Optimum Organic Loading Rates (OLR) for Food Waste Anaerobic Digestion: Study Case Universitas Indonesia

Indonesia currently has a waste generation problem arising from the fact that sixty percent of the waste it produces is organic waste (OW). The scope for anaerobic digestion (AD) of OW has not yet been optimized, with many reactors not functioning properly. The purpose of this study is to determine the suitability of food waste with an organic loading rate (OLR) of 8–14 kgVS/m³day to produce the highest volatile solids destruction (VSD) and methane. A semi-continuous pilot-scale dry AD of 0.5 m³ was run for 134 days in mesophilic conditions. The results showed that the feedstock was suitable for dry AD due to the high total solids (TS) (23.2–27.1%) and organic content (volatile solids of 90–95% TS). Meanwhile, the optimum OLR was 10 kgVS/m³day with a VSD of 92.2% and a methane yield of 127 LCH₄/grVSday. In addition, OLR 10 kgVS/m³day had the highest stability, as shown by the pH value of 6.52 and ammonia concentration of 848 mg/l. The VSD values fell with respect to the OLRs of 12 kgVS/m³day and OLR 14 kgVS/m³day, to 90.42% and 86.73%, respectively.

Biogas; Inhibitor; Municipal waste; Renewable energy

The amount of municipal solid waste (MSW) generated in Indonesia is approximately 175,000 tons/day, where 60–74% is organic waste (OW). OW is generally generated by domestic, commercial, and institutional premises (Dong et al., 2010). The incorrect treatment of OW can lead to serious cases of environmental contamination, including Escherichia coli outbreaks and contaminated ground and surface water. Having organic solids that account for 61% of the waste, a moisture content of 78%, and volatile solids (VS) of 81% TS (Wang et al., 2014) means there is potential for treatment by anaerobic digestion (AD). Based on ecosystem sustainability, AD is the most efficient method of OW treatment. AD offers the ability to reduce a high quantity of OW in a small area and is a relatively faster method than composting. Additionally, AD is considered a source of renewable energy due to the production of methane, which can be utilized as biogas, and it is suitable for sustainable social-economic development (Tetteh et al., 2018). It comprises the four stages of hydrolysis, acetogenesis, acidogenesis, and methanogenesis.

Indonesia has very little experience in the field of AD for MSW. From a previous survey, the very few facilities that are in operation mostly do not produce methane (Priadi et al., 2015). One of the indicators of AD processing is based on the efficiency with which organic matter is decomposed, which is usually measured by volatile solids destruction (VSD). Generally, a higher VSD value indicates higher production of methane as biogas (Nagao et al., 2012). However, VSD has an upper limit, where methane production declines due to the value of VSD exceeds the limit. The decline happens due to inhibitor accumulation caused instability of AD. Fluctuating feedstock and low COD in recirculated leachate may also result in very low biological activity and then lead to low biogas production. These conditions can be overcome by operating AD with the optimum organic loading rate (OLR). Thus, to improve AD performance, the OLR should be adjusted according to the MSW specific to Indonesia so that the waste degradation process can be streamlined and made more efficient.

OLR represents the daily quantity of OW used as feedstock in a continuous system. The optimum value indicates the total nutrients needed for microorganism growth and metabolism (Dong et al., 2015). Excess OLR can induce accumulation of volatile fatty acids (VFA), shock loading on the reactor, and a low pH (Lin et al., 2011; Dai et al., 2013). A low OLR, however, can lead to the microorganisms lacking the required nutrients, unstable growth, death, and the production of inhibitors to generation such as ammonia (Dai et al., 2013; Dong et al., 2015). The value of OLR depends on the type of AD reactor, such as a one- or two-stage and a wet or dry AD. The OLR in wet AD using household OW has been found to either vary within the range of 2–10 kgVS/m³day (Aslanzadeh et al., 2014) or to be below 7 kgVS/m³day (Kothari et al., 2014). Meanwhile, dry AD has a higher OLR than wet AD, with a range of 7–15 kgVS/m³day (Fagbohungbe et al., 2015) or 12–15 kgVS/m³day (Kothari et al., 2014). Dry AD has the ability to attain a higher OLR and process a greater volume of OW by up to > 60% (Mattheeuws & Baere, 2011). There are several advantages of implementing dry AD, such as the small volume, thus making it suitable for hydrophobic substrate (Fagbohungbe et al., 2015), VSD at 40–75%, low hydraulic retention time (HRT), high and consistent biogas production (Kothari et al., 2014), and good tolerance to shock loading. Nevertheless, the optimum OLR for dry AD is needed in order to produce a good level of VSD and methane, specifically in the context of using OW in Indonesia. Accordingly, this study aims to determine the optimum OLR needed to produce VSD using a semi-continuous pilot-scale dry AD.

With a high TS and organic content, a mixture of OW and CM is suitable for use in dry AD. The optimum OLR in this experiment was 10 kgVS/m³day, which gave a VSD of 92.2% or 179 g/l and a methane yield of 127 LCH₄/kgVSday. The operation is likely to be more stable with temperature, pH, and ammonia values of 28.7^oC, 6.52, and 848 mg/l, respectively. Thus, the optimum OLR obtained for AD can produce high VSD and serve as a solution to the problem of OW in Indonesia.

This research was funded by Grant of Indonesian Directorate of Research and Higher Education, Number 1149/UN2.R12/HKP.05.00/2016. Meanwhile, the conference was financially supported by Grant of Indonesian Directorate of Research and Higher Education, Number NKB-1728/UN2.R3.1/HKP.05.00/2019. Special thanks are due to the laboratory of the Environmental Engineering Study Program at Universitas Indonesia.

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