Alkalis and Hydrogen Sulfide
Alkalis are an alternative technology for treatment of individual collection system gravity and forcemains segments with high hydrogen sulfide loading (>10 mg/L), odor scrubbers, ponds and lagoons and solids processing. They have been used in one form or another for over 20 years in hydrogen sulfide control applications and are a well understood technology.
Alkalis raise the pH of the wastewater solubilizing hydrogen sulfide so it cannot volatilize into the atmosphere:
- At pH 7, approximately 50% of hydrogen sulfide is in the non-volatile form.
- At pH 8, approximately 75% of the hydrogen sulfide is in the non-volatile form.
- At pH 9, approximately 95% of the hydrogen sulfide is in the non-volatile form.
The elevation of pH may also inhibit biogrowth, reducing odor generation. Alkalis are fast reacting and are unaffected by hydrogen sulfide levels and oxygen uptake rates. Alkalis provide residual alkalinity that may enhance downstream biotreatment processes. Magnesium hydroxide is a non-hazardous chemical that buffers the pH (typically to pH 8.5 -9), providing some advantages over sodium hydroxide for collection system application.
Alkali Properties and Dosing
The two most commonly used alkalis (magnesium hydroxide - Mg(OH)2 and sodium hydroxide - NaOH) are supplied as a slurry and solution, respectively. Magnesium hydroxide is typically provided in bulk truckloads. Sodium hydroxide is supplied in containers of 55 or 300 gallons, or in bulk shipments of 4,000 - 20,000 gallons.
Alkalis prevent hydrogen sulfide volatilization according to the following equation:
|H2S (volatile) + OH- → HS- (non-volatile) + H2O |
In order to raise the pH to 8, 50 mg/L OH- is required. In order to raise the pH to 9, 150 mg/L OH- is required.
Alkali Pricing and Sources
The most common form of alkalis on the market for hydrogen sulfide control is magnesium hydroxide slurry. Typical pricing for magnesium hydroxide slurry is $2.50 to $3.00 per gallon. Typical dose rates of magnesium hydroxide are between 75-140 gal MgO slurry per million gallons of wastewater. The cost per pound of sulfide non-volatilized is dependent on the loading in the individual segment treated.
Some drawbacks of alkalis use are as follows:
- Limited applicability (typically not cost effective for hydrogen sulfide levels < 10 mg/L)
- Non-volatilization of hydrogen sulfide to low levels may be cost prohibitive
- Not effective for low atmospheric hydrogen sulfide targets, especially in turbulent zones
- Does not destroy hydrogen sulfide - once the treated stream is mixed with other lower pH streams the wastewater is neutralized and hydrogen sulfide is released
- May change or worsen odors
- The use of alkalis in the slurry form may build deposits over time when applied in wet wells and slower moving gravity and forcemains
- Alkali slurries must be constantly mixed in order to avoid hardening in the storage tank (may require back-up power at remote sites to insure continuous mixing)
Note: The use of magnesium hydroxide for odor control may be covered under US Patent.
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