Experimental investigation and modelling approach of the impact of urban wastewater on a tropical river; a case study of the Nhue River, Hanoi, Viet Nam
Résumé
Analyses of water quality and flow regime in combination with laboratory studies and ecological modelling were used to assess the water quality impact of pollution from to To Lich River that drains through Hanoi City and greatly contaminates the Nhue River. With an average discharge of 26.2 m3/s, the Nhue River receives about 5.8 m3/s of untreated domestic water from the city?s main open-air-sewer the To Lich River. The studies during 2002 2003 showed high concentrations of BOD (70 mg O2/l), DOC (15 mg C/l), coliform (2.4e6 MNP/100 ml), total phosphorus (3.5 mg P/l), and total nitrogen (31.6 mg N/l) in the To Lich, while DO level was less than 1 mg O2/l. Such high loads of untreated wastewater impacted water quality in the Nhue River where DO decreased at times to as low as 1 mg O2/l. The accumulation of particulate organic matter and micro-organisms in the sediments of the Nhue represented substantial sources of nutrients and sinks for DO. They are also considerable production of dissolved carbon dioxide at concentrations up to two orders of magnitude higher than pressure. Such pressures (EpCO2) are expected in polluted environments, but the results presented here are new for Vietnam and much of developing countries. A number of factors linked to field monitoring and laboratory measurements clearly indicate the importance of autotrophic over heterotrophic biological processes and sediments. An ecological model for management purposes has been developed that reliably estimates of the pollutant loads. An opportunity was taken to examine the changing impacts and processes when the To Lich was diverted from the Nhue. The monitoring and modelling of this opportunity showed low dissolved oxygen levels even if the impact from the To Lich was lessened. Alternatives are proposed to alleviate problems of water quality in the Nhue. It is concluded that the treatment of the To Lich River?s water is highly recommended; otherwise a reduction to one third of current wastewater discharge is needed to bring water quality back to the environmental standard.