Biotechnology and Waste - Environmental Biotechnology: Theory and Application - page 188

were prone to heavy leachate production in conditions of high rainfall, leading
to concerns regarding localised soil pollution. This was largely an engineering
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 acceler-
ated. 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 5m high and
up to 40 feet in length. There has been some interest in adapting it to deal with
MSW-derived material and one system which has evolved uses huge polythene
bags, a metre or so high and 60-m long into which a special filling machine
packs around 74 tonnes of source separated putrescible material. This particular