Application of ozone in wastewater treatment : For mitigation of filamentous bulking sludge & reduction of pharmaceutical discharge

Wastewater treatment with activated sludge is the most common way of treating wastewater. The process relies on bacteria converting nutrients, BOD and COD into biomass, CO2 and N2. Even though the configuration of activated sludge processes and wastewater treatment plants varies there are common problems which are shared. Filamentous bulking sludge is one problem that is widespread and disturbs the critical sludge separation which can result in reduced throughput, increased cost, sludge release, increased nutrient release and clogging of polishing steps. Another problem is that WWTPs are not designed to reduce the amount of pharmaceuticals commonly found in domestic wastewater and a large portion of them are being released into the environment.Alleviation of filamentous bulking sludge and reduction of pharmaceutical residues are both possible to achieve with ozone. This thesis presents results from two full-scale installations focused on mitigation of filamentous bulking sludge as well as one pilot-scale trial in which ozone was combined with a pre-treatment to remove pharmaceuticals from treated wastewater. Ozone addition into the return activated sludge was very effective in reducing the critical parameters SVI or DSVI to acceptable levels while not affecting the desired biological activity of the main treatment line. The energy consumption of such a system was investigated and found to be 0.044 kWh m-3 for a treatment lasting 45 days. This added operating cost should always be compared to the cost of having filamentous bulking sludge problems. A pre-treatment consisting of coagulation/flocculation/disc-filtration was highly effective in reducing the amount of ozone scavenging compounds. When ozone was added at a dose of 5 g O3 m-3 after the pre-treatment, the total pharmaceutical reduction reached 95% compared to 80% without pre-treatment. When ozone was added after pre-treatment the energy consumption reached 0.165 kWh m-3 year-1 while without pre-treatment the consumption reached 0.212 kWh m-3 year-1