Designing a Reedbed
The following submission to the SA Health Commission (now Dept Health) demonstrates the principles involved in designing a reedbed system. Chas Martin, one of the designers, operates as a consultant for the design and construction of reedbeds in South Australia. The design below was approved by the Health Commission and Gawler Council and built at The Food Forest in 2001.
Significant improvements have been made by designers since this system was proposed but it demonstrates the basics of an application.
Design for an Enhanced Effluent Treatment System at 'THE FOOD FOREST'
Section 52 Hundred of Munno Para
Clifford Rd, Gawler, SA, 5118
Prepared by Chas Martin and Graham Brookman
The development is on a property known as The Food Forest whose owners have aimed to demonstrate sustainable living through the use of recycling of wastes, use of straw bale construction and the adoption of organic farming practices. The property is visited by school groups, university classes, permaculture practitioners, farmers and the public. Visitors are able to observe sustainable farm management practices proven by years of operation.
The proposal is to build onto the eastern end of the existing house and in terms of waste water the plan is to establish an extra bathroom and kitchen. (The existing kitchen and bathroom will remain in use for the children ). The house will remain a 4 bedroom home and no increase in occupancy will occur. The existing septic system will also continue to service the house, as it has since the 50s.
In addition to the existing toilet and bathroom in the house there is another toilet block with two toilets, a urinal and a shower which is used by family and visitors. This SAHC approved facility comprises a Clivus Multrum CM20 combined with a reedbed system. It has been working successfully for 3 years. Visitors stay at the studio, a short distance from the house and almost adjacent to the toilet block.
Note that in respect of the proposed development:
- no spa, auto dishwasher or food waste disposal unit will be installed
- rainwater captured from the roof will be used for hot water
- the aim is to most efficiently treat the effluent and to be able to reuse grey and black water from the new development of the house
- the system will comprise a standard septic tank connected to a reedbed for purification of the water which will then be used for irrigation of a woodlot
The site is a 15 hectare property abutting the Gawler River close to the township of Gawler. Average rainfall is 450 mm per year and evaporation is 1900 mm. The prevailing soil type is a free draining silt. Groundwater is at a depth of five metres or greater at all times of year.
The 1 in 100 year return period flood level is 43.0 metres AHD (Australian Height Datum). The system will be more than 100 metres from the Gawler River.
The property has been developed for 15 years by the owners, Graham and Annemarie Brookman, as a permaculture farm, visitor centre and wildlife sanctuary. The property contains orchards, woodlots, vegetable gardens, poultry runs, bush tucker plantations and a fenced predator-proof area for free-range poultry and endangered native animals. The site is unsewered and is relatively flat, there being a slight fall away from the house to the area where the reedbed is to be established.
Effluent Management Objectives
Owners wish to install an effluent management system which reflects the sustainable theme of the property. Key objectives are:
- to provide better effluent management than a standard septic system
- to produce an effluent suitable for sub-surface irrigation re-use
- to be low maintenance, using minimal energy and no chemicals to treat the water
- to provide an educational experience for visitors
The Enhanced Effluent Management System can achieve all of the above objectives.
System Diagram of Enhanced Effluent Treatment System
Whilst the sizing of the system is to be based on more generous assumptions, (as is required, given that the home could be sold and occupied by a larger and resource hungry group), the current occupants estimate the following real use:
Estimated real effluent volumes for ensuite/kitchen development:
|Source||Appliance||Uses per day||Vol per use||Vol per day||Vol per week|
|Toilet (double flush)||8
Proposed assumption of volumes for ensuite/kitchen development
However it is proposed that the conservative figure of 300 litres of liquid waste per day be used bearing in mind that:
- the development will essentially service two persons
- rainwater will be used for hot water
- the existing system services the whole family currently
- there are other toilets which are used on the property
(The Health Commission can accept lower figures than the standard 6 persons X 150 litres = 900litres per day. For example if the house operates only on rainwater, the per person figure can be dropped to 125 litres per day. Similarly, if the reedbed is in addition to existing waste disposal facilities without any increase in the potential number of occupants then an agreed % of the total effluent can be assigned to the new facility)
Componenets of the proposed new waster water system
Sizing of septic system
A 1650 litre unit will give a retention time of 5.5 days in primary treatment (the standard being 2-3 days for standard soakage-based systems. This gives a very large spare capacity and thus the possibility of adding extra modules to reedbed if that should ever become required.
Sizing of reedbed
Using the Plugflow model as endorsed by the US Environment Protection Agency and allowing for local SA conditions the following formula and values were adopted:
Flow (cubic metres/day) X BOD of inflow
Area of bed = BOD of outflow________
Temperature Constant X beds porosity X depth of water
- Depth of water 0.5 metre
- Porosity of gravel in bed 40%
- Biological Oxygen Deficit (or Demand) of inflow 300
- Biological Oxygen Deficit of outflow from bed 20
- Temperature average of 15 degrees C gives a Constant of 0.473
- Flow 0.3 cu metre/day (300 litres)
Therefore the Area of the reedbed must be 8.52 square metres
Using a width of 1.2 metres length = area/width
The length of the bed must be at least 7.1 metres
Using modules of 2.4 metres length, three modules give a reedbed length of 7.2 metres which is thus adopted.
The volume of this reedbed will give a retention time of a fortnight, well over double the retention time required by many reedbed designers in Australia.
A pump sump will be at the exit end of the reed bed fitted with a submersible pump and irrigation piping to the woodlot.
The proposed constructed reedbed is a subsurface flow unit planted with Typha orientalis (Bulrush or cattail), an endemic species in the Gawler region. The reedbed will be a buried to ground level, filled with aggregate (gravel - average diameter 20 mm) and planted with reeds. Effluent enters one end of the reedbed and exits at the other, flowing through gravel voids. The standing effluent level is 100 mm below the gravel level, ensuring no access to effluent by children or mosquitoes and lessening any odours escaping the reedbed. The reedbed will be at 43.3m AHD (Height above sea level) - this is important in respect of predicted flood flow levels on the Gawler River.
The actual inside dimensions of the reedbed will be 1200mm wide by 600mm deep by 7200mm long.
The excavation will be lined with prefabricated waterproof bins made of 8mm high density polyethylene, welded as per the Australian Standard for welding of HD polyethylene. (note: this technology has since been superceded by the manufacture of 'spun' polyethylene tanks).
The excavation will allow for 50mm of filling sand to be packed between the trench wall and poly film surrounding the bin walls and the floor of the trench will be screeded flat with a bed of 30mm of sand.
Each bin is notionally 2.4m long and 1.2m wide and effluent passes from one to the next via two entry ports which have removable lids for effluent monitoring. Large rocks in the inlet area will nullify the risk of clogging. However if the structure of the bins proves sufficiently rigid it will be made in the form of one bin only, probably with 2 permeable bulkheads (at the joints of the 2.4m sections). See Fig 2
Reeds grow hydroponically in the effluent and transfer oxygen to their roots, creating an aerobic root zone in an otherwise anaerobic environment. This provides sites for aerobic and anaerobic microorganisms to reduce contaminant levels in water. Nutrients are taken up by plants and microorganisms, organic matter is stabilised by microorganisms, suspended solids are physically filtered by gravel and roots, and pathogens are destroyed by competing microorganisms and natural die-off over time. Reeds will take approximately 24 months (using Cyperus spp this is achieved in 12 months) to become well established. Prior to this time treatment will still be occurring due to the growth of bacteria in the reedbed.
Subsurface flow reedbeds are a proven technology for effluent management. Several single household reedbeds have been installed in South Australia. An approved reedbed for four houses has been operating successfully since 1987 at the property of the Village Community Cooperative, Willunga (system is shown in Appendix 2). Many other small reedbeds have been installed around Australia. Mitchell et al (1990) (see Appendix 3) performed trials on small-scale reedbeds ('wetlands') and concluded:
"Single household sewage treatment systems have been designed for individual households to substitute for soak-away facilities below septic tanks.... On present evidence, systems are expected to be long-lived and require little maintenance beyond some annual harvesting of older plant culms.... It can be concluded that, on the basis of experience gained so far, artificial wetlands...coupled to a septic tank, appear to be particularly suitable for the treatment of domestic sewage from individual households in rural and semi-rural conditions."
Some water will be transpired from the reedbed by reeds. Reeds transpire water at approximately the same rate as open water evaporation Reed et al. 1995). In January and February, pan evaporation is approximately 9.3 mm per day. Using an actual evapotranspiration factor of 0.65, daily evapotranspiration is 6.0mm. Hence transpiration from the 8.52 m2 reedbed is approximately 50 litres per day during summer.
The design of the reedbed is shown. Effluent will enter the reedbed through two entry ports which have removable lids for effluent monitoring. Large rocks in the inlet area will nullify the risk of clogging. Effluent will exit the reedbed through two similar exit ports with removable lids.
Effluent will exit the reedbed to a sunken sump with submersible pump which will facilitate the pump-out of treated effluent from the barrel to the subsurface irrigation area. The sump (offset min 2.5 metres from the reedbed) will have a capacity of 500 litres, being a 1200mm length of RIBLOC S2000 stormwater pipe. The pipe is a spirally wound high density polyethylene with a galvanised steel reinforcing hoop (Z600). The pipe is set 50mm into a 125mm thick concrete base. Shrinkage gaps between the pipe and concrete are to be sealed with a copolymer sealant. Concrete to be used is 32MPa reinforced with F 62 fabric 50mm above the bottom.
An 8mm, child-proof chequerplate lid is to be provided with a mosquito proof induct vent. It will be borne on a concrete plinth poured around the top of the pipe(this technology has been superceded by a polyethylene prefabricated sump).
A Grundfos CP 250 submersible pump will transfer the effluent from it via a 100 mesh screen filter to the subsurface irrigation area through a 25mm low density polyethylene main and then 19mm laterals. The filter bleed water will be cycled back to the septic tank via 19mm poly tube.
Total head acting against the pump is calculated at 3.6 m.
The pump-out cycle will be controlled by two float switches in the sump. In addition there will be a hard wired audio/flashing alarm system for high or low levels in the sump.
Pump sump guidelines noted in design of sump
Pump sump construction shall comply with the following requirements:
The sulphate resisting cement reinforced concrete components shall be constructed so as to comply with AS 3600 Concrete Structures and AS 3735 Concrete structures for retaining liquids and achieve 25Mpa at 28 days. Also:
- there shall be no structural failure or undue distortion of the sump, empty or full, due to hydrostatic or other pressures when placed in situ,
- it will be designed to withstand any loading imposed by vehicles, adjoining structures or surrounding soils,
- there will be no structural failure or cracking of the prefabricated pump sump when transported and lifted into the excavation,
- it shall be installed on a solid, level base,
- it will be provided with a cover that is fitted so as to be watertight and have an access opening and cover of at least 500 x 450 mm or DN 500 terminating at surface level,
- to be fitted with a DN 100 induct vent either located in the inlet pipe to the pump sump or on the pump sump cover,
- all connections and joints shall be waterproof,
- setback distances are the same as for septic tanks,
The pump shall:
- be constructed from materials suitable for pumping effluent and may be an above ground or submersible type,
- have a capacity to discharge the maximum daily flow against any physical or imposed head,
- be statically mounted and protected from the elements and be wired to operate automatically,
- The electrical work shall be carried out to the requirements of The Supply Authority Service Rules and AS 3000 SAA Wiring Rules,
- The pipework associated with connection of the pump shall be made with approved material in a workmanlike manner,
- be provided with an audible and visible alarm with muting facilities for the audible component and be in a conspicuous position to warn of pump failure and high-water level.
The subsurface irrigation area will comprise a 5 metre by 20 metre woodlot comprising eucalypts and wattles. Branching from the 25mm submain (see Figure 6) will be five 19mm low density polyethylene laterals with simple jab adaptors with 2mm nozzles inserted every 0.5m. Using l9mm tube, friction head loss will be contained to approx 1 metre of head so watering will be quite even. The area around each jab adaptor will be filled with 20mm diameter gravel to prevent blockage (as per diagram). The whole plot will be permanently mulched to a depth of l20mm to prevent access by mosquitoes, children etc and to maximise the use made of the water by the plants
Application of water below mulch but above ground will allow maximum further treatment of effluent as it percolates through biologically active topsoil. The species elected are known to be a water-hungry, nutrient-stripping plants with a fast growth rate. This will optimise effluent nutrient uptake and produce fire wood and structural material for use on the property.
Water Utilization Area
An allowance of 300 litres per day enters the reedbed and the great majority will exit in winter (250 litres per day in summer). According to the SA Code -Supplement A (page 9), the irrigation area should be sized to absorb 4.5 litres per sq metre per 24 hr period. Using this calculation the disposal area would need to be 66sq metres; however as a safety measure, 100sq metres has been provided. Other models suggest that an actively growing planting of native trees can utilize 10 litres of effluent per square metre per day. A single large citrus tree is known to use 250 litres of water per day during summer.
The flow from the pump will deliver 50 litres per minute operating at a 5 metre head so irrigation will take place in about 6 minutes to handle a peak load of 300 litres of effluent per day (applying 4.5L/sq metre per day).
The capacity of this particular area to absorb the water is more than adequate, the deep loam soil type being identical to that in the irrigation plot associated with the reedbed system at the Learning Centre (that system has been working well for >3 years).
System Management includes operation and maintenance but also response strategies in case of system malfunction.
The proposed system is designed to be relatively passive, with minimal operating requirements. The sump pump is the only moving part. Maintenance includes the regular mowing of the reeds (at least once every 2 years, but ideally each half of the bed is done each spring when new shoots are emerging), inspection of inlet and outlet ports annually, monthly cleaning of the filter and biennial checks on the septic system.
Effluent flows through the system by gravity to the sump and is not expected to clog. If the pump fails the users are alerted by the alarm system. The irrigation lines servicing the water utilization area may become blocked over a very long time this would become obvious through the pump operation and the growth of trees. Annual checking of jab adaptors at the ends of the lines would be sufficient. If adaptors block they can simply have new ones forced into the pipe next to the blocked adaptors.
The owners of the property have operated such a system for three years with no breakdowns and a minimum of maintenance. Very low skill level is required to operate and maintain the system.
Design and installation of the system is by Graham Brookman of The Food Forest and Chas Martin of Edwards Rd, Willunga SA.
Operation will be the responsibility of Graham and Annemarie Brookman of The Food Forest, Clifford Rd, Hillier.