Application of ozone in wastewater treatment : Oxidation of pharmaceuticals and filamentous bulking sludge

Abstract: As pharmaceutical removal was not part of the of the modern wastewater treatment plant (WWTP) design, it is not surprising that WWTPs have been identified as major point sources of pharmaceuticals entering the environment. The oxidation of pharmaceuticals in the WWTP effluent is one of the-end-of-pipe solutions that are considered most ready for full-scale use. However, there are several aspects of its application that must be further researched. One aim of this thesis project was to investigate how effective ozone is when applied to different WWTPs. When ozone was applied to the effluent of Lundåkraverket WWTP, the importance of the organic carbon content was highlighted. A pre-treatment that removed most of the organic carbon (suspended solids) removed 95% of the total pharmaceutical concentration with an ozone dose of five g O3/m3. Without this pre-treatment, the removal reached 80% with the same ozone dose. Moreover, the impact of TOC was substantial when ozone was applied in the same manner at ten different WWTPs; as the TOC content in the wastewater effluent increases, a higher ozone dose is required to reach an 80% reduction in the total pharmaceutical concentration.Filamentous bulking sludge that upsets the clarifying process still constitutes a problem at many activated sludge plants. Ozone added to the return activated sludge has alleviated the problem, but it has not been applied in many trials. The other aim of this thesis was to investigate how ozone addition impacts settling qualities and filamentous bacteria when applied to the return activated sludge. Ozone was tested at two different WWTPs using similar equipment. At Öresundsverket, the specific ozone dosage ranged from 2.8 to 5.0 g O3/kg SS with a constant ozone dosage rate of 900 g O3/h. At Klagshamn WWTP, the specific ozone dose was more variable because the flow of return activated sludge was changed to investigate how it impacted the results. The addition of ozone to the return activated sludge lowered the sludge volume index (SVI) and diluted sludge volume index (DSVI) significantly at both locations. The SVI in the first Klagshamn trial was decreased from approximately 200 mL/g to below 100 mL/g, and the SVI was reduced from 170 mL/g to 100 mL/g in the trial at Öresundsverket WWTP. The biological nutrient removal processes were not affected by the ozone process. A second full-scale trial at Klagshamn WWTP further demonstrated that ozone significantly improves the settling qualities of activated sludge. The DSVI was decreased from 82 mL/g to 54 mL/g with 4.0 g O3/kg TSS. Live/Dead® analysis of the ozonated sludge from that trial revealed that filamentous bacteria protruding outside the flocs are significantly more affected by ozone than bacteria within the flocs. In addition, ozone doses that are applicable to filamentous bulking control will not result in higher methane production from anaerobic digestion or be sufficient to oxidize micro-pollutants in the sludge.