During the operation of the biogas plant further attention has to be paid to keeping the
groundwater clean. Seeping biogas digesters and unprotected slurry storage can pollute
water sources chemically (nitrate poisoning can be fatal for infants) and biologically (mainly
with toilet biogas plants). Reasons may be wrong configuration of security devices like the
pressure relief valves or because of leakage in lower parts of the digester. Smaller cracks,
however, close up in the course of time through particles in the slurry.
Trace metals applied to natural systems do not pose a threat to groundwater quality because
trace metals are usually removed from the percolating water by adsorption or chemical
precipitation within the first few meters of soil, even in rapid infiltration systems with high
hydraulic-loading rates.
Bacterial removal from effluents passing through fine soils is quite complete. It may be less
complete in the coarse, sandy soil used for rapid infiltration systems. Fractured rock or
limestone cavities may provide a passage for bacteria that can travel several hundred meters
from the point of application. This danger can be avoided by proper geological investigations
during site selection.
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