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Table 2 Problems encountered at different steps when anaerobic digestion process is sought to be utilized for crop and other solid waste (adopted from Weiland, 2005)

From: The myth and the reality of energy recovery from municipal solid waste

Process step Problems Consequences
Storage Formation of organic acids during storage (pickle-formation effect); partial digestion i) Amounts to loss of some utilizable portion of the substrate
ii) Increases the risk of inhibition of the subsequent methanogenic process
Formation of mold during ensiling and storage of energy crops i) May cause inhibition of methanogenic activity in the digestion step
Substrate pre-treatment Portions of the substrate may not get broken into sufficiently small pieces i) Would reduce anaerobic degradation rate
ii) Risk of scum formation in the fermenter
iii) Difficulty in the handling of the substrate
Solids feeding Nature of feed makes it impossible to achieve exactly continuous flow i) Reduces process stability
ii) Reduces biogas yield
iii) Can cause H2S-surges to occur in the biogas
Mixing of silage and process water in an external open tank i) Digestion occurs to some extent causing losses of methane to the atmosphere
ii) Mixing consumes a lot of energy
Direct solids feeding by screw conveyor, piston and flushing systems i) Risk of blockage in screw conveyors of diameter < 300 mm
ii) Piston systems cause compacting of long fiber crops
iii) Flushing systems cannot be applied for crops of low density
Fermenter and storage tank Scum formation i) Reduces biogas yield
ii) Causes clogging of the overflow pipe
iii) The entire process can break down
Accumulation of biogas in the fermenter digestate i) Reduction of the gas storage capacity in the top of the fermenter
ii) Fermenter can be operated only at reduced loading
iii) Gas pipe will get clogged
Short-circuiting during the flow of substrate i) Reduces biogas yield
ii) Incomplete degradation of the substrate
Long hydraulic retention time i) Large reactor volumes are needed thereby adversely effecting process economics
ii) Low specific methane productivity
iii) High energy input per ton of substrate for heating and mixing
Formation of biogenic heat by mono-fermentation of energy crops i) Stable mesophilic temperature conditions cannot be achieved
ii) Process failure occurs due to the reduced microbial activity above 42 °C
Open digestate storage tanks i) Uncontrolled methane emissions occur
Biogas upgrading Insufficient biological desulphurization i) Reduces lifespan of the electricity generator (EG)
Entry of surplus air to the fermenter for biological desulphurization i) Reduction of the ignitability of the gas due to the resultant lowering of the CH4 content of biogas
Incomplete drying of biogas The moisture content poses problems in:
i) The transportation of biogas
ii) In the measuring devices in the gas main
iii) In the functioning of the EG
Sizing of equipment Lack of reliable data on the biogas yield of energy crops Insufficient adaptation of fermenter and EG capacity which result in:
i) Reduced electrical efficiency of EG
ii) Increased pollutant emission from EG
iii) Intermittent operation of EG
Lack of reliable data on the degradation capacity of the H2S oxidizing bacteria The efficiency of H2S reduction cannot be estimated properly resulting in oversized or undersized installations.