back to main page A constructed wetland to improve the effluent quality from an oxidation ditch diurnal oxygen pattern | disinfection | effect of HRT     The low oxygen concentration of the STP effluent was hardly influenced in the presettling basin, but a diurnal oxygen pattern was observed at the end of the ditches. Photosynthesis of submerged aquatic macrophytes and macro-algae in the rear ditch sections mainly caused this change, with higher maximum oxygen concentrations at more sunny days.   Clear diurnal oxygen patterns and enhanced daily oxygen maxima of the STP effluent were observed at the end of the ditches most of the year. Development of a thick floating layer of Lemna sp. and macro-algae at the water surface, however, caused a strong decrease of the oxygen concentration in the water. After removal of this floating layer, a fast recovery of the oxygen pattern occurred.