements in regard to posi-tion, and decorative plants such a lilies, cannas or fernswhich grow well in wet conditions can be used to createan aesthetic value.  Typha and Phragmites spp. are notrecommended for domestic installations because of themassive seasonal release of wind-blown seeds (Mitchellet al., 1990).  To dispose 1 m3/d of septic tank effluent,conventional drainfields require a surface area of 50-100 m2 whereas macrophyte trenches need 24-50 m2 ofsurface area with virtually no pollution problems.For an average household a single cell macrophytetrench 0.6 m deep is adequate to treat the septic tankeffluent.  The bottom of the trench should be lined with animpervious layer to prevent seepage if there is no claylayer.  The inlet zone with a buried perforated pipe shouldhave 25-50 mm washed gravel for a length of about 1 m,and for the full depth of the bed.  Similar size gravel canbe used for the outlet zone as well.  The gravel in thetreatment zone should be clean with sizes up to 15 mm indiameter.  Since a low aspect ratio of the macrophytetrench is a very important factor in the hydraulic designof the system, a value of 3:1 or less is recommended (Reedand Brown, 1992).  The cost of a home septic tankmacrophyte trench system can vary above or below thatof the standard drainfields and will depend on conditionssuch as topography, soil type, and the cost of gravelmedia (which represents over 50% of the cost of subsur-face flow macrophyte trenches).  The longitudinal sectionof a single cell subsurface flow macrophyte trench isshown in Figure 1.Treatment of secondary effluent - pilotstudiesThe pilot size subsurface flow macrophyte trenches atWodonga Sewage Treatment Facility in Australia consistof four cells each 27.0 m long x 3.6 m wide x 0.6 m deepcontaining emergent vegetation growing in 0.5 m deepgravel media.  Part of the secondary treated sewage fromthe treatment facility is used as the inflow to each of thefour trenches, three of them planted with eitherSchoenoplectus validus, Juncus ingens or both species ofplants and the fourth serving as an unvegetated controltrench.  The trenches with Schoenoplectus validus andJuncus ingens contain 10 mm and 14 mm size gravelrespectively as bed media while 20 mm size gravel is usedfor the other two trenches.  Monitoring began in Decem-ber 1993 and mean concentrations of BOD5, suspendedsolids, nitrate (NO3), total phosphorus (Total-P), chemicaloxygen demand (COD) and ammonia (NH3) obtained todate from the inflow and outflow are presented in Ta-ble 1.  During this period, flow to the trenches was variedto give different hydraulic retention times with 1 daybeing the lowest value.The quality of the secondary treated wastewater fromthe sewage treatment facility has been inferior because ofthe unusual acceptance of effluents from a meat worksand from a pet food industry to the treatment facilityduring the monitoring period.  This affected the perform-ance of the macrophyte trenches which were less than sixmonths old and in the developing stage.In the vegetated trenches BOD5 removal efficienciesaveraged between 56-63% whereas in the unvegetatedcontrol trench BOD5 removal averaged 79%.  In generalthe trench with both plant species, Schoenoplectus validusand Juncus ingens offered better BOD5 and COD removalthan the trench with either Schoenoplectus validus or Juncusingens.  Since BOD5 removal is enhanced under aerobicconditions, it is reasonable to assume that the superiorefficiency obtained in the control trench was due to thepresence of oxygen in the higher voids component of the20 mm gravel media.  The macrophyte trench (20 mmgravel media) with the mixture of two plant species hadslightly elevated effluent BOD5 levels contributed bysome of the decaying vegetation.Figure 1. Longitudinal section of a Macrophyte trench.E   SANITATION AND WASTEWATER: THOMAS, P.R. and KALAROOPAN163Table 1. Performance of macrophyte trenches at Wodonga.Sampling period:December 1993 BOD SS COD NH3 NO3 Total-Pto April 1994 mg/L mg/L mg/L mg/L mg/L ms/LInfluent 57 50 221 29 26 8Effluent from:Schoenoplectus validus 25 10 80 24 12 7(10mm gravel)Juncus ingens(14mm gravel) 23 6 80 25 5 7Mixed species(20mm gravel) 21 10 72 22 9 7Unvegetated Control(20mm gravel) 12 5 54 18 8 6Suspended solids removal averaged 88% with Juncusingens and 80% with both Schoenoplectus validus, and thecombination of the two species whereas the control trenchshowed a reduction of 90%.  For suspended solids andBOD5, results to date show that removal rates were notimproved with long hydraulic retention times.Reduction in ammonia was poor, 14-24% for macrophytetrenches and 38% for the control trench, whereas fornitrate removal, the overall performance was better withJuncus ingens offering the highest reduction of 81%.  Al-though it is expected to have a significant ammoniaremoval through nitrification-denitrification mechanismsfrom the macrophyte trenches, it is believed that theoxygen in the gravel media was insufficient to convert theammonia to nitrate.  Generally all of the trenches havebeen performing satisfactorily in regard to denitrification,with effluent nitrate levels dropping to as low as 3.5 mg/L in the trench with Juncus ingens.  Phosphorus removalwas low in all of the macrophyte trenches showing aremoval efficiency of about 13%.  During this initialperiod of monitoring.  surface flows were observed at theinlet end of the macrophyte trenches from time to time.This may be either due to excessive organic loading or dueto the aspect ratio of 7.5:1 for each of the trenches.ConclusionsMacrophyte trenches have the potential to provide aneconomically feasible and simple method of polishingpre-treated wastewater where a consistent high qualityeffluent is not always required.  They can be integratedinto septic tank systems for single houses as well as intothe wastewater treatment systems for small communi-ties, particularly in rural areas.  Since the effluent from aseptic tank normally has a BOD5 of about 150 mg/L, eithera two-cell septic tank or a septic tank effluent sand filterbuilt in the system may be useful to reduce the loading onthe macrophyte trenches to prevent any potential clog-ging problems.ReferencesAustralian Water Resources Council (1988).  Low costsewerage options study.  Water Management Series No 14.Australian Government Publishing Service, Canberra.Davies, T.H. (1988).  Reed bed treatment of wastewaters:a European perspective.  Water 15(1), 32-33, 39.Finlayson, C.M. (1983).  Use of aquatic plants to treatwastewater in irrigation areas of Australia.  Proceedings ofAWWA 10th Federal Convention, Sydney.  Australian Wa-ter and Wastewater Association, 25.1-25.9.Mitchell, D.S. (1978).  The potential for wastewater treat-ment by aquatic plants in Australia.  Water 5(3), 15-17.Mitchell, D.S., Breen, P.F. and Chick, A.J. (1990).  Artificialwetlands for treating wastewaters from single house-holds and small communities.  Proceedings of the Interna-tional Conference on the Use of Constructed Wetlands in WaterPollution Control, Cambridge, U.K., September, 383-389.Reed, S.C. and Brown, D.S. (1992).  Constructed wetlanddesign:  the first generation.  Water Environment Research64(6), 776-781.Scholes, J.D., Kerr, R.I. and Nuttall, P.M. (1986).  Treat-ment of wastewater by aquaculture systems.  AustralianWater Resources Council Research Project No. 80/13 FinalReport, Department of Resources and Energy, Canberra.

Macrophyte trenches for septic tank effluent

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Macrophyte trenches for septic tank effluent

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