Odour problems and their control at wastewater treatment works

In this article for the Foundation for Water Research (FWR) June 2021 Newsletter, WRc's wastewater treatment and quality control specialists consider the causes, impacts, mitigation measures and future challenges of odour emissions from sewage facilities.

Wastewater Treatment Works (WwTWs) play an essential role in protecting surface waters from the potential negative impact of wastewater generated from human society. However, effective odour management is vital for ensuring odour emissions from WwTWs (and sewer networks) do not have negative effects on residents and businesses. 

Potential for odour emissions at each process step

Rising mains

Under current climatic conditions in the UK, domestic wastewater flowing through a typical ventilated gravity sewer remains aerobic and does not, in general, become significantly odorous. However, when there is the requirement to pump wastewater through rising mains the wastewater is no longer in contact with air. If the hydraulic residence time is too long (over around six hours) any dissolved oxygen becomes depleted and the wastewater becomes anaerobic. Under this condition, sulphate present in the wastewater is converted to hydrogen sulphide (H₂S) by bacteria within the biofilms on the internal walls of rising mains or within accumulated solids. This can significantly increase the odour of the wastewater discharging from rising mains. Odour generation increases with warmer seasonal temperatures and stronger sewages due to dry weather. Trade effluent can also result in odour problems – it may itself be odorous when discharged into a sewer network, or may for instance increase the strength of the wastewater, resulting in odour generation.

Tanks and channels

Odour is emitted from the surfaces of tanks and channels within WwTWs. However, when odorous wastewater passes through turbulent conditions the emission of odour increases. Such conditions typically occur within the sewer network at locations including where a rising main discharges into a gravity sewer or a pumping station wet well. Within WwTWs wastewater is often turbulent in the inlet works, within channels and chambers that have features that impinge on the flow (eg weirs and penstocks), the weirs of primary sedimentation tanks (PSTs) and interprocess pumping stations. The inefficient removal of sludge from PSTs will contribute to odour generation within these assets. H₂S is the common odorous gaseous species associated within these areas, but there are often other odorous gaseous species present that make up the overall odour, such as mercaptans and disulphides.

Flows in WwTW channel

Sludge processes

The aerobic conditions within biological treatment processes, such as the activated sludge process (ASP), usually remove the odour in wastewater. Although odour emissions may sometimes occur from the inlet zone of an ASP, they tend not to be significant from other parts, or from the final settlement tanks (FSTs) or any downstream processes, such as sand filters.

The storage and processing of sludge, such as screening, thickening to reduce its volume, digestion (with or without pre-treatment to make the sludge more biodegradable) or dewatering, has the potential to release odour emissions if the odours are not effectively contained or treated appropriately. Raw or lime-stabilised sludge tends to be more odorous than digested sludge. H₂S and mercaptans are usually the odorous gases associated with raw sludge, while ammonia is associated with digested or limed sludge. A further consideration for the odour potential is through the handling of sludge cake, as mechanical handling results in the release of considerably more odour emissions than when sludge is static in storage areas. Finally, the resultant liquors from sludge processes are highly odorous and when returned to the front of the WwTW can significantly increase the odour of wastewater flowing through the works.

Other occasional activities at WwTWs can also be the source of odour emissions, such as emptying and cleaning out tanks, the breakdown of treatment processes, or the installation and commissioning of new processes.

The seriousness of odour problems

The dispersion of odour emissions from WwTW is influenced by meteorological conditions. When there are calm winds and low atmospheric mixing, odour tends to hover over or around the WwTW and neighbouring area. The geographical location of the WwTW, therefore, will have a significant effect on odour for local residents and businesses. Historically, many WwTWs would have been located away from populated areas. However, as the demand for housing and business developments has increased, the situation has changed.

The importance of the issue can also change over time – over the last year in response to the global COVID-19 pandemic more people have been exercising in their local area. Footfall past WwTWs and sewer network pumping stations has risen and, as a consequence, operators are under increasing pressure to reduce the odour emissions.

Treatment plant close to a residential area

Odour is a serious issue for customers – and despite operators employing odour management measures, there may be occasions where odours are experienced by local residents and odour complaints are received. In England and Wales, customer satisfaction with water companies is expressed in terms of the customer measure of experience (C-MeX), which can ultimately lead to positive or negative financial payments from the regulator and affect the company’s reputation. Odour complaints therefore will ultimately cause a decrease in the customers’ overall level of satisfaction, making it a serious issue for water companies.

Mitigating odour emissions

Odour emissions from WwTWs are mainly subject to regulation by local authorities. However, sludge treatment processes that are subject to Integrated Pollution Prevention and Control (IPPC) regulations (due to the import of sludge or the use of certain processes) are regulated by the environment protection agencies in the four UK nations.

Odour Management Plans (OMPs)

Odour Management Plans (OMPs) are written for WwTWs to ensure the risks of odour emissions are assessed and appropriate mitigation and management procedures are put in place. OMPs contain a description of the treatment works, the odour control measures used, the complaint handling procedure, the responsibility of management, emergency breakdown procedures, training records and maintenance procedures. Foreseeable events that may lead to odour emissions should also be included.

When an odour complaint is received by a water company’s call centre, details are provided to the water company’s operational staff for them to determine the likely causes of the odour problem in either the sewer networks or WwTW. Sewer air monitoring for H₂S may also be appropriate in some circumstances.

Operators of WwTW play a key role in managing the investigation of the odour complaint because their knowledge of the operation of the WwTW at the right point in time helps to find the root cause.

The first approaches for minimising odour emissions are to practice good housekeeping. In practice this means activities such as cleaning up any accidental spillages or keeping odour control covers closed. It is also important to operate the WwTW processes as intended (such as cleaning storm tanks after storm events and the effective removal of sludge from Primary Sedimentation Tanks) and to maintain plant and equipment (such as odour control systems).

Designing out the risks of generating or releasing odour is beneficial, where possible. Minimising odour generation within the sewer network can be achieved by reducing the hydraulic residence time in pumping station wet wells and rising mains, or applying suitable trade effluent discharge consents. The release of odour emissions can be curtailed by reducing turbulence by, for example, reducing the height of weirs on PSTs.

Chemical dosing

Chemical dosing can also be used as an odour mitigation measure, preventing the generation of H₂S in the sewer network or treating the resultant H₂S. For example, calcium, sodium or iron nitrate can be dosed as a source of oxygen to prevent anaerobic conditions, or H₂S can be removed by the addition of iron salts (such as iron chloride) or oxidants (such as chlorine dioxide or potassium or sodium permanganate). Chemical dosing can be optimised by linking the dosing rate to flow meters or sensors measuring the resultant H₂S in the sewer network. Alternative approaches could be to raise the pH value of the wastewater by dosing with an alkali to reduce the emission of H₂S, or to periodically remove the biofilms inside rising mains that cause the generation of H₂S by a combination of pigging and chemical treatment.

Odour Control Units (OCUs)

Where odour emissions are high from particular WwTW treatment processes and tanks, it can be necessary to cover them, extract the air and treat it in an odour control unit (OCU), before discharging through an outlet stack to assist with dispersion. However, it can be uneconomic to cover large areas and could result in existing mechanical and electrical equipment needing to be upgraded to allow them to function in the resultant operating conditions. It can also make it harder for operational staff to monitor, operate and maintain the covered processes, which can lead to additional odour issues.

Odour Control Units: biofilter (left) and wet chemical scrubber (right)

There are a number of types of OCU used in the water industry, including: activated carbon filters, often impregnated with chemical reagents; dry chemical scrubbers; biological treatment using biofilters or bioscrubbers; and wet chemical scrubbers. 

• Activated carbon filters adsorb odorous gases and impregnated materials to aid the removal of certain gases. They are cheaper to install than other types of OCU but require the replacement of the media approximately once a year.
• Biofilters contain organic or inorganic support material (eg wood chips/bark or lava rock) on which sulphur-oxidizing bacteria grow and treat the H₂S in odorous air that is blown through. Irrigation water is important to keep the media moist and to remove sulphuric acid that is produced. Sometimes a biofilter is followed by a ‘polishing stage’ using an activated carbon filter. Biofilters require a larger footprint than other types of OCU but are cheaper to operate than other systems.
• Chemical scrubbers consist of packed absorption towers in which odorous air is blown upwards and counter-current to recirculating sprays of sodium hydroxide or sodium hypochlorite to remove acid gases, such as H₂S, or acid to remove ammonia. They are effective for treating large volumes of highly odorous air. However, they have become less favourable to water companies because they are more expensive to install, operate and maintain, and require the storage and use of hazardous chemicals.

The future

The increasing demand for housing and land for commercial activities across the UK will lead to the development of land in closer proximity to WwTWs. In addition, predicted UK climate change effects, such as hotter, drier summers and warmer winters are likely to result in higher odour emissions from sewer networks and WwTWs. Both of these issues will increase the risk of odour generation and consequently odour complaints, resulting in the need to apply additional odour abatement measures. Existing mitigation technologies, such as chemical dosing, may still be suitable but will need to be applied in more locations and to a greater extent. This will put pressure on water companies as they seek to balance the increased operational costs associated with odour control, carbon emission reductions and customer expectations.

New technologies

However, there are existing and new technologies and methods available to assist water companies to assess and manage odours in more cost effective ways going forward. Sewer network modelling can be used to model sewage flows and predict the generation of odour in wastewater. Odour dispersion modelling is another important tool to determine the extent of the off-site impact of the odour generated and will provide insight into the potential risk of unacceptable impacts to the public and local environment. As housing developments encroach on the boundaries of WwTWs, odour dispersion modelling used appropriately can provide an unbiased, objective representation of the odour generated on site. It can also be used to model the benefits of applying additional odour mitigation measures, allowing the most cost effective measures to be adopted.

New technologies: odour dispersion modelling example output

The water industry are working together to develop these approaches and meet customer expectations around odour control. Co-ordinated by WRc, an Odour Management User Group with representatives from a number of water companies has met regularly over the past year on the theme of managing odour complaints. During the discussions it was apparent that obtaining appropriate detailed information when odour complaints are received is an important aspect for efficiently and effectively investigating and resolving odour issues. This can be done potentially through improving the collection of information by call centres – smartphone applications now make it easier for the public to readily log complaints and are starting to be adopted in the UK and Europe. They enable water companies to build a better picture of odours arising from their assets.

In the future, smartphone apps and associated software are expected to prove to be powerful tools by incorporating data about wastewater treatment locations, treatment process operational data, odour dispersion maps based on real time odour emissions and weather data, and odour complaints’ locations and information. The apps will also enable fast and effective communication with stakeholders and provide reports, alerts and statistical analysis for validating odour complaints.

Solutions for the future are likely to require a collaborative approach with all stakeholders involved. This includes operators, local residents, industries, regulators and technology providers working together to improve the quality of life of local communities by reducing and mitigating odours from WwTWs.