188 Environmental Biotechnology
more to the vagaries of the weather and makes process control more difficult.
While this might be a problem for some kinds of biowaste, for the typical park
and garden waste treated by this method, it generally is not. However, some early
attempts were prone to heavy leachate production in conditions of high rainfall,
leading to concerns regarding localised soil pollution. This was largely an engi-
neering problem, however, and the almost universal requirement for a suitably
constructed concrete pad and interceptor has made this virtually unknown today.
Limited aeration occurs naturally via diffusion and convection currents, but
this is heavily augmented by a regime of regular turning, which also helps to
mix the composting material, thus helping to make the rate of breakdown more
uniform. Dependent on the size of the operation, this may be done by anything
from front-end loaders on very small sites, to self-propelled specialised turners
which straddle the windrows at larger facilities. The intervals between turning
can be tailored to the stage of the process, being more frequent early on, when
oxygen demand is high, becoming longer as composting proceeds.
Windrows have a typically high land requirement, can potentially give rise
to odour problems and are potentially likely to release fungal spores and other
bioaerosols during turning. Despite these drawbacks, this approach accounts for
the vast majority of centralised composting projects, possibly because it is often
carried out as an addition to existing landfill operations, thereby significantly
reducing the actual nuisance generated.
Static pile
Superficially resembling the previous method, the static pile, as its name suggests,
is not turned and thus does not have to conform to the dimensions of a turner,
allowing the rows to be considerably taller and wider. What mixing is needed can
be achieved using standard agricultural equipment and so these systems tend to be
significantly cheaper in respect of equipment, manpower and running costs. They
do not, however, remove the land requirement, since decomposition progresses
at a slower rate, causing the material to remain on site for a longer period.
In an attempt to get around this, a variant on the idea has been developed,
particularly for the co-composting of food or garden biowaste with manure or
sewage sludge, which relies on forced aeration. With a perforated floor and fans
to push air through the material, the characteristically low oxygen level within the
core of traditional static piles is avoided and processing accelerated. However,
bulk air movement is expensive, so this system tends to be reserved for small
tonnage facilities, often in areas where good odour control is of major importance.
Tunnel composting
Tunnel composting has been used by the mushroom industry for a number of
years, where processing takes place inside closed tunnels, around five metres
high and up to 40 feet in length. There has been some interest in adapting it to