Biotechnology and Waste - Environmental Biotechnology: Theory and Application - page 210

CaseStudy8.2 MixedAnaerobicDigestion(Mantova,Italy)
The potential that AD has for recovering a substantial energy yield frommaterial with
a high organic content inevitably makes it an attractive option for dealing with waste.
As Rivalta Energy's plant outside of Mantova demonstrates, a carefully designed AD
facility can integrate very successfully with a variety of existing environmental
strategies and waste management arrangements.
With a design capacity of 10 000 tonnes/year of municipal/commercial and
agricultural feedstocks, the plant principally comprises three individual 70m 3 pre-
treatment compartments, a complete mix digester and four 350 kW generators,
currently producing 1MW of electricity, but built to be expandable to 2MW.
Municipal and commercial solid wastes are processed into a slurry within their
receiving compartment, with the addition of digestate or dairy manure to aid the
mixing, and a sophisticated biofilter system is used for odour control.
Producing a suitably homogeneous mix for digestion takes between 15 and
60 minutes, depending on the nature of the incoming waste, after which the
slurry, now at around 10-15% solid content, is pumped into a cyclone separator,
where any remaining entrapped non-organic inclusions are removed to feed into
existing recycling arrangements as appropriate. The slurry then flows to the third
compartment, where it is balanced and blended with agricultural manures and
other input feedstocks before being pumped into the digester. The Rivalta digester
is a completely mixed system, consisting of two primary and two secondary cells.
Biogas mixers within the cells ensure that the digesting substrates remain completely
homogenised throughout the process to optimise performance in terms of both
biological action and biogas yield.
Once the digestion process has been completed the material is dewatered using
sequential screw press and roller press separation, with the solids being sent for
final aerobic composting, while the liquid is held in a storage tank and subsequently
used either to wet incoming solid waste in the pre-treatment phase, or applied
to local fields. After composting and maturation, the solid material is also used
for agriculture.
The success of this facility, which was completed and commissioned in 2007,
has led Rivalta Energy to developed three further plants using the same technology.