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A municipal water treatment plant is designed to produce 20million gallons per day (MGD) of potable water. The plant treatssurface water using coagulation followed by sedimentation,filtration, and chlorination. Although the water is hard, it is notpresently softened. The water is coagulated using a 30 mg/L dosageof alum (aluminum sulfate, Al2(SO4)3•14.3H2O, MW = 600). Thischemical reacts in water to form Al(OH)3 as follows:

Al2(SO4)3 + 6HCO3- → 2Al(OH)3(s) + 3SO4-2 + 6CO2

How many mg/L of bicarbonate (alkalinity) will be \"consumed\"(converted to CO2) when the

alumisadded? (AWs: Al=27,S=32,C=12,0=16,H=1,Ca=40)

Estimate the pH of the water following coagulant addition.Assume an initial bicarbonate concentration of 244 mg/L and waterinitially in equilibrium with the atmosphere (i.e., H2CO3* = 10-5M). (Hints: assume negligible loss of CO2 to the atmosphere fromthe deep basins used for treatment; adjust the concentrations ofHCO3- and H2CO3*, based on your answer to Problem 1; then use theexpression for K1 to solve for [H+] following coagulant addition.Assume [OH-] and [CO3-2] are negligible.)

How many mg/L of hydrated lime, Ca(OH)2, would be needed to keepthe pH constant when the alum is added? Hint: the neutralizationreaction can be represented as follows:

Al2(SO4)3 + 3Ca(OH)2 → 2Al(OH)3 + 3Ca+2 + 3SO4-2