Ozonation of a secondary effluent both for fresh water protection and re-use

49 trials were carried out at pilot scale to evaluate ozonation for polishing secondary effluents. Ozone generator was fed with pure oxygen and increasing ozone doses (3, 5, 7.5 and 10 mg O3/L) were tested. Contact time was 10 min for each of the three contact columns. The pilot was fed with the nitrified and filtered effluent of a wastewater treatment plant. In most cases, at 3 mg O3/L, residual ozone concentration was below detection limit after 10 minutes contact time. For faecal coliforms and E. coli, log reduction increased from 3 mg O3/l to higher ozone doses, among which no appreciable difference was observed. No improvement in disinfection efficiency was seen for contact times over 10 minutes. Complete disinfection was obtained only in few cases, but final counts complying with the limits for discharge were always met. COD removal was low, while colour removal was significant and clearly increasing from 3 mg O3/L to higher doses. The removal of surfactants slightly increased with increasing dose for non ionic compounds, but not for anionic ones. Data confirmed that slight increases in ozone dose involve the release of bacterial organic matter which partially counterbalance the removal of COD.