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.
