OXIDATIVE REGENERATION OF IRON FOR TREATMENT PLANT PURPOSES
The Orange County Sanitation Districts (OCSD) currently utilize PRI-SC™ technology to control hydrogen sulfide emissions within its collection system. With PRI-SC™, ferrous chloride (FeCl2) is added for sulfide control and then the combined iron is regenerated
using hydrogen peroxide (H2O2). OCSD also adds ferric chloride (FeCl3) at its treatment plants to enhance solids separation (CEPT). We previously reported on the improved sulfide control efficiencies in the collection system using PRI-SC™; however, little
data were available at the time on regenerating that iron a second time (at the treatment plant) to benefit CEPT. This paper discusses the economic and performance benefits of doing so using hydrogen peroxide and bleach at OCSD Plant No. 2. The study involved beaker tests, jar tests and field
trials, as well as an analysis of the historical plant records.
The results show that maximizing the overall cost performance of all relevant chemical additions at the plant (H2O2, FeCl3, bleach, polymer) requires that the influent sulfide be removed and
the ferrous portion of the influent iron (from PRI-SC™) be oxidized to ferric iron. Significantly, this oxidized ferric iron was shown to have flocculation performance similar to that of FeCl3. It was further shown that (generally) H2O2 is the more cost-effective
oxidant for both sulfide and ferrous iron oxidations. However, since the plant already adds an excess of bleach to the primaries for disinfection, we found it to be practically more efficient to allow the bleach to oxidize the ferrous iron. Thus, the recommendation of this work was to remove
residual levels of sulfide from the influent sewers using H2O2; and to allow the ferrous iron to be oxidized later by the bleach disinfectant. Conversion of ferrous iron to ferric iron was essentially 100%, resulting in a contribution to Plant No. 2 of ca. 7,000 lbs-Fe3+
per day. The cost for additional bleach to oxidize this Fe2+ was ca. $2000, yielding an effective cost of ca. $0.30 per lb-Fe3+. This represents a 40% savings over adding additional FeCl3.
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