Distribution Characteristics And Volatilization Rules Of Volatile Organic Chemicals From WWTP

A variety of volatile organic compounds (VOCs) would volatilize from WWTP during the wastewater treatment process, which cause severe adverse impact on air quality and staff health. In order to research the distribution characteristics and volatilization rules of VOCs, air and water samples collected from WWTP in Harbin were given a qualitative and quantitative analysis. The BTEX (benzene, toluene, xylene) and chlorinated hydrocarbon (chloroform, carbon tetrachloride, trichloroethylene, tetrachloroethylene) were set as research objects. Their change rules of concentrations with time and space were researched. The different mathematical model were applied to calculate volatile rates of BTEX and chlorinated hydrocarbon in screen room, primary clarifier, biochemical treatment unit and secondary clarifier. Meanwhile, their distribution variations in three-phases in aeration tank during wastewater treatment process were also investigated, what's more, the risk assessment on the BTEX volatilizing from primary clarifier was made.Results of distribution characteristics of VOCs revealed that the low molecular weight hydrocarbon, BTEX and chlorinated hydrocarbons were the main components of VOCs volatilizing from WWTP. Gaseous concentrations of VOCs were highest in summer, while their liquid concentrations were highest in winter. The tetrachloroethylene was not detected in gaseous sampings. Benzene, toluene, xylene, chloroform and carbon tetrachloride had the highest gaseous concentrations in primary clarifier, whose values were 80, 99, 172, 23.4 and 11.2μg/m3. The concentration of trichloroethylene reached peak in secondary clarifier at 0.53μg/m3. Liquid concentrations of BTEX and chlorinated hydrocarbons were positively correlated with UV254. The decrease of UV254 value in anaerobic suggested that liquid concentrations of BTEX and chlorinated hydrocarbon were decreased. BTEX belonged to easy-biodegraded substances, and they were quikly degradated in biochemical tank. Chlorinated hydrocarbon belonged to hard-biodegraded substances, and their increasing liquid concentrations in secondary clarifier showed that some absorbed by microbe could come into liquid phase. The staff who worked in WWTP long term would have a slight carcinogenic risk. The volatilization rules of VOCs showed that the volatile rates of xylene and chloroform in screen room were lowest in researched units, which were 12.94×10⁾-6)¹7.24×10^-6g/s and 1.88×10^-62.58×10^-6g/s. In primary and secondary clarifier, the volatile rates in spring were 1.7 times than those of summer. Compared to primary clarifier, the volatile rates of benzene, toluene and xylene in secondary clarifier decreased 40.8% , 46.7% and 95.5%. However, the average volatile rates of chlorinated hydrocarbon average increased 13%. The volatile rates of BTEX and chlorinated hydrocarbon in biochemical treatment unit were the highest in WWTP, which were highest in winter and lowest in summer. The volatile rates of benzene and trichloroethylene were 1.92⁵.22g/s and 16.94¹8.8g/s, respectively. The volatile rates of BTEX in biochemical tank exceeded the <Comprehensive Discharge Standard of Atmospheric Pollutants> (GB16297-1996). During the period of anaerobic section in biochemical tank, owing to the degradation of organic, the total mass of benzene, toluene, xylene, chloroform carbon tetrachloride, trichloroethylene and tetrachloroethylene were increased 20.9%, 20.3%, 24.2%, 26.2%, 12.0%, 11.0% and 7.4%, and at the same time, the 38.7% of benzene, 43.6% of toluene, 38.0% of xylene, 28.8% of chloroform, 24.3% of carbon tetrachloride, 15.3% of trichloroethylene and 20.5% of tetrachloroethylene existed as solid phase. In the hydraulic retention time of biochemical tank, nearly all of BTEX were degradated. What were opposite to BTEX, the average removal efficiency of chlorinated hydrocarbons was only about 40%. At the end of secondary sediment process, the distribution ratios of chloroform, carbon tetrachloride, trichloroethylene, tetrachloroethylene in solid phase were 5.2%, 20.1%, 6.8%, 0%, respectively.The results of VOCs removal efficiency owing aeration of the study showed that during the aeration process, 30.6% of TOC volatilized and didn't be degradated by microbe. They only transferred from liquid phase to gaseous phase. The volatilization ratio of BTEX reached 100%, and the ratio of chloroform, carbon tetrachloride, trichloroethylene and tetrachloroethylene were 27.5%, 39.0%, 42.4% and 38.5% respectively.