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Table 5 Summary of pro and cons of pretreatment strategies to optimize the hydrolysis stage

From: A review on strategies to optimize metabolic stages of anaerobic digestion of municipal solid wastes towards enhanced resources recovery

Strategy

Pro

Cons

Mechanical

• Particle size reduction increases the surface area available for microorganisms resulting in improved anaerobic degradability

• Promotes rapid digestion of lignocellulosic-based OFMSW

• Excessive particle size reductions may result in pH decrease resulting in decreased methane yields

• High energy requirements and the possibility of impurity contaminations during particle size reductions process

Thermal

• Accelerate lignin solubilization of lignocellulosic-based OFMSW and shortening hydraulic retention time

• Removal of pathogens in substrates with subsequent enhancement of digestate handling

• High running costs due to high heating energy requirements for running the process

• High temperatures may result in the creation of chemical bonds and agglomeration of particles

Chemical

• Alkali and acid pretreatment enhances removal of lignin which enhances better contact of substrates and microorganisms

• Alkali pre-treatment with NaOH and KOH enhance COD solubilization of OFMSW which accelerates methane production

• When used at a small scale, chemical pre-treatment has low capital costs

• Use of chemical pre-treatment may lead to the formation of inhibitory products such as phenolic compounds, furans, and carboxylic acids which may inhibit the growth of the methanogens

• Acid pre-treatment may lead to the corrosion of equipment which may be very expensive to repair.

• A high-cost requirement of chemicals in large scale biogas productions

• Digestate produced may require careful handling due to by-products formed

Microbial

• Facilitates removal of lignin and hemicelluloses degradation of lignocellulosic-based OFMS which enhances better contact of substrates and microorganisms

• Due to low or no use of chemicals, there is little corrosiveness and by-product formation

• Can be applied in milder conditions

• May lead to the production of the safe digestate with minimum disposal costs

• Considered to be an environmental friend with low capital costs and energy requirements

• A slow process and hence degradation of lignocellulosic-based OFMSW may take several weeks to months

• Due to the heterogeneous nature of OFMSW, different specific enzymes may be required.

• Require sterile environments