Understanding odours around a wastewater treatment plant

A local authority's “sanitation system” is made up of two main parts: the collection system (the network and its ancillary structures) and the wastewater treatment plant (WWTP).

The collection network

Wastewater usually flows by gravity through underground pipes. There are two main types of network:

• Separate network: wastewater and rainwater flow through two separate pipes
• Combined network: wastewater and rainwater flow through the same pipe, with overflow structures (storm overflows) in case of heavy rain

Diagram: Syndicat Intercommunal d'Assainissement du Bassin de Sallanches

The pumping station

When the terrain does not allow wastewater to flow by gravity to the plant, a pumping station (or lift station) is needed. It is an underground structure in which wastewater accumulates before being pumped to a higher point in the network. These structures are generally compact, often located in urban areas or at low points, sometimes right next to homes. They are usually fitted with manholes (for maintenance) and openings for ventilation.

The treatment plant

A wastewater treatment plant treats the collected wastewater before releasing it into the natural environment. Treatment is generally organised in several stages:

• Pre-treatment: screening (removal of solid waste), grit removal and degreasing (removal of sand and grease)
• Primary treatment: settling of part of the suspended solids, often in large open circular tanks
• Secondary (biological) treatment: breakdown of organic pollution by bacteria, usually in activated sludge tanks
• Tertiary treatment (depending on the site): additional treatment of nitrogen and phosphorus, or disinfection
• Sludge treatment: thickening, digestion and dewatering, before recovery (spreading, composting) or disposal

Pumping station / network vs treatment plant: different odour profiles

The network and pumping stations on the one hand, and the treatment plant on the other, do not present the same odour nuisance risks, in particular because wastewater stays in them under different conditions.

Sources: ASTEE – French scientific and technical association for water and the environment • French national public sanitation portal • INERIS / AIDA – ICPE and IOTA regulations • French water agencies

Even a well-managed sanitation system can at times cause noticeable odours. Most of these situations are occasional and identifiable. The table below lists the main odour sources observed.

Aggravating factors: retention time, weather and substances in transit

Several types of factors strongly influence the intensity of the odours perceived, without this necessarily being a malfunction:

• Wastewater retention time: the longer wastewater stagnates in a pipe or wet well (dry weather, low night-time flow, heat), the more odorous it becomes, in particular through the production of hydrogen sulphide (rotten eggs)
• Light wind: odorous compounds linger near the source instead of dispersing
• Temperature inversion (often in the morning or evening): cold air stays close to the ground, preventing odours from rising and diluting
• Heat: it speeds up the fermentation of wastewater and sludge, and increases the volatility of odorous compounds
• Humidity: it helps odour molecules persist in the air
• Type of substances: sewer networks generally collect domestic wastewater, but in some neighbourhoods businesses, tradespeople or restaurants may also use them. Sometimes certain substances are poured into the network (e.g. grease, paint, solvents, oils), which can generate other odours (solvent, paint) or maintenance problems (e.g. the network being blocked by grease)
• Concentration of effluents: in dry weather or in summer, effluents can be more concentrated and generate more odours because of higher biological activity and potentially longer retention times (e.g. the time between two successive pumping cycles)

Sources: ASTEE – odour management in sanitation • INERIS – air quality and odour nuisance • French national public sanitation portal

Odours linked to sanitation cannot be reduced to a single olfactory signature. Depending on the structure involved, several types of odour can be perceived, associated with different chemical compounds.

The compounds in a nutshell

• H2S (hydrogen sulphide): produced by bacteria in the absence of oxygen in stagnant wastewater; detectable by the human nose at very low concentrations, well below any danger threshold
• NH3 (ammonia): produced when nitrogen-rich matter breaks down, typical of sludge and biological treatment
• VOCs (volatile organic compounds): a broad family of volatile carbon-based molecules, behind many fermentation odours
• Volatile fatty acids (VFAs): intermediate products of anaerobic breakdown, responsible for sour or “vinegary” notes
• Mercaptans (thiols): these include ethanethiol, methanethiol and methyl mercaptan, which are detectable at extremely low thresholds. This means they are noticed even at tiny concentrations (odourless natural gas contains them for safety reasons)

Sources: ASTEE – characterising odours in sanitation • INERIS – air quality • French water agencies

In France, the vast majority of networks and plants treating domestic wastewater fall under water regulations (the IOTA regime – Installations, Structures, Works and Activities), under heading 2.1.1.0 of the “water” nomenclature of the Environmental Code, and are governed by the Order of 21 July 2015 on public sanitation systems. Some plants receiving a large share of industrial effluents are also subject to the regulations on Installations Classified for Environmental Protection (ICPE), in particular headings 2750 (industrial plants) and 2752 (mixed plants).

The Order of 21 July 2015 explicitly states that the structures of the sanitation system (network, pumping stations, storm tanks, treatment plant) must be designed, located and operated “so as to minimise the emission of odours” and other nuisances for neighbours. The owner (local authority or operator) must keep an up-to-date register of incidents and breakdowns, as well as a preventive maintenance schedule for the structures.

Main obligations relating to odours

• Design and siting of structures so as to minimise odour emissions for neighbours
• Regular cleaning of the network to limit deposits and stagnation
• Ventilation and, where appropriate, odour control of pumping stations and pre-treatment structures
• Monitoring and maintenance of electromechanical equipment (pumps, aeration systems) to avoid prolonged breakdowns
• Controlled management of storm tanks and overflows, designed to protect residents from odour nuisance
• Keeping a register of incidents and a preventive maintenance schedule

Sources: Légifrance – Order of 21 July 2015 • AIDA / INERIS – ICPE headings 2750 and 2752 • French national public sanitation portal – regulations

Noticing an odour near a network, a pumping station or a wastewater treatment plant is not abnormal in itself: some occasional emissions (cleaning, maintenance, very hot spells) are expected and generally brief. On the other hand, an intense, recurring or unusual odour may reveal a malfunction that is worth reporting.

Situations that are usually normal

• A brief odour during cleaning or maintenance work
• A stronger odour at a pumping station during a very hot spell or periods of low night-time flow
• An occasional odour at a storm tank after heavy rainfall

Situations to report

• A strong odour lasting several hours or days, particularly at a pumping station or manhole
• A new, unusual odour, or one very different from what is usually noticed
• An odour associated with repeated nuisance, especially at night or early in the morning

Every report, even for a nuisance that seems modest, helps to pinpoint the structure causing the nuisance (network, pumping station or plant) and, where necessary, to guide corrective action by the operator or inspections by the relevant services. The precision of the report (location, date, time, duration, intensity, weather conditions) is decisive for effective action.