Bioremediation of Phenolic Compounds in Circulating Packed Bed Bioreactor

Wastewaters containing phenolic compounds such as phenol and cresols pose a high risk to human health and natural environment. Compared to physicochemical methods, bioremediation is an attractive alternative for removal of phenolic compounds from contaminated waters. This research aimed to evaluate bioremediation of phenolic compounds in batch systems and in continuously operated circulating packed bed bioreactors (CPBBs). The biodegradation of individual phenol, o-cresol, and p-cresol and mixtures of these compounds (binary and ternary mixtures) were studied. In addition to creating kinetic data on biodegradation of these contaminants, toxicity of the treated effluents generated under various conditions were assessed. Effects of initial concentrations and temperatures were investigated in batch system. Work in continuous flow CPBBs focused on the impact of phenols concentration and loading rate on the removal percentage and removal rate for influents containing individual and mixture of phenols (binary and ternary) for a wide range of conditions. The toxicity of treated effluent samples generated under various conditions was determined and compared with the employed influents to evaluate the potential risk of releasing the effluents into natural water bodies. In batch systems, a linear relationship between biodegradation rate and initial concentration of p-cresol or o-cresol was observed for the range of initial concentrations evaluated. The optimum temperature for biodegradation of p-cresol and o-cresol were 35 and 25 °C, respectively. In a binary mixture, the presence of phenol enhanced p-cresol biodegradation. During both binary- and ternary-biodegradation, p-cresol was the preferred substrate and utilized first, while phenol and o-cresol were used simultaneously (if both were present) upon complete exhaustion of p-cresol. The interaction of phenols in the ternary mixture was more complicated, as a result, polynomial models were used to describe the impact of initial concentration on biodegradation rate. It was shown that increase in p-cresol and o-cresol initial concentrations had positive effects on biodegradation rate of all three phenols, but their interaction appeared to impact the biodegradation rate negatively. In batch system the maximum observed biodegradation rates for phenol, p-cresol, and o-cresol were 17.8, 8.9, and 7.2 mg L-1 h-1, respectively. In continuous flow CPBBs, the maximum removal rates of phenol, p-cresol, and o-cresol were 82.6, 107.2, and 73.8 mg L-1 h-1 at the loading rates of 104.7 (residence time: 4.7 h), 183.9 (residence time: 2.8 h), and 163.9 mg L-1 h-1 (residence time: 1.8 h), respectively under mono-substrate biodegradation. For binary-substrate biodegradation, the presence of o-cresol had a negative impact on phenol removal rate, while p-cresol did not impose the same effect. The maximum removal rates of phenol and p-cresol during binary-substrate biodegradation were 89.2 and 78.4 mg L-1 h-1 at their respective loading rates of 137.9 and 123.9 mg L-1 h-1. The maximum removal rates of phenol and o-cresol during binary-substrate biodegradation were 119.9 and 70.3 mg L-1 h-1 at the respective loading rates of 209.8 and 112.9 mg L-1 h-1. When all three substrates were present in the influent, the maximum removal rates of phenol, p-cresol, and o-cresol were 129.2, 135.3, and 108.0 mg L-1 h-1 at their corresponding loading rates of 179.3, 195.9, and 165.7 mg L-1 h-1. It was also shown that p-cresol was the preferred substrate, followed by phenol and o-cresol. In case of untreated influents, p-cresol presented the most toxicity, followed by o-cresol, with phenol presenting the least toxicity among these three compounds. Toxicity evaluation of effluents obtained under various operating conditions revealed that overall treatment in CPBBs reduced the toxicity of influent containing phenolic compounds, although the decrease in toxicity differed pending on the operating conditions such as nature of phenolic compound, its influent concentration and loading rate