Cost-effective advances in odour control from urban MBT plants
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by Bob Maloney
Odorous emissions are a serious concern relating to biowaste treatment facilities and affect the likelihood of planning permission being granted particularly in urban areas. Depending on weather and topographical conditions, the odours from these plants may be carried several kilometres. They are most likely to be significant within 500 metres of the site and this is reflected in most countries’ planning legislation that specifies minimum distances between a treatment facility and the nearest houses. There is also some debate as to whether airborne micro-organisms from composting processes can be carried in the exhaust air from the process. As the need for alternative treatment mechanisms for MSW grows, minimizing the environmental impact of such facilities can only help to speed up the construction process. Research has shown that odour is one of the prime concerns from urban residents where MBT plants are proposed.
Combating odours through biofiltration
There is a wide range of technologies to treat odours, including expensive chemical and thermal technologies. According to Bord na Móna, a company based in the Republic of Ireland with expertise in odour control technologies, the most reliable and effective technology to treat odours from composting facilities is biofiltration. And in the UK this is reflected in its status as a recognized Best Available Technique (BAT).
Biofilters use a media – often peat (see box) or heather – to provide an environment on which bacteria flourish. Odours in the air are passed through the filter and are degraded by the bacteria. Biofiltration converts compounds in an air stream to carbon dioxide, water and biomass by specially selected, harmless, naturally-occurring bacteria supported on a carrier media. There are potentially 1400 to 1600 different varieties of bacteria that can be used within a biofilter to accomplish this.
A conventional biofilter operates by engineering the correct environment in a closed vessel, ensuring that the bacteria within it are cultivated to effectively biodegrade the compound in the air that passes through it.
What type of odour?
Before looking more closely at biofiltration and odour control, it is important first to understand the constituents of the waste and its state of degradation. Composting plants treating source-separated green waste and low quantities of kitchen waste emit a completely different set of odorous compounds to MSW with high organic fractions. Fresh green waste with low concentrations of kitchen waste (approximately 10%) emits odours high in alcohols and terpenes, predominantly derivatives of limonene or lemon oil. In contrast MSW emits odours more akin to those found on a landfill site: they have low solubility and higher volatility with alkanes and short chain volatile fatty acids predominating. Alternate bin collections leads to MSW being held in plastic bags before being collected for disposal resulting in some waste reaching a treatment facility in an advanced state of anaerobic decomposition.
Odour control for MBT plants should start by considering the handling of the waste, odour containment and ventilation rates to ensure that odour is effectively captured for treatment. Cost-effective treatment requires the selection of appropriate technology, but the temptation to just consider capital costs rather than a whole life cost that incorporates running costs can be an expensive mistake. Biological systems have significantly lower operating costs with low consumables and energy consumption.
The Bord na Móna system: Mónashell and Mónafil
Bord na Móna has developed a patented system of enhanced biofiltration technology. Two of its main products in this sector are Mónashell and Mónafil.
 Peat biofiltration can treat odours from MSW efficiently and cost-effectively. |
The former uses shells coated with bacteria as its filtration media. The media is supported on a plenum floor that allows exhaust air to be transported evenly though the media bed. This improves the contact between the bacteria and the odour molecules being treated. Recirculation of the airstream improves the rates of degradation by the bacteria.
The shell media has several advantageous characteristics. These include:
- A surface structure that is excellent for supporting bacteria and has the ability to retain moisture in the event of temporary water failure.
- A self-supporting structure with good packing characteristics that ensure there is minimal pressure drop across the media bed (thus reducing running costs).
- The right conditions to encourage bacteria to metabolize more than grow which minimizes sludge production and hence associated disposal costs.
Whilst Mónashell is used primarily in industrial applications, including treating odours from sewage treatment works, of greater interest to MBT facilities is the Mónafil technology, which was developed to overcome two significant drawbacks of conventional biofilters, namely a large footprint and relatively short media life.
Mónafil is produced from decomposed peat. It is highly stable with a uniform granular particle size (10-20 mm), so it can be packed 3m high without compromising its performance, delivering a greatly reduced footprint that lends itself to urban based MBT plants where space is at a premium. The size of the granules also keeps operating costs low. Air needs to pass uniformly through the media in a biofilter and the effectiveness of airflow can be determined by the pressure drop across the filter. As with the Mónashell technology, low pressure drops signify good performance with associated lower energy requirements and lower energy bills (more on this to follow).
Mónafil in practice
Mónafil material has been used for a number of years in Italy for the treatment of air from the composting of municipal waste. One of the earliest and largest of these applications took place in the city of Milan. The disposal of municipal solid waste in Milan became a critical issue in the 1990s. The municipality leased a site near the centre of the city to four private companies to install four composting plants on the site. As the site was very close to residential areas, severe odour limits were imposed by the environmental agency in Italy (200 OU/m³ at the biofilter outlet).
 A Mónafil biofilter |
Of the four companies that installed the odour control facilities on those composting plants, two used Mónafil as the primary odour control media. These filters, using a total of 3000m³ of Mónafil media, passed all odour tests undertaken on them to meet the strict criteria laid down by the municipality, and were in operation for seven years without media replacement (prior to the closure of the facilities in 2004).
Low pressure drop means reduced running costs
One of the principal and defining characteristics of the peat media produced by Bord na Móna is the very low pressure loss associated with moving air for treatment through the filters. This is due to a number of important parameters:
- very low fines content (typically <5%)
- large void spaces (typically>75%)
- resistance to breakdown (life expectancy in excess of five years).
If we consider that with 8000 m³ of biofiltration media it is possible to treat around 800,000 m³ per hour of air, it is possible to compare power requirements for air handling using different media types. Bord na Móna found that its Mónafil media uses 545 kW per hour, compared to 760 kW per hour for other media. This equates to an annual saving of around £32,000.
Media performance may also be assessed in terms of greenhouse gas emissions. According to calculations from the Sustainable Energy & Economy Network, this may have a potential positive impact on CO2 emissions from power generation of oil of 1410 tonnes of CO2 saved.
These factors, when taken with the other minimal cross-media effects of the Mónafil material, make using this peat-based media in biofiltration an environmentally sound and sustainable method for air pollution abatement. Expected media life of the media is about five years. After five years it may be possible to screen the media for removal of the fines fraction. The larger fraction (up to 60%) can be re-used as biofiltration media. The fines fraction can then be used in horticulture as a soil conditioner or as a mixer in the composting process.
Even at the end of the seven-year period treating the Milan plant the condition of the media was still relatively good, with very low back pressure across the media and very low running costs. The media has since been removed, rescreened and used in other plants.
In conclusion
As the need for new waste management infrastructure across Europe becomes more prominent, in many cases – particularly where priority is not given to thermal treatment and waste to energy – MBT plants are likely to have a warmer reception than other technologies. Countries that require significant investment in waste infrastructure, such as the UK, can learn a lot about effective odour control from other countries in mainland Europe where more widespread investment in such technologies has already occurred.
Peat biofiltration offers a cost effective odour control option for application in MBT facilities, where the filtration system is dealing with a waste stream with predominantly different odour characteristics to standard green waste (making woodchip media less effective). And with improvements to peat media that extend media life, reduce footprints and lower operating costs, biofiltration is set to reliably control odours from MBT and help reduce the environmental impact of urban waste management infrastructure.
Bob Maloney works with from Bord na Móna Environmental Products, UK
Peat biofiltration
Peat biofiltration is well respected as a treatment for odorous emissions from MBT plants. As a result, Bord na Móna began investigating how peat biofiltration could be enhanced through improving the engineering of the media to treat odours from MSW efficiently and, importantly, cost-effectively. The company has participated in research conducted in Milan, Italy, where composting MSW has been underway for a number of years. Studying the control of odours with biological systems treating high air flows up to 300,000 m3/h has led to an innovative development based on insights gained in operating these facilities – see main text for more information.
