Working out how to control odours

Regulation of odour emissions generally results from a nuisance complaint at the site boundary. So, an effective odour control system must ultimately achieve a defined boundary condition. Odour sampling and dispersion modelling allows an emission standard to be defined and monitored ensuring set odour concentration limits at the site boundary are not exceeded.

The parameters used to define this emission standard are important. Most technology providers use H₂S to measure their technology’s performance. Whilst this is relevant for municipal treatment, it may not be adequate for industrial odour emissions or landfill and compost sites disposing of municipal solid waste.

Defining odour in terms of H₂S is like defining a fine wine by its alcohol content. The odour from leachate treatment or compost results from a range of compounds. High ammonia content, ketones and carboxylic acids contribute to the odour. Simply removing H₂S would not solve the problem.

Olfactory testing

Using odour units to define emissions at the site boundary provides a comprehensive measurement involving olfactory testing. It is also sensible for sewage treatment plants. At some works mercaptans can be the cause of the odour. We have encountered sites where H₂S was as low as 0.3 ppm while odour units were in excess of 20,000. H₂S is easy to measure to describe removal performance but it will often lead to under sizing of technology and its likely running costs if used as a design parameter.

While odour units are a good performance indicator, likely to be supported by the Environment Agency in future guidance publications, selecting the correct technology requires an understanding of the compounds causing the odour and the source of the odour in relation to the site boundary.

Modelling dispersion of the odour from the ensures that suitable technologies can be selected accounting for the proximity of the stack to the site boundary. Also, understanding the character of the gas stream causing the nuisance odour is important to provide meaningful performance guarantees. While the constituents of odour from sewage treatment are well established and relatively consistent this is not the case for plants treating landfill leachate and for some industrial sites.

The change in emphasis to BAT under IPPC requires greater attention to the environmental impact of the technology being installed. This includes energy efficiency, consumption of raw materials and the length of time required to install it. Selecting BAT is efficiently achieved by constructing a matrix for each specific application to assess key technical features. Applying these criteria to selecting BAT for odour treatment may significantly alter the balance of technologies traditionally used.

Chemical scrubbing is historically favoured with two or three scrubbing stages being used. This technology may suffer under IPPC as it has low removal efficiencies for organic compounds and very high running and maintenance costs. In addition, concerns surrounding the environmental impact of the chemical scrubbing reagents raise questions over its suitability under BAT.

Thermal oxidation is also popular. However, it is very energy intensive. Its use at landfill sites can be cost effective if methane vented from the site is used as a fuel. The economics and environmental impact can also be improved by either using regenerative oxidisers or by utilising the heat output for space heating. Controlling nuisance odours with thermal oxidisers becomes less attractive considering the resultant SOx and NOx emissions in relation to the environmental impact of the nuisance odour itself.

Activated carbon physically adsorbs molecules on to its high surface area by intermolecular attraction. It is a useful technology where the average concentrations of odorous compounds are low or high loadings are intermittent. The media needs to be replenished when breakthrough occurs but otherwise the unit is easy to control and operate. However, media replacement costs are expensive. The type of activated carbon used dictates its removal efficiency. Impregnated activated carbon is moisture passive and performance is often enhanced by high humidity, contrary to popular belief.

Biofiltration is increasingly popular for treating nuisance odours. The technology has improved considerably and biofilters are robust, reliable and offer low running costs. This is mainly due to extensive research on support media and biofilter operation delivering compact technology with excellent removal efficiencies and low environmental impact. Today’s “enhanced biofilters” are designed to treat up to 500,000 ou/m³ (approximately 250ppm H₂S) with an efficiency of 99.9% reduction.

Installing the correct odour treatment technology should have a net positive impact on the environment. A guidance document for odour control will be published by the Environment Agency later next year. In the mean time, careful assessment of BAT for a particular site can ensure an efficient nuisance odour control system is commissioned.

Table 1: Cost Comparison Matrix for Odour Abatement Techniques

Calculations based on an air flow rate of 50,000m³/h, typical media lifetimes, media replacement costs, electricity at 3p per kWh, gas at 20p per Therm, 6240 operational hours per annum.

Table 2: Typical Matrix comparing attributes of Odour Abatement Techniques

Note: *impregnated carbon may improve with humidity where condensation is not washing out the impregnant.