Waste-free anaerobic bioconversion of organic waste into biogas, digestate and related products

The analysis of the theoretical foundations of anaerobic digestion of organic waste shows that the efficiency of this process depends on many factors, such as temperature, pH, carbon to nitrogen ratio (C/N), activity of microbial consortia, duration of hydrolysis and methanogenesis. Successful application of such biotechnologies requires an integrated approach to the preparation of raw materials, selection of the optimal type of bioreactor, control of environmental parameters and high-quality purification of the final product – biogas. From an environmental perspective, anaerobic digestion reduces environmental pollution through the efficient processing of organic waste, which, if used irrationally, leads to the generation of harmful greenhouse gases and soil pollution. Biogas produced from the anaerobic digestion of organic waste contains not only methane (CH4), the main energy component, but also a significant proportion of impurities that reduce its energy value and cause corrosion of equipment. The by-products that need to be removed include the following components: CO2 (carbon dioxide), which reduces the calorific value of biogas; H2S (hydrogen sulphide), which is toxic and causes severe corrosion; NH3 (ammonia), which is formed during the fermentation of protein-containing substrates; moisture and dust, which form acidic solutions in contact surfaces. Particular attention is paid to technologies for biogas purification from carbon dioxide, hydrogen sulphide and ammonia, which are critical for increasing the energy value of biogas and ensuring safe operation of equipment. Physical, chemical and biological methods of purification, including adsorption, membrane technologies, scrubbing, biofiltration, which have proven their effectiveness in industrial practice, are considered. Digestate, a valuable by-product of the bioconversion process, has a high content of macro- and microelements, organic matter and microbiological activity, making it a competitive organic fertiliser. Its proper storage, dosage and application to the soil can improve soil structure, increase crop yields and help reduce the use of chemical fertilisers. The digestate is enriched with microbial organic matter by 30–50 %, and in the process of mineralisation, mineral components available for plants are formed in the following amounts for the liquid fraction (Nitrogen – 6.6 kg/m3, Phosphate – 1.9 kg/m3, Potassium – 6.2 kg/m3, Zinc – 8.2 mg/kg, Copper – 14.1 mg/kg and Cobalt – 7.2 mg/kg); for the solid fraction (Nitrogen – 6.8 kg/m3, Phosphate – 3.1 kg/m3, Potassium – 2.7 kg/m3, Zinc – 12.5 mg/kg, Copper – 34.5 mg/kg and Cobalt – 18.1 mg/kg).