WET News 2

ODOURsim® - accurate and dynamic odour emission modelling from Water Innovate

Dr Steve Callister
Developed at Cranfield University’s School of Water Sciences, ODOURsim® is a revolutionary odour emission modelling software package that predicts formation and emission of H2S from sewage treatment works.

ODOURsim® software is unique in producing dynamic predictions that demonstrate variation of emission rate over time in response to changing flow, load and meteorological conditions. It can help generate accurate and meaningful odour contour plots in dispersion modelling packages.

ODOURsim® provides an essential design tool to help operators develop defensible odour management plans and abatement strategies for existing and planned installations. It has been released into the international water market following the recent launch of Water Innovate Limited, a spin out from the School of Water Sciences. Water Innovate is bridging the innovation gap by providing a conduit for technology transfer out of the laboratory and into the water industry.

Water Innovate’s initial focus is on marketing ODOURsim®, N-Tox® (a new nitrification toxicity monitoring technique) and a novel high performance chemical additive for water and wastewater treatment. It also has three advanced tertiary treatment process technologies in development.

The use of dispersion models (such as ISC and Aermod) to predict odour concentrations originating from sewage treatment works is commonplace. The resulting odour contour plots are often used in support of planning applications to show nuisance will be minimal. The models can be used to make decisions about site-specific odour control technologies.

However, it is important to consider that each element in the odour annoyance pathway (see below) is equally important. An over-emphasis on dispersion modelling, where other pathway elements are treated simplistically, can give inaccurate predictions.

One of the main problems in modelling the annoyance pathway is the difficulty in measuring odours. Odorant concentration can be measured analytically or the effect of odorants on the sense of smell can be evaluated using dilution methods.

Because of the over-emphasis on dispersion modelling and the lack of attention given to inputs to dispersion models, the School of Water Sciences developed ODOURsim®. Using H2S, the software concentrates on accurate mechanistic modelling of formation and emission of odour.

The relationship between odorant concentration and perceived odours is complicated because sewage emissions contain many different odorants. H2S is commonly used as a proxy indicator of overall odour strength because it is easily measured and it is one of the most common odorants associated with wastewater.

The software employs a liquid-phase H2S model and uses mass-transfer calculations for various process components such as weirs or sedimentation tanks. The software allows the impact of flow, quality and meteorological variables on emission rates to be examined over time.

Hence, ODOURsim® avoids the inaccuracies inherent in using a constant H2S dispersion rate, through mechanistic modelling of variable H2S formation and emission at source.

Because of variations in wastewater flow and quality at a treatment works inlet, significant differences result in influent odorant concentrations and subsequent emissions from downstream treatment processes. To investigate these effects, ODOURsim® has been used to model a sewer feeding a set of primary sedimentation tanks.

The simulation was engineered so the sewer flows full for periods, with conditions becoming anaerobic in the sewer, allowing H2S to form. During low flow conditions the sewer fills part full and aerobic conditions return, allowing H2S to be oxidised.

ODOURsim® reveals emission peaks at high flow conditions because the sewer flows full, allowing H2S to form, and emissions from the sedimentation tank weirs are enhanced by high flows.

These large variations have implications for when odour measurements are taken. If spot emission rate measurements were being inputted into dispersion models, the time of measurement would be crucial (see graphs below). As ODOURsim® is dynamic, the impact over time of variables on emission rates can be predicted.

Above Left: Odour Emmission Variation At 09:00
Above Right: Odour Emission Variation At 16:00

Where there are significant variations in emission rate, deviations can radically alter odour footprint. Therefore, it is essential for emission variations to be included in odour modelling exercises, and integrated odour modelling approaches developed.

No other commercial software modelling tool is currently available that deals with H2S formation and emission to accurately calculate odours arising from different wastewater unit processes. Because ODOURsim® is compatible with existing dispersion models its adoption is relatively simple. The improved level of confidence from dispersion modelling exercises will be important to water utilities, plant operators and regulatory authorities alike in the cost effective management of odour abatement.

Authors’ Note

Steve Callister is Managing Director at Water Innovate Limited. Visit www.waterinnovate.co.uk or telephone 01234 756014 for further details. 

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