Pretreatment of petrochemical wastewater by coagulation and flocculation and the sludge characteristics.

In the present study, coagulation-flocculation was investigated as a pretreatment process for the treatment of purified terephthalic acid (PTA) wastewater. The effect of various inorganic and organic coagulants on the treatment of wastewater collected from flow equalization tank of an effluent treatment plant was studied. The settling and filtration characteristics of the sludge were also studied. The jar tests revealed that the wastewater was best treated when 3000 mg l(-1) of ferric chloride was dosed at pH 5.6. At optimum conditions, COD of the wastewater was reduced by 75.5%. The results of the gravity filtration of the treated wastewater showed that the addition of cationic polyacrylamide (175 mg l(-1)) to ferric chloride coagulation improved the filtration characteristics and reduced the specific cake resistance. Scanning electron microscopy and energy dispersive spectroscopy studies were also conducted to know the sludge structure and composition, respectively. Thermal analysis of the sludge showed that the oxidation of the sludges in the present study is a three step process. However, with the addition of C-PAA to ferric chloride coagulation system, the oxidation was found to be a two step process.