Evaluation of wastewater solids and their relevance in ultraviolet disinfection

As ultraviolet radiation (UV) emerges as the primary alternative to chlorination in wastewater disinfection, the primary factors affecting UV disinfection efficiency must be investigated thoroughly. Research shows that the suspended solids content of wastewater has a major impact on UV disinfection efficiency. Yet attempts to correlate disinfection efficiency with common measurements of solids, such as TSS and turbidity, have not been successful.; This dissertation develops and employs a new method of evaluating wastewater solids and their relevance in ultraviolet wastewater disinfection. A fractionation technique was developed to separate particle-associated bacteria into small, medium, and large fractions based on particle size. A blending technique was incorporated to release bacteria from their particle associations. Wastewater from two wastewater treatment plants employing different treatment processes, pure oxygen activated sludge and rotating biological contactors, was used throughout the study.; The results of these techniques show that particle size is significant in UV disinfection efficiency. The small particle size fraction of the wastewater (<5 μm) consistently produced a faster rate of disinfection than the large fraction (>20 μm), with the whole unaltered sample in between the two extremes. The medium fraction disinfection rate was variable, but generally close to the small fraction. Tailing was observed only in the large fraction. Approximately one percent of the measurable bacteria in the large fraction contributed to tailing.; Evidence of particle shading was observed with only one wastewater. Evidence of particle shielding was observed with both wastewaters. The relative significance of particle shielding was almost double that of particle shading where both phenomena existed.; Reconstruction of the original sample's disinfection curve by a weighted addition of the three size fractions was largely successful over the log linear section of the dose-response curve. Reconstruction exercises using a correlation between the initial bacterial concentration of the large fraction and TSS to estimate the percent contribution of the large fraction also were largely successful.; The study data support two methods of pretreatment prior to UV disinfection. The benefits gained from using filtration and the blending technique as pretreatment methods were similar.