Composting & Maturation Ventilation
EVS Ltd provide General Extract Ventilation Systems for Composting Plants to ensure the containment and control of odourous air generated and released within the Reception, Maturation and Process Areas is achieved through negative pressure. Contaminated air removed from the area is treated therefore neutralised via either a Bio-Filter System or Chemical Scrubber to ensure clean air is dispersed to
Contaminated air within Composting Plants has a high Relative Humidity and a temperature rise of up to 40ºC above ambient during warm periods, and with the presence of ammonia the environment within the building is corrosive, requiring system components to be manufactured from PVC, UPVC and/or Polypropylene.
We can offer a full service package from initial design through to installation, commissioning and
documentation, ensuring compliance of current UK Legislation.
For more information visit www.hse.gov.uk/waste/composting
Odour is one of the most common, immediate and potentially damaging emissions at composting sites. It should not come as a surprise that it is also one of the most widespread sources of public complaint about their environment. Odour is also well represented in concerns raised during public consultation exercises and may be a determining factor in planning applications, permit determinations and even prosecutions.
There will always be the potential for odourous releases at composting sites. As such, site operators will need to instil confidence with both neighbours and the relevant regulatory authorities that sites can be managed without causing unacceptable levels of odour. Any failure of individual sites to realise this important standard reflects badly on the industry as a whole and can make communities
understandably reluctant to accept new sites in their neighbourhoods.
In order to manage odour effectively, knowledge of what odour is, how individual people react to it and how it can be measured is essential. It will be the responsibility of site operators to assess individual circumstances and consider how best to take advantage of the opportunities for odour prevention and control which exist.
Section 33(1) (c) of the Environmental Protection Act 1990 makes it an offence to manage waste in a manner likely to cause harm to human health or the environment, including offence to the senses. As with statutory nuisance, this does not apply to sites that are regulated under the Pollution Prevention and Control Regulations. The Pollution Prevention and Control (PPC) Regulations 2000 (as amended) primarily apply to large and complex composting operations, such as MBT facilities. These sites
operate under a permit which requires the operator to use best available techniques (BAT) for the
control of odours. In England and Wales the Environment Agency has issued draft PPC guidance H4.
The first priority of any site should be to identify and take advantage of all reasonable opportunities to prevent the formation of odours in the first place. Good practice should be carried out as routine, however, additional measures may be necessary from time to time, or where certain feedstocks are concerned. The odour treatment technologies discussed here may be necessary and useful additions to measures to reduce the formation of odourous chemicals discussed earlier. However, relying entirely on treating odours is a risky and potentially costly strategy. Some of the considered solutions may
involve significant capital expense, but this in itself is no guarantee that the outcome will be
satisfactory. Keep in mind, however, that the cost of non-compliance in relation to odour generation can, in some instances, lead to site closure, which helps to put expenditure on odour prevention and mitigation into perspective.
In order to get the most out of any odour abatement system, it will need to be well maintained and
actively managed. As in all aspects of biowaste processing, there needs to be a recognition that both the materials and ambient conditions are continuously changing. For this reason the operator must never be complacent with the system that is in place and be prepared to make modifications as
Bio-filters are organic, microbially active substrates that filter odorous air. They are used as a primary mechanism for converting odorous compounds into less odorous or odour- free compounds. As the air passes through the aerobic bio-filter substrate, odourous chemicals dissolve in the water layer and micro-organisms within the substrate metabolise the odour molecules before the air is discharged to atmosphere. Bio-filters are biologically active and will require active management. The micro-
organisms within the bio-filter are fed through metabolising odourous compounds. If a bio-filter is
accustomed to coping with moderate loadings on a continuous basis, in the event of a sudden loading occurring, it will not be able to respond quickly to this dramatic change in condition.
There are two types of bio-filters, both of which work on the same principle:
- Closed bio-filters, where the filtration medium is housed within a structure. This will typically have an inlet port for the dirty air, and an exhaust port for the treated air that has been through the filter medium.
- Open bio-filters, where the medium is exposed to atmosphere, sometimes at the sides as well as the top. Gaseous emissions will escape freely from this on all exposed surfaces.
The latter system will have a lower capital cost and may be used periodically in the event of particularly odourous feedstocks being admitted to the processing facility (for example, as part of a contingency plan).
Some closed systems pass the filtered air and steam through a condenser unit to provide a further layer of environmental protection. This will also ensure that no steam plume is visible coming from the treatment plant as this may cause concern to neighbours.
A bio-filter’s effectiveness depends on the bio-filter substrate being actively managed. Bio-filters are static in design and closed units will normally be constructed with a concrete or steel retaining wall. The efficiency of a bio-filter can be measured by dynamic dilution olfactometry of the inlet and outlet. Bio-filters will have some odour of their own, but when properly maintained this should be minimal and not particularly offensive. All Bio-filters generally work at 85–95% efficiency levels providing that they are proactively maintained. Take nothing for granted and ensure that standard operating procedures are in place for their monitoring and maintenance.
Bio-filters, in a wide range of configurations, are by far the most popular and common method of odour scrubbing at composting sites. They are generally quite successful in this role because of their potential for high removal efficiencies and low cost. However, bio-filters are not particularly effective with highly variable odour streams, and will require a relatively large area and need to be actively
maintained by competent personnel. Because of these limitations, there may be circumstances where wet chemical scrubbing is a suitable supplement or replacement for bio-filtration in the removal of
Wet chemical scrubbers provide intimate and prolonged contact between the treated air stream and an aqueous absorbing solution. A wide range of variations are possible including:
- Re-circulating and single-pass scrubbing solutions
- Acidic or alkaline scrubbing solutions
- Oxidising scrubbing solutions
- Packed column, plate or spray towers
The removal of dust can help to reduce the overall odour burden but de-dusting units on their own are not generally effective for the removal of odourous volatile chemicals.
Capital costs for these systems can be relatively high, particularly when equipped with systems to re-circulate and chemically dose the scrubbing solutions. However, their compact size, high potential removal efficiency and ability to handle highly variable air streams can make them cost effective in some cases, particularly where high volumes of air require treatment (e.g. building ventilation).
Careful design and knowledge of the odourous chemicals present is also critical for their effective
application. For example, acidic scrubbing solutions may effectively remove ammonia but have no
effect on other odourous chemicals such as volatile fatty acids (e.g. acetic and propanoic acids).
Effective scrubbing may therefore require two or more stages. Flow rates must also be designed to
allow sufficient residence time and to prevent excessive carry-over of scrubbing solution into the treated air stream.
For a free quotation without obligation please contact our offices on Freephone 0800 228 9973.
composting ventilation Nelson