Biodegradation of Isothiazolone Biocide by a Laboratory-Scale Rotating Biological Contactor

Abstract

The aim of this study was to investigate the effects of operating conditions (organic loading rate, OLR; hydraulic loading rate, HLR and hydraulic retention time, HRT) as well as the presence of other carbon sources (lab-lemco broth) in the synthetic wastewater contaminated with isothiazolone on COD removal and biocide degradation by the RBCs. Biofilms had been established on the RBCs and then exposed to the wastewater containing 6 ppm isothiazolone under various operating conditions. The results showed that a period of cell acclimatization under biocide stress, the amount of growth substrate and the operating conditions were important parameters to increase the treatment efficiency of the system. When the normal synthetic wastewater containing 6 ppm isothiazolone was introduced to the units at HRT of 36 min, after acclimatization period COD removal was 16.49 ± 1.55% and biocide removal was rarely occurred. Isothiazolone degradation increased with increasing OLR of a growth substrate and/or HRT. Acclimatized biofilms could degrade isothiazolone via co-metabolism using lab-lemco broth as the growth substrate. At OLR (of lab-lemco broth) of 17 to 20 g COD m-2 d-1, the acclimatized biofilms degraded approximately 73.11 ± 5.01 to 77.38 ± 6.66% and 45.76 ± 5.01% of 6 ppm isothiazolone at HRT of 72 and 36 min, respectively. When the OLR of the growth substrate was doubled, isothiazolone degradation increased to 59.33 ± 5.58% at an HRT of 36 min. The results also showed that biodegradability of isothiazolone did not totally relate with the susceptibility of cells to the biocide indicating by minimum lethal concentration (MLC) values. The resistant bacterium to isothiazolone was tentatively identified as predominantly of the species Burkholderia cepacia.