Effect of ethoxylate number and alkyl chain length on the pathway and kinetics of linear alcohol ethoxylate biodegradation in activated sludge

Batch activated-sludge die-away studies were conducted with various pure homologs to determine the effect of ethoxylate number and alkyl chain length on the kinetics of primary and ultimate biodegradation of linear alcohol ethoxylates. The C-14- (ethoxylate) homologs C14E1, C14E3, C14E6, and C14E9 were used to investigate the effect of ethoxylate number, and C-14-(ethoxylate) homologs C12E6, C14E6, and C16E6 were used to examine the effect of chain length. Activated sludge was dosed with a trace concentration (0.2 muM) of each homolog, and the disappearance of parent, formation of metabolites, production of (CO2)-C-14, and uptake into solids were monitored with time. Ethoxylate number had little effect on the first-order decay rates for primary biodegradation, which ranged from 61 to 78 h(-1). However, alkyl chain length had a larger effect, with the C-16 chain-length homolog exhibiting a slower rate of parent decay (18 h(-1)) compared to its corresponding C-12 and C-14 homologs (61-69 h(-1)). Ethoxylate number affected the mechanism of biodegradation, with fission of the central ether bond to yield the corresponding fatty alcohol and (poly)ethylene glycol group increasing in dominance with increasing ethoxylate number. Based upon the measured rates of primary biodegradation, removal of parent during activated-sludge treatment was predicted to range between 99.7 and 99.8% for all homologs except C16E6, which had a predicted removal of 98.9%. Based upon the measured rates of ultimate biodegradation, removal of ethoxylate-containing metabolites was predicted to exceed 83% for all homologs. These predictions corresponded closely with previously published removal measurements in laboratory continuous activated-sludge systems and actual treatment plants.