Comparison of lab-scale and pilot-scale hybrid anaerobic solid–liquid systems operated in batch and semi-continuous modes
Introduction
Food waste, collected from food-processing companies, restaurants, food courts, markets and households in Singapore, amounted to approximately 0.5 million tonnes in 2002 [1]. Almost all food waste is currently disposed of by incineration. Biological treatments of organic solid wastes, such as composting and anaerobic digestion, are popular methods for food waste utilization. However, composting of food waste is not suitable for countries with limited land resources like Singapore. Compared to incineration, anaerobic digestion appears to be a more promising food waste disposal method. Anaerobic digestion reduces the volume of food waste, generates fuel biogas, mainly methane, and produces organic residue that can be used as soil conditioner or fertiliser [2], [3], [4]. To improve process of anaerobic digestion of organic materials, the two-phase system was developed [5], [6], [7]. In a two-phase anaerobic system, organic matter is degraded to volatile fatty acids (VFA) by hydrolytic and acidogenic bacteria in an acidogenic reactor.
It was reported that application of two-phase anaerobic systems is an effective way for the treatment of solid food wastes [8], [9], [10], [11]. VFA are converted then into methane by methanogens in a methanogenic reactor. The advantages of two-phase systems in comparison with one-stage system for anaerobic degradation of organic waste are as follows: better process stability due to lower risk of methanogenesis inhibition by pH drop, lower total digestion time, higher methane yield and content in biogas [12], [13], [14], [15]. Two-phase anaerobic systems usually are operated in batch mode [16], [17] or with periodical addition of organic matter (semi-continuous) mode [11], [18], [19].
The hybrid anaerobic solid–liquid (HASL) system was developed to improve food waste bioconversion in conventional two-phase anaerobic digester [15], [20], [21]. The HASL system includes an acidogenic reactor (Ra) to treat solid food waste and an upflow anaerobic sludge blanket (UASB) methanogenic reactor (Rm) to treat liquid leachate from acidogenic reactor (Fig. 1). The effluent from the methanogenic reactor is divided into two streams. Part of the effluent from the methanogenic reactor is used for the dilution of acid effluent from the acidogenic reactor to maintain optimal pH for methanogenesis. The flow rate ratio of the stream, which is recycled into the acidogenic reactor, to the stream, which is used for dilution of acidogenic leachate, is 1:4. The rest of the effluent from methanogenic reactor is recycled into acidogenic reactor to reduce the volume of the effluent to be discharged from the HASL system and to avoid addition of water for food waste hydrolysis. The HASL system is expected to be used on industrial scale to minimize the amount of food waste for disposal in Singapore.
The main objective of this research was to study and compare the operation of the lab-scale and pilot-scale hybrid anaerobic solid–liquid systems in batch and semi-continuous modes.
Section snippets
Feedstock, anaerobic microbial sludge and microbial granules
Food waste was collected from a canteen of the university. Waste was shredded into particles with average size of 6.0 mm in a Robot-Coupe Shredder (CL50 Ultra, France). The composition of food waste used in the experiments for lab-scale and pilot-scale HASL systems are shown in Table 1.
Anaerobic microbial sludge, used as inoculum for the acidogenic reactor Ra, was collected from an anaerobic digester of a local water reclamation plant. The concentrations of suspended solid (SS) and volatile
Operation of the lab-scale HASL system in batch mode
The acidogenic reactor was fed with 2.0 kg of food waste and connected to the methanogenic reactor immediately after filling. The pH value dropped from 6.6 to 5.0 on the first day of operation, meanwhile, the VFA and COD concentrations increased to their peak values (Fig. 2a and b). Concentration of VFA in the acidogenic reactor began to decrease from 11,500 mg l−1 (day 2) to 430 mg l−1 (day 10), which led to pH increase from 5.0 (day 2) to 7.0 (day 10) (Fig. 2a). The COD concentration in leachate
Conclusions
The hybrid anaerobic solid–liquid system was proposed for food waste bioconversion. High efficiencies for conversion of food waste into biogas were shown for both lab-scale and pilot-scale HASL systems. Semi-continuous mode of HASL system, estimated by methane production, l l−1 day−1, and VS removal, g day−1, was more effective than batch process. Total VS removal efficiency was high in all experiments and consisted from 77% to 80%. The average content of methane in the biogas produced was similar
Acknowledgments
The research was supported by a grant from the National Environment Agency of the Ministry of the Environment, Republic of Singapore.
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