Table 4 Previously conducted research reviews on the use of biochar as an additive toward an enhanced AD performance with emphasis on the roles of biochar in promoting DIET

Compare AD amended with different CCMs, concerning CCMs type, particle size, dosage, electrical conductivity (EC), redox properties, and AD operational factors, such as temperature and organic loading rate (OLR)

• BC is the second most used CCM next to GAC

• Literature reported that CCM addition enhanced CH₄ production

• Some BCs like wood chips caused inhibition of CH₄ production which is related to its lower quinone and hydroquinone content

• BC with high electron donating capacities is important

• VFA accumulation due to high OLR resulted in an imbalance in microorganisms, pH drops, and process failure was mitigated by CCMs

• During hydrogen build-up causing VFA accumulation and pH drop, CCM can alternatively accept electrons during the process to proceed with VFA degradation

• NH₃, a necessary nutrient for microorganisms but at high concentration (1700–1800 mg l⁻¹) can cause inhibition but methanation and lag phase will still be improved with CCM amendment

• Other inhibitors like sulfate have been classified to react in a way that its corresponding reducing bacteria will compete with methanogens over OM resulting in reduced CH₄ but this can be mitigated through CCM

• BC is rich in FG giving redox properties that allow electron transfer other than EC (2.1–4.4 μS cm⁻¹) explaining why BC is more efficient to promote DIET even though its EC is extremely low compared to GAC (3000 μS cm⁻¹)

2022

[35]

Properties and functionality of BC and their role in AD and economic challenges

• Properties of BC are affected by the pyrolysis environment such as temperature and retention time

• Temperature and biomass types determine the formation of FGs (carboxyl, hydroxyl, phenolic hydroxyl, carbonyl groups) of BC

• Microbial activity and relative abundance are greatly affected by the properties of BC and this can be done by meeting the optimum nutrient requirement in the AD

• BC is a multi-function agent such as an inhibitor adsorbent and serves as an environment for microbial colonization, electron conductor, and pH buffer

• NH₃ or NH₄⁺ ions (FAN) released at high concentrations in the AD inhibit methanogens that will cause the VFA to accumulate. TAN should then be pinned to a safe level (1500 mg/kg) and this can be more economically and easily mitigated using BC addition as compared to pH and temperature adjustment, maintaining C:N and pre-treatment

• VFA accumulation and sudden pH drop are caused by unbalanced acidogenic and methanogenic microorganisms and VFAs accumulation can be controlled with the introduction of Syntrophomonas spp. and Syntrophobacter spp.

2021

[27]

Investigate the production process of BC and

its physicochemical characteristics; and identify the mechanism of BC that improves AD

• Feedstock types and the synthesizing parameters are major factors that influence the yield and characteristics of BC

• Synthesizing environment and the feedstocks types affect the electron transfer capability of BC

• The ability of biochar to shuttle electrons is positively influenced by increased temperature

• VFA inhibition is coupled with the higher H₂ partial pressure

• Electro-active microorganisms are enriched with the BC addition and the VFA metabolism shifts from IHT to DIET

• BC with larger size (2–5 mm) quickly alleviate NH₃ inhibition and led to a high CH₄ production at reduced time, followed by medium-sized (0.5–1 mm) particles, while the slower response in smaller size biochar (75–150 μm)

2020

[26]