Study On The Toxicity Of Urban Sewage Disinfection To Zebrafish Embryos

According to regulations, urban sewage needs to be disinfected before it returns to use. The disinfectants will have chemical reaction with the trace dissolved organic matters in the reclaimed wastewater along with the disinfection process, then produce a variety of disinfection byproducts (DBPs). Many studies have shown that, DBPs have a potential threat on human's health and the ecological environment. Conducting biological toxicology study can provide a scientific basis for the hazard evaluation of reclaimed water disinfection and the selection of the disinfection methods of urban reclaimed water.In this paper, the zebrafish was used as a model organism, and the bio-toxicological test methods was adopt. The embryonic toxicity test was used for investigating the effects of different disinfection approaches on the growth of Zebrafish embryos. The disinfection was managed by chloramine, chlorine dioxide, sodium hypochlorite, ozone, and their combinations with ultraviolet (UV) for treating the effluent of secondary sedimentation tank from a municipal wastewater treatment plant in Hainan Province. At the same time, the water samples which disinfected by chlorine dioxide were diluted to different times and exposed to zebrafish embryos to explore the trends of water toxicity with the increased dilution. A hazard rating method was established based on the toxicological response of the embryos and analytic hierarchy process (AHP) for evaluating the hazard of the disinfection approaches. The conclusion of this stydy showed as follows:(1)The results indicated that no any toxic effect on the growth of the embryos caused by the effluent of secondary sedimentation tank, while the effluents disinfected by chloramine, chlorine dioxide, sodium hypochlorite, ozone, and UV induced toxicological responses to the embryos with the abnormality of yolk sac, decrease of heartbeat, reduction of pigmentation, delayed hatch and capuchin.(2) Different disinfection approaches can cause different zebrafish embryo mortality, the hazard of the different disinfection to the embryo growth was as following:chlorine dioxide<UV<effluent (unsterile,ck)<chloramine<ozone<sodium hypochlorite.(3)The hazard rating showed that chlorine dioxide and UV constituted light hazard as level I; chloramine constituted moderate hazard as level ?, and; sodium hypochlorite constituted relatively strong hazard as level ?.(4) The combination of chemical disinfectant and UV could decrease the hazard of reclaimed wastewater. Among them, the combination of UV with sodium hypochlorite have the most obvious effect; a combination of UV with chloramine disinfection could decrease the hazard slightly, but the effect is not so significant; the hazard of the combination of UV with chlorine dioxide had no significant difference with chlorine dioxide disinfection; the combination of UV with ozone could decreased the hazard of reclaimed water;(5) The toxicological effects of zebrafish embryos of the diluted water sample disinfected by sodium hypochlorite showed:although the toxicity of DBPs which produced by disinfection was strong, but the dilution effect was obvious still. The hazard index was gradually reduced with the increasing dilution, when the sample was diluted to8times, the hazard level dropped to I, and then, the ecological damage had been relatively minor. So, when the outfall of the disinfected water set to the open water bodies, it will not cause great ecological harm.