A whiff of success
Odour control schemes have come a long way in the past two decades. But, as John Hobson says, there are still challenges ahead.Since privatisation in 1989, the issue of odour control in the water industry has steadily grown in significance.
Although the building blocks of odour control, chemical scrubbing, activated carbon and biological odour control were established 20 years ago, there were few examples of successful odour control schemes because of a lack resources, poor application and knowledge gaps.
So what has been achieved and what new challenges lie ahead? Today there are numerous examples of successful odour control schemes. This is the result of increased funding for odour control due to increased legislation and the industry's desire to remain on the right side of an increasingly vociferous public.
Chief amongst the legislative drivers are the serving of abatement notices requiring the use of "best practicable means" to prevent any statutory nuisance due to odour. In parallel are pollution prevention and control (PPC) regulations enforced directly by the Environment Agency on industrial processes.
Thermal drying and incineration will always fall under PPC regulations, which require the use of "Best Available Techniques" for controlling odour, as will large sludge treatment centres where the treated sludge is not used beneficially such as in agriculture. Any work requiring planning approval is also now likely to attract conditions relating to odour.
Good practice today includes good housekeeping on a treatment works, obeying simple operational procedures, preparing and adhering to an odour management plan and timely response to complaints.
Defra's voluntary code of practice on odour nuisance from sewage treatment works outlines a more detailed and quantitative approach when enhanced measures are required for more significant abatement schemes.
Following a quantitative approach like this greatly increases the chances of providing a successful odour control or abatement scheme.
Wastewater is generally characterised by a dozen or more parameters, but it is not infrequent to see odour control systems designed on little more than the concentration of H2S.
Additionally, we need to know the levels of organic sulphur compounds, of ammonia, of non-methane volatile organic compounds (nmVOCs) and the levels of carbon dioxide where they are expected to be high. These nmVOCs contribute to chemical requirements in scrubbers and can dictate the lifetime of activated carbon.
CO2 increases chemical demand and can make scaling issues worse. Even the concentration of water vapour in the air is significant as it affects its heating or cooling ability and can have a significant impact on the uptake of odours by activated carbon.
Even if a designer is happy to set performance requirements in terms of H2S removal, many of these other parameters will influence system performance even for H2S removal.
Odour concentration in ouE/m3 in the treated air is usually the main design requirement. The level of odour in the air to be treated is "nice to know" but is not a significant input into the design process.
Frequently it is necessary to design an odour control system before the offending air streams have been created. Then it is necessary to estimate values for all of the above parameters. With so many parameters and with an odour control project often dealing with a number of different sources, this is becoming quite a challenging task.
An area that directly influences cost is "how much air" should be collected and treated'. The reliance of the water industry on setting the number of air changes per hour to be extracted from a process is difficult to justify and can often lead to over design.
The (often) correct rate of extraction is the minimum amount necessary to prevent leaks. This should drive the quality of enclosures upwards and lead to even lower requirements for extraction rates.
An eye has to be kept on the quality of the air inside the enclosure in terms of corrosion and toxicity.
If these are to be used to set extraction rates, however, they should be done so explicitly, not by relying on rules about the required air changes per hour. If you know the mass emission rate of a source and the leakage rate from the covers then you can set the extraction rate to prevent leakage, quantify the likelihood of corrosion and obtain a more cost effective design.
The UKWIR publication, BPM - A Guidebook for Odour Control at WwTW, suggests measuring the benefit of an odour control scheme as the number of properties removed from odour impact.
Impact is defined as exceeding a benchmark odour concentration that can be identified by combining dispersion modelling with the locations from which complaints have been received. A curve of the numbers of properties removed versus the cost for a number of options can be used to identify the 'best' scheme. In Fig 1 those schemes on the line are the most cost effective. The circled scheme probably reflects the best value for money.
If it were possible to ascribe a monetary value to odour impact then the least-cost scheme as defined by the cost of the scheme minus the cost of the residual impact could be identified. The publication stopped short of identifying the value of odour impact.
Expenditure that is not designed to meet numerical environmental standards, such as on odour control, should be justified on cost benefit grounds. A method to do this should be welcomed but the regulators would not like any overly prescriptive approach.
The water industry has made great advances in odour control over the past two decades, but there are many new challenges. Despite successes there are still examples of failing installations. There is also a suspicion that some successful installations may have been over designed and unnecessarily costly.
I look forward to many new and as yet unenvisaged advances in the next few years let alone the next decade. n
John Hobson is air and odour expert and principal process engineer at MWH.
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