Nitrate Applications for Hydrogen Sulfide Control
Nitrates are a proven technology for medium to long-duration hydrogen sulfide control in collection system gravity and force mains. They have been used for over 15 years in hydrogen sulfide odor control applications and are a well understood technology. Nitrates can be used in certain applications where hydrogen sulfide odor is already present (curative) or to prevent the formation of hydrogen sulfide (preventative) odor.
Nitrates react mainly in the water column providing electrons for oxidation. In the curative application mode, a minimum retention time of 2-3 hours is required in order to be effective. In the preventative mode, nitrates condition the biofilm for anoxic process, preventing sulfate from being reduced to hydrogen sulfide in anaerobic sewer conditions.
A minimum retention time of 3 hours is necessary in order to be effective. When applied properly, control to low hydrogen sulfide levels is possible. Bioxide may provide some in-line BOD reduction in the collection system. Most calcium and sodium nitrate formulations are considered a non-hazardous material and can be stored in simple feed systems.
Nitrate Properties and Dosing
Nitrate is available as either in either granular or liquid form, and either as a calcium or sodium salt. The granular product comes in 100-lb bags, 2000-lb supersacks, or bulk hopper cars. The liquid product is the more commonly used form, and is available in bulk shipments of 2,000 - 4,000 gallons. Typical calcium nitrate formulations contain 4.3 lbs NO3 per gallon of solution. Typical sodium nitrate formulations contain 3 lbs NO3 per gallon.
Nitrates prevent or remove hydrogen sulfide according to the following equations:
Curative (Biomediated Oxidation):
|5 H2S + 8 NO3 → 5 SO42- + 4 N2 + 4 H2O + 2 H+|
|8/5 NO32 + [2 O2] (approximate representation)|
Theoretical dose ratios in the preventative mode of nitrate to H2S are 2.6 lbs NO3:lb H2S. Practical preventative dose rates for calcium nitrate vary anywhere from 2 to 8 gallons per pound H2S depending on the application. Theoretical dose ratios in the curative mode of nitrate to H2S are 0.7 lbs NO3:lb H2S. Practical curative dose rates for calcium nitrate vary anywhere from 1 to 2 gallons per pound H2S depending on the application.
Nitrate Pricing and Sources
The most common form of nitrate on the market for hydrogen sulfide control is liquid calcium nitrate solution. Typical pricing for calcium nitrate solutions is $2.00 to $2.50 per gallon. In the preventative mode, the cost per pound of sulfide treated is anywhere from $4.00 to $20.00. In the curative mode, the cost per pound of sulfide treated is anywhere from $2.00 to $5.00. Nitrates for sulfide control are mostly obtained from fertilizer manufacturers.
Some drawbacks of nitrate use are as follows:
- Inefficient for gravity lines
- Increases nitrogen levels in the wastewater
- Resultant N2 gas (or residual NO3) may present problems downstream at the wastewater plant
- Costs for prevention mode may be excessive in long retention time lines
- Biomediated oxidation mode may require several hours, making nitrates less effective in certain applications (e.g. manifolded force mains)
- Costs may be high in applications with high oxygen uptake rates (higher temperature, etc...)
- Costs for prevention mode impacted by high BOD levels
Note: The use of nitrates for the removal of odors present at the point of addition (biomediated oxidation mode) may be covered under US Patent.
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