Effects of Sub-lethal Concentrations of Isothiazolone Biocide on the Performance of Rotating Biological Contactors

Abstract

The aim of this study was to investigate the effects of operating conditions on chemical oxygen demand (COD) removal and biocide degradation in a synthetic wastewater by rotating biological contactors (RBCs). The RBC operating parameters included the organic loading rate (OLR), hydraulic loading rate (HLR) and hydraulic retention time (HRT). The presence of another carbon source (lab-Lemco broth) in the synthetic wastewater contaminated with isothiazolone (IT) biocide at sub-lethal concentrations was also examined. Biofilms were established on RBC discs and then exposed to the wastewater containing 6 ppm of IT under various operating conditions. After an acclimatization period with an HRT of 36 min, the COD removal was 16.49 / 1.55% and biocide removal was negligible. IT degradation increased with the OLR of a growth substrate and/or HRT. Acclimatized biofilms exhibited IT degradation through co-metabolism when Lab-Lemco broth was included in the medium. At an OLR (using Lab-Lemco broth) of 17 to 20 g COD/m2.d, the acclimatized biofilms degraded 73.11 / 5.01 to 77.38 / 6.66% and 45.76 / 5.01% of the 6 ppm of IT at HRT values of 72 and 36 min, respectively. When the OLR of the growth substrate was doubled, IT degradation increased to 59.33 / 5.58% at an HRT of 36 min. The IT resistant bacteria were tentatively identified as predominantly of the species Burkholderia cepacia. Biofilms could be developed in the presence of 3 ppm of IT after an acclimatization period, and the degree of COD removal and biocide degradation depended on the HLR and HRT values. Biochemical oxygen demand (BOD) measurements were not appropriate for investigating treatment of wastewater contaminated with IT due to severe bio-oxidation inhibition. This was true even when using adapted seed and/or increased incubation time.