## 8 Biochar as an Additive to Enhance the Quality of AD Main Products
### The Role of Biochar in Agronomic Valorisation  
Additionally, when biochar is added to the AD process, there are other factors to consider, including biogas yield. Biochar can also play an important role in digestate properties for further agronomic valorisation [7; 36]. Particularly, biochar's ability to interact with organic and inorganic compounds through the functional groups on its surface enhances the digestate quality after AD [65; 72]. Numerous processes, including adsorption, electrostatic contact, ion exchange, and precipitation, contribute to this. The bioavailability of the nutrients in the biochar-digestate matrix for crop development and nutrient uptake may be impacted by such dynamics [72; 73]. Additionally, it could improve the soil's nutritional balance by stopping nutrients from percolating through the soil into streams and waterways, which would boost plant productivity, change microbial habitats, and have an effect on the bacterial communities' activity and composition in the soil [73; 74]. Biochar addition is expected to help the immobilization/entrapment of the organic and inorganic contaminants. Because of its absorption capabilities, biochar's application to soil has also been shown to shield plants from hazardous organic compounds, reduce nitrous oxide (N\({}_{2}\)O) and methane emissions from soil, as well as adsorb NH\({}_{4}\)+-N through chemisorption and physical adsorption [74]. According to [PERSON] et al. [74], applying biochar to soil may affect the activity of the methanotrophic microbial community by acting as a substrate for microbial growth, affecting nutrient availability, and fostering microbial metabolism. All things considered, applying biochar to AD and digestate management has several hygienic advantages, such as decreased pathogens, the adsorption of toxic materials, odour control, and enhanced digestate stability, in addition to the enhancement of electron transfer between microbial species involved in AD. According to [PERSON] et al. [20], soil can be directly treated with the digestate from the fermentation process where biochar is added to enhance its qualities without endangering the environment. [PERSON] et al. [23] state that it may be more advantageous to combine biochar with digestate to act as an organic fertilizer compared to biochar alone, reporting that biochar by itself is insufficient in providing nutrients. According to that study, one way to boost the organic matter linked to aggregates and minerals in a reasonably short amount of time is to coapply biochar with organic fertilisers, like digestate. [PERSON] et al. [73] obtained similar outcomes, showing that the combination of digestate and biochar increased plant growth, soil nutrient availability, water use efficiency, and physiological parameters, making it a very effective fertilization approach when compared to mineral fertilizer. The combination of digestate and biochar was also found to boost the rate of photosynthesis. This could be because biochar helps to improve the net photosynthetic rate by lowering soil respiration, soluble salt concentration, and CO\({}_{2}\) emissions [73]. [PERSON] et al. [7] tested the use of biochar and digestate from biogas production to increase soil biological activity. The authors concluded that the digestate application to soil resulted in an improvement in the soil's chemical and biological characteristics. The co-application of digestate and biochar mixed led to a lower CO\({}_{2}\) production when compared to digestate application, reduced the soluble organic compounds, limited the amount and activity of microorganisms, and enriched the C-sink of the soil. However, there is a lack of information regarding the enrichment of digestate with biochar for agronomic valorisation, especially, if one focuses on digestate from the AD of PS. Theoretically, applying biochar and digestate together will raise the amount of soil organic carbon in more physically stable fractions because of the co-metabolite present and the formation of organic coverings [36].