Thermophilic aerobic biological treatment of high-strength wastewater
Thermophilic aerobic biological treatment was studied as a potentially advantageous technology for high-strength and high temperature waste streams. Thermophilic treatment was proposed as advantageous because of elevated biokinetic rates and low growth yields while retaining the robust nature of conventional aerobic biological treatment to simultaneously degrade a multitude of pollutants. Thermophilic systems, however, were known to support a non-flocculating biomass that resisted particle separation by conventional means, and were suspected to support a microbial community of limited diversity that adversely affected performance. This research was undertaken to verify or refute these proposed advantages and disadvantages. Biokinetic rates were investigated at temperatures from 25–65°C during the biological treatment of a synthetic wastewater containing gelatin and α-lactose as principal organic constituents. Measurement of the maximum specific substrate utilization rate in batch culture revealed that biodegradation rates were relatively constant over the temperature range studied. Also, biological treatment at thermophilic temperatures was less adept at degrading multiple pollutants simultaneously and achieving chemical oxygen demand (COD) removal. This latter trait was also demonstrated during the biological treatment of pharmaceutical wastewater as residual COD levels increased by an average of 60 mg 1−1 per °C increase over the temperature range of 30–60°C. Investigation of microbial community structure by nucleotide sequence analysis of 16S rRNA genes suggested that the underlying mechanism for reduced metabolic ability at thermophilic temperatures was a reduced phylogenetic diversity. Thermophilic treatment in continuous-flow bioreactors was also studied in chemostats and membrane-coupled bioreactors (MBRs). Chemostat analysis demonstrated that the bacterial maintenance requirement exhibited by the microbes supported by thermophilic reactors was similar to that previously reported for mesophilic systems. This suggests that thermophilic systems do not benefit from low cell yields as predicted. Thermophilic MBRs were successfully demonstrated to maintain high biomass levels and produce a high quality effluent despite the lack of bioflocculation at thermophilic temperatures. In conclusion, thermophilic aerobic biological treatment does not offer many advantages compared to conventional mesophilic processes. The underlying mechanism for the poor performance of thermophilic bioreactors is likely a reduced microbial diversity that results in reduced bacterial function.
Alleman, Purdue University.
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