| dc.description.abstract | The wastewater treatment system, installed at a 6th generation of TFT-LCD manufacturing plant, was designed to meet the goal of reaching 90 % process water recovery and 85 % total water recovery. The high percentage of water recovery resulted in losing the dilution efficiency, thus, the effluent phosphorous concentration was much higher compared with the effluent qualities discharged by similar industry.
According to the investigation of phosphorous distribution in various sources of this case plant, it found that there are two major sources of phosphate containing wastewater. One is directly discharge from cleaning process of Al-etching. The other is RO reject (ROR) in which the RO system followed on the anoxic/aerobic coupled with membrane filtration (A/O MBR) process for treating the mixture of raw wastewater from cleaning process of Al-etching and low organic concentration wastewater from other manufacturing processes.
This study intent to investigate the optimum operation conditions for removing the phosphate from the above two sources of wastewater by chemical coagulation jar test using calcium chloride, aluminum chloride, and ferrous chloride as coagulant, respectively. The experimental results revealed that the optimum operating conditions were: (1) pH value was 8 and Ca/P dosage ratio was 2 for calcium chloride; (2) pH value was 5 and Al/P dosage ratio was 1.4 for aluminum chloride; and (3) pH value was 6 and Fe/P dosage ratio was 1.8 for ferrous chloride. The optimum results obtained in this study were fully agreeing with the past studies for aluminum chloride and ferrous chloride as coagulant. However, the phosphate removal efficiency was decreased as the pH value was greater than 9 for calcium chloride as coagulant in this study. This result was completely disagreeing with using lime as common coagulant which the best phosphate removal efficiency generally occurs at pH was above 10. Based on the comparative evaluation of chemical cost and operating condition for the three coagulants, it suggests that calcium chloride is the proper coagulant for removing phosphate in the case plant.
Comparing the chemical cost and waste sludge treatment cost for treating these two phosphate containing wastewater streams, there are insignificant differences for installing chemical coagulation process at either site of wastewater source. However, the wastewater from cleaning process of Al-etching after pretreatment by chemical coagulation, its water qualities may cause clogging of the RO membrane and decreasing the benefits for water recycle and reuse. Meanwhile, the construction cost of chemical coagulation process for phosphate removal from Al-etching wastewater was high. Consequently, the better location for phosphate removal by chemical coagulation process in this case plant should focus on ROR stream.
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