Written for the Big Green Gathering programme, August 2009

WET system

Planting up the WET system at Fernhill Farm – Photo: Jay Abraham, Biologic Design.

An exciting new development on the Big Green Gathering site this year is the recent installation of an extensive biological sewage treatment system. This means that the BGG’s ‘poo miles’, arising from the need to pump sewage out of the portaloos and dispose of it off-site, can be reduced to little or nothing.

The WET System was designed, constructed and planted by Biologic Design, a Permaculture design business based in Herefordshire. It is a constructed wetland about 1.2 acres in area – much larger than is required for just the farm’s use. The total capacity is sufficient to process the sewage from the 50 or so people who are on site (farm/camping barns) all year round, as well as the 60,000 gallon ‘shock loading’ which is generated by the Gathering.

The system is based on the capacity of water to be renewed and purified by natural means. Water is the most abundant, the most abused, yet the most accommodating of our natural resources. It is filtered through the soil and subsoil on its way to wells and springs, and supports animals, plants and micro-organisms which together act as a biological purification system.

This closely integrated community of living things, together with the physical properties of its environment, constitutes an ecosystem. This utilises the photosynthetic abilities of plants to absorb solar energy and the power of the microbial population to absorb chemical nutrients from soil and water. WET systems function by harnessing the natural productivity of wetlands, creating a purification process which uses the waste water as a resource, converting the nutrients into plant biomass and soil.

As wetlands are the most productive and species-diverse ecosystems to have evolved, the potential for a high biomass yield is great. It is not a simple Reedbed Treatment System, although it does contain reeds; there is also a variety of willow types and wetland tree species, as well as a range of aquatic and marginal plants. 

The system comprises specially designed and constructed earth banks and ponds, which creates a ‘plug-flow’ type of purification kinetic. The waste flow passes through the longest possible distance from the inlet to the final polishing pond. As it flows through the ‘swales’, with a high ratio of planted edge to their volume, it is both purified by micro-biological action and transpired by growing plants.

Around 35 wetland and marginal species are planted in and around the lagoons, ponds and reed beds. The topsoil which was on site has been carefully stripped away and then replaced onto the swale banks. The soil contains the microbes which break down and process the nutrients in the sewage – this is a very rapid process. 

Coppicing the willow on a one, two and three year cycle gives a wide variety of useful products including willow for basketry work, hurdle making, and binders for hedge laying, as well as living willow structures. The larger varieties will probably be harvested and used as fuel in the farm’s log boiler.

The purification processes occur in the soil/root zone. Each year, as new soil is created by the growing plants shedding their leaves, the system matures. The root zone expands and the purification potential increases. 

The inlet ditch will, by the time of the BGG, be totally covered with Glyceria maxima – Reed Sweet Grass – which is very effective at negating possible odours when planted as a ‘reed raft’. 

The roots hang down into the channel, and the reeds absorb any odiferous compounds such as ammonia and hydrogen sulphide. The surface area upon which the active biomass can live has been increased by adding woodchip to the inlet swale ditch.

The farm is located in a very sensitive aquifer area – it feeds the reservoirs which supply Bristol. We have therefore used a Geosynthetic Clay Liner (GCL), which comprises a 10mm thick Bentonite clay layer sandwiched between 2 layers of porous, woven, geomembrane. Bentonite is a clay which swells by 3-4 times when in contact with water. The GCL membrane is equivalent to a 1 metre layer of compacted clay.

Each roll of the GCL is 200 sqare metres and there are 32 rolls. The advantage in using the GCL is that it is a natural product which will stay in place forever – it will not degrade – and will seal itself if it should be accidentally punctured. Nevertheless the Environment Agency and the Planners required a leak detection system – a series of underdrains – which will enable the farmer to visually check the system for leaks.

The portaloos will be emptied by tanker in the normal way, and the sewage transferred to a holding tank and then pumped gradually into the WET system. At some time in the future we hope there will be toilet blocks with pipes leading directly to the holding tanks, doing away with the need to pump out the toilets – which can sometimes be smelly.

• The ‘blue’ used in the portaloos is biologically based and will add to the active microbes in the system.
• No toxic chemicals (such as formaldehyde) will be used. Please do not tip any chemical products, cleaning fluids or other inorganic materials down the toilets. Also no plastics, which will look unsightly if they litter the system.
• In general, people are asked not to put anything down the toilets (apart from toilet paper) that they haven’t eaten or drunk first!

Fernhill farm WET system

The Fernhill Farm WET system, June 2009 – Photo: Jay Abraham, Biologic Design.