Effect of Biochar Addition in Two Different Phases of the Co-Composting of Green Waste and Food Waste: An Analysis of the Process, Product Quality and Microbial Community

Abstract Green waste (GW) contains lignocellulosic substances that are resistant to degradation and prolong the composting process. Different strategies have been used to improve GW composting, including adding co-substrates and additives like food waste (FW) and biochar (Bc). FW offers advantages such as its constant availability in MSW streams, its high content of readily degradable organic matter, and its contribution of essential nutrients like nitrogen and potassium. On the other hand, Bc is incorporated at the beginning of the process. However, the effect of its addition in the phase of greatest biological activity has not been studied. This article compares the effect on the process, product quality, and the compost microbial community of adding 2% Bc in two different phases of the co-composting GW with food waste (FW). Two treatments were evaluated using the same mixture of substrates (wet/ weight) (i.e., 48%GW + 35%FW + 13%Sawdust + 2% Phosphate Rock + 2%Bc), but Bc was added in the mesophilic phase for T1 and the thermophilic phase for T2. With the addition of Bc at the mesophilic phase, higher temperatures were reached in the thermophilic phase, with a maximum temperature of 52.5 °C for T1 and 47.7 °C for T2. T1 final product had higher nutrient content (i.e., 28.30% TOC, 1.91% TN, and 4.69% TP in T1 compared to 15.80% TOC, 1.14% TN, and 4.29% TP in T2) and achieved a higher fertility index. Streptomyces and Nonomuraea were more abundant in T1 than T2, while Rhizobium , Roseiflexus , and Chloroflexus were significantly more abundant in T2 than in T1. These results suggest that T1 presents a microbial community that is more robust in degrading complex organic materials. This article demonstrated increased biological activity, lignocellulosic degradation, and nutrient content in the product when biochar is added from the beginning of the process.