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Management of the Process in the
Digester
Organic waste (wet from differentiated
collection, agri-food industrial discards, zootechnical
excrement etc.), suitably mixed with a pre-established part of
ligno-cellulose fraction (chipped vegetable waste), is fed
into the digester with a mechanical shovel. On completion of
loading the frontal hatch is closed and a first aerobic phase
is activated, thanks to the singularity of the AmBiogas system
which has been designed in such a way as to support, through
appropriate adjustments, both the aerobic and anaerobic phases
of the process. Supply of an adequate quantity of oxygen to
the mixture guarantees rapid triggering of the thermophilic
bioxidation reactions for the activity of aerobic
micro-organisms which in a few hours bring the temperature of
the mass up to 50°C and beyond. Having reached these values,
insufflation is suspended and the air conduction pipes are
closed by special valves, but the aerobic microbic assault
continues its oxidation activity until there is no oxygen left
in the digester. During insufflation the process air extracted
from the reactor is purified in dedicated bio-filters.
When the available oxygen runs out, the aerobic microbic
population disappears and is progressively replaced by
anaerobic bacteria. In this context it is useful to employ,
mixed with “fresh” organic materials, a portion of digested
material (recirculation) with the function of inoculation. To
facilitate development and bacterial activity, the temperature
of the process material is maintained around 35°C through
heating the reactor floor and walls and sprinkling previously
heated percolate. Use of the percolate, produced by the
materials in digestion and collected in a special tank, also
has the function of microbic inoculation. Clearly, the
possibility of increasing initial material temperature with
the foregoing procedure (biological aerobic process) means
considerable energy savings.
Under these conditions the biogas production process (absence
of oxygen and constant mesophilia) evolves through a first
phase, known as acidogenic, of demolition of the organic
molecules and their transformation into fatty acids and
acetate, and a second phase, called methanogenic, of
degradation of the products from the acidogenic phase into
methane, carbon dioxide and water. The quantity of biogas
produced depends on the biomasses treated: for example, using
the organic fraction of the waste we can obtain between 150
and 180 m3 of biogas for every tonne of waste. The
percentage of methane in the biogas varies from 50 to 70%.
The operational cycle is carried on until the quantities of
biogas produced are maintained on economically advantageous
values. On average each cycle lasts 4 weeks.
On completion of the digestion phase a second aerobic phase is
activated, following removal of residual methane from the
reactor by insufflation of inert gases, this in order to avoid
the formation of potentially explosive gaseous mixtures
(oxygen/methane). Insufflation of air into the reactor favours
development of the aerobic microbic assault (bacteria and
fungi) for triggering a new phase of bioxidation reactions
which, in about 15-20 days, lead to good stabilisation of the
material, making it compatible with the subsequent phase of
maturation, the last link in the chain of converting organic
waste into compost.
It need not be underscored how the possibility of managing the
entire biological cycle inside the same reactor means
considerable economic savings, a notable simplification of
management and a drastic reduction in malodorous emissions:
all this by avoiding the transfer of huge quantities of
organic masses from anaerobic reactors to ventilated
bio-cells.
The AmBiogas digester’s sophisticated technological equipment
permits automated management of all process phases, constantly
guaranteeing optimal ecological conditions for the
micro-organisms and safety for the operators.
The reactor is equipped with probes and instruments for
indicating temperature, pressure, oxygen, methane, CO2
and H2S at various points of the system.
AmbiControl software records data, processes them and can set
the process phases in autonomous mode.
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