Treating wastewater using an internal circulation anaerobic digester (IC-AD)

Raijmakers, Natasja
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Anaerobic treatment technologies are becoming increasingly more popular for the treatment of high organic strength wastewater. Investigating the Internal Circulating Anaerobic Digestion (IC-AD) technology is of importance as it is more cost effective, the reactor aids in the production of high quality effluent which reduces the cost of a polishing step and produces an effluent that complies to discharge limits. The following parameters were used to evaluate the daily operation of the Internal Circulating Anaerobic Digestion reactors; temperature, pH, alkalinity, COD (Chemical Oxygen Demand) and Volatile fatty Acids (VFA). This study presents the anaerobic treatment of synthetically prepared FischerTropsch (FT) process wastewater using Laboratory Scale (LS) and Pilot Scale Internal Circulating Anaerobic Digestion (PS IC-AD) reactors. The reactors were inoculated with granular sludge obtained from reactors that were treating brewery wastewater and pulp and paper wastewater, respectively. The results indicated that the IC-AD technology was most likely a viable option for treating high strength COD wastewater at higher Organic Loading Rate (OLR) compared to other conventional anaerobic technologies. During this study the function and structural integrity of the granules were investigated. From the granules used to inoculate the LS reactors, 90% (d90) of the granules sampled consisted of granules smaller than between 1248 µm and 1348 µm. Granules sampled from the LS IC-AD after treating synthetically prepared FT process wastewater for 100 days, indicated that in Run 1, 90% (d90) of the granular sludge sample consisted out of granules smaller than 874 µm, whereas 90% (d90) of the granular sludge sample during Run 2 consisted out of granules smaller than 660 µm. Visual observations (SEM images) demonstrated a probable shift in the microbial population relative to the granules used during the inoculation process; this shift was probably ascribed to the substrate being fed to the reactors. The reactors were probably not able to sustain the formation of granules, increase the granular size and form new granules. Furthermore, the reactors had to be re-seeded each time operational upsets occurred, this probably resulted in the microbial population within the granules not being adapted to the synthetically prepared FT process wastewater and possibly compromised the structural integrity of the granules. The LS IC-AD reactor studies were evaluated for a period of 100 days and divided into two sections, Run 1 and Run 2. In Run 1 the influent COD concentration was maintained constant at approximately 22000 mg/L, whereas in Run 2 the influent COD concentration was started at about 1000 mg/L and increased with time to about 6000 mg/L. Although the reactor in Run 1 was not able to achieved stable operation, due to OLR being increased continuously (state of ramp up), the assumption was made that stable operation was achieved from day 80 onwards. This assumption was based on the fact that both IC-AD reactors had obtained a similar OLR of approximately 10 kg COD/m3.d. The IC-AD reactor operated during Run 2 achieved stable operation on day 51. During Run 1 the reactor obtained an average OLR of 10 kg COD/m3.d with a removal efficiency of 98%. In Run 2 the reactor obtained an average OLR of 11 kg COD/m3.d with a removal efficiency of approximately 94%. During the Pilot Plant Internal Circulating anaerobic Digester (PS IC-AD) evaluation period of 544 days the reactor could obtain an OLR of between 30 to 40 kg COD/m3.d. The PS IC-AD yielded an effluent COD of 732 mg/L with a removal efficiency of >90%
A Dissertation submitted in the fulfillment of the requirements for degree of Master of Science to the School of Chemistry and Environmental Analytical Chemistry Research Group, University of the Witwatersrand, Johannesburg