4.6.1 Pretreatment of Municipal Solid Waste
To upgrade and homogenize the feedstock for digestion and to remove inert and
non-biodegradable material pretreatments are essential. They can be manual,
mechanical, thermochemical, biological, etc. There are several ways in which this
can be accomplished.
First of all bulky materials and specific hazardous waste materials are removed
prior to mechanical processing from municipal solid waste. The best pretreatment
process is source separation which provides clean waste with some fraction of
plastic. Metal, glass, plastics, paper, etc. are recovered regularly at waste collection
beans and disposal sites by waste pickers. Though this process of scavenging
reduces the total volume of the waste and enriches the waste with high organic
concentrations, this waste contains a number of items such as dust, foils, some
plastics, metals, papers, discarded construction materials, grits, ash, broken
ceramics, etc.
To further process manually cleaned municipal solid waste, it is being passed
through trommel screens where oversized materials and other foreign materials are
separated followed by hammer crushing to break down larger raw materials to small
pieces and thereby making it more accessible to bacteria which in turn reduces
retention time. The grinder also acts as a mixer. Then the municipal solid waste
passes through a drum magnetic separation mechanism, where a strong magnet
separates the ferrous metals. A hydro-pulper then sorts incoming solid waste into
heavy and light fractions of nonorganic material as well as creates mixed organic
waste. For thorough mixing of the waste and water (slurry), a slurry mixture machine
should be fitted in the inlet of a digester. It is also necessary to remove inert materials
such as stones from the inlet before feeding the slurry into the digester. Otherwise,
the effective volume of the digester will decrease.
Chemical pretreatment has been tried in a variety of temperature regions and over
a variety of time periods, from 15 to 120 min. These strategies particularly help with
the degradation of fats, which is troublesome because of their insolubility in water
and their semi-solidification. For fats hydrolysis, they must be emulsified to enhance
their bioavailability in water. Pretreatment with sodium hydroxide, lithium hydroxide, or potassium hydroxide increases the hydrolysis rate. Lime, sodium hydroxide,
and ammonia are the least expensive of these chemicals. For biological method,
bacterial growth (anaerobic microorganisms) is stimulated by the addition of some
organic compounds (e.g., amino acids, cofactors, cell content) in the inlet tank of the
digester.
Anaerobic digestion of solid waste has been demonstrated as a technically
feasible process, duly deserving further consideration in any integrated waste management concept addressing municipal solid waste. Anaerobic digestion provides an
important opportunity to generate 100% renewable energy from biodegradable
waste. The conversion of a sizeable part of organic waste into a convenient source
of energy, i.e., biogas, is a precious asset, not in the least in times of oil scarcity and
of economic support for renewable forms of energy. The simultaneous generation of
digest, which can be turned into a soil amendment, may be an added advantage
116
G. Monika et al.
To upgrade and homogenize the feedstock for digestion and to remove inert and
non-biodegradable material pretreatments are essential. They can be manual,
mechanical, thermochemical, biological, etc. There are several ways in which this
can be accomplished.
First of all bulky materials and specific hazardous waste materials are removed
prior to mechanical processing from municipal solid waste. The best pretreatment
process is source separation which provides clean waste with some fraction of
plastic. Metal, glass, plastics, paper, etc. are recovered regularly at waste collection
beans and disposal sites by waste pickers. Though this process of scavenging
reduces the total volume of the waste and enriches the waste with high organic
concentrations, this waste contains a number of items such as dust, foils, some
plastics, metals, papers, discarded construction materials, grits, ash, broken
ceramics, etc.
To further process manually cleaned municipal solid waste, it is being passed
through trommel screens where oversized materials and other foreign materials are
separated followed by hammer crushing to break down larger raw materials to small
pieces and thereby making it more accessible to bacteria which in turn reduces
retention time. The grinder also acts as a mixer. Then the municipal solid waste
passes through a drum magnetic separation mechanism, where a strong magnet
separates the ferrous metals. A hydro-pulper then sorts incoming solid waste into
heavy and light fractions of nonorganic material as well as creates mixed organic
waste. For thorough mixing of the waste and water (slurry), a slurry mixture machine
should be fitted in the inlet of a digester. It is also necessary to remove inert materials
such as stones from the inlet before feeding the slurry into the digester. Otherwise,
the effective volume of the digester will decrease.
Chemical pretreatment has been tried in a variety of temperature regions and over
a variety of time periods, from 15 to 120 min. These strategies particularly help with
the degradation of fats, which is troublesome because of their insolubility in water
and their semi-solidification. For fats hydrolysis, they must be emulsified to enhance
their bioavailability in water. Pretreatment with sodium hydroxide, lithium hydroxide, or potassium hydroxide increases the hydrolysis rate. Lime, sodium hydroxide,
and ammonia are the least expensive of these chemicals. For biological method,
bacterial growth (anaerobic microorganisms) is stimulated by the addition of some
organic compounds (e.g., amino acids, cofactors, cell content) in the inlet tank of the
digester.
Anaerobic digestion of solid waste has been demonstrated as a technically
feasible process, duly deserving further consideration in any integrated waste management concept addressing municipal solid waste. Anaerobic digestion provides an
important opportunity to generate 100% renewable energy from biodegradable
waste. The conversion of a sizeable part of organic waste into a convenient source
of energy, i.e., biogas, is a precious asset, not in the least in times of oil scarcity and
of economic support for renewable forms of energy. The simultaneous generation of
digest, which can be turned into a soil amendment, may be an added advantage
116
G. Monika et al.