Plant-biofilm oxidation ditch for in situ treatment of polluted waters

Abstract Eutrophication of surface water bodies is a problem of increasing environmental and ecological concern worldwide and is particularly serious in China. In the present study, oxidation ditches were connected to a lake receiving municipal sewage sludges. Two 24 m 2 (width 2 m, length 12 m) parallel plastic oxidation ditches material were installed on a lake near the inlet of the municipal sewage. Zizania caduciflora and Canna generalis were grown in the ditches with plastic floating supporters for the removal of N and P from the sewage. The experiment was conducted firstly with municipal sewage in autumn–winter seasons for about 150 days under the following conditions: 2 m 3 /h influent flow, 0.75 kW jet-flow aerator (air/water of 5), 18 h HRT (hydrological retention time) and a return ratio of 10. Then it was run with the polluted lake water in summer–autumn for about 160 days with an aerator of 1.25 kW and an influent of 6 m 3 /h (air/water 3.3, HRT 6 h). The performance was quite stable during the experimental period for the municipal sewage treatment. The average removal rates of COD (chemical oxygen demand), SS (suspended solids), TP (total phosphorus), NH 4 + -N and inorganic-N were 70.6, 75.8, 72.6, 52.1 and 50.3%, respectively. For the polluted lake water treatment, the average concentrations of COD, NH 4 + -N and TP were 42.7, 13.1 and 1.09 mg/L, respectively, in the influent and were 25.1, 6.4 and 0.38 mg/L, respectively, in the effluent. The capacity of the plants to remove N and P by direct uptake was limited, but the indirect mechanisms also occurred. The proposed process, transforming the natural lake into a wastewater treatment plant, could evidently reduce the costs of the sewage collection, the land space requirement and the construction compared with conventional sewage treatment plants, and is especially suited to conditions in south China and south-east Asia.