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Reedbed (second-stage wastewater treatment) systems
Reedbed systems are a second stage treatment for onsite wastewater systems - mostly for greywater systems. Before installation, an application for the approval of a reedbed system must be submitted to the Department of Health. Individual site or environmental considerations identified by Departmental officers when assessing the application may require the applicant to provide more details. (Guides are also available for greywater systems and composting toilets or other blackwater systems.)
This guide specifies the information that must be provided with the application for assessment by DH for approval. Some treatment options have been provided.
1 Location & site characteristics
- A plan showing detailed directions to locate the property.
- A scaled plan of the property and all nearby land and buildings and the land use of nearby properties.
- A site plan showing the location of the house/building, boundaries, recreation areas, reedbed area, disposal area of reedbed effluent and blackwater treatment and/ or disposal unit.
- Details of site characteristics, such as topography, proximity to any watercourses, bores, marine environment, etc.
2 Treatment & disposal of all wastewater
A reedbed system is part of an overall property wastewater management system. The following information on the management of the whole system is required. Method of black water (toilet waste) treatment and disposal Supporting information and engineering calculations should be provided.
Options include:
- a septic tank (for a minimum of six persons) with effluent disposal by subsoil soakage, connection toan SA Water sewerage system, OR
- connection to an SA Water sewerage system or a private or a council scheme, OR
- a composting toilet or other type of blackwater treatment / disposal system approved for installation in SA.
Greywater pre-treatment
For a domestic property, a tank with a minimum volume of 1620 litres is required for the greywater pretreatment before reedbed disposal.
Plumbing plan
The plan of the plumbing layout for the wastewater/greywater collection system must comply with Australian Standard AS 3500.5.
3 Design & installation
Number of persons
The system must be designed for a minimum of six persons. Reedbed surface area Generally a surface area of 3m2 - 6m2 per person is needed. Engineering data must be included to confirm the surface area size. The actual figure is dependent on the BOD, hydraulic flow, gravel void space and liquid depth of the bed.
Liquid depth
Reedbed water depth will be between 300-700 mm with at least 100 mm cover over the liquid.
Gravel substrate
The reedbed is to be constructed using a 20 mm gravel substrate. The gravel is to be washed prior to placing in the bed. Larger gravel/stones are required to minimise clogging around the inlet and outlet.
Total porosity
The total porosity of the reedbed (void space of gravel where the wastewater lies) is usually calculated on a basis of 35- 50%.
Length to width ratio
For the length to width ratio of the reedbed area, a ratio of between 3:1 and 5:1 is preferable.
Reed bed retention time
A minimum detention time of 5 days is required.
Lining
The lining of a reedbed should be impervious, durable and able to resist penetration by macrophyte roots. If using plastics, it is advisable to lay sand or geotextile under and over the plastic liner to prevent piercing by gravel, subsurface rock or tree roots.Alternative lining materials and installation methods must be supported with engineering specifications.To minimise the chance of punctures, thin liners (i.e. 0.2mm or less) must not be planted with species such as Phragmites which have spear-like rhizomes. Rigid plastic modules must not be planted with Typha, a species with considerable expansive strength, due to the risk of rupture when the module becomes packed with growth.
Acceptable liners include:
- a double layer of construction grade PVC liner (minimum 0.2mm thickness per layer)
- a suitable thickness of reinforced concrete.
Setbacks
Distances for setbacks from watercourses, dams, bores, buildings or other similar features provided in the Standard for the Installation and Operation of Septic Tank systems in South Australia for soakage systems are to be considered when determining setbacks. A lesser setback for lined beds may be considered where information, including calculations from a suitably qualified engineer is provided indicating that any installed lining will not be breached (eg. by lateral tree roots).
Inlet/outlet structure
Inlet/ outlet structure details for discharge to sub surface must be provided.
Slope
Any slope in the design is not to exceed 1% and may be zero.
Construction of collection sump
The construction of the collection sump must not compromise the structure of the reed bed and must contain at a minimum of 50% of the daily flow or 500 L, which ever is the greater. The sump must be constructed in accordance with the requirements for sumps in Standard for the Construction, Installation and Operation of Septic Tank Systems in South Australia.
Plant species and planting layout
Suitable species
Where possible, use only indigenous native Australian species. A list of appropriate species may be found on page 4, but this is not comprehensive and is only intended as a guide. Many suitable natives are not commercially available, but may be harvested from existing local wetlands provided a permit (public lands collection only) or permission from the local landholder is obtained. Several introduced species used in reedbeds (e.g. Elodea Canadensis, Salvinia molesta, Alternanthera philoxeroides, Eichhorniz crassipes and Lagarosiphon major) have caused major stream clogging, and should be avoided. Check the national weeds database (www.weeds.org.au/noxious.htm), consult with your local nursery, or phone the Animal and Plant Control Board before confirming species selection.
Typha spp., Phragmites spp., and Schoenoplectusspp. are widespread, and have deep, vigorous root systems which generate essential aerobic conditions and encourage nitrification. Schoenoplectus validus and Phragmites communis are superior in removing nitrogen. Typha spp. require less nutrients and do well in greywater only reedbeds. Bolboschoenus caldwellii, Schoenoplectus pungens and Schoenus nitens tolerate variable salinity. In general, Juncus, Baumea, Bolboschoenus, Cyperus, Isolepsis, Lepidosperma, and Schoenoplectus species flower profusely, and are high nutrient assimilators.
Seed versus seedlings
Planting seedlings or clumps is easier than planting seed as rhizomes can be placed the correct way up to ensure successful establishment. Set rhizomes in the gravel deep enough to be in contact with subsurface water.
Planting density and layout
Select 2-5 different species, grouping each species together. Shallower rooted species should be located near the inlet because of higher influent temperatures and nutrient levels. Deeper rooted species should be located toward the end of the reedbed. Planting a mix of annuals and perennials will ensure that nutrient assimilation occurs across all seasons. The planting density can range from 0.3m to 1.0m between the centres of each plant. Higher densities accelerate the development of a mature and completely functional reedbed, but also increase the cost. Rows should be 0.45m apart, planted perpendicular to the direction of flow and staggered 0.25m.
Establishment time
A minimum establishment period of 6 weeks is required to allow new plants to settle and grow. Planting in spring provides the most successful results for seedlings, root-rhizome stock or clumps.
4 Disposal of reedbed effluent (reclaimed water)
Effluent disinfection
Surface irrigation for reedbed effluent for household systems requires adequate disinfection of the effluent in accordance with:
- SAHC Code Waste Control Systems, Standard for the Installation and Operation of Septic Tank Systems in South Australia, and
- Supplement A ~ Aerobic Sand Filter Systems.
Other disinfection methods may be acceptable, depending on the proposed system, its design installation and operation details provided in the application.
For larger installations, reclaimed water reuse must be in compliance with the standards set out in the South Australian Reclaimed Water Guidelines – Treated Effluent.
Effluent discharge to the irrigation area
Pumped discharge to the irrigation area is required to achieve even pressure to all parts of the irrigation system.
Disposal outlets
The disposal area pipe work outlets should be protected from clogging. This may be achieved by the installation of a self-cleaning filter. Backwash discharge from the filter must be forwarded to the primary chamber of the septic tank via a
separate pipe work system.
Effluent storage
Any effluent storage details should be included in the application, such as the method of storage, tank type and size, and detention time.
Irrigation /subsoil surface area
Engineering assessment of soils should be supplied for the irrigation area (surface / sub surface disposal). The irrigation area is to be sized using 4.5 L/m2 per day or to engineer's assessment and requirements.
The Australian Standard AS/NZS 1547:2000 - Onsite
Domestic Wastewater Management may be used as a
guide to the design of alternative land disposal or
irrigation systems.
Surface water diversion
Details showing adequate diversion of surface waters around the reedbed and the irrigation area are
required.
Reedbed maintenance
Where any property is vacant for extended periods, arrangements will be to be made to ensure plants within the reedbed are suitably watered.
More information
ABN 97 643 356 590
Wastewater Management Section
Environmental Health Service
Department of Health
CGU Building 150 Grenfell St ADELAIDE SA 5000
PO Box 6 Rundle Mall ADELAIDE SA 5000
Telephone (08) 8226-7100
Fax: (08) 8226-7102
Email:
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Web: www.health.sa.gov.au/pehs/environ-health-index.htm |
