Characteristics of raw spent wash (RSW), Reactor effluent, and overflow effluent were analyzed as per the standard methods for the examination of Water and Wastewater (17). The COD and BOD values were observed to be 126000 mg/L and 57000 mg/L respectively resulting in COD to BOD ratio 2.2 which indicated the spent wash is highly suitable for biological digestion.
Effect of OLRs on CSTR parameters:
The results are reported in Fig. 2–6, which summarizes the performance of CSTR at steady-state conditions. Methanogenesis is sensitive to both high and low pH and occurs between pH 6.5 and pH 8 hence reactor pH maintained between this ranges. From the commencement of the process, the temperature of the reactor was observed as the stability and efficiency of the anaerobic treatment process are greatly influenced by temperature (18)(19). Reaction rate, the dominance of certain biochemical pathways, and microbial activity are some of the areas known to be affected by temperature (20). Hence, paying attention to the reactor temperature is essential, since small temperature variation can considerably influence the reactor performance and the biogas (21).
The growth and decay rates are different at different temperatures hence; Mixed Liquor Suspended Solids (MLSS) concentration was correlated to temperature variations (Fig. 2.). The steady growth of solids was observed for constant reactor temperatures whereas higher growth was recorded during rise in reactor temperature. Highest COD removal efficiency of 72% was recorded at temperature 37 ± 1 oC when MLSS was around 36000–44000 mg/L. This performance is slightly at the lower side as comprised with membrane anaerobic bioreactor which results in 76% COD removal efficiency at 37 oC (18). Figure 2 summarizes the variation of the MLSS and Temperature. Reactor temperature increased up to 37 oC. Further, a decrease in HRT, results in reduction in COD removal efficiency, this could be a combined effect of high substrate availability and low net biomass growth rate. Further studies on CSTR needs to be conducted to reduce the COD of overflow effluent. The drop in temperature was predicted after further increase in HLRs.
For variable organic loading rates (OLR), the performance of reactor was examined in terms of biogas generation and % COD reduction. The optimum organic loading for highest COD removal from the spent wash has been examined and shown in Fig. 3. It was observed that COD removal reached a maximum of 73% when OLR is 0.11 kg COD per day with 9.17 kg COD/ m3/ d. Some studies have reported optimum conditions for COD exertion of the spent wash to be between OLR of 8 and 10 kg COD / m3/ d and on further increasing the OLR, hydraulic shock loading conditions would result with the rapid drop in COD reduction activity in case of Up-flow Anaerobic Sludge Blanket Reactor (UASBR) with fixed film (22). CSTR performance was on the slightly lower side in terms of the COD reduction under a similar range of OLRs. This may be because of a better reaction rate shown by fixed film reactors. At higher OLRs, it was observed and predicted that there is a gradual drop in COD reduction, unlike the fixed-film reactors which shows a sharp drop in COD removal efficiency. An increasing volume of biogas was observed during the treatment process which indicated the presence of a growing number of methanogenic bacteria. Characterization of seed present in the digester at different stages of bio-digestion is required to be done to better understand the role of microorganisms in performance of CSTR.
Generally, the volume of biogas can be calculated as; Volume of biogas = α Q (Sin – Sout). Where Q the feed-flow rate in m3/d, Sin, and Sout are the influent and effluent substrate concentration (kg/ m3) and α is the conversion coefficient of the substrate in biogas. For biogas produced by the degradation of COD as substrate a conversion coefficient α = 0.45 applies (23). In our study, from the recorded biogas quantities from full-scale CSTR, the conversion coefficient was calculated and it was found to be 0.405 and the same is used to calculate the biogas generation to get relevant results. It was observed that optimum biogas produced was in the range of 29 L to 32 L. The anaerobic process is very sensitive to temperature as mentioned earlier; it was observed that temperature increases from 32 oC to 37 oC from the commencement of process study to produce maximum COD exertion of 73% for HRT of 14 days.
During the present study, COD variations observed between 120000 mg/l to 130000 mg/l. Figure 4 shows variation in HRT with OLRs and the corresponding removal of COD. It was observed that the highest COD exertion occurs when OLR increased from 0.10 kg/d to 0.11 kg/d this causes reduction in HRT from 15 d to 14 d. these observations are similar to the study reported by Benabdallah El-Hadj (2007) (24). Further, it is observed that the decrease in HRT from 14 d to 8 d, COD removal decreased from 72.6% to 68%. Reactor shows VFA to alkalinity ratio at this stage as 0.34.
Volatile Fatty Acids and Alkalinity of reactor samples were observed and are shown in Fig. 5. The effluent alkalinity is higher than the influent alkalinity. This indicates that adequate buffering capacity was present in the reactor due to which reduction in pH was not observed even after an increase in the concentration of VFA in the reactor, particularly at high OLRs (above 0.10 kg/d). This indicates that the efficiency of the reactor decreased due to sulfide inhibition rather than VFA inhibition. The increase in VFA concentration in the reactor represents the incomplete conversion of VFA into the final end product (CH4) may be due to the reduction in retention time or due to sulfide toxicity to the methanogenic bacteria. Similar results were reported by Gupta and Singh with an anaerobic hybrid reactor. This indicates biogas production is strongly correlated with the OLR (25). The impact of VFA accumulation was reflected in the decrease in COD removal.
VFA has been identified as one of the very important characteristics during anaerobic digestion and is considered a vital parameter for anaerobic treatment (26). The study shows that methanogenesis appears to be an alkalizing process, as it consumes hydrogen and H3O- ions (27). Figure 6 shows the variation of pH and VFA to alkalinity ratio for all ranges of OLRs. It is extremely difficult to maintain the pH of the reactor constant. VFA production was found to be increasing with an increase in organic loading due to the high metabolic activity of acid-forming bacteria and the Alkalinity of digester is considerably affected by the organic loading rate. The decrease in alkalinity with an increase in OLR can be attributed to an increase in VFA concentration in the reactor effluent. Further, better results could be achieved by increasing the buffering capacity of the reactor.
Kinetic Modeling
The kinetic model proposed by Stovere Kincannon relates the organic substance utilization rate as a function of OLR at steady-state conditions Eq. (1).
The model applied to the reactors data where S is digester substrate concentration (kg COD /m3 in the Eq. (3)), dS/dt is substrate removal rate (kg/m3 per d), Umax is maximum removal rate constant (kg/m3 per d), Kb is saturation value constant (kg/m3 per d)
If (dS/dt)−1 is considered as V/Q (Si-Se), which is the inverse of the loading removal rate, and this is represented and plotted with the inverse of the total loading rate V/ (QSi), it will produce a straight line. The intercept of these lines is 1/Umax, and the slope of the lines is Kb/Umax, respectively. Figure 7, shows the results on the graphs. The graph represents Q (Si-Se)/V versus QSi/V for all the HRT considered for the study. Table 4 shows, Stover and Kincannon model table showing values for X-axis and Y-axis for the CSTR lab model. The maximum removal rate constant Umax (kg/m3/d) and is a saturation value constant Kb (kg/m3/d) was determined as below
Table 4
Stover and Kincannon model table showing values for X-axis and Y-axis for CSTR lab model
V (m3) | Si (kg COD/m3) | Se (kg COD/m3) | Q (m3/d) | V/(QSi) (m3d/kg) | V/Q (Si-Se) (m3d/kg) |
0.012 | 122 | 58 | 0.00008 | 1.230 | 2.344 |
0.012 | 122 | 57 | 0.00016 | 0.615 | 1.154 |
0.012 | 122 | 55 | 0.00025 | 0.393 | 0.716 |
0.012 | 122 | 53 | 0.00033 | 0.298 | 0.527 |
0.012 | 130 | 52 | 0.00038 | 0.243 | 0.405 |
0.012 | 130 | 49 | 0.00046 | 0.201 | 0.322 |
0.012 | 130 | 45 | 0.00054 | 0.171 | 0.261 |
0.012 | 130 | 40 | 0.00062 | 0.149 | 0.215 |
0.012 | 130 | 39 | 0.00069 | 0.134 | 0.191 |
0.012 | 128 | 35 | 0.00078 | 0.120 | 0.165 |
0.012 | 128 | 35 | 0.00086 | 0.109 | 0.150 |
0.012 | 128 | 37 | 0.00094 | 0.100 | 0.140 |
0.012 | 128 | 36 | 0.00102 | 0.092 | 0.128 |
0.012 | 119 | 36 | 0.00118 | 0.085 | 0.123 |
0.012 | 119 | 37 | 0.00126 | 0.080 | 0.116 |
0.012 | 119 | 38 | 0.00134 | 0.075 | 0.111 |
The important characteristic of the plot is the steady loss in efficiency with raised organic loads. The kinetic constants Kb and Umax can be calculated as 33.471 kg/m3/d and 17.123 kg/m3/d from Fig. 7. The regression line had an R2 of 0.9995, verifying the accuracy of Eq. (2).
The obtained values of Umax and Kb can be used to find the reactor volume to reduce the organic matter from Si to Se. It is also used to find Se (effluent concentration) for known or given V and Si.
Mass balance for an anaerobic reactor can be written as
{Mass in X volume of media} = {Mass out X volume of media} + {Mass biodegraded}