Annelidic conversion (AC)
The use of a variety of annelid worm species is one alternative approach that has received fairly regular reawakenings of interest over the years, having been variously termed worm composting, vermicomposting, vermiculture or our pre-ferred annelidic conversion, a term first attributed to H. Carl Klauck of Newgate, Ontario. The description worm composting and its like is somewhat misleading, since the process from both biological and operational criteria is quite distinct from true compost production in two significant ways.
Firstly, as we have seen, in traditional composting, breakdown is brought about by the direct actions of a thriving microbial community. Within a worm-based system, while micro-organisms may contribute in some way to the overall biodegradation, their role in this respect is very much incidental to that of the worms themselves.
Secondly, in worm systems, biowaste is typically laid in much shallower layers than is the case for windrows or static piles, frequently being deposited on the surface of an underlying soil bed. This is a major difference, principally because it reduces the natural self-heating tendency within the decomposing matrix.
Worms of various species can be present in traditional compost heaps, even in thermophilic piles, but they avoid the genuinely thermophilic core, being found at the significantly cooler edges of the heap. In addition, under such conditions the resident annelid population is, in any case, many magnitudes smaller than in the deliberately high-biomass levels of AC systems. While in common with all poikilothermic organisms, worms do require some warmth to remain active, which for most species means a lower limit of 10 ◦C, they do not generally tolerate temperatures in excess of 30 ◦ C and death occurs above 35 ◦ C. Most species have an optimum range of 18 – 25 ◦ C, which makes the point very clearly that the highly exothermic conditions encountered as part of the ‘true’ composting process would be impossible for them to survive, and certainly not in any sizeable numbers.
Annelidic conversion is similar to composting in the sense that it can be scaled to meet particular needs and, as a result, it has been promoted in various forms for both domestic and municipal applications over the years. Again, like composting, particularly in respect of home bins, this has not been entirely free of problems, since all the difficulties regarding bin design, operator diligence and issues of compliance apply if anything, more rigorously to AC as to traditional composting. While some recycling officers have found that these projects have been widely welcomed and effective, others report ‘considerable’ drop-off rates in usage.
In the case of commercial scale treatment, worm systems have sufficient inbuilt flexibility to be tailored to suit. However, since the beds must be significantly shallower than an equivalent windrow, accommodating the same amount of mate-rial for treatment necessitates a much larger land requirement, which may itself prove a constraining factor. Thus, for each tonne of biowaste to be deposited weekly, the typical bed area required is around 45 m2. Hence, for a typical civic amenity site annual production of 4000 tonnes, and allowing for the seasonal nature of its arising; around half a hectare, or one and a quarter acres, of ground is required simply for the beds themselves. This rises to more than double to provide the necessary service access between and around the wormeries.
Worm systems are essentially biomass intensive, with an initial population den-sity, typically exceeding 500 animals per square metre and a cumulative annelid biomass production rate, once established, of 0.07 kg/m2. Clearly, this demands careful control of the local environmental conditions within the beds for opti-misation of system performance, particularly since the physical and biological needs of the organisms involved lie within more precisely defined limits than those of the microbes responsible for composting. Bed design is partly influ-enced by the temperature tolerances discussed previously, but the large surface area to volume ratio typical of this method also allows for the ready aeration of the biowaste matrix, especially in the surface layers, where many of the worm species used preferentially reside. Design is further constrained by the need for adequate moisture to permit gas exchange across the annelid skin, which must be achieved without waterlogging, since this reduces pile aeration and may, further actively drive the resident worms to leave in search of drier conditions. Unsur-prisingly, many commercial scale systems make use of extensive drainage works to help avoid this. Well-ventilated covers are also often used, particularly in out-door installations, which help overcome the rigours of the weather while also producing continuously dark conditions. Accordingly, the worms are encouraged to be active for much more of the day than would otherwise be the case. This brings additional bonuses, since the burrowing of the animals themselves both promotes enhanced aeration and has beneficial effects on odour control, partic-ularly in respect of sulphide concentrations, which have been reported as being reduced by a factor of 100 or more.
In common with trends in many other biotechnological interventions, there has been some interest over the years in developing in-vessel systems. This principally arises as an attempt to create circumstances in which process control can be maximised, but has the additional benefit of also giving rise to a modular and highly portable approach, which has helped annelidic conversion penetrate areas that might otherwise have remained closed to it. One of the reasons for this is that this approach gets around the need for a permanent installation, which may be an important consideration for some applications. However, the unit processing cost is consequently higher than would be the case for a simple land-based system of similar operational capacity.
A number of different species of worms are used in vermicultural operations around the world, but in general terms all of them can be placed into one of two broad categories, namely redworms and earthworms. Although some uncer-tainty exists as to the absolute validity of this division, it is a useful tool, at least at the functional or morphological level, to aid understanding of the whole approach. The true earthworms are burrowers, and generally speaking consume dead biological material from within the soil itself rather than directly assimi-lating the biowaste. The nutrient value of the organics so treated is returned via worm casts. Hence, initiatives reliant on earthworms are probably best regarded as a form of worm-enhanced composting. By contrast, redworms, which are also sometimes termed manure or compost worms, rapidly and directly feed on the biowaste, consuming half or more of their own body weight per day. This influx of material is turned into increased worm biomass, both in terms of individual growth and population increase. As a result, annelidic conversion has tended to make use of redworm species such as Dendrobaena, Helodrilus and especially Eisenia as the mainstay of these operations. In nature, these animals are naturally found amongst the fallen plant material of woods and forests, where they are commonly associated with the production of leaf lit-ter. Their use in worm beds stands as another example of making use of an organism’s natural abilities to achieve the biotechnologist’s desired result. When the artificial environment of the worm bed is well enough managed and condi-tions suitably optimised, the redworms decompose and mineralise the biowaste extremely efficiently, effectively reprising their role in nature under these engi-neered conditions.
Despite the predominance of redworms within vermiculture for the reasons outlined, there have been a number of cases that have used true earthworm species, with varying degrees of success. Thus, members of the genera Lumbricus and Amynthas have featured, but perhaps one of the most notable successes was with Pheretima elongata, a deep burrowing worm, native to India, which was used to great effect as part of the Bombay plague prevention project, which was founded in 1994 in response to an earlier outbreak of the disease. The growing amount of waste in the city had been strongly implicated in attracting and harbouring the vector rats. The use of vermiculture proved very effective in reducing the biowaste problem (Menon 1994) and hence was seen as a major preventative measure against a recurrence of the plague.
The use of worms to break down waste is not a particularly novel idea, though like many aspects of environmental biotechnology, it frequently seems to fall first in and then out of favour on a fairly regular basis. The advantages of worm-based biowaste treatment are fairly easy to see. In the first instance, vermiculture offers a high potential volumetric reduction, often exceeding 70% while producing a well-stabilised final product. Secondly, this product itself is rich in potassium, nitrogen, phosphorous and other minerals, which is presented in an ideal form for plant uptake, and hence represents a high fertiliser value. The market for this product has already been successfully established in some parts of the world and many others seem set to follow. This is an important issue, as it has been estimated that for every one tonne of biowaste deposited on the bed, around half a tonne of worm casts is produced (Denham 1996).
Finally, the rapid growth potential of worms under idealised conditions has the potential to provide a harvestable biomass resource, typically either as seed populations for other vermiculture operations or for direct sale into the fishing market. The reputation of the bait outlet has been tarnished in the light of various spectacular collapses, most notably the Californian pyramid franchise back in the 1970s and other similar operations in the UK more recently. Often aimed at farmers seeking diversification ventures, it is unlikely that worm production will truly be the salvation for all of them. However, the fact remains that there are a number of long-established businesses in Britain, the USA and elsewhere successfully trading in live worms for various purposes.
The sequential combination of various approaches into treatment trains has become one of the major themes of environmental biotechnology over recent years. Annelidic conversion has particular potential for use in this way and this may prove of increasing importance within the waste management industry in the future. There is a particularly logical fit between this method and tradi-tional composting, since a period of precomposting permits the thermophilic inactivation of pathogens, while secondary worm action offers a high quality product more rapidly.
There are many of advantages to this method, most obviously in that it allows for the input biowaste to undergo established sanitisation procedures without detriment to the worms themselves, which are, as discussed previously, temper-ature sensitive. A less commonly appreciated benefit of this approach is that the period of initial composting significantly reduces worm ammonia exposure, to which, again, they are very sensitive. However, as with so much of these combined approaches, there is a need to manage the treatment conditions care-fully to produce the optimisation desired. There is evidence to suggest that a precomposting phase has a negative effect on worm growth and reproduction rates (Frederickson et al. 1994) which obviously represents a direct reduction in the overall rate of worm biomass increase. Obviously this has an effect on the overall rate of stabilisation and processing, particularly since it has been demon-strated that the enhanced waste stabilisation achieved under worm treatment is only attained under conditions of high resident worm biomass (Frederickson et al. 1994). It seems reasonable to suggest, then, that to maximise the effectiveness of the combined treatment approach, the initial composting period should be no longer than the minimum necessary to bring about pathogen control of the input biowaste. Though this represents the kind of compromise balancing act so typical of much of environmental biotechnology, it is one which holds much promise. The combination of annelidic conversion with composting permits both enhanced stabilisation rate and product quality, with the additional bonus that the volatile organic content is also significantly reduced. It is also possible that the natural ability of worms to accumulate various hazardous substances within their bodies will also have implications for waste treatment, particularly if it proves possible to use them deliberately to strip out particular contaminating chemicals.
Annelidic conversion is currently very clearly a minority technology in this role, but the potential remains for it to play a role in the future biological treatment of waste, either as a standalone or, as seems more likely, as part of an integrated suite of linked processes. This seems particularly likely if the characteristically superior product derived from this process can be shown to be consistent, since specialist materials in the horticultural and gardening market generally tend to offer better returns. However, only time will tell whether this will prove to be sufficient financial incentive to offset the costs of production and encourage wider adoption of the technology.