Many confined-space accidents have happened in wastewater treatment tanks, mainly caused by hazard gases. To identify the factors affecting the distribution of toxic and harmful gases in wastewater treatment tanks, in this study, we collected data on confined-space accidents occurring in wastewater treatment tanks in China and analyzed accident types, the substances that caused the accidents and the purpose of entry. We carried out field tests to detect the concentrations of oxygen, hydrogen sulfide, combustible gas and carbon monoxide in 222 wastewater treatment tanks from 28 industrial enterprises and investigated the influence of wastewater treatment tank type, cover type and industry type on gas distribution. Through continuous monitoring, the concentrations of hydrogen sulfide and carbon monoxide in the regulating tanks of two industrial enterprises were monitored for a few days. The mechanism of harmful gas generation and control approaches were explored and analyzed. The results showed that more than 90% of confined-space accidents in wastewater treatment tanks were poisoning accidents, and the levels of harmful gas in wastewater collection tanks, regulating tanks, hydrolysis acidification tanks, sedimentation tanks and sludge tanks were high, qualifying them as high-risk wastewater treatment tanks prone to accidents. Without disturbance, there is basically no harmful gas in wastewater treatment tanks with completely uncovered tops. In addition, the concentration of toxic and hazardous gases in wastewater treatment tanks is not always stable, instead fluctuating greatly with time. The main purposes of this study are to identify the factors affecting the concentration of toxic and harmful gases in wastewater treatment tanks and to assess the risks of using wastewater treatment tanks.
Yang et al. (Tue,) studied this question.