The ever-expanding urbanization from rapid population growth has accelerated the generation of municipal solid waste worldwide. In China, the main composition of domestic waste in urban and rural areas is food waste (FW), accounting for up to 40%. Due to its exceptionally high content in both moisture and fermentable organic matter, inappropriate disposal of FW will lead to bacteria breeding, odor and toxic gas emissions, groundwater contamination and other problems, causing serious environmental pollutions. The total amount of FW has been increased up to 100 million tons per year with an annually increase of more than 10% in recent decades (Guo et al. 2019), which puts significant pressure on the handling capacity of recycling facilities. At present in China, the dominant disposal strategy of FW is landfill or incineration, accounts for 52% and 45% of the total amounts respectively (Lu et al. 2017). Due to the high moisture content of FW, landfills are prone to produce leachate and lead to water pollution, while incineration need high extra-heat investment and produces dioxin and other harmful gases (Jiang et al. 2015). Therefore, to relieve or distribute the environmental burden, construction of more efficient disposal facilities and strategy for the processing of FW is in urgent need.
Most recently, microbial treatment strategies of FW such as anaerobic digestion, aerobic compost and biological drying have received more attentions and are confirmed to exhibit higher efficiency in both mass reduction and re-utilization (Meng et al. 2022; Lu et al. 2022; Yuan et al. 2019). Anaerobic digestion has a high resource utilization capacity based on the biogas generation, which can be used as a new energy supply. However, this technology has some limitations, such as large project area, long processing period, high equipment investment and high requirements for pretreatment and operation (Pramanik et al. 2019). Traditional composting is a kind of aerobic decomposition process that organic waste is transformed into humus through natural degradation by endogenous microorganisms. This treatment method has good versatility and is now widely applicated in the treatment of American courtyard FW (Dung et al. 2014). However, due to the low degradation efficiency, traditional composing takes a long time, generally as long as one or two months, and the degradation rate can be easily affected by the variations in environmental temperature, dissolved oxygen, FW composition, pH value and endogenous microbial structure (Zhai et al. 2015; Wang et al. 2017). Moreover, the high proportion of land occupation in traditional composting also significantly hampered its application in the growing demand of urban domestic waste mass reduction (Kumar Awasthi et al. 2019; Guidoni et al. 2018; Guo et al. 2018). Bio-drying technology removes water from FW and the remaining materials can be used as fuel. However, the longtime duration in the pretreatment of FW with high moisture (7–15 days) (Mohammed 2017) and the high extra-heat investment (Jiang, et al. 2015) are still the main barriers noticeably for its wide application.
Processing the increased FW via centralized treatment approaches as discussed above require longer time as well as inefficiency, generating more waste collection and transportation (Taşkın et al. 2020). A closer look at the amounting quantities of FW and the related emissions along the food production-consumption chain brings to light both the transportation and terminal disposal stage of FW faces difficulties in high investment (Govindan 2018). Cost of the collection and transportation in urban areas accounts for 34% of the total cost for food waste disposal (Oh et al. 2017), to diminish the transport requirements and related pollutions, in-situ disposal strategy of FW is therefore considered (Sakarika et al. 2019), which has been suggested as one way to relieve the burdens of conventional disposal facillities through mass reduction of FW (Manu et al. 2019). To reduce the considerable volume of FW in urban areas of Korea, a pilot-scale on-site system combing biological treatment and a drying stage was applied to achieve mass reduction (Jeon et al. 2020). In addition, a zero food waste system for sustainable residential buildings in urban areas reduction strategy of food waste was constructed and food waste was pretreated by a complex enzymes degrading in a reactor operating at 30–40°C, achieving a total mass reduction of 94% and producing fertilizer as the final porduct (Oh, et al. 2017). Apart from the weight reduction monitor in the decentralized systems, the entire disposal approach should be inexpensive, require low instrument investiment and easy handling for maintenance. A continuous mode is suggested to perform during digestion process, variation in regular waste composition and changes in physico-chemical parameters such as moisture content, pH value and structure of microorganism should be also monitored (Perin et al. 2020; Waqas et al. 2018).
The synergetic degradation potential of microbial consortium in FW decomposition has attracted more attention on the facilitating mass reduction of organic matters within food waste. Inoculation of a mixture of 71 strains in chicken manure and corn stover compost accelerated the degradation of organic matter (Zhang et al. 2020). Our previous study indicated that a combination of gram-positive and -negative bacteria largely improved lipid degradation owes to the cooperative activity of different microbial isolates (Ke et al. 2021) and high-temperature-resistant oil degrading microbial consortium was further applied in the biodegradation of oily food waste (Ke et al. 2021). Several artificial microbial consortia with excellent characteristics in the degradation of FOG (fats, oils and greases) have been successfully applied for the co-digestion of municipal wastewater sludge (Awasthi et al. 2014) and FW (Deaver et al. 2020), which can potentially improve the efficiency and effectiveness in organic matter decomposition.
In addition, temperature conditions are critical in the biodegradation of food waste. Food waste processed at higher temperatures and longer heating times can break down both organic and inorganic compounds more efficiently (Jin et al. 2016). However, prolonged heating and high temperature conditions mean accelerated consumption of electrical energy, and this amount cannot be ignored. Therefore, to develop a new degradation process that can reduce energy consumption while keeping the degradation efficiency of food waste unchanged is vital for its practical application. Our previous work demonstrated that the electricity cost accounts for the major proportion (above 50%) of total expenditure for equipment maintenance under high temperature (up to 55°C ) (Zhou et al. 2022). Accordingly, heat treatment of food waste may be the most suitable pretreatment method, while its energy consumption was as high as 49.3 MJ/t (Jin et al. 2015). At the same time, food waste has a high moisture content (up to 70%-90%), which is not suitable for direct microbial degradation, and often requires dehydration. Due to its poor dehydration, it takes an energy input of 137MJ/t to dry to 60% water content (Zhang et al. 2021). Therefore, it is in urgent need to develop new processes with lower energy consumption and higher efficiency.
The primary object of this study was to develop a rapid in-situ mass reduction of FW via microbial degradation strategy. Specifically, bacterial isolates with superior biodegradation towards starch, protein, fat and cellulose were screened and further compounded for both lab- and pilot-scale application. Besides, a mesophilic-thermophilic temperature-phased bio-degradation processing were adopted in fed-batch processing and key parameters was respectively monitored to evaluate the effectiveness of microbial facilitated FW decomposition, the practical application prospect was finally confirmed by the byproduct composition analysis.