WET systems for waste purification and resource production
Sewage and slurries need not be awkward pollutants; they can be fantastic resources, as Jay Abrahams explains
Wastes such as sewage, farm slurries, silage liquor, dairy wastes and washdown water, as well as a range of biodegradable wastes from certain industries, can be looked upon either as a problem or as a valuable resource. The former point of view is the current, conventional one and usually results in a costly and complex mechanical or chemical treatment system. The latter perspective leads to the design of a system which takes the waste as an unused resource and converts it into useful, valuable, or at least non-polluting products.
Pollution incidents often occur when effluent reaches a watercourse in an untreated state. The seriousness of the event is, more often than not, directly related to the strength of the waste - its Biological Oxygen Demand (BOD) value.
Conventional treatment systems usually create an oxygen rich environment using motorised aerators and powerful mixers in large concrete or steel tanks. It is within these vessels that the microbes degrade the waste and lower its BOD value.
Some conventional systems are based on oxygen free or anaerobic biological processes and although these do have the advantage of producing methane (a potential fuel gas), as a by product, they are still very expensive to build.
They can also be problematic to run, especially if a high throughput of waste is being treated. This is because the bacterial populations which carry out the anaerobic breakdown of the waste do not respond at all well to rapid changes in their environment i.e. if the temperature, pH or nutrient load changes too fast the bacteria die off and the process fails.
Energy Hungry Systems
Large quantities of energy are needed to keep both of these types of process going (for heating, mixing and aeration) and very often complex control systems are necessary to maintain the required level of treatment. In most cases back-up power supplies are also needed in case of mains power failure.
The resultant treated material is then often discharged into estuaries or the sea. So the valuable nutrient contained in the waste waters is lost, and the oceans used as a dustbin. By 1997 the dumping of this sludge into the sea will be made illegal under an E.U. directive and already many of the water companies have started to install incinerators to burn the sludge instead - transferring the problem from the sea to the atmosphere!
The technological approach of using mechanical/chemical systems continues for the most part unabated, leading to yet more environmental problems as well as ongoing costs. A more considered response would be to look at the purification processes found within the natural world and apply these as our solution.
Many plant species have evolved symbiotic relationships with micro-organisms and are able to both oxygenate and purify waters with high BOD values. Wetlands have evolved to be some of the most productive and species-rich ecosystems on the planet. The multitude of animal, plant and microbial species which inhabit this type of environment have one of the highest capabilities to absorb and transform nutrients entering the system. A Wetland Ecosystem Treatment (WET) System can have benefits apart from the initial primary objective of pollution control. These various benefits are directly related to the number and diversity of the species introduced onto the WET site in the design.
The WET System is not a simple reed bed, although it will contain reeds, and it is not a pond system either, although it will often include ponds too. Typically a WET System will contain up to 40 different aquatic and marginal species, many varieties of willow and other tree species, depending upon what yields the designer wishes to gather from the site.
Working With The Earth
In a WET System a series of earthworks take the flow of waste water into soil banks which have been very closely planted with willow and other wetland species. It uses a dryland strategy for water harvesting, the swale, to act as an interceptor so the waste water can feed the root systems of the trees and other plant species as it percolates through the soil as groundwater recharge.
Using relatively inexpensive earthworks such as interceptor banks and cross slope swales, as well as rock filled gabions and spoiled straw bales, the flow of nutrient rich water is harvested. The resultant groundwater recharge is absorbed by the roots of the plants on the earthworks and transformed into plant growth.
The purification process, as in conventional systems, is microbiological and is accomplished by the plethora of bacteria and fungi which inhabit the root zone of the planted species, the plants supplying the microbes with an oxygen-rich micro-environment, as well as root exudates which contain nutrients such as sugars from which the microbes derive energy.
The microbes break down the organic matter in the waste water, supplying the plants with minerals, and in their turn secrete mucopolysacheride gels which enhance the water retentive capacity of the soil, a simple but effective symbiotic relationship in which all of the partners benefit. The purification processes are carried out within the soil and as one of the products of the plant/microbe interaction is soil creation, the WET System, by creating more soil, becomes more effective at purification as time goes by.
A Working Example
An example of a large scale WET System can be found at the Westons cider mill in Herefordshire, England. This family run company was fined by the National Rivers Authority (now the Environment Agency) for allowing high strength wastes to enter a local stream and so looked at the various options open to it to prevent a similar occurrence. The family decided to enlarge an existing bog in one of its fields so that it would become an effective treatment area for the mill's waste waters rather than install a conventional mechanical/chemical waste treatment system.
The design of a waste purification system for the cider mill had several problems which would have been either difficult or very expensive to deal with using conventional methods. The waste waters are mildly alkaline for most of the year, due to tank and vat washing procedures, but during the 12-14 week cider making season the usual 2,000 gallons per day of alkaline water increases to 80,000 gallons, with a pH of 3.5 and a BOD of 4,500. A conventional system would have to include a huge amount of storage capacity, in very expensive tanks, which for most of the year would not be used! It would also require a chemical dosing system to neutralise the acidity before very large motorised aerators and mixers could begin to treat the waste water. This would mean that a large power supply would be required and both the running and maintenance costs would be high.
Good Design - Using Existing Resources
During the initial site survey a detailed contour map of the site was compiled and several deep trial pits were dug to ascertain the soil profile. Some of the very useful information and site characteristics highlighted during this survey were that:
The mainly clay based substrate could be puddled to hold water so that any ponds or interceptor ditches would not need to be lined. The clay subsoil excavated when the ponds were dug could be used to form a retaining bund wall around the whole of the WET site preventing seepage into the streams which flow on three sides of the site and into which the wastes had gone originally causing the firm to be fined.
There were deposits of limestone gravel in the top part of the field over which the acidic waste waters could move. This would have the effect of neutralising the low pH wastewater without the need for an expensive chemical dosing system.
The whole of the site could be fed by gravity from the factory outfall so no pumping was required to get the waste water through the WET site.
A large quantity of organic matter was available to be used as a mulch for tree planting on the WET site, this included large sheets of cardboard from the pallets of empty bottles arriving at the works to be filled (this excellent sheet mulch was normally burnt), and several tons of apple pomace - the solids left after the juice has been extracted from the fruit - was used to weigh down the cardboard to prevent it blowing away in the wind.
The philosophy behind the design was to try to use the existing properties, materials and features already on the site to their best advantage and by so doing reduce the cost and increase the efficacy of the system. One 'by-product' of this philosophy was the fate of the cardboard from the pallets initially used for sheet mulching. When Westons' management realised the volume of cardboard collected each week, rather than burning it, they found a recycling company to buy it. Once again a problem has been converted into a resource of benefit to the firm.
Overview Of The System
As the waste water leaves the mill it is fed into a deep anaerobic lagoon which was created by dredging out and enlarging an existing pond in one of the firms orchards. This lagoon has a capacity of around 450,000 gallons and the BOD of the waste water is significantly reduced by this first, anaerobic, treatment stage. The lagoon acts as a buffering stage in which any fluctuation of the waste's composition is ameliorated due to its huge volume and the sloping sidewalls aid the settlement of solids, thus protecting the rest of the system from silting up.
Anaerobic degradation of the waste takes place in the lower reaches of this 12m deep pond and a potential problem with this stage of the process is the breakdown or deamination of the amino acids - the building blocks of proteins - by anaerobic Clostridia bacteria which can produce foul smelling gases. The chief odour producing compounds are methylmercaptan and hydrogen sulphide (from amino acids containing sulphur) and cadavarine (from the breakdown of lysine).
The major source of proteins entering the system comes from the 'tank bottoms' or lees which contain a very high concentration of yeast cells from the fermentation tanks. During the design stage it was agreed that the lees would not be put down the drains to limit potential odour formation. During the first season the lees were however not excluded due to a breakdown in communication and a smell nuisance was reported.
The odour problem has now been addressed as the lees are being tankered from the factory site and used as a source of high protein pig food rich in B vitamins and so a major problem (a source of potential odour forming compounds) has been transformed into a resource.
It is planned to install a Phragmites reed raft to cover the whole of the surface of the anaerobic lagoon to preclude any potential odour problems as well as keeping the top 3 or 4 feet of the waste water well oxygenated.
From the anaerobic pond the waste water flows through a pipeline to the remainder of the WET System entering a Facultative pond which, being shallow and having a large open surface area, enables the deoxygenated water coming from the anaerobic pond to become recharged with oxygen from the atmosphere.
From the Facultative pond, the water passes into the first of the swales and so starts to infiltrate the soil. Planted on the swale bank are thousands of acid tolerant, biomass type willow varieties which are coppiced annually to maintain the rapid growth necessary for waste transformation.
Beyond the swale is a shallow and meandering pond, densely planted with a range of reeds and rushes, in which the water flows over the limestone gravel which increases the pH before it enters the second swale.
The second swale, densely planted with osier willow varieties for basketry work, again causes the water to pass into the soil from where it feeds the remaining three ponds. The ponds give the WET System the hydraulic capacity required to handle the large volumes of waste water entering the system. The ponds are planted with around 40 species of indigenous aquatics, marginal waterplants and reeds. The planting has been carried out with the expected pH and nutrient levels in the various ponds, earth banks and ditches in mind.
The swales, being densely planted with willow, act on the water passing through them in two ways: first the root zone microbes mineralise the high nutrient load of the waste, and secondly the volume of water passing through the site is reduced considerably by evapotranspiration. This takes the groundwater and releases it as water vapour to the atmosphere. The lower ponds were situated around the existing boggy area which already had a vigorous population of reedmace. This is very good at cleaning up nutrient-rich waters and is quickly colonising the new ponds as expected.
Home Grown Resources
The WET System at Westons cider mill converts a potentially polluting organic load into a wide variety of plant products including:
35,000 willow trees (50 varieties) which are being sustainably managed as short rotation coppice to yield osier willow whips for basketry as well as biomass varieties to fuel the mill's boilers.
2,250 traditional coppice wood trees (14 different species) which will be managed under various coppice rotations to yield a variety of products for use on the Westons farm or for sale.
The grazing field which has been modified by the WET Site has now become a wildlife haven and local conservation officers have remarked that its biodiversity will probably attract Site of Special Scientific Interest (SSSI) status within a couple of years.
The WET System is not a static entity and, as with all wetland environments, it will tend to evolve from open water (ponds) through marsh to carr and then on to woodland unless it is managed positively. It is the management regime which keeps the WET System operating at its peak efficiency since by regularly coppicing the trees, keeping them growing at a high rate, the organic matter in the wastes is transformed into plant biomass most effectively.
A WET System, which uses natural ecologies to treat waste, creates resources and enhances the environment, is a far more preferable way of dealing with a problem and turning it into a solution than conventional treatment plants.
Jay Abrahams is a microbiologist and permaculture designer who specialises in creating WET Systems for domestic sewage, farm based enterprises and certain industries. A comprehensive list of species which can be used in a WET System is available. Please contact: Biologic Design, Archenhills, Stanford Bishop, Worcestershire WR6 5TZ England.