The characteristics of multi-substrates (low and high C/N) anaerobic digestion: focus on energy recovery and the succession of methanogenic pathway

Highlights

• The characteristics of anaerobic digestion of HF, FW and LG were analyzed.

• Methanosaeta was the dominant archaea in low HF ratio treatment.

• Methanobacterium and Methanosarcina dominated in high HF ratio treatments.

• Co-digestion could reduce the absolute number of pathogenic bacteria in HF.

• Energy and nitrogen could be recovered by anaerobic digestion.

Abstract

Anaerobic digestion of common rural wastes (human feces (HF), food waste (FW) and lawn grass (LG)) were studied considering the specific methane yield, process parameters and microbial characteristics (mainly microbial community and pathogenic bacteria). The results showed that co-digestion of multiple substrates obtained high digestion performance when the total solid (TS) was 4%. The optimal co-digestion ratio of HF, FW, and LG was 33–56%, 21–38% and 20–40%, respectively. The digestion system containing HF underwent ammonia inhibition, which leads to the succession of the methanogenesis pathway from the acetoclastic pathway to the hydrogenotrophic pathway. Simultaneously, the dominant methanogenic archaea changed from Methanosaeta to Methanobacterium and Methanosarcina. Co-digestion reduced Salmonella's absolute concentration. The recovered energy and nitrogen could meet 52–109 % energy demand of rural community and all nitrogen demand in lawn fertilization, respectively. The main rural organic wastes could be recycled by anaerobic digestion, considering the flexibility of substrate ratio.

Introduction

Compared with city, the distribution of rural communities is relatively scattered, which leads that large-scale centralized collection and treatment of organic waste is challenging and uneconomical. Until now, most of the rural communities don’t reasonably treat organic wastes such as food waste (FW), human feces (HF), and grass. Therefore, the mode of microcirculation of rural organic wastes should be constructed. Anaerobic digestion is a traditional way to treat organic wastes and produce bio-energy (Cai et al., 2018). It has apparent advantages in constructing microcirculation of rural organic wastes. First, anaerobic digestion can treat all kinds of organic wastes, especially those with high water content. Second, the scale of anaerobic digestion ranges from household biogas to large-scale biogas projects. Therefore, it can meet the need of communities with different population sizes. Third, the products and by-products can be supplied to rural communities to realize the entire cycle and zero waste of rural organic wastes.

In China, the rural population reached 560 million (Lee et al., 2015). Although the amount of HF varies from person to person due to the difference in diet, the average consumption of HF per person per day is about 400 g based on fresh weight, which means that about 82 million tons of HF are generated in rural area each year (Kim et al., 2019, Colón et al., 2015). HF was the main crop fertilizer before chemical fertilizers were popularized in China. However, the direct use of HF as fertilizer has certain environmental risks. This way is contrary to the increasing requirements of people for the quality of the living environment. In 2019, China proposed a “toilet revolution” action plan for rural livability. The original toileting way (dry latrine) will be changed, and centralized treatment of HF will replace the previous way (mainly random stacking). In some villages, wastewater source separation systems (separating domestic wastewater and HF) are gradually promoted and applied. Source separation system provided support for the centralized treatment of HF in the countryside.

HF consists mainly of fat, protein, and undecomposed food. Therefore, it is theoretically feasible to treat HF by anaerobic digestion. Some previous studies have demonstrated the anaerobic digestion characteristics of HF (Kim et al., 2019, Qin et al., 2010). For instance, Kim et al. (2019) reported that methane production potential of HF reached 350 mL g⁻¹ VS_feed. However, the anaerobic mono-digestion of HF still faces two issues, which should be taken seriously. First, the carbon to nitrogen (C/N) of HF is less than 10, which is lower than the optimal C/N (20–30) (Qin et al., 2010). Low C/N may lead to high total ammonia nitrogen (TAN) concentration and then cause ammonia inhibition (Fuchs et al., 2018, Zheng et al., 2021). The most effective strategy to alleviate ammonia inhibition is co-digestion with other high C/N substrates (Cai et al., 2021a). LG (lawn grass) and FW are common high C/N substrates in rural areas (Cai et al., 2021b). The feasibility and characteristics of multi-substrates digestion still need to be further explored. Second, previous study showed that HF contained intestinal pathogens such as Salmonella and E. coli, which pose a huge threat to human health (Hanabara and Ueda, 2016). Anaerobic digestion can remove pathogenic bacteria to a certain extent (Pandey et al., 2016). In the digestion system, microorganisms compete for substrates. Theoretically, pathogenic bacteria will be in a vulnerable situation when the digestion efficiency is high. Therefore, co-digestion of these substrates should have higher removal efficiency of pathogenic bacteria than mono-digestion of HF. Although relevant studies have reported the characteristics of co-digestion of HF with other substrates such as straw, cattle manure and food waste. However, these previous studies generally focus on digestion performance. There are few comprehensive studies about the effect of substrate ratio on the succession of microorganisms and methanogenesis pathways and the removal rate of pathogenic bacteria.

This research aims to explore the possibility of constructing a microcirculation system of organic wastes (mainly HF, FW, and LG) using anaerobic digestion in rural communities. The digestion characteristics of mono- and co-digestion of HF, LG and FW, including the methane production potential, the change of metabolites and physicochemical properties, and the feature of microorganisms (mainly the composition of bacteria and archaea and removal rate of pathogenic intestinal bacteria), were studied. These experimental results will offer theoretical support for the recycling and zero waste of organic wastes in rural areas.