Initially the molasses stillage sample was characterized for its composition. The COD of the sample was so high, which was measured to be above 2,32,000 mg/L with a BOD of 133,000 mg/L. However, the AD was started at a lower OLR of 120 mg/L CODsol by diluting the original sample.
3.1.1. The startup periods
After digester setup was completed, a lab preserved inoculum of 1.5 L was added to each of the already 40% scoria packed anaerobic digesters. Afterwards, the viability of the system was monitored using pH and methane production for three days, which was followed by feeding a glucose solution formulated in the lab as presented in table one under the method section.
Among other issues, process stability in AD behaves as a function of substrate type, system susceptibility to xenobiotics, the mode of operation and temperature, either continuous or batch or thermophilic or mesophilic, and pH 32. Thus, towards reaching stable operation, glucose solution and glucose stillage mix have been fed to digesters. However, pH drops of scale over 1.5 have been recorded in just four days during the second week that also followed poor methane generation with lower percentages (10% methane). Consequently, a mix of anaerobic sludge from a working waste activated sludge AD was obtained and supplemented to the current experiment. Generally, stability in pH has been monitored for over 40 days and the results are show in Fig. 5.
The slightly lower pH on the second day of the pseudo-steady operation of the reactor ‘A’ was associated with the relatively higher initial COD and the subsequent hydrolysis and acidogenesis activities happening in the system. Though the slight variation in pH between the two systems persisted nearly all the time, the pH of both systems was maintained between 6 and 8 during the entire start up period. Maximums of 8.0 and 7.7 and minimums of 6.7 and 6.1 pH have been recorded for the digester ‘A’ and ‘B’ subsequently. However, the pH monitored showed a significant difference between reactors ‘A’ and ‘B’ (p-value = 0.00) based on a t-test performed in R. Despite their major disparity in pH, the two digesters were within desirable ranges of pH except the slight increase recorded by digester ‘A’. After the establishment of a relative stability in pH, the test runs were started aiming at the COD removal efficiency of both digesters fed simultaneously and the OLR as well as feed were changed over time.
3.1.2. COD removal efficiency variation with feed pretreatment
Average values from both digesters were taken for nearly two months after establishing pseudo-steady state conditions whereby OLR was varied between 2 to 3 weeks. The entire runs were repeated except changing the feed type from the raw molasses ethanol stillage to a WAO pretreated one. The COD after the stabilized pH was showing negative removal for the first two days, which could be due to leaching from the packing material, release of extracellular polymeric substances in the sludge inoculum, poor mixing in the system as well as the use of undiluted samples for the COD analysis. Despite the accumulating COD, the pH of both reactors was on a slight increase that would be due to the effect of the packing material.
After giving some time to run both systems, the two reactors were compared against feed type for COD removal efficiency with respect to the OLR applied. Upon establishing a steady operating conditions, the OLR and the COD removal efficiency in percentage is compared between the WAOp and the raw feed. The averaged organic/stillage loading rate against the CODsol removed has been compared between the wet air pretreated (CODr (mg/L) _WAO) and the unpretreated (CODr (mg/L)) for which the graph shows a relatively better performance of the earlier. Furthermore, the two systems showed inverse relation in COD removal against loading from 1667 mg/L-d upwards. Further, based on a simple linear regression modelling of the COD removal a visible difference between the WAOp feed and the raw feed has been observed; which is CODr = 0.022x OLR + 2.7381 and CODr = 0.0167xOLR + 0.6264 respectively. Indeed, the difference in the regression constants as well as the regression coefficients was tested for statistical significance. Based on Wilcoxon signed rank test with continuity correction, both variables have proved to be different (p-value = 0.036) with an average of 13% difference in COD removal.
Despite the decline on the average COD removal at 1667 mg/L OLR, the WAOp feed showed increased removal. The improved removal continued even on the highest OLR of 2000 mg/L that signals the opportunity to higher loading possibility of such pretreated feeds in AD while maintaining better efficiency in the COD reduction. On the contrary, the unpretreated fed showed steep decline in COD removal with such an increase in OLR (Fig. 6). In fact, physicochemical pretreatments are reported to recover more methane thereby removing more COD in AD at various loading rates.33,34 Though the data obtained in this study is for comparison purpose, the issues with continuous operation of the systems, air interference and hence poor mixing contributed to the overall low performance of both systems.
Though the focus of the current study was to see the effect of the stillage pretreatment on the removal of the COD in AD with respect to varied OLR, the methane content of both systems was also followed periodically. As a result, a negligible variation in percentage methane content was obtained between the digesters and among OLR, ranging between 37% and 40% on average. However, a relatively better percent methane (up to 42%) was frequently recorded by the digester which was fed with a WAOp stillage. In a related experiment, the reject water from the AD testing was moved to a polishing treatment, the aerobic degradation.
3.2. Aerobic batch digestion of the post anaerobic digested pretreated stillage
Though there is a relatively recent attraction in AD, aerobic degradation is used to be the main systems in wastewater treatment since earlier times. The oxygen molecule being the terminal electron acceptor, the degradation of organic molecule results in the transformation of organic molecules to cell masses and mineral byproducts, including the endogenous respiration. In such schemes, the rate of oxygen consumption is stoichiometrically linked the organic utilization rate to cell mass buildup in the system 35. Aerobic systems are good for their shorter hydraulic retention time. Though, the process demands many variables with a higher biomass yield resulting in a huge mass of sludge.
The mechanical supply of air/oxygen, nutrient system supplement as well as diversification of the consortia of organisms as degrading crew among other environmental factors is used to be a process performance determinant. However, aeration cost and the resulting biomass buildup that gives up secondary waste are among the influencing parameters to prefer anaerobic systems to it. On the other hand, the application of either of the two systems alone is less efficient, especially in the treatment of complex industrial waste including distillery stillage 32. Therefore, the coupled application of both systems is reported to have a desirable effect on the degradation of stillage including the application of different pretreatments. The aerobic degradation potential of the stillage after the WAOp AD is performed in the current study to see if the final removal is desirable to meet discharge limits, which is becoming widely and increasingly stringent.
With the perspective that the coupling of aerobic and anaerobic digestion of organic matter increases the removal efficiency 36, the anaerobically treated stillage is moved to aerobic digesters. Aerobic reactors, glass bottles of one-liter volume were constructed to run the experiment. Rubber corks, plastic tubes and air blowers were used to mechanically supply air, insert the feed and remove samples as well as for monitoring the systems, which were running in duplicates for over eight days. During those experimental days, the COD removal of the feed, including the glucose concentration supplement, was monitored along with pH and DO (Fig. 7).
Even though there were drops in DO up to 2–3 mg/L in the systems during the initial and when glucose supplement was performed, mostly read DO was around 6 mg/L. Though the optimal pH was around 7 and a pH of 6–9 is tolerable 35, there were times when the pH of the systems went over 9, especially during the first three days of the experiment. Consequently, regulating the rise in pH was done by adding more inoculum on the third day and by using a 2% HCl solution, which was added to a volume of 5–10 mL to each reactor.
Unfortunately, the nonbiodegradable COD remained resistant even after the aerobic degradation period was over. The best COD removal (68%) in the process was attained in just eight hours. In fact, the final corrected COD after aerobic degradation is still around 2278 mg/L; however, it is far better when compared to the removal of the same, which was obtained from prior studies and of course that is far better than the same COD measured after the AD was completed 28. During the aerobic test, the least COD measured is attained within four days. Besides, it has to be noted from the graph that the negative removals were representing the effect of the glucose solution added on the fifth day and it has been understood that its addition had no effect on enhancing the COD removal despite the expectation that it could have done otherwise (Fig. 7).
The aerobic degradation has improved the overall average COD removal efficiency of the integrated treatment in the current study. Thus, the elimination efficiency has brought above 90%, which is very much closer to the set distillery wastewater (DWW) discharge limit set by the authorities, by considering the concentrated stillage alone without mixing another component wastewater in the sector. More importantly, the current integrated biological treatment has completely removed the BOD of the stillage feed. In Ethiopia, the discharge limits are set to be able to join the natural water streams, which is established at 250 mg-COD/L and 60 mg-BOD/L effluent or less for the ethanol industries.
Despite the existing reality of the local river pollution, which is already becoming open channels of wastewater of either industrial, municipal or agricultural origin because of the absence of well laid infrastructure for sewage management, industrial discharge limits can have alternatives. For instance, the discharge limits can be set at two levels: one for connecting to the local municipal sewage system and the other for joining water bodies or just the natural environment including the land, the former being at a relatively high level. In such cases, discharge limits are set at two stages whereby linking to sewage can easily be met.
Considering the huge water portion in the entire DWW, this degree of reduced COD level along with total BOD removal can meet the discharge limits on its own even without the need to join other municipal waste streams as far as COD and BOD are concerned. Moreover, it is not only pollution minimization but the recovery of water also has to be given large emphasis as the sector is part of a major water consuming category, that is the industry and agriculture, that consume 90% of the overall global freshwater 37. Such removal efficiency is superior compared to reports based on coupled treatment studies 19,38,39.