1. Introduction
City councils are faced with managing increased amounts of food waste (FW) nowadays, which could potentially derail sustainable economic development. Pramanik et al. [
1] reported that annual FW generation reached 278, 74.7, 51, 157, and 44 kg per person in America, United Kingdom, India, Japan, and China, respectively. Another report developed by Edward [
2] noted that according to the Solid Waste Corporation Management of Malaysia (SWCorp), Malaysia disposed of 16,687 tons of FW daily. SWCorp also noted that 55% of municipal solid waste disposed at landfills mainly consisted of FW. Several treatment processes including incineration, composting, and landfill have been widely used to manage FW [
3,
4]. However, these processes are not economically feasible, as they incur high energy losses and increased environmental pollution [
3,
4,
5]. This challenge of managing FW whilst protecting the environment has spurred the need to develop new and innovative techniques that would allow FW to be used for other purposes—based on the concept of a circular economy.
FW has a high organic content and excellent biodegradability. Therefore, it can be treated using anaerobic digestion (AD), an environmentally friendly technology that is also able to recover energy from the FW treatment in the form of biogas. The stability and efficiency of AD depend on many factors such as feeding mode, moisture content of the FW, reactor configuration, and operating conditions [
3,
6]. Reactor configuration has an important impact on FW treatment [
7]. The configuration includes single-phase, two-phase, or multiple-phase. Of these, the single-stage AD process has been proven to have many advantages including recirculation adaptability, a simple design, less technical failure, and low cost [
6]. It is important to note that most of the AD systems operate in single-phase systems, for example, 95% of European full-scale plants typically function based on a single-phase AD process to produce biogas from organic waste, such as food waste [
3,
4,
5,
6,
7], manure [
8], and biological sludge [
9], where hydrolysis, acidogenesis, acetogenesis, and methanogenesis occur simultaneously in one reactor. Ganesh et al. [
10] compared the process performance and reactor stability of one-stage and two-stage AD of fruit and vegetable waste. They found that the single-stage process produced a higher methane yield, volatile solid (VS) destruction, and energy yield, compared to the two-stage process. Most of the studies have used lab-scale AD set-up for biogas production from FW [
4,
7,
10,
11,
12,
13,
14,
15,
16,
17,
18,
19,
20,
21,
22]. However, the performance of a semi-pilot scale anaerobic biofilm single-stage reactor in producing biogas from typical FW, especially from the cafeteria, needs to be investigated.
The performance of a single-stage AD system is affected by operational conditions including hydraulic retention time (HRT), organic loading rate (OLR), inflow rate, and duration. The OLR is an important parameter for the AD process since it indicates the amount of VS to be fed into the reactor every day. The biogas production may decrease if the feeding rate in the reactor is beyond the optimal level, and then, system failures can occur due to overloading [
6]. HRT is another critical parameter in the AD process, as it indicates the time required to complete the degradation of the FW. Furthermore, HRT affects biogas production, AD operation stability, kinetic model parameters, and biomass concentration [
23]. HRT depends on the OLR, process temperature, and substrate composition and is connected to the bacterial growth rate [
24]. A longer HRT and a lower OLR are the best options for achieving constant and maximal methane yields. Meanwhile, a significant accumulation of volatile fatty acid (VFA) could occur at a shorter HRT and a higher OLR, leading to AD system failure [
24]. Kim et al. [
11] investigated the AD of FW at an HRT of 10 days and 12 days under mesophilic and thermophilic conditions. They found that the methane gas yield in the mesophilic and thermophilic conditions increased with an HRT of 12 days more than an HRT of 10 days. Shi et al. [
25] reported that average biogas production increased from 55.2 mL/g VS to 105.2 mL/g VS while HRT increased from 20 days to 60 days. The author pointed out that the HRT of 20 days displayed lower stability compared with the HRT of 40 days and 60 days. Bouallagui et al. [
12] investigated the production of biogas from fruit and vegetable waste at HRTs of 12 days, 15 days, and 20 days under mesophilic conditions, and found that the HRT of 20 days displayed stable performance with the highest biogas yield and highest VS reduction efficiency. However, more studies should be done to investigate the effectiveness of a semi-pilot anaerobic biofilm single-stage reactor in producing biogas from FW at different successive operating stages.
Bacteria washout is the most familiar issue faced by most conventional AD processes [
26]. However, this issue can be solved by using a biofilm in the reactor since biomass attached to biofilm carriers can move freely in the water volume inside the reactor and are contained inside the reactor via screens at the reactor outlets. As mentioned previously, FW contains high concentrations of complex components such as carbohydrate, lipid, and protein; thus, an anaerobic biofilm reactor can be used to treat FW while achieving a more stable growth process and operation because this type of reactor enhances the interaction between substrate and bacteria [
26]. From the literature review conducted in this study, no study has yet investigated the use of a biofilm-based AD process for biogas production.
The mathematical kinetic model used for the AD process plays a vital role in optimizing, predicting, simulating, and monitoring process performance under various conditions [
27]. The models help in the prediction of kinetic parameters as well as in clarifying the digestion process. Deepanraj et al. [
13] compared two kinetics models—the modified Gompertz model and the logistic model—to determine the kinetic parameters of the reaction of FW as a feedstock under different total solid (TS) concentrations. They found that the modified Gompertz model yielded better performance and better described the process kinetics compared to the logistic model. However, the use of kinetic models for biogas production from FW at different HRTs has not been widely investigated.
Based on the identified gaps in previous studies, the objective of this study is to investigate the biogas production efficiency of a semi-pilot (160 L) anaerobic biofilm single-stage reactor using FW at different successive operational stages under mesophilic conditions. This study also evaluated the degradation performance of FW (i.e., total solids, VS, and chemical oxygen demand (COD)). Three mathematical kinetic models (i.e., first-order kinetic model, modified Gompertz model, and logistic model) were used to determine the biogas production potential, the maximum biogas production rate, and the lag time for AD by fitting the measured biogas yields. See the end of the document for further details on references.
4. Conclusions
The AD of FW at HRTs of 124 days, 62 days, and 35 days were investigated under mesophilic conditions. The highest biogas and methane yield were obtained in Stage-1 with an OLR of 0.51 kg VS/m3/day and an HRT of 124 days. It was observed that the biogas and methane yield decreased when HRT decreased from 124 days to 62 days. When HRT was decreased to 35 days in Stage-3, the AD process became unstable and the biogas production decreased sharply due to VFA and ammonia accumulation. The shock loading, temperature fluctuation, irregular mixing, and stepwise feeding type (i.e., three days in a week) may be the reason for the VFA and ammonia accumulation inside the reactor. However, AD process failure, VFA and ammonia accumulation, low buffer capacity, foaming problem, and high financial cost are different economic and technical challenges for single-stage AD of FW waste. Therefore, multi-stage and temperature control reactor, co-digestion, continuous feeding and mixing, FW pre-treatment, and addition of micro-nutrients and antifoaming agents are preferentially recommended to improve the performance of the single-stage AD reactor. Among three kinetic models, the modified Gompertz model was the most suitable model (R2 = 0.997) for fitting the measured biogas yield and it could be used to describe the kinetics of the AD process more reasonably. The calculated parameters displayed that AD of FW at high loading have low hydrolysis rate and long lag phase. The modified Gompertz model could be used for practical applications to optimize process parameters to improve the design and operation of an AD process.