Source-sorted waste sorting plants
Some
plants are designed exclusively to handle, clean and dry paper from private
households and paper and cardboard collected from offices. At such plants the
material is received in a tipping hall and a front end loader is used to move
the material onto a belt conveyor leading to the processing area. Manual
sorting at the belt conveyer may either be negative, i.e. unwanted materials such
as plastic bags can be removed from the main stream, or positive, i.e. the
recyclable paper and cardboard materials can be diverted to give different
quality fractions. The sorted fractions are transported to balers and the bales
are shipped off to paper mills.
Compost and biogas producing plants
Several
different systems for low and high technology composting exist. The biodegradable
materials are in general shredded before composting, and when the compost
process is finished, the compost is sieved to remove large items including
materials which are not biodegradable.
In
the simple low technology composting plants, the piles are established outdoors
with no equipment for aeration. After a period of several months to years
during which the piles are agitated mechanically, the compost is sieved and
subjected to final composting. Due to the low content of structural materials
in the biodegradable fraction of domestic waste, it is difficult to maintain an
efficient composting process in this fraction. Therefore, the biodegradable
fraction of domestic waste is often mixed with garden waste or with other
biodegradable fractions such as sewage sludge and slurry from livestock
buildings.
The principal steps
in biogas production are shredding of the biodegradable waste and mixing with
water. The mixture is fed to the reactor for anaerobic digestion at reactor
residence times of 10-20 days. The digester effluent is dewatered. The liquid
is reused for the process and the sludge is shipped either to disposal, e.g. as
‘earth’ cover materials at landfills, or used as material for compost
production.