Example 2. Chemical phosphorus removal and/or CEPT using hydrogen peroxide regenerated iron
When applied to plant influents, hydrogen peroxide destroys dissolved and/or total sulfide that otherwise scavenges Fe3+ added for CEPT and/or chemical phosphorus removal. The foundational aspect of this PRI-TECH application is that one lb of total sulfide theoretically consumes 3.5 lbs Fe3+ and thus represents an additional cost of $2 - 3 per lb influent sulfide.
2 HS- + 2 FeCl3 → So + FeS + Fe2+ + 2 H+ + 6 Cl-
FeS + 2 FeCl3 → So 3 Fe2+ + 6 Cl-
On the other hand, pre-oxidizing one lb of influent sulfide with hydrogen peroxide (as commonly done for headworks odor control) typically requires 1.2 - 1.5 lbs hydrogen peroxide and so represents a cost of $0.75 - 1.25 per lb influent sulfide. For a 10 MGD plant with 3.0 mg/L influent sulfide, this difference could produce savings of around $150,000 per yr, with the unavoidable side-benefit that odor control is improved (i.e., less dissolved sulfide in the clarifier influent/effluent).
An important secondary benefit of adding Fe to primary clarification is that the Fe captured into the settled solids goes to anaerobic digesters where it can control biogas H2S and struvite scaling. Consequently, it is important that the PRI technology not deprive the digesters of this needed Fe. Key in this regard is the form in which the Fe is delivered to the digesters, and in particular, the ‘availability' of that Fe to provide benefit. The diagram below illustrates the partitioning of Fe through primary clarification under two scenarios: if influent sulfide present and allowed to consume Fe3+; and if influent sulfide is pre-oxidized by hydrogen peroxide (before Fe3+ addition).
An important and under-appreciated aspect in these figures is that Fe flowing over the primary weirs is incorporated into the secondary biomass. Because most of this biomass passes through anaerobic digestion, that Fe is less available to control biogas H2S levels and struvite scaling. The effect of PRI-TECH™ application is thus to increase the capture of Fe into the primary settled solids, and in a form that's available to provide digester benefits.
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