TOXICITY OF SOME CONTAMINATED SITES IN HOCHIMINH CITY AND SUGGESTION OF MONITORING PROGRAM

Abstract

The goal of this project was to conduct a first assessment of the environmental risk of two contaminated sites, by applying ecotoxicological tests, and measuring heavy metal presence as well as standard parameters, such as BOD and COD. The first site studied is a septic sludge disposal and composting area. One this site, septic sludge is transformed into compost through a dewatering and carbon enrichment process. The dewatering of the sludge is made in earth wall basins from which water seeps and is collected in small canals that flow directly into the environment. Samples from these canals revealed acute toxicity to D.magna. Heavy metals were also found in concentrations exceeding limit values for wastewater. The sample taken in a pond on the other side of the unloading area showed no acute toxicity and lower metal concentrations. The composting process used does not enable mineralisation of the dewatered sludge enriched with rice skin which also means that the high temperatures needed to kill pathogen bacteria are not reached. Suggestions were made to improve the composting process, the quality of the compost and to limit the impact of leachate waters on the environment. Measurement should be carried on compost to control heavy metal concentration. Leachate should be concentrated in lagoons, where it could be treated and finally composting should be done using windrows. The second site is a 40 ha landfill, in which municipal, as well as industrial and hospital wastes have been deposited. Samples from the leachate lakes, a leachate canal and a fresh water canal all had an acute effect on the test species (D. magna, C.cornuta and V. fisheri). Heavy metals concentrations were not high, therefore ammonia was suspected to induce a toxic effect. The toxicity of the samples together with the very large volume of leachate collected around the landfill, suggest that the volume of leachate should be reduced in first priority. Impermeabilisation and the related problems of this method are discussed briefly. Health and safety practices should also be of first concern.