A local municipal drinking water plants utilizes a plug flow
reactor to disinfect water prior to distribution. It has been
determined that if the process can achieve a 4-log reduction of
Cryptosporidium under normal conditions, they will never be in
violation of water quality standards for fecal coliforms. The
operators are Alabama grads who hate chemistry and have hired a
contractor (you) to help them determine what chlorine dose they
should use in order to handle the pH variability in their normal
operations. Chlorine is used as a disinfectant and is added in the
form of sodium hypochlorite (NaOCl), which dissociates completely
to Na⁺ and OCl^–. The form of chlorine is then
determined according to the known equilibrium reaction: HOCl ↔
H⁺ + OCl^– with K_a =
10^-7.54. The relevant parameters for the system are the
following:

The volume is 425 m³

The flow rate is 0.35 m³/s

HOCl has a coefficient of specific lethality of 0.209
L/(mg*min)

OCl^– has a coefficient of specific (k) lethality of
0.0158 L/(mg*min)

1. If the pH of the treated water is 4.8, what will the ratio of
HOCl to OCl^– be?

2. What total chlorine dose (in mg/L) will be required to
achieve the 4-log reduction of Cryptosporidium at pH 4.8? (You may
neglect one form of chlorine if it is less than 1 % of the
total)

3. What mass of NaOCl must be added every day to meet this
dosage requirement calculated in part (b)?