1206 URBAN RUNOFF
TABLE 38
Cost data on chlorine gas and hypochlorite disinfection (ENR = 5,000)Location, agent, source Capital cost ($) Operating cost ($/yr) Cost/lb available chlorine ($)
Akron, OH*
Sodium hypochlorite Purchased 1,104,000 58,200 0.37–0.67
Cambridge and Somerville, MA
Sodium hypochlorite Purchased — — 0.97
On-site generation — — 0.51
New Orleans, LA†
Sodium hypochlorite
On-site generation 1,448,000 724,000 0.30
Saginaw, MI‡
Chlorine gas 406,000 5,800 0.87
Sodium hypochlorite
Purchased 49,000 16,000–30,000 0.45–0.78
On-site generation 239,000–403,000 11,700–13,000 0.69–1.0
South Essex Sewage District, MA§
Chlorine gas 1,667,000 582,000 0.09
Sodium hypochlorite
Purchased 1,054,000 913,000 0.12
On site generation Seawater 4,200,000 406,000 0.09
Brine 4,200,000 758,000 0.13*^ CSO disinfection.
† Storm sewer discharge disinfection.
‡ CSO disinfection at use rate of 42,000 lb/yr of chlorine.
§ Sewage treatment plant effluent disinfection at use rate of 24,000 lb/day of chlorine.TABLE 39
Comparison of estimated capital costs for three different disinfection methods (ENR = 5,000)Disinfection method Capital cost ($/mgd)
2-min ozone contact (chamber with once-through oxygen-fed ozone generator) 32,530
2-min chlorine contact (chamber with hypochlorite feeder) 3,800
5–10-min conventional chlorine contact 4,230or by specially designed plug flow contact chambers con-
taining closely spaced, corrugated parallel baffles that create
a meandering path for the wastewater.
High-rate disinfection was shown to be enhanced beyond
the expected additive effect by sequential addition of Cl 2 , fol-
lowed by ClO 2 at intervals of 15–30s (EPA-670–2-75–021;
EPA-600/2-76-244). A minimum effective combination of 8
mg/l of Cl 2 followed by 2 mg/l of ClO 2 was found as effective
in reducing total and fecal coliforms, fecal streptococci, and
viruses to acceptable target levels as adding 25 mg/l Cl 2 or
12 mg/l ClO 2 individually. It was surmised that the presence
of free Cl 2 in solution with chlorite ions (ClO- 2 [the reduced
state of ClO 2 ]) may cause the oxidation of ClO
2 back to its
original state. This process would prolong the existence of
ClO 2 , the more potent disinfectant.
Ozone has a more rapid disinfecting rate than chlorine
and also has the further advantage of supplying additional
oxygen to the wastewater. The increased disinfecting rate of
ozone requires shorter contact times and results in a lower
capital cost for a contactor, as compared to that for a chlorine
contact tank. Ozone does not produce chlorinated hydro-
carbons or a long-lasting residual as chlorine does, but it is
unstable and must be generated on-site just prior to applica-
tion. Thus, unlike chlorine, no storage is required. In tests on
CSO in Philadelphia (see “microstraining and disinfection”
reports listed in the references at the end of this section),C021_002_r03.indd 1206C021_002_r03.indd 1206 11/23/2005 9:45:49 AM11/23/2005 9: