Can earthworms provide the answer to the sludge treatment problem?
Vermistabilisation is the use of earthworms to treat sewage sludges. Dr Piers Clark of WS Atkins Water explains how vermistabilisation, with its low capital cost and relatively simple operation, may represent the ideal solution to some rural UK sites, where raw sewage has traditionally been disposed direct to agriculture.
In April 1996 the Royal Commission on Environmental Pollution (RCEP) published a report on the sustainable use of soils. One of the many recommendations made in this report was that the use of untreated sewage sludge on agricultural land ought to be phased out. The UK government is expected to respond to the RCEP report within the coming months.
Approximately 50 per cent of all sludge produced in the UK is recycled to agriculture. On large works most of this sludge is anaerobically digested and dewatered prior to disposal (in accordance with the UK Code of Practice). On small rural works however the quantities of sludge produced are relatively small and thus the cost of extensive sludge treatment prohibitive. Since the availability of suitable agricultural land around small rural works is usually high, raw sludges are often applied directly. It is estimated that approximately 25 per cent of all the sludges disposed to agriculture in the UK are raw sludges derived from small rural works.
Clearly if the recommendations made by the Royal Commission are adopted by the UK government the implications for sludge treatment on rural sites are considerable. Traditional rural works treatment technologies, such as lime stabilisation remain costly and capital intensive. In recognition of this WS Atkins, in collaboration with a number of water companies, is investigating and developing alternative treatment technologies such as vermistabilisation.
Earthworms in the natural environment are responsible for the turnover of large quantities of organic matter. There are in excess of 3000 species, although detailed scientific knowledge is only known for about five per cent. Populations as great as two tonnes per hectare have been recorded in the natural environment, with densities 10 times this achievable under controlled laboratory conditions.
The principle function of the earthworms in vermicomposting is to increase the surface area available for micro and macro-organisms involved in the decomposition and stabilisation of organic matter.
The oligocheata Eisenia foetida (also known as brandling or tiger worms) has been the focus of much of the research done on using worms to treat sewage sludges. The species grows rapidly, achieves sexual maturity in a short time (within 10 weeks) and multiplies prodigiously. It is an ubiquitous species and many organic wastes become naturally colonised by it.
Other worm species also offer potential in sewage sludge systems. For example the African nightcrawler Eudrilus eugeniae can grow twice at fast as E. foetida but they cannot withstand temperatures below 12 degrees C and are thus not suited for UK climates. The earthworm Allobophora chloritica can withstand starvation better, breeds all year round and can survive in wetter conditions. Aquatic (as opposed to terrestrial) worm species have also been considered by researchers. The aquatic polycheate Neris japonica is tolerant of variable salinity, temporary oxygen deficits and hydrogen sulphide levels.
The advantages associated with vermistabilisation are well known and documented:
fragmentation of sludges, converting them into castings and increasing the surface area available for drying and microbial decomposition;
improved aeration due to tunnelling action of worms;
increased concentration of available phosphorus, potassium and magnesium;
reduction in carbon: nitrogen ratio;
decrease in particle size and thus increase in moisture holding capacity of sludges (to a level comparable with peat);
reduction in odour characteristics of the waste;
bioaccumulation of heavy metals; and
enhanced sludge destruction.
Maintaining the correct physico-chemical environment for the worm population is vital. Specific conditions vary for different worm species but in general the compost media must be aerobic with a pH within the range 5.5 to 8.5. Bed moisture concentrations should range between 70 to 90 per cent. If too dry aerobic microbial composting can occur producing localised ‘hot spots’ which can kill the worms.
The optimum temperatures for vermicomposting range bet-ween 15-25oC. Temperatures above this will kill the worms. Conversely, little growth occurs below 10oC. In maintaining the correct temperature range it may be necessary to protect the worm bed by installing a heating system (such as underbed heating cables) or enclosing the bed (in poly tunnels with adequate ventilation).
The concentration of soluble ions present in the sludge is also an important parameter. Chloride levels in excess of 0.5 per cent are detrimental and certain ammonium ions and copper salts are toxic at levels above 0.1 per cent. Wastes which contain considerable concentrations of free ammonia, such as poultry litter, may not be suitable for vermistabilisation.
Vermicomposting bed systems can be classiÞed as: outdoor static beds (or windrows); static beds with controlled conditions; or enclosed continuous beds.
Outdoor static beds/wind-rows. Outdoor static beds/wind-rows are conceptually very simple. Sludge, usually mixed with a bulking agent, is laid out on an open hardstanding and seeded with a worm population. After a suitable acclimatisation period more sludge is added to the bed and the worms move into the new layer. The windrow system is similar in principle, except that the fresh sludge is added to the end of the row, lengthening it. In climates such as the UK uncontrolled static beds are not practical or economic, due to the cold winter months during which the worms are inactive.
Controlled static beds. In controlled static beds the temperature, moisture content and aerobic nature of the beds is managed. Enclosure of the beds and/or underbed heating is necessary. Ensuring aerobic conditions is usually achieved through blending the sludge cake with a suitable bulking material.
Enclosed continuous beds. The most complex vermicomposting system in terms of equipment is the enclosed continuous beds developed by the Rothamsted Experimental Station and marketed by British Earthworm Technology (BET). In this system the fresh sludge is added to the top of the bed and the worms move into the fresh waste. The composted sludge is removed from the bottom of the bed via a specially designed discharge ßoor. The advantages of this system included a faster process time and reduced labour requirements. BET however is believed to no longer be trading.
One of the major disadvantages associated with the cake-based vermicomposting systems described above is that expensive items of capital equipment are required to dewater the sludges. On the small rural sites (at which vermicomposting may represent the most economic treatment solution) the installation costs for the dewatering equipment are prohibitive, producing a Catch 22 situation. Furthermore the polyelectrolytes used in the dewatering process are, in many instances toxic/detrimental to the processing abilities of the worms.
In response to this WS Atkins, funded by a collaborative group of UK water companies, is undertaking a programme of research to develop a vermistabilisation system for treating liquid (as opposed to cake) sludges. It is anticipated that results will be published within the coming year.
The three areas of particular interest when considering the economics associated with a vermistabilisation system are: the worm worked waste (the ‘vermicompost’), the worm bodies (as primary proteins) and the ‘avoided’ costs (due to reduced tankering or a reduction in waste disposal costs). These avoided costs can only be reliably quantified and assessed on a site specific basis since the viability and costs associated with the various potential disposal options will vary significantly from site to site.
Vermicompost is usually a finely divided material with excellent structure, porosity, aeration, drainage and moisture holding capacity, resembling peat in appearance and characteristics. However for successful marketing the product should be screened, dried, blended and packaged.
The worm bodies represent a valuable source of protein that could be utilised for livestock (chickens, fish) or even human consumption. Research in Japan for the use of worm proteins in beefburgers has been reported. Worm protein, based on its amino acid, fatty acid, mineral and vitamin content has a similar value to fish, pig and poultry feeds. However harvesting the worms represents a considerable challenge and, once harvested the worms must be washed (to remove pathogens and parasites) and preserved.
The vermistabilisation of sewage sludges has been widely studied in recent years and a number of pilot and full scale systems have been operated across the world. Vermistabilisation offers a variety of potential benefits when compared to other more conventional treatment processes including the production of a high quality compost, the production of primary proteins and the avoidance of additional disposal costs.
More research is required to identify, quantify and optimise the key processing parameters (such as sludge loading rates and worm stocking densities) since these will ultimately determine the economic success or failure of vermicomposting. A detailed programme of research is currently being undertaken to address these issues and it is anticipated that results will be published within the next 12 months.
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