Milan Research Solves MBT Odor Problem
The growing emphasis on being less dependent on landfill is impacting on both commercial and domestic waste streams. Mechanical Biological Treatment (MBT) plants treating residual waste will emerge in response to the need to meet LATs targets and comply with European legislation. Constructing MBT plants in urban areas is likely to raise concerns in the community particularly relating to odor emissions.
Chemical or thermal technologies to treat odors are often expensive, so Bord na Mona (Anua in the U.S.) is investigating how biological systems can be harnessed to treat odours from MSW efficiently and, importantly, cost-effectively. Work has been conducted in Milan, Italy where composting MSW has been underway for a number of years. Studying the control of odors 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.
When developing cost-effective treatment systems for odor control of MSW it is important 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 odors high in alcohols and terpenes, predominantly derivatives of limonene or lemon oil. Bord na Mona has experience of treating these wastes cost-effectively with its woodchip based media. 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. Here woodchip media is simply not effective.
Odor control for MBT plants should start by considering the handling of the waste, odor containment and ventilation rates to ensure that odor 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, so biofiltration was selected to trial and optimise with MSW composting plants in Milan.
Biofilters use a media to provide an environment on which bacteria flourish. Odors in the air are passed through the filter and are degraded by the bacteria. However conventional biofilters have two significant drawbacks: a large footprint and relatively short media life. Bord na Móna has addressed these problems by developing an engineered biofiltration media, called MÓNAFIL, 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. Low pressure drops signify good performance with associated lower energy requirements and lower energy bills.
Trials with MÓNAFIL ran on four composting plants treating Milan’s municipal solid waste. Two of the plants ran with MÓNAFIL while two other plants operated with a standard peat based media. A stringent odor limit of 200 odor units/m3 at the biofilter outlet was achieved with 3000m3 of MÓNAFIL medium continuously operating for seven years without the need for media replacement. The field trial results show that MÓNAFIL has a longer media life than other media, particularly when treating air flows over 100,000 m3/h. Also the media is designed to minimize environmental impact through out its lifecycle. At the end of its life MÓNAFIL can be screened, graded and up to 50% of it reused, reducing disposal costs. Its low fines content and large void spaces means the filter operates at one third of the pressure drop than conventional filters with media such as woodchip. This translates to using 30% less power and equates to annual savings of £32,000 for an average sized biofilter using 8,000 m3 of media: equivalent to 1410 tonnes of CO2 saved in energy production.
Biofiltration offers the most cost effective odor control option but needs to be designed to treat a waste stream with predominantly different odor characteristics to green waste making woodchip media less effective. Peat biofiltration is regarded as a BAT (best available technology) for the treatment of odorous air from MBT plants. With improvements to peat media, such as MÓNAFIL, that extend media life, reduce footprints and lower operating costs, biofiltration is set to reliably control odors from MBT and help reduce the environmental impact of urban waste management infrastructure.