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)

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.