Optimization of Selective Ozonation for Wastewater Treatment

Sammanfattning

The increasing strain on global water resources necessitates the reuse of wastewater for agriculture. However, disinfection processes must balance pathogen removal with the preservation of valuable plant nutrients. This thesis investigates the optimization of selective ozonation for treating reject water (a nutrient-rich side stream from anaerobic digestion) to meet agricultural reuse standards while retaining agronomic value. A 2³ factorial screening design was employed to evaluate the effects of pH (6.0 and 8.0), temperature (15°C and 25°C), and ozone dose (5 and 15 mg/l) on E. coli inactivation and the retention of total nitrogen (TN), nitrate (NO₃⁻), orthophosphate (PO₄³⁻-P), and total phosphorus (TP). Ozone was generated on-site using a corona discharge ozone generator. Results demonstrated that the combination of high pH (8.0), high temperature (25°C), and high ozone dose (15 mg/l) achieved the greatest E. coli reduction (80 MPN/100 ml), meeting EU Class B reclaimed water standards (≤100 MPN/100 ml). While the target Class A standard (≤10 MPN/100 ml) was not attained, nutrient retention remained exceptionally high. Post-ozonation samples retained TN concentrations of 266–313 mg/l and exhibited increased nitrate levels (up to 41.4 mg/l), indicating controlled oxidation of ammonium to a more plant-available form without significant total nitrogen loss. The addition of H3PO4 acid for sample acidification made it difficult to draw strong conclusions about TP and orthophosphate preservation or loss during ozonation. These findings demonstrate that selective ozonation can effectively balance practical disinfection with nutrient preservation for agricultural applications. The study provides a scientific basis for designing energy-efficient, selective ozonation strategies that transform reject water into a valuable irrigation resource, contributing to circular water management and resource recovery.