Biotechnology and Waste 201surface 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 m^2. 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/m^2. 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