Integrated Environmental Biotechnology - Environmental Biotechnology: Theory and Application - page 243

Integrating biowaste products
The potential for nutrient and humus recycling from biowaste back into the soil,
via composted, digested or otherwise biologically treated material was mentioned
in Chapter 8. Without digressing into detailed examination of the general options
open for the utilisation of such soil amendments, they do have water holding
applications and form another example of the natural potential for environmental
biotechnologies to self-integrate.
Much of the evidence for this has come from the field, with research conducted
throughout the UK highlighting the major water holding benefits to be gained
by large scale use of biowaste compost. It has been shown that at an application
rate of around 250 tonnes of composted material per hectare, the land is able to
hold between 1000 and 2500 tonnes of rainwater (Butterworth, 1999). Perhaps
the most significant evidence in this respect comes from the trials of large-scale
compost treatment in the loose, sandy soils of East Anglia, which seem to suggest
that this would allow SRC crops to be grown without any further watering in all
but the most exceptional of years (Butterworth, 1999). According to the same
study, even under such circumstances, the additional irrigation required would
be very greatly reduced. The same work established that relatively immature
composts are particularly effective in this respect, as they can absorb and retain
between 2 and 10 times their own weight of water. The situation appears similar
for dewatered AD digestate, when applied to soil and permitted to mature in situ .
Digestate sludges are often aerobically stabilised in a process sometimes rather
inaccurately termed 'secondary composting'; this approach simply extends the
same idea. The end result of this process is a high humus material, with good
micro-biological activity and excellent water retaining properties, which appears
to match the performance of 'true' composts at similar application levels. More-
over, it would also seem that biologically derived soil amendment materials like
these, applied appropriately to soils either as a surface mulch or ploughed-in,
cannot only lower supplementary watering demands enormously but also largely
offset any tendency to drought-stress in the growing biomass. In addition, the
leaching of nitrate from the soil is also lessened significantly.
As an aside, it is interesting to note that this ability to retain large amounts
of water, together with its naturally high organic content has led to the use of
compost in the construction of artificial wetlands. The US has been particularly
active in this area, in part due to the fact that federal environmental regulations
encourage the creation of this type of habitat as a means of water treatment.
This approach, which has been discussed more fully in an earlier chapter, has
as its main goal the manufacture of a wetland which behaves like a natural
system in terms of both its hydrology and biology. To achieve this, a humus
rich, biologically active medium, which closely replicates the normal physical
and chemical properties of local soils is required. Biowaste derived composts
have been found to contribute well as constituents of manufactured wetland soils,
often allowing vegetation to become established on such sites more quickly than
usual (Alexander, 1999).