Odor control within sanitary sewers has been practiced for over 50 years, yet only recently have substantive advances been made. Where once the choice of chemical treatment was either chlorine or iron salts, safer and more environmentally benign technologies based on nitrates, hydrogen peroxide, and/or magnesium hydroxide have gained acceptance. These new alternatives, however, can increase treatment costs substantially and present limitations in themselves. Therefore, it makes sense to explore new avenues for lessening the adverse impacts of the older, cheaper mainstays, particularly iron salts (ferrous/ferric chloride or sulfate) that provide other benefits to wastewater treatment operations.

US Peroxide, Inc. has addressed this need through development of its proprietary technology called PRI-SC^TM (Peroxide Regenerated Iron-Sulfide Control^TM, patent pending).  PRI-SC^TM is a combination treatment that integrates iron salts with hydrogen peroxide (H2O2) in a synergistic fashion. At the most basic level, PRI-SC^TM treatment may be viewed as an oxidant (hydrogen peroxide) regenerating the spent iron salt (FeS) in-situ, yielding fresh ferric (Fe3+) iron and colloidal sulfur.  The combined treatment provides cost benefits superior to either chemical alone, while allowing greater flexibility as to the placement of storage and dosing facilities. For example, practical control of sulfides to very low levels (e.g. < 0.1 mg/L dissolved sulfide) is afforded with minimal reaction time (< 5 minutes). Other benefits are accrued to the reaction, including: reduced solids (FeS) loadings, accelerated reaction rates and enhanced flocculation (in clarifiers). The process has particular benefit when employed for gravity interceptors, force main discharges and headworks treatment.

The conceptual basis of PRI-SC^TM is to use the iron for primary sulfide control, and use the H2O2 to regenerate the iron from FeS. The process can be represented as a regenerative cycle (below), where a working inventory of iron is maintained with H2S as the input, elemental sulfur as the output, and H2O2 as the driver.

The process occurs in three steps: 1) iron complexation with dissolved sulfide; 2) H2O2 oxidation of the FeS complex to provide elemental sulfur and free iron (hydrous ferric iron); and 3) oxidation of additional sulfide by the ferric iron to produce elemental sulfur and FeS. The net reaction requires 0.67 lbs Fe (or 1.45 lbs FeCl2) and 0.6 lbs H2O2 per lb-Sulfide. This yields a theoretical control cost of about $0.50-0.75 per lb-Sulfide, which is less than one-half that of alternative nitrate or magnesium hydroxide treatments.

Step 1:      2H₂S + 2FeCl₂ ---> 2FeS + 4HCl
Step 2:      2FeS + 3H₂O₂   --->  2S₀  + 2Fe(OH)₃
Step 3:      2Fe(OH)₃ +  3H₂S  ----> S₀  +  2FeS  +  6H₂O
Net:          5H₂S + 2FeCl₂ + 3 H₂O₂   --->  3S₀  +  2FeS  +  4HCl  +  6H₂O

PRI-SC^TM in Practice

In practice, PRI-SC^TM involves adding an iron salt at the upper reaches of the collection system and using hydrogen peroxide to regenerate the "spent" iron (FeS) at one or more points downstream (see below). The analogy is one of a capturing agent (iron salt) added at the top of an interceptor, which then absorbs dissolved sulfide as it moves down the line. Intermittent hydrogen peroxide injection serves to regenerate the capturing agent thereby allowing additional sulfide to be absorbed downstream. Incorporating a final regeneration site at the treatment plant provides rapid oxidation of sulfides to very low levels, effectively reducing the H2S loading to the influent scrubbers, and hydrous ferric iron that can be used for additional odor control in primary treatment or for enhanced clarification.

 

Practical Benefits of PRI-SC^TM

While the theoretical cost of PRI-SC^TM is similar to that for either iron or H2O2 (in the oxidation mode), there are practical benefits that lower the actual costs. Among these is the reduction (or elimination) of the disadvantages of using only iron e.g., solids production is reduced by > 40%, salinity levels by > 60% and acidity levels by > 60%. PRI-SC^TM can also be engineered in such a way that actual costs are closer to theoretical costs. For example, where iron is being used to control sulfides for > 4 hours duration (thereby increasing iron requirements by 2-4 fold), the feed rates can be reduced to control for < 4 hours duration. H2O2 can be injected prior to the discharge to oxidize the remaining sulfide, affording residual sulfide levels of < 0.1 mg/L within 1-2 minutes. This is something that is not afforded by either chemical alone, even at 2-4 fold excess doses. Further, if this discharge were into the headworks of a treatment plant, the iron would enter the plant as free (hydrous) ferric oxide, which enhances odor control and settling through the primary clarifiers. Residual free iron in the primary solids would also enhance control of H2S and/or struvite in the digesters. Thus, the combination treatment accentuates the advantages of H2O2 and iron while minimizing or eliminating their respective disadvantages.

 

Full-Service Program

US Peroxide, Inc. offers PRI-SC^TM as a full-service program that includes comprehensive applications development, integrated storage and dosing systems, complete chemical inventory management and on-going system maintenance and performance monitoring.  PRI-SC^TM is ideally suited for large municipalities with gravity interceptors and/or force main systems that are characterized by high sulfide loading rates and atmospheric H2S problems.  Our turn-key program allows the municipality to implement PRI-SC^TM on a limited resource budget, making the solution to a complex problem a lot simpler.

PRI-SC^TM is a proprietary technology from US Peroxide, Inc., patent pending

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