Materials derived from Amaranthus cruentus L. used as co-substrates can intensify methanogenesis during bioconversion of organic waste

Methane fermentation (biomethanogenesis) performed by a multicomponent microbial consortium under anaerobic conditions results in a mixture of approximately 65 % CH4, 30 % CO2, 1 % H2S and minor amounts of N2, O2, H2 and CO. The peculiarity of biomethanogenesis lies in the ability to convert almost all classes of organic compounds, household, agricultural and some industrial waste into biogas. We were the first to assess the efficiency of the biogas production from organic waste as influenced by various materials derived from amaranth ( Amaranthus cruentus L.) which were used as co-substrates. Our findings indicate that optimization of the substrate organic matter composition by using dry phytomass of amaranth plants or amaranth pulp which remains after removing all practically valuable substances makes it possible to produce biogas from sewage sludge. This facilitates solving ecological problems of waste disinfection and utilization, and gives us an alternative, cheap and renewable source for fuel. Cultivated A. cruentus is a high-yielding protein-rich crop. Its biomass serves as a reproducible raw material. In our previous works, we reported the technology for rutin, vegetable protein and pectin production from A. cruentus plants, and suggested a scheme for complex processing which includes extraction of these substances from amaranth dry phytomass in a single technological cycle. The pulp obtained after extraction of all valuable compounds was proposed as a co-substrate for organic waste anaerobic fermentation. We modeled the effect of amaranth-derived substances on biogas production in the laboratory bioreactor using large-tonnage urban sewage sludge as a substrate. It was shown that the doses of the additives affected the process, i.e. the excess of amaranth plant mass (74 % and 87 %) suppressed methanogenesis. The thermophilic (50 °C) fermentation was found to be superior to the mesophilic one (37 °С), with the biogas production of 354 ml per gram of dry matter, when large-tonnage sewage sludge after filter press (45 % humidity) was fermented using amaranth pulp as the co-substrate. Moreover, in the presence of amaranth pulp, the biomethanogenesis under the mesophilic conditions also increased, the lag phase was almost absent, and the CH4 level throughout the experiment was about 60 %. As a result, the specific biogas yield reached 251.9 ml per gram of dry matter that is equivalent to ~ 0.25 m3 of the resultant biogas from 1 kg of organic raw material dry matter. In order to search for the active fraction of amaranth phytomass, we used solvents of different polarity, i.e. dichloromethane, 70 % aqueous ethanol and distilled water. It was found that the lag phase reduced to 10 days with the CH2Cl2 and EtOH extracts, which was comparable to that in the presence of dry amaranth phytomass. Obviously, these extracts contain components which either undergo rapid destruction by microorganisms able to turn them into biogas, or contribute to bacterial growth. The dichloromethane extract added to the substrate led to the most efficient biogas production, which is consistent with the literature data. Our findings indicate the ecological and economic feasibility of using amaranth pulp for organic waste bioconversion.