As bulking agents (BA) affect the composting process, this work examined the impact of combinations of different organic components in order to obtain an efficient co-substrate for food waste (FW) in-vessel composting. To boost the occurrence of microorganisms inhabiting the compost, mature compost was firstly coupled with wheat straw, added to FW, and considered as a control (BC₀). Then, two trials (BC₁₀, BC₂₀) including 10% and 20% of biochar were monitored. The results indicated that the temperature of the amended bioreactors was notably increased compared to the unamended one. Thermophilic temperatures were achieved at 14, 34, and 78 h after the experimental setup for BC₂₀, BC₁₀, and BC₀, which lasted for 14, 17, and 12 days, respectively. When it came to an assessment of maturity and stability, the quality of the compost was evaluated against several indicators and compared with the compost quality standards of the UK, France, Canada, the USA, Poland, and Germany. BC₁₀ illustrated a high-quality product in relation to the heavy metal concentration, a C:N ratio which reached 14.97, an AT4 which was lower than 6 (4.36 mg O₂/g TS), and a nitrification index of 2.61 (<3). Consequently, the addition of 10% of biochar as a co-substrate showed an improvement of the process evolution and the characteristics of the biofertilizer produced.

The use of enzyme additives in anaerobic digestion facilities has increased in recent years. According to the manufacturers, these additives should increase or accelerate the biogas yield and reduce the viscosity of the digester slurry. Such effects were confirmed under laboratory conditions. However, it has not yet been possible to quantify these effects in practice, partly because valid measurements on large-scale plants are expensive and challenging. In this research, a new enzyme product was tested under full-scale conditions. Two digesters were operated at identic process parameters—one digester was treated with an enzyme additive and a second digester was used as reference. A pipe viscometer was designed, constructed and calibrated and the rheological properties of the digester slurry were measured. Non-Newtonian flow behavior was modelled by using the Ostwald–de Baer law. Additionally, the specific biomethane yield of the feedstock was monitored to assess the influence of the enzyme additive on the substrate degradation efficiency. The viscosity measurements revealed a clear effect of the added enzyme product. The consistency factor K was significantly reduced after the enzyme application. There was no observable effect of enzyme application on the substrate degradation efficiency or specific biomethane yield.