7.3 Odor Control & Wastewater Chemical Addition

7.3 Odor Control & Wastewater Chemical Addition

Odor complaints are one of the fastest ways for a wastewater utility to lose public trust, and the same septic conditions that cause odor also corrode concrete and endanger workers (Sections 9.1 and 12.1). The WPI Wastewater Treatment outlines list odor control devices (biofilters, scrubbers) as equipment and chemical dosing for coagulation/flocculation, nutrient removal or enhancement, odor control and pH adjustment as a treatment process.

Where Odors Come From

Odors rise sharply wherever septic wastewater is turbulent: force main discharges, drop structures, channels with free falls, grit chambers and splashing weirs release dissolved gases into the air.

Preventing Sulfide in the Collection System

Sulfide forms when sulfate-reducing bacteria in slime layers work without oxygen, especially in long force mains with long detention times and warm temperatures, which describes much of Alabama for most of the year.

1. Calcium nitrate supplies nitrate as an electron acceptor, so bacteria use nitrate instead of sulfate and sulfide is not produced.
2. Iron salts (ferrous or ferric chloride) react with dissolved sulfide to form insoluble iron sulfide. The theoretical iron dose is about 1.74 mg of iron per mg of sulfide, and practical doses are higher.
3. Magnesium hydroxide raises pH toward about 8.5, keeping sulfide in the non-volatile $\text{HS}^-$ form so less hydrogen sulfide gas escapes.
4. Oxidants such as hydrogen peroxide or sodium hypochlorite destroy sulfide already formed; they work fast but must be dosed close to the odor point.
5. Air or oxygen injection keeps force mains aerobic.
6. Periodic caustic slug dosing at very high pH can strip slime layers that produce sulfide.

Worked example. A force main carries 0.80 MGD with 5.0 mg/L of dissolved sulfide to remove. Using the theoretical ratio of 1.74 and a practical factor of 2:

$$\text{Fe dose} = 5.0 \times 1.74 \times 2 = 17.4\text{ mg/L as Fe}$$ $$\text{Fe feed} = 0.80 \times 17.4 \times 8.34 = 116\text{ lb/day as Fe}$$

The feed of commercial ferrous chloride solution is then found from its iron content per gallon.

Controlling Odors at the Plant

• Keep it fresh. Pump primary sludge often enough to prevent septic blankets (Section 4.2), avoid long storage of raw sludge and keep wet wells and channels free of deposits.
• Reduce turbulence at drops and weirs where possible.
• Cover and ventilate the strongest sources and send the foul air to treatment.

Treating Foul Air

Wet chemical scrubbers. Foul air rises through a packed tower against a recirculated scrubbing solution.

• Caustic (sodium hydroxide) with sodium hypochlorite absorbs and oxidizes hydrogen sulfide and mercaptans. Operators control pH (commonly about 9.5 to 11) and oxidation-reduction potential, watch for scaling and keep chemical feeds and blowdown in balance.
• Acid (sulfuric acid) scrubbers remove ammonia and amines, which are basic gases.

Biofilters. Foul air passes slowly through a bed of moist organic media such as wood chips or compost, where bacteria oxidize odor compounds.

• Keep the media moist (roughly 40 to 60 percent moisture) with irrigation; dry media cracks and channels.
• Monitor media pH. Bacteria oxidize hydrogen sulfide to sulfuric acid, so the media becomes acidic over time; very low pH slows treatment and calls for washing or media replacement.
• Provide adequate empty-bed contact time (EBCT), commonly tens of seconds to a minute or more:

$$\text{EBCT} = \frac{\text{Media volume (ft}^3)}{\text{Airflow (ft}^3\text{/min)}}$$

For a 40 ft by 30 ft bed with 4 ft of media (4,800 ft³) receiving 6,000 cfm, EBCT is $4{,}800 \div 6{,}000 = 0.8$ minute, or 48 seconds.

Biotrickling filters use inert media with recirculated, nutrient-fed water. Activated carbon adsorbers (often impregnated for hydrogen sulfide) polish low-concentration air; operators track breakthrough and replace carbon on schedule.

Chemical Addition in the Treatment Process

Chemical Handling Rules

• Store incompatible chemicals apart: hypochlorite must never contact acids or ammonia compounds, and bisulfite or other reducing agents must be kept away from hypochlorite.
• Provide secondary containment sized for the largest tank, label lines and tanks, and keep Safety Data Sheets current (Section 12.3).
• Verify each delivery against the order and certificate of analysis before unloading.