Effect of suspended solids on wastewater disinfection efficiency by chlorine dioxide

Effluents enriched with high suspended solids concentration were tested for disinfection with chlorine dioxide. Following disinfection with various chlorine dioxide concentrations from 0 mg/l to 52.78 mg/l and contact times from 2 up to 24 h, half of the experimental samples were crushed (by Ultra-Turax) and tested for survival of indicator microorganisms. The crushing process revealed that a certain fraction of indicator microorganisms were left intact as a result of chlorine dioxide disinfection. This fraction was found to be able to regrow as was shown for all bacterial indicators, such as coliforms, fecal coliforms, enterococci and heterotrophic count, despite high disinfectant concentrations. The study results indicate that some microorganisms fraction entrapped into suspended flocs can survive disinfection with chlorine dioxide, depending also on indicator type, therefore their prior removal by coagulation, sedimentation and filtration is a major prerequisite for successful disinfection.