Environmentally friendly exhaust air purification using biofilters

The biofilter method has been used for biological waste treatment for many years. The advantages are obvious: low investment and operating costs and a high level of environmental compatibility. For some 20 years, exhaust air from liquid detergent and cleaning product production at the Mibelle Group in Frenkendorf has been purified using biofilters.

  • The level of the nutrient solution, which provides the trickle filter with adequate minerals, is being checked.

  • Temperature control: the optimum trickle filter temperature is 25-30°C. The biofilter is most effective at this temperature.

  • The biofilter system consists of a fibreglass-reinforced polyester container, weighing 17.5 tonnes. Its contents consist of wood chips, compost, expanded clay and chalk layers.

  • The system is capable of purifying 1500 Nm3/h of exhaust air per hour. The air given off via the chimney smells slightly earthy. The solvent removal performance is at a constant > 95%.

  • The graphic shows the biofiltration system used at the Mibelle Group in Frenkendorf.

The idea of biological purification of exhaust air is not new. Biofiltration is a relatively simple and cheap method of purifying exhaust air. Microorganisms such as bacteria, fungi and yeasts destroy the substances in the air and convert them into harmless, odour-neutral products. 20 years ago, the Mibelle Group’s specialists considered an environmentally friendly way to purify air. “The requirements at the time were for cleaning performance to be consistently high and system maintenance low”, explains Fredy Kühnis, Head of Safety & Sustainability at Frenkendorf.

Exhaust air purified with two-stage biofilter
The Mibelle Group manufactures detergents and cleaning products such as Total, Elan and Potz at the Frenkendorf site. Production produces around eight tonnes of pollutants per annum that are contaminated with solvents ethanol and isopropanol. A biofilter is supposed to reduce these pollutants effectively to a legal level. The Mibelle Group decided to construct a two-stage biofilter. A large proportion of pollutants are removed at the first stage, the trickle filter. An organic carrier (wood chips) removes the remainder of the organic solvents and fragrance compounds at the second stage. The organic exhaust air contents are converted into CO2 and are used to form new biomass.

“It was important for production to run independently of the biofilter system, especially in the case of breakdowns. This is why we built the plant outside the production building”, says Fredy Kühnis. Production exhaust air is fed into the plant with the help of a constant volume flow regulator. When certain production lines are not running, the hot air from the hall is fed into the biofilter via a regulated valve. This prevents the biofilter from freezing up in winter.

95% removal and low energy consumption

A biofilter is favoured because of its low energy consumption - it only requires electricity for the ventilator and pumps - and the low water consumption compared to other purification methods. Also, it does not deposit pollutants in waste water. The thought put into the choice back then has paid off. Fredy Kühnis is pleased to report that “We remove more than 95% of pollutants from exhaust air using biofiltration.”

Sustainability facts about biofiltration in Frenkendorf:

  • Construction year: 1999/2000
  • Exhaust air rate: 1‘500 Nm3/h
  • Pollutants (Solvents): Ethanol, Isopropanol
  • Production exhaust air: 800 kg solvents per year
  • Clean gas after biofiltration: 40 kg solvents per year
  • Performance: > 95%
  • Investment CHF 300‘000.-

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