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.
