Odour reduction at wastewater treatment plants — causes and treatment methods
At wastewater treatment plants, the anaerobic (oxygen-deficient) decomposition of organic matter leads to the formation of various putrefaction gases. These gases cause characteristic unpleasant odours that are not only disturbing for nearby residents but may also affect workers’ health and damage equipment.
Which Gases Cause Unpleasant Odours?
The most important gases formed are:
- Hydrogen sulfide (H₂S) – “rotten egg” smell; highly toxic and corrosive.
- Ammonia (NH₃) – pungent, irritating ammonia odour; can cause respiratory problems.
- Methane (CH₄) – colourless and odourless, but explosive.
- Carbon dioxide (CO₂) – odourless, but in high concentrations displaces oxygen.
- Volatile organic compounds (VOCs) – various odorous substances with characteristic unpleasant notes.
Where Are Odours the Strongest?
The most odour-intensive areas of a wastewater treatment plant are those where anaerobic conditions favour putrefaction.
1. Incoming wastewater and screening units: very strong odour (typical “sewer smell,” reminiscent of rotten eggs).
2. Grit and grease removal units: moderately odorous (mainly greasy and sulfurous smells).
3. Primary clarifiers: very strong odour (ammonia smell is typical).
4. Biological treatment stage: oxic zones are generally not odorous (mild, earthy, slightly sulfurous smell). Anoxic zones are moderately odorous.
5. Secondary clarifiers: usually not odorous (slight sludge smell may be noticeable).
6. Sludge handling (thickening, dewatering): very strong odour sources due to high organic content and often anaerobic conditions (strong ammonia and sulfurous putrefaction smells).
7. Anaerobic digesters: odours are only noticeable externally in case of leakage (although large amounts of gas are produced during anaerobic digestion, this takes place in a closed system).
Odour and Gas Treatment — Methods and Technologies
1. Source-Level Prevention
Often the most effective solution is addressing the problem at its source:
Use of well-sealed, covered tanks during plant design. Closed systems reduce gas emissions to the environment.
Optimised operation: adequate aeration in biological reactors reduces the formation of anaerobic zones, thereby limiting the production of putrefaction gases.
Continuous monitoring: installation of gas detectors (e.g. H₂S and NH₃ sensors) is essential for safety, odour control, and early identification of technical or operational issues.
Regular maintenance: clean ventilation ducts, filters, and extraction systems ensure efficient operation.
2. Gas Collection and Controlled Extraction
Generated gases are collected and treated using extraction systems:
Extraction ducts and suction systems: gases are conveyed under built covers, membrane roofs, or in enclosed channels.
Fans and HVAC systems: ensure continuous airflow and effective gas collection.
3. Physico-Chemical Removal
After collection, odorous and harmful components are reduced or eliminated using various technologies:
Activated carbon filtration: gas components are adsorbed onto the large surface area of activated carbon, reducing odour and toxicity.
Wet scrubbing: gases are washed with water or chemical solutions, removing ammonia and hydrogen sulfide.
Catalytic or thermal oxidation: at high temperatures, odorous compounds such as VOCs and H₂S are converted into less harmful substances (e.g. CO₂ and H₂O).
4. Biological Odour Control
Microorganisms in biological filters and biofilm systems are used to degrade odorous gases:
Biofilters: moist, lightly ventilated filters filled with organic media (e.g. compost or wood chips), where microorganisms break down odorous compounds.
Biological trickling systems: biological conversion of ammonia and other odorous substances under aerobic conditions.