Influence of operating parameters on Hydrolysis and Acidogenesis
Main end product of the first phase of the biomethanation is the formation of the volatile fatty acid (VFA). The formation of VFA is influenced by many factors such as pH, retention time, and composition of the substrate; therefore it becomes imperative to understand the influence of these parameters as this will help in better designing of the process. In this research, performance of the dry fermentation unit for different retention time was analyzed for the quality of leachate obtained. The variations in the pH, VFA, COD of leachate from reactors R1, R2, and R3 were studied and are presented in the Figs. 2,3,4,5 and 6, respectively.
Variation of pH
pH of the reactor plays crucial role in the degradation of food waste through microbial activity. Ideal pH range for hydrolysis and acidogenesis for the degradation of organic fraction of solid waste is considered to be from 4 to 6 (Li and Fang 2007). The variation of pH of the leachate obtained through dry fermentation unit is shown in Fig. 1. It was observed that in the initial stage, the pH decreases up to 3.51 and then increases gradually to 5.5 in R1, which is in consistent with the earlier result reported (Selvam et al. 2010; Xu et al. 2011; Xu et al. 2012). Whereas a minimum value of pH of 3.52 and 3.48 was observed in leachate from R2 and R3 on day 8. The lower pH values of the reactors are attributable to the production of formic acid and lactic acid due to their low pKa values, i.e. 3.77 and 3.86, respectively. Furthermore, due to presence of high concentration of the soluble organics in kitchen waste result in pH drop. Formation of the intermediate products such as acetate, lactate, formate, succinate and ethanol occur during acid fermentation, as a result of the enzymatic hydrolysis by the members of the family Enterobacteriaceae, Clostridia and lactic acid bacteria (LAB). LAB are the dominant microbial archea during the degradation of food waste in the mesophilic temperature regime (Wang et al. 2001). However, if pH is adjusted using alkaline solution it reduces the dominance of LAB by enriching it with the coliform bacteria and Clostridia.
Variation of VFA concentration of leachate during operation Period
Rate of the volatile acids production is directly proportional to the digestion period. In the initial stage, rapidly degradable materials like grains are liable to cause VFA accumulation. This brings about severe inhabitation in acidic fermentation by reducing the pH of the reactor, thus ultimately results in the inhabitation of biogas production (Shin et al. 2001). Variation of VFA concentration of leachate during operation period is shown in Fig. 2. The first peak on day 3 was the result of grains degradation (12.993g/l). It was observed that the degradation of cellulose increases at high retention time and the degradation of protein increases at both high retention time and neutral pH (Noike et al. 1985). However, with the improvement of the pH of the system, VFA production again shoots up to (11.257g/l), indicating the enhanced degradation of vegetables. From the graph, it can be concluded that the reactor with shorter retention time achieves better acidification rate as it does not support the accumulation of VFA. The decline trend of VFA production after day 17 may be attributed to the shortage of the sufficient substrate available for the microbial consortia. The variation of cumulative VFA during the digestion is represented by Fig. 3. It indicates that the rate of VFA production increases during the initial period and then the rate decreases which is mainly attributed to the pH and acidification efficiency of microbes, as at lower pH it results in the activation of the enzymes necessary for the microbial community to perform degradation (Tembhurkar and Mhaisalkar 2007). The cumulative value of VFA in R1 indicates a smoother trend with higher accumulated VFA, whereas the VFA generated in R2 is comparatively less and R3 is least. The similar trend was exhibited in the past studies by (Tembhurkar and Mhaisalkar 2007; Xu et al. 2011).
Variation of COD of leachate during operation Period
From Fig. 4, a biphasic pattern of COD leaching can be observed. COD concentrations of the collected leachate during initial days of operation were found to be higher ranging from 72 g/L to 185 g/L in reactor R1, after that they start depleting gradually. The initial high rate of COD leaching was partially attributed to the water-soluble fraction of food waste itself and it also depends on the substrate concentration. Decline in the trend of the COD may be attributed to the wash out of the water soluble fraction of the food waste from reactor (Xu et al. 2012; Xu et al. 2014). The value of COD from the leachates obtained from all the reactors show sudden peaks may be because of accidental slipping of the hydrolyzed waste. It was observed that the higher value of COD during initial days is because of the hydrolyzed material getting washed as the leachates which impart high rate of COD. The rate of COD accumulation of the leachate can be better studied by plotting the cumulative diagram COD which is shown in Fig. 5. The rate of accumulation of COD of R3 is lowest than R2 then R1 due to the conditions prevailing in these reactors and it is mostly attributed to the lower pH of the system which will hamper degree of acidification of the system and it will be in the order of R3 < R2 < R1.
Solubilization rate of kitchen waste during hydrolysis
The maximum amount of energy in the form of methane obtainable from a solid organic residue by biomethanation depends on the extent of solubilization of the organic residues. The yield is directly related to the extent of solubilization of a substrate, but the rate of gas production will depend on the slowest of the three steps, namely, solubilization, acidogenesis and methanogenesis. Methanogenesis is the slowest step, whereas hydrolysis and solubilization may be the rate limiting steps (Jash and Ghosh 1996).
The solubilization rate decreases with the operation period. It may be due to the high content of rapidly degradable matter. The solubilization rate of substrate decreased in the following order: R1 > R2 > R3. The rate of solubilization decreases with an increase of substrate concentration due to lowering of pH. It is also evident that at higher substrate concentration, the solubilization rate is initially higher, resulting in a higher concentration of soluble substrate amenable to acidogenesis leading to lowering of ph. Because of lowering of pH, the solubilization rate slows down subsequently steps (Jash and Ghosh 1996).
The solubilization rate of the kitchen waste was determined for reactor R1, R2 and R3. The first phase of hydrolysis and acidification of the kitchen waste was assumed to follow the first order rate of reaction, which is given as equation,
-dC / dt = kC,
t = - (1/k) In (C/Co),
Where k = first-order specific rate constant (day− 1), C = COD concentration of kitchen waste (mg/l) at time t, Co = initial COD concentration of kitchen waste (mg/l) at zero time.
The data is plotted in Figs. 6, 7 & Fig. 8 to determine specific rate constant for R1, R2, R3, respectively. The specific rate constant was found to be maximum 0.063 for R1 and that is for R2 is 0.054 and R3 is 0.027 indicating that daily addition of water improves the solubilization process. The solubilization rate of the kitchen waste of reactor R1 is higher than of reactor R2 and R3, respectively.