anaerobic (upflow anaerobic sludge blanket) aerobic (completely stirred tank) reactor system at increasing organic loading rates

The purpose of this study was to determine the treatability of 2,4 dichlorophenol (DCP) in an anaerobic/aerobic sequential reactor system when molasses was used as carbon source. Laboratory scale upflow anaerobic sludge blanket (UASB) reactor/completely stirred tank reactor (CSTR) were operated at different 2,4 DCP loading rates and constant hydraulic retention times (HRTs) to investigate the COD removal efficiencies, accumulation of volatile fatty acid (VFA), total, methane gas productions and methane percentages. Studies were carried out in continuous mode and the effluent of the UASB reactor was used as feed of the CSTR reactor. 2,4 DCP removal efficiency decreased from 99 to 78.7% when the initial 2,4 DCP concentration and 2,4 DCP loading rates were increased from 5 to 120 mg/l and from 0.006 to 0.144 g/l.d, respectively. The maximum COD removal efficiency was achieved as 77% at a 2,4 DCP loading rate of 0.042 g/l.d. When the 2,4 DCP loading rate was increased to 0.12 g/l.d, the maximum measured VFA concentration was 1000 mg CH3COOH l–1 in UASB reactor. In the aerobic reactor, the COD removal efficiencies varied between 60 and 90% depending on UASB reactor treatment efficiencies and to COD concentration coming from the UASB reactor. The effects of sludge retention time (SRT) on the 2,4 DCP and COD removal efficiencies were also investigated in the aerobic reactor. The 2,4 DCP and COD removal efficiencies were observed as 90 and 85%, respectively, at a SRT of 25 d. 86.67% 2,4 DCP removal efficiency was obtained in the whole sequential anaerobic/aerobic (CSTR) reactor system at an initial 2,4 DCP concentration of 120 mg/l.

In this study it was shown that 2,4 DCP and COD can be successfully degraded using a sequential anaerobic UASB and aerobic CSTR reactor system. SMA tests showed that no decreases in methanogenic activity were observed even at high 2,4 DCP loadings since the granular structure protect internal anaerobic bacteria from exposure to DCP toxicity resulting in acclimated granules. Anaerobic conditions are essential for 2,4 DCP dechlorination by granulated anaerobic microorganismis while the remaining intermetabolites and phenol mineralized under aerobic conditions (data not shown). 120 mg/l of 2,4 DCP with molasses as carbon source in synthetic wastewater was effectively degraded in continuous operation of laboratoryscale UASB reactor at 2,4 DCP loading rates ranging between 0.006 and 0.144 g-2,4 DCP/l.d. Over 66% 2,4 DCP was removed at an HRT of 20 h. Maximum COD removal efficiency was achieved as 77% at a 2,4 DCP loading rate of 0.042 g-2,4 DCP/l.d. Maximum methane percentage in total gas was observed as 70–75 %. When the 2,4 DCP loading rate was increased to 0.072 g-2,4 DCP/l.d, the VFA concentrations did not exhibit significant increases. The maximum measured VFA concentrations were 1000 mg CH3COOH/l at a 2,4 DCP loading rate of 0.12 g2,4 DCP/l.d. The bicarbonate alkalinity consumed and decreased from 4500 to 2500 mg/l at this loading rate due to buffering requirements of anaerobic microorganisms and providing the neutral pH. The SMA increased from 0.79 g CH4-COD/g SS.d to 0.92 g CH4-COD/g SS.d when the 2,4 DCP loading rate was increased to 0.144 g/l.d from 0.006 g/l.d. High 2,4 DCP loading rates such as 0.084 g/l.d affected the methanogenic activity of the biomass therefore the SMA decreased to 0.26 from 0.99 g CH4-COD/g SS.d. Over 65% COD removal efficiency was obtained in the aerobic (CSTR) reactor. As the 2,4 DCP loading rate increased from 0.06 to 0.144 g/l.d, the 2,4 DCP removal efficiency reduced to 37% from 99%. In the meantime COD removal efficiency decreased to 75% from 86%. The COD removal efficiency did not significantly change as the SRT increased from 14 to 35 d in the aerobic reactor The 2,4 DCP removal in the anaerobic (UASB)/ aerobic (CSTR) sequential reactor system occurred mainly in the anaerobic step. 86–99% 2,4 DCP and 86–95% COD removal efficiencies were obtained, respectively, depending on 2,4 DCP loading rate in the anaerobic/aerobic sequential system. As the 2,4 DCP loading rate increased from 0.006 to 0.144 g-2,4 DCP/l.d, the COD removal efficiency decreased from 95 to 86%, while the 2,4 DCP removal efficiency reduced to 86% from 99%. 76.00 and 99.54% 2,4 DCP removal efficiencies were obtained in anaerobic and aerobic effluents while 99.90 % total 2,4 DCP removal was obtained in the whole system.