Due to the infrequent nature of combined sewage overflows, a combined sewage disinfection facility with the lowest life cycle cost is one that minimizes the cost of the contact tank, at the expense of incurring a higher cost for more mixing power and higher disinfectant dose. The
concept to minimize the contact tank size includes introducing a relatively high concentration of disinfectant at the influent of the swirl concentrator with powerful mixers. To support this concept, a bench-scale disinfection pilot was completed to test the effectiveness of peractic acid
(PAA) and sodium hypochlorite at high doses and short contact times. The effluent goal for the disinfection pilot was a fecal coliform density of 200 counts/100 mL. Results indicate the biocide effect of PAA against fecal coliforms was similar to that of sodium hypochlorite. These data show
that significant reductions in fecal coliform (4-log) can be achieved at short contact times (one and two minutes) with high doses (30 mg/L) of disinfectant. The benefit of this approach is to eliminate or significantly reduce the size of the disinfection contact tank, thereby reducing
the life cycle cost of a combined sewage disinfection facility.
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