# Biochar for Wastewater Treatment--Conversion Technologies and Applications
## 5 Environmental Application of Biochar for Wastewater Treatment  
### As a Support Media during Anaerobic Digestion  
Anaerobic digestion is one of the most promising treatment technologies considered as an eco-friendly process for wastes management because of its ability to combine bioremediation and energy recovery. Performing the anaerobic digestion process under stable conditions with high energy recovery and efficient pollutants removal is a major concern for scientists since some constituents of the feedstock or their metabolic intermediate products could inhibit microorganisms and hinder their activity. Different approaches have been proposed so far to counteract the inhibition in anaerobic reactors such as the acclimation of bacterial cells, the adoption of thermophilic operating conditions and the reduction of the concentration of inhibitors either by dilution or by co-digestion with other substrates. The use of packing materials in anaerobic digestion has been proved as an advantageous alternative to reduce the mobility or bioavailability of inhibitors while allowing for the development of a large bacterial biomass within the digester. Biochar as a porous, bio-stable, and available material has earned an increasing interest as an alternative to adsorbents like activated carbon and zeolite, and its application in anaerobic digestion is gradually increasing [163]. Moreover, the residual biochar in digestate can be directly reused as amendment to improve the soil properties without environmental risk [164]. The sorption capacity of biochar toward different organic and inorganic contaminants has been widely reported in the literature, but a lack of information with regard to its behavior toward inhibitory compounds during anaerobic digestion still exists. [PERSON] et al. [22] showed that biochar can adsorb NH\({}_{4}\)\({}^{+}\) and remain stable in ambient air. Equally, [PERSON] et al. [165] reported that biochar mitigates NH\({}_{4}\)\({}^{+}\) inhibition during anaerobic digestion of glucose solution at an NH\({}_{4}\)\({}^{+}\) concentration of 7 g/L. [PERSON] and [PERSON] [166] performed batch tests to investigate the anaerobic digestion of aqueous pyrolysis liquid. The results showed poor performance of the anaerobic process and underlined the inhibition of biological process even with the nutrient supplementation, whereas it was found that the biochar addition increased the yield of CH\({}_{4}\) and improved the reaction rate. In another study, [PERSON] et al. [167] investigatedthe effect of biochar addition on H\({}_{2}\) and CH\({}_{4}\) production in a two-phase batch anaerobic digestion set-up. Their results showed that biochar addition increased H\({}_{2}\) and CH\({}_{4}\) yields and enhanced volatile fatty acids generation during H\({}_{2}\) production and VFAs degradation during CH\({}_{4}\) production. Compared to zeolite, [PERSON] et al. [168] reported that biochar showed less process stabilizing capacity. According to the same authors, further optimization can be assumed for the effective use of chars in anaerobic digestion. The addition of biochar to increase the buffering capacity within digesters is another option that make beneficial the application of biochar during anaerobic digestion process. The acidification of digesters is often prevented by the addition of alkaline agents such as lime. However, some studies showed that the continuous addition of alkaline biochar could increase the buffering capacity of the anaerobic system [169; 170]. [PERSON] et al. [164] compared biochar and non-biochar incubation using glucose as a substrate and showed that for the tests performed by biochar, the CH\({}_{4}\) yield increased by about 90% while the acidification is reduced. Equally, [PERSON] et al. [167] reported that in addition to supporting microbial metabolism and growth, the application of biochar also buffered pH during biohydrogen production.