Maximization of the methane production from durian shell during anaerobic digestion
Introduction
Anaerobic digestion (AD) is one of the oldest and most widespread used technologies which could convert different organic matters and biomass wastes to biogas (a kind of clean energy mainly consisting of CH4 and CO2) with limited environmental impacts (Ariunbaatar et al., 2014). Biogas generated through AD has been efficiently used in some European cities such as Linköping and Stockholm in Sweden, Lille in France, and Oslo in Norway (Koksin et al., 2016). However, the development and application of AD in developing countries is far from enough. In Asia, most of the developing countries are still suffering from open dumping and simple landfilling, and their environmental problems with organic wastes are well known (Chiemchaisri and Visvanathan, 2008, Menikpura et al., 2013). In the areas where AD has been paid increasing attention recently, most medium and large-scale biogas plants are badly in need of continuous and stable feedstock supply (Zheng et al., 2015). Thus, in order to solve the problem of supply shortage, materials which have been proved to be desirable for AD should be collected and fully applied. And it is equally important that more research should be conducted to explore and discover the potential wasted materials which were never tried for AD before.
Durian (Durio zibethinus Murr.) is one of the most important fruit commodities in South-East Asia. The top 4 durian producers in the world (Thailand, Indonesia, Malaysia, and Philippines) are all located in this area and they had a combined durian production of more than 1,600,000 tonnes annually (Manshor et al., 2014, PAEDA, 2013). Hundreds of thousands of fresh durian will be exported from these countries to China and many other countries every year. Despite the delicious and unique flavor of the flesh, durian shell occupies a large proportion (approximately 60%) of a durian (Chandra et al., 2007). The direct discharge of durian shell is certainly one of the reasons to cause the problem of agricultural waste excess and environmental pollution. Many researchers in South-East Asia are now devoting themselves to exploring the further economic value of wasted durian shell in the interest of environment and the research has been yielding good results. For example, wasted durian shell has been proved to be a good material to produce activated carbon adsorbent for the removal of acid or basic dye from aqueous solutions (Nuithitikul et al., 2010, Ong et al., 2012), and it could be converted into desiccant for air condition system, biochar, and a lot of other material via different processes (Futrakul et al., 2010, Khedari et al., 2003, Manshor et al., 2014, Prakongkep et al., 2015). However, no one else has tried wasted durian shell for AD so far. Compared to the mentioned utilization approaches of durian shell, AD has an advantage of large quantities treatment and continuous application.
AD process is very sensitive to the concentration and composition of the substrate (Zhou et al., 2011). It is reported that an appropriate increasing of the substrate concentration could enhance the AD efficiency. But an excessive amount of the substrate could probably result in the accumulation of total ammonia-nitrogen (NH3-N) and volatile fatty acids (VFA), which has a strong inhibiting effect on biogas yield (Fernández et al., 2008, Kun et al., 2015, Sánchez et al., 2002). Meanwhile, the content of inoculum is also an important factor when estimating the biomethane potential of a feedstock or running a large scale batch digester (Hashimoto, 1989, Liu et al., 2009). With a worse feedstock to inoculum (F/I) ratio selection for the AD system, the methane production would decrease or even stop. The optimal F/I ratio for different feedstock during AD process is different. For example, for ball-milled straw, Hashimoto (Hashimoto, 1989) found out that the methane yield would decrease if the F/I ratio exceeded 4. While for marine, herbaceous, woody, and municipal wastes, the optimal F/I ratios were obtained from 0.5 to 1.0 by Chynoweth (Chynoweth et al., 1993). As to anaerobic co-digestion of food waste and distillers’ grains, the highest methane yield was shown at the F/I ratio of 0.4 (Wu et al., 2015). So, determining the optimal organic loading and F/I ratio of a substrate is particularly important, especially when exploring a novel material for AD.
Response surface methodology (RSM) was adopted in this study to estimate the best-matched organic loading and F/I ratio of durian shell during batch AD. As a simple, effective, and reliable method, RSM has been applied to the research of AD and many other fields over the years (Ahmad et al., 2009, Nam and Capareda, 2015, Nourani et al., 2016, Shekarriz et al., 2014, Wang et al., 2012).
From the above, the objectives of this study were to: (1) make a systematic research on the AD performance of a novel substrate: durian shell; (2) maximize the methane yield of durian shell by optimizing organic loading (OL) and F/I ratio during AD; and (3) evaluate the stability of AD system under different OL and F/I ratio.
Section snippets
Substrates and inoculum
Fresh durian was purchased from a supermarket in Beijing, China. After the flesh was taken out, durian shells were put into a refrigerator at −18 °C to prevent deteriorating. Hours before batch AD tests were performed, durian shells were unfrozen and ground by a high-speed grinder (XINGSHILIHE, Beijing, China) into a paste. The anaerobic sludge used as inoculum was taken from Donghuashan Biogas Plant in Beijing, where only pig manure was fed as a substrate.
Analytical methods
Total solid (TS) and volatile solid
Characteristics of durian shell
The characteristics of substrate and inoculum are shown in Table 3. As a kind of fruit residue, durian shell had a high moisture content. It is shown in Table 3 that the TS and VS content of durian shell were only 29.4% and 27.7%. The VS/TS ratio reached 94.4%, which indicated high organic and low ash content of TS, and this was desirable for AD. A C/N ratio of 27.8 was obtained by elemental analysis. It has been reported that during AD process, the optimal range of C/N is from 15 to 30 and
Conclusion
The methane production performance during anaerobic digestion of durian shell was systematically evaluated. With the F/I ratio going down from 2 to 0.2, the EMY kept increasing. While for OL, the EMY showed a sharp rise at first followed by a decline as the OL went up to 27 g VS/L. According to the results of RSM and together with the practical application requirement, 0.5 was set as the optimal F/I ratio, and the maximal methane yield of 165.0 ml/g VS would be obtained at the OL of 20.45 g VS/L.
Acknowledgements
This study was supported by the Teaching Reform Project for Higher Education of Beijing (2015-ms035), Teaching Reform Program in Graduate Education (G-JG-PT201603) and Undergraduate Education (B201418) at Beijing University of Chemical Technology, and the National Key Research and Development Program of China.
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