Optimising ventilation and covers for controlling odour

It's a fine balance between not enough air and not enough cover when it comes to fugitive odours, but WRc has come up with software that can make the calculation for you explains consultant engineer David Sivil

Odour emissions from sewage treatment works can be the cause of complaints from local residents. Odour emissions from works can sometimes be reduced by
adopting better house-keeping measures, making changes to processes or dosing chemicals. In some cases, however, it is necessary to invest considerable sums of money in covering odorous processes and extracting and treating the air from under the covers.

After the installation of such systems, it can be difficult to determine whether the covers and ventilation system are adequate for preventing fugitive odour emissions or are excessive and costing more to operate than they ought. WRc has developed the Odour Assessor software to model the performance of covers and ventilation on sewage treatment processes.

WRc carried out a collaborative research project with a number of water utilities to investigate, via an experimental programme on real works, how covers and ventilation operate and then to create software to model such systems.

Finding an effective cover
During the project, WRc developed a new test to measure the effectiveness of different covers. The procedure involves injecting the tracer gas sulphur hexafluoride (SF6) under a cover, turning off the ventilation and monitoring the concentration of SF6 under the cover over a period of time as it escapes through holes into the surrounding atmosphere. The half-life decay for different types of covers ranged from around five minutes for ineffective covers to over five hours for very effective covers. The half-life was used to calculate the resultant flow rate air out from under the cover at zero ventilation.

The Odour Assessor software estimates the concentration of hydrogen sulphide under covers for different air extraction rates. It also estimates the fugitive odour emission rate that would escape from under the cover if insufficient air extraction is used and the odour emission rate that would enter an odour control system.

The model was compared with values measured during the experimental programme and found to correlate well. The minimum air extraction rate for preventing fugitive odour emissions is expected to be similar to that estimated from the SF6 decay half-life.

The SF6 decay test has been used in a consultancy project at a sewage works to
determine the effectiveness of covers on an imported sludge tank and a picket fence thickener. The SF6 decay half-life was over five hours for the imported sludge tank, which showed that the cover was very effective. However, the half-life for the picket fence thickener cover was only six minutes, which showed that it was not effective. This ineffectiveness was due to badly fitting inspection hatches around the periphery of the tank.

Models were set-up for each covered tank using the Odour Assessor software. The models were used to estimate the fugitive odour losses during the normal air extraction rate. These found that the air extraction rate for the sludge import tank, which had a very effective cover, was adequate for preventing fugitive odour emissions. However, for the picket fence thickener, which had a less effective cover, the air extraction was insufficient to prevent fugitive odour emissions. Therefore, it was recommended that the cover be made more effective by sealing the inspection hatches and the air extraction rate increased.

The consultancy project showed the benefits of using the SF6 decay curve test and Odour Assessor model software for determining the performance of covered systems and investigating how they can be optimised to prevent fugitive odour emissions and minimise operating costs.

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