Biofiltration solves municipal refuse odour abatement problem on sites across Italy

Compound Formula Molecular
weight Odour
characteristic Limonene

C16H16 136 Lemon-like Acetone C3H60 58 Mint-like Methylethyketone
(MEK) C4H8O 72 Moderatly
sharp Cyclohexane C6H12 84 Mild
sweet Xylene
(mixed isomars) C8H10 106 Aromatic-like Benzene
derivatives – – Paint
thinner-like

Bord na Mona conducted a comprehensive study into choosing the correct abatement technology required to solve the odour problems generated at these composting plants. Bord na Mona has access to a broad range of technologies including biofiltration, carbon adsorption, wet scrubbing and thermal/catalytic oxidation. Other niche technologies include condensation, cryogenic condensation and membrane separation. The assessment of this range of technologies is normally achieved by developing a matrix for a particular application that assesses the key technical features against the selection criteria outlined above. The conclusion of that assessment will indicate the Best Available Techniques (BAT).

Selection criteria

The selection of the appropriate odour control technology and its successful application, relied upon adopting a problem solving approach. A thorough investigation of the sites was conducted by Bord na Mona personnel, utilising our extensive laboratory facilities in Ireland to conduct chemical analysis, odour audits and odour modelling.

A knowledge and understanding of the abatement technologies available needed to be combined with applications experience if the installed plant was to function effectively. The diverse range of Bord na Mona’s experience embraces sewage treatment, municipal refuse, chemical/pharmaceutical, painting/finishing-off gases, rendering plant odours and food and poultry processing. Bord na Mona was able to use these experiences in a specific sector to assist in the evaluation.

In order to optimise the efficiency of the abatement process, certain parameters were taken into consideration, these included:

• Type of application
• Emission requirements
• Qualification and quantification of the odour contaminants
• Total air flow and inlet temperature
• Location of equipment and available space
• Economic considerations and a net present value (NVP) assessment.

Delivery of refuse and the removal of the composted material both potentially exposed the process area to atmospheric emission and odour nuisance. Therefore, containment of the odours as well as its subsequent abatement, was of utmost importance to these applications. These technical challenges were overcome by including double door entry and exit arrangements for vehicles and personnel, and by ensuring that the atmosphere within the production buildings was extracted under negative pressure before being presented to the odour control equipment. This was achieved by designing the system with odour abstraction fans on the exhaust side of the control unit thereby ensuring that any leakage was clean air into the system rather than untreated odorous air expelled from it.

Extensive chemical determination of the odours from the municipal refuse composting plants in Italy used both GC-MS analysis and Forced Choice Dynamic Olfactometry. The results of these tests revealed a wide variety of different chemical compounds with the main constituents shown in the table below. The total inlet concentration of these VOCs was measured between 50-70 mg/m3 resulting in an odour concentrations of between 10,000-20,000 ou/m3.

Evaluation

During the selection process for composting odour control, biofiltration and chemical scrubbing were considered. The chemical scrubbing process oxidises the odorous compounds and normally consists of two or three stages. The first stage utilises sulphuric acid followed by second stage hydrogen peroxide scrubbing. Occasionally a final stage scrubbing with sodium hypo-chlorite is incorporated in the flow sheet. The scrubbing technology is well known and is widely used, but suffers from high running and maintenance costs and typically low removal efficiencies of organic compounds. In addition, there is increasing environmental impact concerns regarding the use of these chemical scrubbing reagents.

All of these considerations were assessed and a final decision was made to use biofilters for both of these applications in Italy due to the low capital cost of the system, linked to very competitive whole life costs. In addition, the nature of the odours which arise from municipal biological composting, lend themselves naturally to the use of biofilters for efficient removal.

Plant design

Biofilters remove organic compounds by providing an enclosed environment in which selected bacteria rapidly biodegrade the VOC compounds. These bacteria are held within an appropriate carrier which is packed into the biofilter.

Bord na Mona has developed specialised peat-based media, offering exceptional durability, high mechanical strength, regulated moisture content and high organic removal efficiencies. This media, known as MONAFIL, proved the key to the successful application of biofilters for the composting projects by demonstrating high removal efficiencies for a wide range of organic compounds. The patented MONAFIL media is produced from high density (500kg/m3 @ 50% moisture content) well decomposed sod peat with a particle size distribution of 10 – 20mm. The patented process enables media depths of up to 3 metres to be incorporated in the design compared with other media fills – generally restricted to 1m depth due to the problems of compaction.

Recent odour abatement performance tests on the MONAFIL biofilter at one of the composting plants in Italy, demonstrated a consistently high efficiency of the biofilter in removing odours generated by the composting process.