Biogas - Sludge Management
Sludge storage
To retain the maximum fertilizing quality of digested slurry, i.e. it’s nitrogen content, it should
be stored only briefly in liquid form in a closed pit or tank and then applied on the fields.
Preferably, it should be dug into the soil to prevent losses on the field.
Sludge storage is normally effected according to one or the other of the following three
techniques
• Liquid storage
• Drying
• Composting
Liquid storage
The effluent outlet of the biogas system leads directly to a collecting tank. Loss of liquid due
to evaporation or seepage must be avoided. Just before the sludge is needed, the contents
of the tank is thoroughly agitated and then filled into a liquid manure spreader or, if it is liquid
and homogenous enough, spread by irrigation sprinklers. The main advantage of liquid
storage is that little nitrogen is lost. On the other hand, liquid storage requires a large,
waterproof storage facility entailing a high initial capital investment.
The practice of spreading liquid slurry also presents problems in that not only storage tanks
are needed, but transport vessels as well. The amount of work involved depends also on the
distance over which the slurry has to be transported. For example, loading and transporting
one ton of slurry over a distance of 500 m in an oxcart (200 kg per trip) takes about five
hours. Distributing one ton of slurry on the fields requires another three hours.
Drying
It is only possible to dry digested sludge as long as
the rate of evaporation is substantially higher than
the rate of precipitation. The main advantage of
drying is the resultant reduction in volume and
weight. Drying can also make the manual spreading
easier. The cost of constructing shallow earthen
drying basins is modest. On the other hand, drying
results in a near-total loss of inorganic nitrogen (up
to 90%) and heavy losses of the total nitrogen
content (approx. 50%).
Composting
Figure 42: Drying of digested
sludge and sludge disposal in
Thailand
Photo: Kossmann (gtz/GATE)
Nitrogen losses can be reduced by mixing the digested sludge with organic material. As an
additive to crop residues for composting, biogas sludge provides a good source of nitrogen
for speeding up the process. At the same time it enriches the compost in nitrogen,
phosphorus and other plant nutrients. Furthermore, the aerobic composting process, by it’s
temperature, effectively destroys pathogens and parasites that have survived the anaerobic
digestion treatment. The ready-made compost is moist, compact and can be spread out by
simple tools. With most available transport facilities in developing countries, it is easier to
transport than liquid manure.
Composition of sludge
Process of biomethanation
Anaerobic digestion draws carbon, hydrogen and oxygen from the substrate. The essential
plant nutrients (N,P,K) remain largely in the slurry. The composition of fertilizing agents in
digested slurry depends on the fermented substrate and can, therefore, vary within certain
limits.
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