Historically, these waste byproducts have also contained a high concentration of free acid, along with significant levels of heavy metal impurities (e.g., chromium, nickel, lead). While most reputable suppliers now remove much of the heavy metal impurities, acidity levels of 1-6% are not uncommon in many of the products, including NSF and AWWA grades. In one midwestern community which used unrefined waste pickle liquor, the free acid content was nearly 20% w/w.

Hydrogen peroxide (H2O2) is a high-purity chemical manufactured from hydrogen and air. Generally sold as a 35% or 50% aqueous solution, H2O2 has undergone rapid growth as a environmentally-friendly replacement for chlorine in the manufacture of e.g., pulp and paper, textiles, and food products, as well as a range of other uses related to chemical processing, electronics, mining and metallurgy, pollution control, and personal care.

Where are iron salts and H₂O₂ used in wastewater treatment?

The three common uses for iron compounds in municipal wastewater treatment are (in order of occurrence):

1. as a flocculant to enhance solids separation in clarifiers;
2. as an odor and corrosion control agent within collection systems and solids processing units; and
3. as a precipitant for free phosphate removal (during secondary clarification). However, these applications are necessarily distinct -- i.e., "spent" iron (tied up as FeS) will neither enhance clarification nor contribute to phosphate removal.

Hydrogen peroxide is used primarily as an odor and corrosion control agent - within collection systems, at headworks facilities, in odor scrubbers, and in solids processing units. It is also used to control incidents of filamentous bulking.

How do iron salts and H₂O₂ control wastewater odors?

Iron salts control only those odors related to hydrogen sulfide (H2S) - they will not control complex organic odors such as amines, aldehydes or volatile acids. With regard to H2S, the primary mechanism of control involves the conversion of dissolved sulfide to an insoluble (and non-volatile) FeS precipitate. When added to a wastewater which already contains sulfide, ferric salts may also provide some degree of oxidation:

In contrast, H2O2 controls wastewater odors in three ways:

1. direct oxidation of odors - in the case of H2S, to elemental sulfur;
2. bio-mediated oxidation of odors - by providing dissolved oxygen to endogenous bacteria; and
3. by prevention of septic odor formation - by oxygenating the wastewater.

What are the shortcomings of these two mechanisms of control?

Significantly, the precipitation of H2S by iron is an equilibrium reaction, meaning that it is reversible - i.e., increasing a reactant on one side of the equation will drive the reaction to the other side. The desired effect is to drive the equilibrium to the right, increasing the conversion of H2S to FeS by adding free Fe2+. However, because an increase in H+ will have the opposite effect of converting FeS back to H2S, the efficiency of the reaction depends on the pH of the wastewater -- the lower the pH, the less efficient the iron. This dilemma is compounded by two other factors:

1. the high acidity contained in iron solutions; and
2. the increased tendency of sulfide to volatilize as pH decreases:

Taken together, these aspects limit the effectiveness of iron in controlling H2S emissions to low levels. Increasing iron feed rates - the normal response to achieve better H2S control -- decreases the efficiency of the treatment and may actually increase H2S emissions.

In contrast, oxidation with H2O2 irreversibly destroys the H2S. The elemental sulfur end-product is largely inert to subsequent chemical or biochemical action, and will not revert to H2S.

What are the quantities of each chemical needed?

Theoretically, removing 1.0 lbs-Sulfide will require 1.6-1.7 lbs-Fe or 1.0 lbs-H2O2. The actual amount needed will be greater depending on the degree of removal needed, the available reaction time, and the duration of control required. Under optimal dosing conditions for each chemical, the actual requirements are typically 25-50% greater than theoretical.

What are the optimal dosing conditions for each chemical?

Iron salts are best applied upstream of H2S formation, where they can control H2S release for several hours downstream. In contrast, H2O2 is best used to:

1. destroy H2S that has already formed (allowing 5-10 minutes reaction time); or
2. prevent downstream H2S generation for periods of 1-2 hours.

What the conditions to definitely avoid in using each chemical?

Iron salts should not be used where expedient removal of the FeS precipitant cannot be guaranteed. This typically occurs in low velocity collection lines where the precipitate accumulates in low spots and harbors sulfide-producing bacteria. In such cases, adding iron may actually worsen H2S generation. Also, using iron will increase solids loadings within the treatment plant. Controlling 1.0 lbs-Sulfide with iron will typically add 5-10 lbs (dry wt) to the solids stream. Also, iron should also not be used if the underlying organic odors will present problems.

Hydrogen peroxide should not be used either where immediate control of odors is needed (within 1-2 minutes) or where control is needed for > 2-3 hours downstream (e.g., in long duration force mains).

What becomes of the residual or excess chemical added to the wastewater?

The FeS byproduct will typically settle in clarifiers and enter the solids processing stream where it remains "inert" (i.e., it stays as FeS and provides no benefit with regard to subsequent odor control, flocculation, or phosphorus removal). Some carry-over of FeS into secondary treatment operations is likely, however, where the FeS will consume dissolved oxygen in being converted to ferric iron and sulfate ion. This regenerated iron is then available to enhance secondary clarification.

Excess iron (from overdosing) will hydrolyze in the wastewater to form a Fe(OH)x ^. xH2O colloid which may provide ancillary benefit in terms of improved clarification, phosphorus control, or subsequent odor control. However, realizing these side benefits requires careful study; otherwise, the excess iron may merely increase solids processing and disposal costs, and contribute to fouling and corrosion of in-line instrumentation.

Because of their derivation (from metal-bearing wastes) and ultimate disposition (into the biosolids stream), iron salts (and their contaminate residues) may restrict land application rates for the processed biosolids. Before implementing widescale application of iron salts, the resultant increase in heavy metal content in the biosolids should be determined. As a result, more than one municipality has limited iron dosing in its collection system.

In contrast, the reaction products of H2O2 are elemental sulfur and water. The sulfur (1.0 lbs per lb-Sulfide) occurs as a inert colloid which enters solids processing stream either as free sulfur or entrained into cell mass. In either case, its volume is insignificant and it poses no effect on subsequent treatment or disposal practices.

Excess H2O2 (from overdosing) will decompose within 30-45 minutes to yield dissolved oxygen. Two mg/L H2O2 will produce 1 mg/L D.O. This will aid in controlling septicity in downstream collection lines and clarifiers.

How does the cost of iron compare to that of H₂O₂?

Bulk prices for both iron salts and H2O2 are typically $0.50 - 0.60 per lb active ingredient (Fe or H2O2). Under optimal dosing conditions for each chemical therefore, the cost for using H2O2 will be less.

What are the comparative storage and handling issues?

All of the commonly used iron salts are listed as CERCLA hazardous substances, which necessitates strict permitting and spill containment/reporting procedures, along with the associated liabilities for soil remediation. Iron salts are classified as highly corrosive due to their high acidities, which translates into high O&M costs for storage and dosing equipment. Because of the high dose ratios and low solubilities, product use-rates are high (such as 2-3 gallons per lb-Sulfide), which means larger storage requirements and/or greater frequency of product deliveries - both increase the overall risk. Further, certain iron salts (e.g., ferrous or ferric sulfate) may freeze in cold weather, thereby limiting year-round use in some geographic areas.

50% Hydrogen peroxide is classified as a 5.1 Oxidizer, but is non-flammable and not listed as a CERCLA hazardous substance. Since its decomposition products are oxygen and water, H2O2 leaves no residue on soils - spills may simply be diluted with water since H2O2 concentrations of < 8.5% are non-classified. However, as an Oxidizer, storing and handling 50% H2O2 requires specialized equipment and procedures. For this reason, FMC generally supplies and maintains storage/dosing modules for its customers. And since 50% H2O2 freezes at -52 deg-F, outside storage is no problem even in the coldest areas.

Conclusion

The differences between iron salts and hydrogen peroxide as odor control chemicals are many. The environmental compatibility of H2O2 reduces product risks; lessens impact on solids transport, processing and disposal; and typically affords more complete control of odors at a lower cost.

Comparative Summary

Iron Salts: Controls only H2S-related odors. Works by precipitation not destruction. Reaction is pH dependent, becoming less efficient and even reversible under slightly acidic conditions. By-product is black FeS precipitate which presents a range of problems (color, TSS, solids deposition, increased H2S production, increased solids loading, biosolids contamination, etc.) Reaction exerts a high oxygen demand which contributes to septicity and can be detrimental to systems with low dissolved oxygen (DO) capacity. Product is stain-forming and highly corrosive which results in high O&M costs for storage and dosing. Product is on CERCLA list of hazardous materials, with low reportable quantities and high remediation costs for spills. Hydrogen Peroxide: Destroys both H2S and related septic odors - reactions are irreversible. End products are elemental sulfur, water and oxygen - no increased solids production. Excess product decomposes to provide dissolved oxygen downstream. Most cost-effective alternative when properly employed. Volume requirements are relatively low (allowing compact storage and dosing modules) and product is non-corrosive (permitting long-lived use of equipment). Product is not on CERCLA list of hazardous materials - spill remediation consists of dilution with water.

---Technical Data Sheet

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