Norwalk virus, a human calicivirus of the norovirus genus, is an important cause

Question

Norwalk virus, a human calicivirus of the norovirus genus, is an important cause of epidemic gastroenteritis, which is commonly transmitted by water. At pH 6.0 and a contact time of 1.0 min, a total chlorine concentration (CT) of 5 mg/L is required to achieve 99.99% inactivation of Norwalk virus in water. Note: CT = [HOCl] + [OCl—].

(a) If HOCl is the active disinfectant for Norwalk virus (i.e., effect of OCl— can be neglected) what is the total chlorine concentration (CT, in mg/L) required for 99.99% inactivation of the virus at pH 7.0? The pKa of HOCl is 7.5.

(b) If the rate of virus inactivation is (pseudo-) first-order with respect to virus concentration when CT is kept constant at 5 mg/L, what is the virus inactivation rate constant (in min^-1 ) at pH 6.0?

(c) You want to inactivate the virus with chlorine using the conditions in (b), in a continuous, steady-state CSTR system. What would be the difference in total reactor volume if you use 3 identical CSTRs in series, instead of 1 single CSTR, to obtain the same (99.99%) efficiency?

Explanation / Answer

a. pH = pKa + log10 concentration of chlorine/ concentration of acid

6-7.5 = log10 concentration of chlorine/ concentration of acid = -1.5; so, concentration of chlorine/ concentration of acid =0.031

Now, at pH =7

7 - 7.5 = log10 concentration of chlorine/ concentration of acid = -0.5

Concentration of chlorine/ concentration of acid = 0.31; so, concentration of chlorine/concentration of acid should be reduced 10 times to achieve 99.9% activation. So, now chlorine concentration should become 0.5mg/L

Total chlorine concentration = concentration of added acid * 0.31

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