Iron Salts (Ferric and Ferrous)
Iron Salt Applications
Iron salts are a proven technology for long-duration hydrogen sulfide control in collection system gravity and forcemains, solids processing units, solids transfer lines and anaerobic digesters. They have been used for over 30 years in hydrogen sulfide control applications and are a well understood technology.
Depending on the wastewater plant unit process configuration, iron salts may also provide improvements in clarification, phosphate removal, struvite control, solids dewatering and anaerobic digester performance. Iron salt performance is not impacted by oxygen uptake rates but they do remove dissolved oxygen from the water.
Iron Salt Properties and Dosing
Iron salts are supplied as liquid solutions containing 5-13% ferrous or ferric iron as either a chloride or sulfate salt. They are supplied in containers of 55 or 300 gallons, or in bulk shipments of 4,000 - 20,000 gallons.
Iron salts bind with hydrogen sulfide according to the following equations:
3 H2S + 2 FeCl3 → S + 2 FeS + 6 HCl
Theoretical dose ratios of ferrous iron to H2S are 1.75 lbs Fe2+:lb H2S with practical dose rates anywhere from 2 to 6 depending on the application. Theoretical dose ratios of ferric iron to H2S are 1.2 lbs Fe3+:lb H2S with practical dose rates anywhere from 1.5 to 4 depending on the application.
Iron Salt Pricing and Sources
Iron salt pricing is highly regional and dependent on the local sources and demand for the product. Iron salts for sulfide control are mostly obtained from spent pickle liquor from the steel industry. More recently, newer sources have been manufactured from scrap materials and acid in order to meet regional demand.
Iron Salt Drawbacks
Some drawbacks of iron salt use are as follows:
- Removes dissolved oxygen from the water
- Precipitate settles out in low-velocity sewers (< 2 fps)
- Iron films form on pipe walls and instrument sensors
- Ineffective for (non-sulfide) organic odors
- Hard to achieve low sulfide limits (pH dependent)
- Does not destroy sulfide (H2S may volatilize if the pH is lowered)
- Product purity may impact biosolids re-use (heavy metal contamination)
- Messy to handle
- CERCLA rating may restrict dosing sites (persistent environmental hazard)
- High dosages may cause solids carry-over from clarifiers
- Solids production (> 3 lbs/lb - Sulfide) increases processing and disposal costs
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