Reducing odour emissions from wastewater treatment works (WwTWs) and pumping stations is often essential for reducing the impact of the installation on local residents and businesses and preventing complaints. Considerable investment is required to install and operate effective odour control measures.

There are a number of key design aspects that need to be considered in selection of appropriate mitigation measures, says David Sivil CEng CEnv MIChemE BSc (Hons) Chem Eng, Engineering Consultant, The WRc Group.

Installation of odour control

Before selecting the odour control system, it is critical to accurately define the odour problem that needs to be addressed. This includes establishing the odour standard that the mitigation measures must achieve and to fully understand the causes of the odour emissions. Decisions made on installing odour mitigation measures without due consideration of the causes could result in investment in measures that do not work or cost more than is necessary. For existing works, design should be based on real data collected under appropriate conditions that enable accurate quantification of the odour emissions.

For new facilities the nature and quantity of odour emission can be estimated from experience and reference to emission rates from similar installations. The impact of the odour emissions on the local environment can then be determined by modelling. An atmospheric dispersion model allows different options for mitigation of odour emissions to be considered to determine the most effective solution.

Prevention of odour emissions at source is often better than mitigation and it can be more cost-effective to design systems to avoid odour generation. For example, a common source of odour is the return of odorous sludge liquors to the primary settlement tanks (PSTs): redirecting the return liquors to the inlet of the downstream activated sludge process reduces the odour potential of the settled sewage and, hence, the odour emission rate from the PSTs.

Reducing turbulence within processes, for example weirs or open pipe discharges, can also decrease the release of odorous compounds into the air and, therefore, reduce odour emissions.

However, if there is no choice but to cover odour sources, then there is still scope to achieve cost-effective design improvements. The extraction of air and emissions from within the covers can be optimised to minimise the rate (and cost) of air extraction but still prevent fugitive odour emissions. This needs to be balanced against the risk of accumulating hydrogen sulphide concentrations within covered areas causing problems with corrosion or health and safety. WRc has developed a computer software program called Odour Assessor which it uses at WwTWs to optimise the rate of extraction of air from covered processes and determine the concentration of H2S in the extracted air.

Components of odour control systems

An overall odour control system is built up of all or a number of the following components and can be a considerable investment:

• Covers.

 • Air ducts.

 • Air extraction fan.

 • Odour control unit (OCU).

 • Outlet stack for OCU.

 • Storage tanks for certain OCU types (e.g. wet chemical scrubbers).

 • Ancillary equipment for certain OCU types (e.g. irrigation water system for odour biofilters and air heaters prior to some activated carbon filters to reduce the relative humidity).

The OCU is where the odorous components of the gas are destroyed and there are a wide range of types of odour control unit (OCU) available and used by the water sector. These include but are not limited to: wet chemical scrubbers, dry adsorbent media (e.g. activated carbon), odour biofilters and odour bioscrubbers.

In order to select the most appropriate and cost-effective OCU for a particular situation it is important to consider the following:

 • The flowrate and inlet concentration of the air.

 • The variation in the input concentration.

 • The performance standard that needs to be met.

A small flowrate of odorous air requiring a low outlet odour concentration could be treated by a dry adsorbent media OCU; a wet chemical scrubber is likely to be more costeffective for a larger flowrate. Getting the right OCU requires a good understanding of the different factors that affect performance and whole life costs. There is no onesize- fits-all solution.

Maintaining odour control equipment

Odour control equipment represents significant capital investment and ongoing operational costs. It is important that it is maintained to consistently provide the required level of performance for the life of the equipment; failure to do so can lead to increased costs and potential failure to meet required standards.

Odour Management Plans are required to include procedures for checking the performance of odour control equipment; many companies incorporate these procedures into the operational staffs’ list of routine tasks.

Good practice would include, in addition to any supplier checks and servicing recommendations, the following routine checks:

 • Correct operation of the air extraction fan.

 • Adequate levels in tanks of chemical reagents and that the chemical reagent is within its recommended storage life.

 • Air ducts and covers have no holes and any hatches are closed.

 • Measurement of the inlet and outlet concentrations of parameters such as hydrogen sulphide and ammonia and comparing the readings with the design specification for the equipment. The measurements can indicate whether media might need to be changed or other remedial work is required.

 • Non-toxic theatrical smoke can be used to test the ability of covers or enclosures for containing odorous air (this is particularly good for enclosures around sludge thickening and dewatering facilities).

 • Monitor important operating parameters for OCU systems. For example, for biofilters, the pH of recirculation liquid should be checked; at very low pH values the performance of the odour biofilter would deteriorate.

 • H2S and NH3 measurements can be backed up by occasionally collecting outlet air samples and analysing them for odour concentration using olfactometry.

Although expensive compared to H2S, olfactometry enables the impact of OCU on all odorous components to be determined. This is important if odour markers such as H2S are removed by an OCU but other odour is not.

Conclusions

Odour control systems need to be correctly designed and selected. They should not be an afterthought. To obtain the most cost-effective solution it is important to:

• Define the odour problem and the odour standard that needs to be met.

 • Determine the causes/sources of odour emission.

 • Use the best data, collect new data if needed.

 • Consider prevention measures in addition to mitigation options.

Odour control systems work best when well maintained and operated; checks on performance should be part of the routine, not part of the investigation of why they have stopped working.

For further information, contact David Sivil via e-mail: david.sivil@wrcplc.co.uk or visit: www.wrcplc.co.uk

This article was brought to you by Water and Sewerage Journal

Related Articles:
UK AD & Biogas 2010 – The UK’s first anaerobic digestion and biogas trade show!
With the Coalition Government committed to "a huge increase in energy from waste through anaerobic digestion", UK AD & Biogas 2010 is the event of the year.

CEEQUAL: rewarding environmental and social care in the water and sewerage industry
Not all sequels are bad. CEEQUAL – the Civil Engineering Environmental Quality Assessment and Awards Scheme – focuses on improving the sustainability of civil works.