Optimization of Mix-chemical Coagulants in Water Purification Technology

Abstract

Coagulation and flocculation followed by clarification are the most used and important process in the purification of turbid water containing colloidal particles. Salts of aluminum and iron are the most commonly used chemical coagulants in water treatment. The study was aimed at optimizing mix-chemical coagulants in water purification technology. An experimental comparative study where controlled factors were evaluated under various experimental set up. Jar tests were conducted to assess the efficiency of alum and ferric chloride coagulants in dual (1:1 and 3:1 alum to ferric chloride) combinations as well as separately. The highest percentage turbidity removal performance of alum, ferric chloride, 1:1 and 3:1 alum-ferric chloride combination were 98.7%, 99.1%, 98.7% and 97.8%, respectively. The highest percentage COD removal performance of 71%, 58.1%, 63.6%, and 50.9% were demonstrated for alum, ferric chloride, 1:1 and 3:1 alum-ferric chloride combination, respectively. And the highest percentage TDS removal performance of 55.8%, 72.6%, 81.4% and 81.4% were exhibited for alum, ferric chloride, 1:1 and 3:1 alum-ferric chloride combination. The ferric chloride coagulant demonstrated highest (94.8%) average varying initial turbidity removal. Whereas, 1:1 alum-ferric chloride coagulant combination shows highest (80.8%) concurrent TDS, turbidity and COD average removal. The coagulation experiments indicated that coagulation process effectively removed turbidity from water using 15 to 25 mg/L dosage at 7 to 8 pH range of raw water for the applied coagulants. The selected optimal doses were more effective for medium (150 NTU – 300 NTU) initial turbidity. The study demonstrated strong positive correlation (r = 0.93) between coagulant dose and residual aluminum ion concentration which was statistically significant (P < 0.05). Also strong negative correlation (r = -0.97) was observed between the water pH and residual aluminum ion concentration which was similarly statistically significant (P < 0.05). The residual aluminum and ferric ion concentration were greatly reduced when alum and ferric chloride coagulants were used in combinations than used separately. The regular monitoring of residuals in water when using chemical coagulant should be practiced as well as control of coagulant dosage and water pH should be considered in water treatment plants.