Bioconversion of solid livestock waste using a consortium of organotrophic microorganisms: results of the pilot experiment

Disposal of agro-industrial wastes is one of the major unresolved environmental problems of our time in many countries of the world, and it is associated with adverse impacts on ecosystems due to their improper storage. Composting is considered one of the most attractive ways of using a wide range of organic wastes due to its environmental safety, low cost and the possibility to obtain useful products. In addition to fresh animal and poultry litter, organic wastes such as lignin and cellulosic materials from crop production, wood processing, and others can be used in co-processing systems through microbial bioconversion to produce organic fertilisers or soil mixes. Many existing methods of controlled composting of waste are based on the use of expensive technological techniques and equipment, which significantly increases the cost of the product. Therefore, in addition to increasing the rate of composting, it is very important to simplify the process of obtaining the finished product and also reduce labour and energy costs. The aim of this study was to isolate and investigate new strains of organotrophic microorganisms, develop a microbial consortium for accelerated composting, and conduct industrial-scale trials of combined composting of animal waste (poultry manure as an example) and wood processing (conifer sawdust) under the conditions of Western Siberia. During this work, the necessity of aeration of the mixture for successful composting of waste was also investigated. Sampling of organic wastes (poultry manure, swine manure, and pine sawdust) for research purposes was carried out in January-February 2020 at the relevant enterprises of Tomsk and Tomsk district. The obtained samples of poultry and swine manure served as a source for isolation of thermophilic and thermotolerant heterotrophic microorganisms – organic matter decomposers. A total of 7 pure cultures of organotrophic bacteria were isolated from animal waste and identified by molecular methods, most of which were represented by Gram-positive spore-forming cells (see Fig. 1). The only strain that did not form spores was characterised by its ability to degrade cellulose (see Fig. 2). The bacterial strains isolated from animal waste belonged to the families Paenibacillaceae and Bacillaceae within the phylum Bacillota (see Table 1). The strains were selected for the consortium based on their ability to degrade organic matter, including complex polymers, and general physiological properties that determine the biotechnological potential for waste bioconversion. As a result, the consortium included thermophilic and thermotolerant strains of organotrophic non-pathogenic bacteria that produce hydrolytic enzymes: Bacillus amyloliquefaciens PL-1, Aneurinibacillus thermoaerophilus PL-5, Brevibacillus brevis PM-3, and Pseudoxanthomonas taiwanensis PM-cell. The consortium also included a thermotolerant organotolerant strain Anoxybacillus kamchatkensis AS-GOS-2, previously isolated from activated sludge of municipal wastewater treatment plant in Tomsk. The physicochemical conditions for the application of the developed consortium are presented in Table 2. Pilot tests of the technology of joint composting of animal waste (poultry manure) and wood processing waste (coniferous sawdust) using the developed microbial consortium were conducted in October-November 2021 for about 6 weeks. Composting of raw materials was carried out in piles (dimensions 3×3×2 m, volume 18 m3 ) designated as “PB-1” – experimental pile with consortium application and use of forced aeration, “PB-2” – experimental pile with use of forced aeration, and “PB-3” – no effect control pile. All treatments were carried out with a 1:2 ratio of poultry manure to sawdust. Forced aeration of the mixtures in the experimental piles was carried out by discrete air supply at a rate of 0.35 m3 /min, starting from the first day of composting in the 12-hour mode after a 12-hour rest. Throughout the experiment, the temperature of each pile mixture was measured at 10 points at a depth of 50 cm (see Fig. 3). The obtained data on the temperature changes in the piles during composting testify to intensive biological processes in the PB-1 sample related to the development of thermophilic and thermotolerant microorganisms in the consortium. The physico-chemical and agrochemical parameters of the obtained compost were evaluated for compliance with the normative requirements for organic fertilisers based on animal waste (see Table 3). The content of pathogenic and opportunistic microorganisms, the presence of protozoan cysts and oocysts, as well as helminth eggs and larvae were evaluated to control sanitary and hygienic indicators (see Table 4). Contrary to the control samples, the compost obtained with the participation of the microbial consortium not only complied with the norms as regards the content of organic matter and other nutrients, but was also characterised by elimination of the eimeriosis pathogen together with a reduction in the number of opportunistic microorganisms to the normative level. Thus, the combined composting of poultry manure and conifer sawdust in the ratio of 1:2, carried out for 46 days at ambient temperatures from +3℃ to -15℃, showed a positive effect of the developed consortium on the quality of the obtained compost. The developed consortium can be recommended for composting organic waste in a wide technological corridor of pH values and temperatures.