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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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