Guidelines for Successful Reedbed Wastewater Systems.

By Admin on 20-07-2015

Introduction

Approximately 12% of Australian households are not connected to centralised sewerage systems and hence use on-site technologies for the management of their wastewater (Geary and Gardner, 1996 in [1]). In 2001 the failure rate for on-site sewage management systems (eg septic tanks) in the Lismore City Council area was found to be 44% [2]. In Europe, constructed wetlands (reed beds) are increasingly being adopted [1]. Given these figures and the many studies confirming the success of reed bed systems (see references list), it is likely Australia will see an increase in the use of reed bed wastewater systems.

A reed bed is essentially a channel, lined with an impermeable membrane that is filled with gravel, planted with macrophytes and used to treat wastewater [2]. Reed beds are a second stage wastewater treatment and are becoming more common in domestic applications. They are usually used in conjunction with:

· Primary collection system. This may be a septic tank, grey water tank, or aerated wastewater treatment system (AWS);

· Secondary treatment system. More than one reed bed may be used. Depending on the situation, a sand filter may augment the reed bed. Alternatively, an AWS undertakes both the primary and secondary treatment roles; and

· Land application area. Treated water may be reused as subsurface irrigation or disposed of via a trench.

The role of reed beds in this ‘treatment train’ is to detain the wastewater for a period of time so it slowly passes the roots and stems of the macrophytes (or reeds) where it is treated via a number of physical, chemical and biological processes.

Compared to other technologies, including the commonly used AWS, reed beds have a number of advantages. They are cheap to build, require no power to operate and very little personal effort or money to maintain [3]. Reed beds can be installed on sites with soils usually not suitable for on-site wastewater disposal such as sands, clays or steep slopes[1]. Reedbeds do not use chemicals [4], relying instead on natural processes. Council inspections are usually annually – compared to quarterly inspections required for AWS [4]. In addition, a reed bed can become an aesthetically pleasing, functional part of a garden [3].

Design

Common reed bed specifications are listed below to help you get the idea. Reed beds are generally designed by specialist contractors and require Council approval. Council wastewater management codes differ may differ, so check with your local Council prior to designing your reed bed.

  • · Detention time – generally 5 to 7 days
  • · Lining – concrete or polyethene pipes, water tanks or Duroplas moulds. Many Councils no longer recommend the use of plastic liners as they can be pierced by macrophyte rhizomes.
  • · Substrate – in most cases 10mm gravel is chosen. Alexandrina Council in SA [5] specify 20mm washed gravel. Larger stones (eg rail ballast 60-80mm) are placed in the first metre of the bed (inlet zone) and sometimes adjacent to the outlet structure [2] to reduce clogging.
  • · Water Depth – ranges of 300 – 700mm are generally specified. Reed beds tend to be designed to a depth-dependent upon the rooting depth of the planted macrophytes [1]
  • · Slope – may be zero to 1%.

It is notable that [2] found earthworms useful in cleaning reed bed substrate. De-clogging it if you like. A management outcome of this finding could be the inoculation of reed beds with earthworms to clean the substrate and prolong their useful life [2].

All about microbes really!!!

Like all plants, macrophytes in reed bed wastewater treatment systems require nutrients for their continued growth. Simply by growing, they remove considerable amounts of nutrients, particularly prosperous and nitrogen.

This relatively high rate of nutrient uptake is, however, often insignificant compared to the loading rate to which most reed beds are subjected Cooper et al, 1986 in [1].

It seems other processes contribute to the wastewater treatment process. One of the most significant is the degradation of pollutants undertaken by the microbial populations attached to wetland substrate (gravel) and the roots, rhizomes and submerged stems of the macrophytes. Planted macrophytes not only provide the physical structure for these microbes to grow, they transfer oxygen to their root zone, providing microscopic aerobic sites for micro-organisms. These oxygenated microenvironments enhance many, if not all, pollutant removal processes within reed bed systems (IWA, 2000 in [1]).

Reed beds also provide a great buffering against fluctuating air temperature, a factor which is thought to affect the rate of many pollutants removing processes (Kadlec and Knight, 1996; Hill and Payton, 1998 in [1]).

In selecting macrophyte species for reed bed systems, the nutrient uptake capacity of the plants is not the primary selection criteria. Species should be selected based on their capacity to grow well in local conditions (which may mean survive frost) and maintain abundant surface biomass. Stem density may also be a consideration for the reed bed designer.

Species

Until recently, Phragmites australis (the Common Reed) and Typha orientalis were the main reeds used in reed beds on the NSW North Coast [2]. However, with the understanding of reed beds growing continuously, many new plant species are being used, such as Lomandra hystrix, Baumea articulata and Schoenoplectus mucronatus [2].

The aggressive nature of the Phragmites australis rhizome system and a tendency for senescence in the top growth (leading to a rather ragged appearance) in the winter months have prompted a search for other species [3]. Some Council’s, such as Alexandrina Council in South Australia [5] specify that rigid plastic modules must not be planted with Typha orientalis, a species with considerable expansive strength, due to the risk of rupture when the module becomes packed with growth. A greywater reuse operator in WA [6] suggests Typha orientalis and Phragmites australis should not be used in domestic wastewater treatment systems in Australia because of the massive seasonal release of wind-blown seeds.

Many Council wastewater management codes recommend the use of locally indigenous species, presumably as these are guaranteed to perform well under local conditions.

Lismore City Council [2] list 9 suitable species for northern NSW (from L. Davison and T. Headley, 2003 in [2]), which includes both Typha orientalis and Phragmites australis.

Unfortunately, most lists that I have seen do not include Lomandra hystrix, a species used with great success in reed beds on the mid-north coast of NSW and one suggested by [2] and used extensively by [4].

Bluedale’s recommendation for reed bed species selection is to use locally indigenous species that maintain a reasonable level of growth throughout the year. The following list is suitable for coastal areas of northern NSW.

Species Name

Common Name

Baumea articulata

Jointed Twig Rush

Baumea rubiginosa

-

Bolboschoenus fluviatilis

Marsh Club Rush

Eleocharis sphacelata

Tall Spike Rush

Lepironia articulata

Grey Sedge

Lomandra hystrix

Slender Mat Rush

Schoenoplectus mucronatus

 

Schoenoplactus validus

River Club Rush

Many of these plants are available to purchase online at www.theplanthub.com.au

References

[1] Davison, L., Bayley, M., Kohlenberg A. and Craven, J. (2002) Performance of Reed Beds and Single Pass Sand Filters with Characterisation of Domestic Effluent: NSW North Coast. Septic Safe Research Report, NSW Department of Local Government, 112 pp. accessible athttp://www.dlg.nsw.gov.au/Files/Information/R14-Lismore%20City%20Council.pdf

[2] Lismore City Council (2005) The Use of Reed Beds for the Treatment of Sewage & Wastewater from Domestic Households. Accessible at www.lismore.nsw.gov.au/page.asp?f=RES-PEY-54-48-10

[3] L. Davison, L., Headley, T. and Pratt, K. (2005) Aspects of design, structure, performance and operation of reed beds – eight years experience in northeastern New South Wales, Australia Water Science and Technology 51, 10, pp 129-138, 2005.

[4] Midcoast Reedbed Wastewater Systems website http://midcoastreedbeds.com.au/

[5] Alexandrina Council. Environmental Health Fact Sheet What is required with an application for a Reedbed (second-stage wastewater treatment) system? Accessible athttp://www.alexandrina.sa.gov.au/webdata/resources/files/Reedbeds.pdf

[6] Greywater Reuse Systems websitehttp://www.greywaterreuse.com.au/

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