Various parameters like temperature, pH, TS, VS and inoculum to substrate ratio affect the biodigestion of the solid matter. These values were recorded and corresponding biogas was measured for different I/S ratio.
Temperature of the Digester
Anaerobic digestion of the organic matter is largely affected by the temperature. There are three main temperature regime namely psychrophilic (0oC to 15 oC), mesophilic(20oCto 45oC) and therrmophilic (45oC to 60oC).Higher the temperature more will be the microbial activity and thus results in higher production of biogas but serves to be uneconomical as it has to be aided with the heating jacket. The temperature of each reactor was measured twice a day at 11 am and at 6 pm IST using thermometer. In present case, this study was carried out at mesophilic regime and the variation in the maximum and minimum temperature recorded for the day is as shown in Fig 2. Optimal temperature during the period of study was found to be 38oC.
pH of the reactor
pH control of the reactor is most crucial parameter for biotransformation of organic matter, thereby maximizing the biogas yield (Mata-Alvarez et al.1992). Moreover, according to previous studies FVWs typically have a pH of 5.5 to 6.5, which is perfect for putrefaction to start early and continue unhindered(Ravi et al. 2018; Pavi et al. 2017; Chakraborty and Mohan,2019).Many anaerobes perform best in the pH range of 5.5 to 8.5(Fang et al., 2010)but optimally in the range of 6.8 to 7.6 (Rittmann and McCarty, 2001), and the rate of methane production can be reduced if the pH is lower than 6.3 or higher than 7.8 (Gerardi, 2003). The initial pH of the reactors R1, R2, R3, R4 and R5 were 5.6, 5.5, 4.8, 5.1, 5.0 respectively.A sharp drop in pH of the treatment systems were observed during initial two weeks of fermentation which may be attributed to high volatile fatty acid (VFA) formation (Haider et al. 2015; Mata-Alvarez et al.1992; Bouallagui et al. 2003, 2009). Moreover the inconsistencies in the pH drop may be attributable to the presence of slowbiodegradable and hydrolyzable chemicals, such as polyphenols or flavonoids present in hard skin/covering /peel of the fruits. These which are extensively prevalent in many fruits and vegetableslike onions, peas, eggplants, spinaches, radishes etc.Polyphenols are mostly degraded by the acids, alkalis and enzymes present in the system, therefore the degradation of such compounds typically begins when the surrounding pH has dropped significantly. This also promotes the hydrolysis of the organic matter present which is essential for the elimination of orthophosphate, TKN, and SO4-2 over time (Sanchez et al. 2000).The pH increased to its normal operating value after VFAs metabolised. pH of the reactor was measured regularly at an interval of three days. The average pattern of pH demonstrated by all experimental mixing ratios for the digester operating under stable condition is shown in Fig.3.
Total Solid of the reactor:
The variation of total solids (TS) and volatile solids (VS) of the substrate in the reactors are depicted in Fig. 4 and 5. Presence of higher VS indicates the biodegradability of the waste which is affirmed from the VS content of the substrate. The initial total solids of the reactors R1, R2, R3, R4, and R5 were 7.73, 7.63, 8.0, 11.10 and 12.07 respectively, whereas initial VS content (as % TS) were 6.61, 6.9, 7.8, 10.85, 11.41 respectively. Performance efficiency of the different reactors were accessed in terms of solids reduction result which shows that highest TS reduction was obtained for Reactor R1 followed by R2, R5 , R3 and R4 respectively. Percentage removal in TS were 49.02,47.05, 46.06 ,42 and 39 respectively for reactors R1, R2, R3, R4 and R5 however VS reductions were evaluated to 56.12 ,54.20 ,67.05 ,65.06 and 70 respectively. For the reactors the reduction in TS is in agreement with (Vats et al. 2019), moreover this alone cannot guarantee about the stability of the reactor.
Daily biogas production at different inoculum to substrate ratio
The daily biogas yield and cumulative biogas yield for different inoculum to substrate ratio, over a period of digestion is as presented in Fig. 6 and 7.Results shows for almost all reactors the biogas production started from day 1 except for rector R5 which is due to presence of least % of substrate. Early biogas production may be attributed to putrescible nature of substrates and showed similarity with the study of Browne and Murphy, 2013. They analyzed the effect of various inoculums on methane production from food waste. Sharp drop in the daily biogas production was noticed in the reactors after first week of operation which may be due to inhibition of the microbial activity (Vavilin et al. 2008).Moreover high biodegradability of the substrate results in rapid hydrolysis of the Intermediate compounds such as Glycerol and LCFAs, which may results in accumulation of LCFAs and thus causing inhibition of biogas production (Angelidaki and Ahring, 1992).Highest daily biogas yield was obtained for inoculum to substrate ratio (30:70) i.e. reactor R2 which is equivalent to 440 ml on day14, after which it begins to decline and this may be due to lack of trace elements required for the performance of the microbial community (Thamsiriroj et al. 2012).Moreover, the lowest biogas yield was registered for the inoculum to substrate ratio (80:20) i.e. reactor R5 this may be due to low % of substrate present thus lack of the essential nutrients present in the reactor which in turn results in inhibition of the biogas production (Angelidaki and Sanders, 2004).
Cumulative biogas Production at different Inoculum to substrate ratio
Cumulative biogas yield for different inoculum to substrate ratios (I/S) are presented in Fig. 7. Highest cumulative biogas was registered for reactor R2 followed by R3 as 11.378 L/day and 9.66 L/day respectively, however lowest yield was registered for R5 equivalent to 6L/day. Presence of the substrate plays a vital role here higher S/I results in lower biogas production which may be due to presence of the lower % of inoculum and thus low microbial activity and more risk of inhibition (Angelidaki and Sanders, 2004).