Understanding and Controlling Gaseous Emissions from Gutters, Sumps, and Manholes: A Technical Perspective on Odor Control and Air Pollution Mitigation

Urban infrastructure such as gutters, sewage sumps, manholes, and lift stations form the unseen backbone of modern sanitation systems. However, these enclosed or semi-enclosed structures also become significant sources of noxious and hazardous gases, contributing not only to unbearable odors but also to public health hazards, environmental degradation, and safety risks.

This article explores the origin, composition, and impact of sewer gases, and presents technically robust and sustainable solutions for their removal and treatment.

1. Origins and Sources of Gaseous Emissions in Sewer Systems

Contents

1 1. Origins and Sources of Gaseous Emissions in Sewer Systems
- 1.1 Common Emission Points:
2 2. Major Gases Released and Their Properties
3 3. Odor Issues and Human Health Impacts
- 3.1 3.1 Health Hazards
- 3.2 3.2 Structural Corrosion
4 4. Measurement and Monitoring
5 5. Engineering Solutions for Odor Control and Gas Treatment
6 6. Integration of Odor Control into Sewer System Design
7 7. Regulatory Standards and Emission Limits
8 8. Conclusion: A Multi-Tiered Approach is Essential
- 8.1 Investing in odor control yields:

Sewers and sumps are biological and chemical reactors. Wastewater collected from domestic, industrial, and commercial sources contains a mix of organic matter, inorganic substances, fats, oils, greases (FOGs), surfactants, and sometimes hazardous chemicals. Under anaerobic (oxygen-deficient) conditions, especially in stagnant or low-flow systems, microbial activity and chemical reactions generate a cocktail of dangerous and malodorous gases.

Common Emission Points:

Manholes and inspection chambers
Domestic and industrial lift stations
Overflow sumps in basements
Rainwater collection drains (if connected with foul water)
Septic tanks and grease traps

2. Major Gases Released and Their Properties

Gas	Chemical Formula	Typical Concentration	Properties & Risks
Hydrogen Sulfide	H₂S	1–500 ppm (can go higher)	Highly toxic, corrosive, smells like rotten eggs, heavier than air
Methane	CH₄	5–15% in air	Explosive in air, odorless, lighter than air
Ammonia	NH₃	5–100 ppm	Pungent odor, irritant to eyes and lungs
Carbon Dioxide	CO₂	>1000 ppm in confined spaces	Asphyxiant in high concentrations
Volatile Organic Compounds	VOCs	Trace to several ppm	Source of odors, many are toxic
Nitrogen Oxides	NOₓ	Variable	Respiratory irritants, contribute to smog
Sulfur Dioxide	SO₂	Trace levels	Acidic, contributes to respiratory distress
Indole & Skatole	–	Low ppm	Extremely pungent, present in fecal matter
Mercaptans (Thiols)	R-SH	ppb–ppm	Rotten cabbage odor, extremely odorous

3. Odor Issues and Human Health Impacts

The primary problem with these gases is odor nuisance, which significantly affects quality of life and is a frequent cause of complaints to municipalities. However, the risks go far beyond just bad smells:

3.1 Health Hazards

Hydrogen Sulfide (H₂S) at concentrations >10 ppm can cause headaches, dizziness, and nausea. At >100 ppm, it is life-threatening.
Methane (CH₄) poses explosion risks in confined spaces and contributes to climate change.
Ammonia (NH₃) and SO₂ are irritants that affect mucous membranes and lungs.
Prolonged exposure to VOCs is linked to liver and kidney damage and carcinogenic effects.
Asphyxiation risk due to displacement of oxygen by CO₂ and CH₄.

3.2 Structural Corrosion

H₂S oxidizes to sulfuric acid in presence of moisture and oxygen, causing acidic corrosion of concrete in manholes, pipelines, and lift stations—leading to high maintenance costs and system failure.

4. Measurement and Monitoring

Before implementing any solution, proper gas measurement and risk assessment is critical. The following instruments are commonly used:

PID sensors for VOCs
Electrochemical detectors for H₂S, NH₃, NO₂, CO
Infrared sensors for CH₄ and CO₂
Multi-gas analyzers for confined space entry checks
Odor threshold panels and olfactometry to quantify smell intensity

5. Engineering Solutions for Odor Control and Gas Treatment

To mitigate these issues, a combination of passive and active odor control and gas treatment systems are used. The correct choice depends on the volume of emission, gas concentrations, space availability, and regulatory requirements.

5.1 Passive Odor Control Filters (Point-Source)

Ideal for manholes, sumps, and isolated chambers.

Activated Carbon Filters

High surface area adsorbent for H₂S, VOCs, mercaptans
Can be impregnated with KMnO₄ for oxidizing capacity
Life span: 6–12 months depending on load

Biofilters for Small Volumes

Biodegradation of odorants by biofilms (bacteria + media)
Media: peat, compost, synthetic fibers
Best for low VOC/H₂S load applications
Limited use for small-scale odor sources

5.2 Chemical Scrubbers for Lift Stations / Sumps

For higher gas flow and concentrations, wet scrubbing is preferred.

Packed Bed Wet Scrubbers

Process: Contaminated gas passes through a packed tower while liquid (water + chemical) flows counter-current
Packing Material: Polypropylene, ceramic rings, or pall rings
Chemicals Used:
- NaOH (Sodium Hydroxide) to neutralize acidic gases like H₂S and SO₂
- NaOCl (Sodium Hypochlorite) for oxidation of VOCs and mercaptans
- H₂O₂ (Hydrogen Peroxide) or KMnO₄ for advanced oxidation
Removal Efficiency:
- H₂S: >95%
- NH₃: >90%
- VOCs: 60–85% depending on type

5.3 Biological Odor Control Systems

Trickling Biofilters (Biotrickling Filters)

Structured packing with biofilm, sprayed with nutrient-rich water
Microorganisms metabolize H₂S and VOCs into CO₂, water, and sulfate
Ideal for continuous gas flow with moderate loads
Low operational cost, sustainable, but startup time needed

5.4 Air Dilution and Ventilation

Used in combination with chemical or biological systems
Employ ID (induced draft) blowers to direct gases into treatment units
Prevent stagnation in chambers using jet aeration or diffused air systems
Essential for basement lift stations where natural ventilation is not possible

5.5 Ozonation and UV Treatment

Ozone oxidizes odorants at a molecular level
UV destroys VOC chains and reduces microbial content
Often used for air polishing as a final stage after scrubbers
Require safety precautions due to ozone toxicity

5.6 Sealing and Design Modifications

Install sealed FRP or HDPE manhole covers with venting ports connected to filters
Use trap seals or U-bends in wastewater pipelines to prevent backflow of gases
Modify grit chambers and sumps with gas collection hoods
Ensure proper slope and hydraulic design in sewers to minimize anaerobic zones

6. Integration of Odor Control into Sewer System Design

An ideal setup includes:

Gas Collection Hoods or Ducts
- Collect gases directly at the source
- Made of corrosion-resistant PP or SS-316
ID Blower with Variable Frequency Drive
- Draws air from multiple sources
- Maintains negative pressure to prevent leaks
Scrubber System or Biofilter Unit
- Sized based on CFM and contaminant load
- Includes pre-filter section for particulates
Control Panel with Sensors
- PLC-based control with H₂S/CH₄/NOx sensors
- Remote monitoring of gas levels, blower speeds, and pH/ORP of scrubber solution
Emergency Alarm & Vent Stack
- For critical gas buildup and high-pressure relief

7. Regulatory Standards and Emission Limits

Most countries follow permissible exposure limits (PELs) as defined by OSHA, ACGIH, or local pollution control boards.

Pollutant	OSHA PEL	Short-Term Exposure Limit (STEL)
H₂S	20 ppm (ceiling)	50 ppm
NH₃	50 ppm	35 ppm (TWA)
CH₄	Explosive above 5% in air	N/A
VOCs	Varies by compound	0.5–2 ppm (benzene: 1 ppm)

PME – Dammam KSA, CPCB – India, and EPA – USA require outlet concentrations of treated gases to be within safe discharge limits.

8. Conclusion: A Multi-Tiered Approach is Essential

Eliminating odor and hazardous emissions from manholes, sumps, and gutters is not merely a matter of comfort—it is a necessity for worker safety, environmental protection, and infrastructure longevity.

An effective odor control system combines: