Odour surveying leads by the nose

Surveying can be the most practicable and cost-effective way of delivering smell-free wastewater treatment in residential areas, argues Bryan Pickering, principal process engineer at MWH

In recent years, water companies have been under increasing pressure to tackle the issue of odour emissions from wastewater treatment works (WwTW). Prior to about 1980, there were few WwTWs where any attempt had been made to control the odours released during the biological oxidation of the effluent or the treatment of sludges.

This is hardly surprising because the majority of WwTWs were remote from residential areas. Where this was not the case, odour complaints did not result in enforcement action becauuse legislation dating back to the 19th century was ineffective.

Treatment methods in those earlier times were less intensive than in modern works, and, in general, the sites were large with the released odours dispersing easily.

Over the intervening years, people have become more vociferous when compaining about problems with their environment. And, as residential areas expand, it is not uncommon to find developers providing housing, retail outlets and leisure facilities at the site boundary of a WwTWs.

These points have had significant impact on both the capital and operating costs of improving existing and new WwTWs.

Control costs
The 1990 Environmental Protection Act (EPA) was intended to supersede all existing legislation, and cover all aspects of air quality and nuisance. A number of high-profile actions have been taken by local authorities, which have been robustly but unsuccessfully defended by the water companies.

These have confirmed that the 1990 EPA applies to WwTWs, and that Defra's Code of Practice, which imposes requirements for dealing with odour, must be adopted. The result is that all sites now need to have an odour management plan. If odours are excessive, they have to be reduced using the best practicable means and by taking into account cost/benefit analysis.

The cost of providing containment and extraction to odour control equipment can be high. It will also increase power consumption and maintenance costs. Consequently, it is in the interests of water companies and customers to maximise the cost-effectiveness of odour control facilities.

UK Water Industry Research has recently initiated a project to develop a methodology for applying cost benefit analysis to odour control. But, until this is completed, the existing methods of site surveys and dispersion modelling give the best method of satisfying environmental health officers, planners and residents.

Sourcing odour
It is an unfortunate fact of life that sewage smells. All stages of effluent treatment have the potential to release odours. The concentration of these odours will be at different levels and this will have a different impact off site.

To resolve odour nuisance, the first step is to establish where it is coming from and at what concentration. Odour surveys are not cheap, but they can provide comprehensive data on which to base a strategy for minimising odour emissions, on or off site.

Both of these survey techniques report on the odour measured at a moment in time for the prevailing conditions. If good on-site data is available, then dispersion modelling can be used to predict off-site impact.

The methodology for dispersion modelling is well established. There is also sufficient data published for emissions from individual processes to identify potential odour sources from planned works before measured data becomes available.

This data enables dispersion modelling techniques to be employed to predict the impact of individual or combinations of odour sources beyond the boundary fence. This establishes where the installation of odour control equipment will have the greatest impact. The odour survey data and baseline dispersion model can be used in a number of ways, depending upon the outcome being sought.

If agreed by an environmental health officer that a phased approach is acceptable, then the odour sources can be ranked according to intensity, characteristic, hedonic tone (how pleasant or not a smell is) and frequency. From this ranking, priorities can be allocated to each odour source and the list of sources split into a number of phases, usually two to four.

The principle of this is that phase one will be completed, and the impact determined by a second odour survey, prior to reassessing the requirements for subsequent phases. For each of these phases, or when odour is to be addressed in a single phase, then a number of scenarios can be developed, providing different levels of odour reduction.

Odour technology
There are a variety of odour control technologies, which individually can achieve odour removal efficiencies of between 50% and 99.99%. When used in combination, they can achieve in excess of 95%. Not all technologies are suitable for all applications, and care needs to be exercised when selecting solutions.

For example, biological methods of odour control perform best under stable conditions, with H2S concentrations of less than 100ppm. These systems are less suitable for applications where the peak-to-mean ratio and rate of change is high.

In addition to offering different removal efficiencies, each technology has different capital and operating costs. Combining cost and odour reduction levels for each method, the most cost-effective solution can be identified.

Best practicable means
To conclude, when costs are attributed to each of the solutions being considered, it is then possible to compare benefits in terms of reduction in the number of residents affected, and the costs of providing the benefit.

This will go some way towards satisfying the requirements of Defra's Code of Practice in establishing best practicable means, while assessing costs attributable to benefits achieved. It will also help defray mounting public pressure on water utilities regarding unwanted odours.

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