How proactive investments mitigate smelly situations

A defining characteristic of wastewater is its noticeable odor. Commonly perceived as “foul,” or “unpleasant,” wastewater odors can create concerns of health and well-being within communities and may lower the quality of life for residents by deterring investment and development in the affected areas.

Additionally, odorous compounds can have damaging and costly effects if left untreated. Common odorous compounds found in wastewater collection systems include:

• Hydrogen sulfide (H₂S): Rotten egg smell.
• Ammonia (NH₃): Pungent, sharp odor.
• Volatile organic compounds (VOCs): Solvent-Like or chemical odor.
• Mercaptans: Skunky or rotten cabbage smell.
• Fatty acids: Rancid or sour smell.
• Sulfur dioxide (SO₂): Burnt match smell.
• Ketones and aldehydes: Fruity or solvent-like smell.
• Chlorinated compounds: Bleach or medicinal odor.

Hydrogen sulfide (H₂S) is one of the most prevalent odor-causing compounds in wastewater collection systems and has been identified as the primary cause of odor complaints and corrosion damage. H₂S gas is detectable by the human nose at concentrations down to 0.01 ppm.

At approximately 100 ppm, exposure to H₂S in enclosed, not well-ventilated areas causes throat and eye irritation as well as altered breathing and can no longer be detected by the human nose. Prolonged exposure at concentrations of 100 ppm and greater can be life threatening, and at 1000 ppm, it can cause a nearly instant death.

In addition to its adverse health effects, H₂S-related corrosion of sewer mains causes extensive pipeline deterioration and results in an estimated loss of $14 billion annually in sewer assets in the U.S. Therefore, it is important to understand H₂S formation within wastewater collection systems and proactive approaches to odor mitigation.

Sulfide production in sewers

Sulfide is readily formed in wastewater collection systems through reduction of sulfate (SO₄^2-) to sulfide (S^2-) by sulfate-reducing bacteria (SRB). Sulfate reduction primarily takes place within the thin layer of biological slime inhabiting sewer pipe walls below the water surface or in solids deposits on the pipe invert when oxygen (O₂) and nitrate (NO_3-) are depleted.

Sulfide is then released into the wastewater where it takes the form of a sulfide ion (S^2-), bisulfide ion (HS^-), or aqueous hydrogen sulfide (H₂S (aq)). The relationship between the presence of S^2-, HS^- and H₂S is controlled by the pH. Around a pH of 7.1, there is approximately a 50/50 proportional relationship between HS^- and H₂S. If the pH is lowered, H₂S becomes the most dominant form of sulfide, and if the pH is raised, HS^- and S^2- become the dominant forms. H₂S (aq) is the only sulfide species that can directly off-gas from the wastewater and cause odor and potential corrosion problems.

Sulfate, SRB, and anaerobic conditions are present in virtually all collection systems making this phenomenon inevitable if left untreated. Additionally, certain wastewater and collection system conditions, such as high biological oxygen demand (BOD₅), low dissolved oxygen (DO), low pH, high temperature, long detention times, and turbulence are known to accelerate sulfide generation and H₂S off-gassing.

Once H₂S is released to the sewer atmosphere, it poses a serious risk to pipeline damage through corrosion. Thiobacillus bacteria inhabit sewer walls above the water surface and oxidize H₂S gas into sulfuric acid (H₂SO₄) in the presence of moisture and O₂.

Sulfuric acid attacks concrete sewer walls producing calcium sulfate (CaSO₄), a soft, paste-like compound with significantly less strength than concrete. The calcium sulfate paste is also easily washed away with wastewater flow, exposing fresh concrete vulnerable to acid attacks leading to thin sewer walls and eventual catastrophic collapse if not treated.

Finding the root of the problem

A proactive approach to treating wastewater odors is crucial to maintaining community quality of life as well as the integrity of wastewater collection system infrastructure. The first step to odor mitigation is identifying target areas by tracking and mapping odor complaints made by residents through a city’s designated odor reporting hotline. Topics of interest for each complaint could include location of odor detection, date and time, and type of odor perceived (e.g. rotten egg).

Key areas can then be determined from the hotspots identified by the odor complaints, and a collection system odor monitoring program can be developed to quantify the baseline odor levels and characterize the wastewater to help identify causes of the odor complaints and potential mitigation strategies.

Monitoring programs generally include continuous H₂S gas, differential pressure, and flow monitoring. Wastewater grab samples are also typically collected. Continuous H₂S gas and flow monitoring provides insight into diurnal curve patterns. Similar patterns often indicate that peak H₂S results from peak flow amplifying H₂S off-gassing at points of turbulence. Understanding the temporal variation of H₂S is important for determining appropriate treatment technologies and potential dosing strategies (e.g. dosing curve).

Continuous differential pressure monitoring reports the difference between sewer headspace pressure and atmospheric pressure in inches-of-water (in-H₂O). Negative readings indicate the collection line is under vacuum conditions preventing release of H₂S from the sewer headspace. Positive readings indicate H₂S gas is escaping from the sewer headspace to the atmosphere at pick holes of unsealed manholes leading to potential odor complaints in the affected areas.

Wastewater grab samples are often analyzed for pH, temperature, oxygen reduction potential (ORP), DO, total sulfides (TS), and dissolved sulfides (DS) to understand potential odor causing characteristics.

Strategies for odor control

Appropriate odor mitigation strategies can be developed after analysis of the field data collected during the monitoring program. Treatment of collection system odors is often accomplished through a combination of liquid phase and vapor phase odor control systems.

Liquid phase odor control

Liquid phase odor control focuses on preventing the formation of odorous gases by introducing chemicals into the wastewater collection system while wastewater is in transit. It is also possible to inject chemicals directly at the source if industrial users were identified to be significant contributors during the monitoring program.

Local limits as defined in a city’s Industrial Pretreatment Program can be enforced or implemented, if not existing, to put pressure on users to make corrections to wastewater effluent prior to discharge into the collection system. There are various chemical approaches that are proven mechanisms commonly used for sulfide control:

• Phase changers – Adding chemicals to increase the pH and thereby reduce the percentage of DS that will exist as aqueous H₂S.
• Preventers – Adding chemicals that interfere with the ability of the microorganisms in the slime layer to produce sulfide out of sulfate.
• Oxidants – Adding chemicals that oxidize the wastewater sulfide into a non-odorous form.
• Binders – Adding chemicals that bind the wastewater sulfide into a non-odorous compound that precipitates out as a solid.

Choosing the right liquid phase approach

Suitability of each chemical approach is dependent upon wastewater characteristics, collection system configuration, cost, familiarity with handling, chemical safety concerns, and constraints such as transporting, storing and applying the chemical.

Field testing is a reliable technique for determining optimal dosing rates since the process involved in sulfide formation is dynamic with significant temporal and spatial variation. Acceptable DS and H₂S concentrations will vary depending on sensitivity to odor complaints and tolerance to potential corrosion, which is influenced by protection measures such as coating systems.

However, in general, recommendations to prevent sulfuric acid formation and limit corrosion in unprotected infrastructure is to maintain DS concentrations of ≤0.5 mg/L and vapor phase H₂S concentrations of ≤5 ppm.

Vapor phase odor control

Vapor phase odor control involves pulling foul air from the sewer headspace to maintain a vacuum pressure and conveying the foul air to a treatment technology that removes odorous compounds. The vapor phase approaches summarized below are industry standards known to be successful in controlling collection system odors.

• Biofiltration
  • Biofilters: Foul air is collected from the sewer headspace and sent through a media bed containing microbes that consume, break down, and oxidize H₂S and other odorous gases into non-odorous compounds as part of their metabolic process.
  • Biotowers: Operates similarly to biofilters except irrigation fluid containing nutrients for the support and growth of microbes is either continuously recirculated (bioscrubber) or sprayed intermittently over the top (biotrickling filters) of inert media.
• Chemical scrubbers: Operates similarly to biotowers except the inert media is continuously irrigated with chemical solutions.
• Carbon adsorption: Foul air is collected from the sewer headspace and sent through a bed of dry activated carbon where the activated carbon’s porous structure physically traps odorous compounds.

Choosing the right vapor phase approach

Suitability of each vapor phase approach is dependent on wastewater characteristics, collection system configuration, cost, chemical safety concerns, maintenance expectations, and constraints such as space, media life, and media disposal.

Performing an extraction fan test is recommended to determine the design air flow rate of the selected vapor phase technology to ensure an adequate vacuum pressure is maintained in the collection system (-0.02 inches-H₂O).

Vapor phase treatment must also be sufficient to comply with regulations related to odors and gaseous contaminants at the property boundary, however, higher standards are often administered within cities to align with being a “good neighbor.”

The bigger picture

Effective odor control is a critical aspect of system management. The key? Know your collection system, test solutions, and scale what works.