2,4,6-trichlorophenol is widely used as insecticides, herbicides and fungicides as well as preservatives for wood, glue, paint, vegetable fibers and leather. It is included in the effluent from pulp mills and contaminate soils around wood-preserving industries and groundwater environment due to surface runoff or direct industrial waste discharges. PVA (polyvinyl alcohol) and HEC (hydroxyethyl cellulose) have been used in the chemical industry, especially in the expandable polystyrene manufacturing process. They remained in the environment for a longtime and could not be effectively destroyed by conventional wastewater treatment processes. Molecular weights of PVA and HEC are more than 25,000 amu.
The objectives of this research are as followed. First, it is to determine the parameters and performance of anoxic-oxic process used for complete degradation of persistent organic compounds, esp. PAV and HEC in petrochemical wastewater and 2,4,6-TCP. Second one is to suggest the method for determining biodegradability. The third is to evaluate the effects of ozonation on biodegradability. Last one is to define the toxic effect of 2,4,6-TCP under anoxic condition and find out the parameters of inhibition model.
In the study of biodegradation using anoxic-oxic process, 2,4,6-TCP was completely removed at recycle rate of 1.5Q and HRT of 18 hours. However, the removal efficiency of 2,4,6-TCP decreased as recycle rates increased. The wastewater from expandable polystyrene (EPS) process was treated using anoxic-oxic process. PVA was completely removed and the removal efficiency of COD and HEC reached to 83% and 89.4% at recycle rate of 2.5Q, HRT of 30 hours and organic loading rate of 0.9 g COD/L·day. When ozonation was applied to petrochemical wastewater, post-ozonation was more effective than pre-ozonation. The removal efficiencies of COD and HEC increased to 92% and 93% at ozone feed rate of 0.18 mmole/min.
Biodegradability was estimated using methane potential, $BOD_5$/COD ratio and toxicity. Biodegradability index (B.I.) was suggested for the estimation of biodegradability of persistent organic compounds. B.I. means the distance from the origin in the three-dimensional space with the axis of methane potential, $BOD_5$/COD and toxicity.
PVA was effectively removed by ozonation in 20 min. The removal rate of PVA increased by increasing pH. However, HEC did not removed at pH 7. The removal rate of HEC could increase by increasing pH. COD removal during ozonation of PVA and HEC was very low. Molecular weight distribution tests were conducted for finding out the reason. It was concluded that ozonation of PVA and HEC was only the breakdown of molecules not the mineralization. 2,4,6-TCP was also removed by ozonation. As pH increased, the removal efficiency increased. From these results, it could be concluded that ozonation was followed by indirect pathway.
2,4,6-TCP could be degraded under anoxic condition such as denitrifying and sulfate reducing conditions. 0.18 mM $\approx$35mg/L) of 2,4,6-TCP was completely removed under anoxic condition in 24 hours. Intermediates such as 2,4-DCP, 4-CP and phenol were not detected during the biodegradation of 2,4,6-TCP. It was thought that the degradation pathway under anoxic condition was different from the reductive dechlorination under anaerobic condition. Inhibition of 2,4,6-TCP followed Haldane inhibition model of which inhibition coefficients ($K_I$) under denitrifying and sulfate reducing conditions were 21.4 and 0.93 mg/L, respectively.'
