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