Food waste (FW) is generated in high volumes and is a serious threat to the environment if utilized improperly or left without control. Conventional methods of FW disposal include combustion, landfilling, aerobic composting, partial recycling, and other ones. Anaerobic digestion (AD) is the most environmentally friendly of all the methods as it is beneficial to the populace, and food waste is a suitable material for biogas production.
Confectionery waste has rarely been utilized by anaerobic digestion (AD) so far. In this paper, the use of waste wafers (WF) in co-digestion with sewage sludge (SS) is proposed for the first time. Annual volumes of production of WW and SS are expressed in hundreds and thousands of tons, respectively. The materials are generated in high amounts and on an ongoing basis, which is a very important factor regarding potential biogas plant investment projects. The objective of this paper was to analyze the AD process of the test substrates in terms of process stability and biogas capacity. The studies have shown that both the waste wafers as the individual material and with sewage sludge (as the co-substrate) are suitable feedstocks for biogas production. The experiments were carried out for the individual material and for a system with a cosubstrate in the form of raw sewage sludge (SS). In both cases, a digested sewage sludge was used as the inoculum. The studies were performed in a laboratory scale using anaerobic batch reactors under controlled (mesophilic) temperature and pH conditions. The highest yields of biogas and methane were obtained for waste wafers (980.1 m³ Mg VS^-1 and 492.6 m³ Mg VS^-1, respectively) and the lowest for raw sewage sludge (349.1 m³ Mg^-1 VS and 177.9 m³ Mg^-1 VS, respectively). After mixed the wafers with the sewage sludge, the material (WF_SS) produced less biogas (667.9 m³ Mg^-1 VS), including methane (387.5 m³ Mg VS^-1), than for the wafers (WF). The differences in biogas production for the samples were primarily connected with the substrates’ composition, including with the content of readily degradable organic compounds and C/N ratio. The combination of waste wafers with raw sewage sludge has appeared to be beneficial, as evidenced by the results of microbiological and biochemical analyses. The sample WF_SS was rich in microorganisms with high metabolic activity, which resulted in the production of biogas with high methane content (58%).