BIOLOGICAL FILTER

Abstract: A biological filter for purifying gases, in particular malodorous gases, comprises a bed of compost screened from initial compost. The bed consists of the bigger particles of the substantially active fraction of the initial compost mixed with a substantial inactive constituent having consid­ erably bigger particles than the active fraction.

Description: The invention relates to a biological filter for purifying gases, in particular stinking gases, in which. a bed of compost screened from initial compost is provided. It is generally known that compost filters should comprise a compost bed having a certain dimen­ sion in the direction of flow of the gas to be purified, said dimension usually being the height. The developed pressure drop over the filter bed defines the energy consumption. In order to maintain said energy consumption within defined limits filters having a large area are necessary. The initial compost may be screened for remov­ ing the fraction having the smallest possible particles. Said screened compost provides only a low reduction of resistance with respect to the initial compost. The invention has the object to provide a filter of above mentioned type in which the resistance is reduced even more. According to the invention, said object is achieved by the features of the characterizing portion of claim 1. Surprisingly it is found that by said feature of the invention the pressure drop over the filter may be reduced by a factor of 20. Thereby the necessary size of the filter may decrease by a factor 2 up to 5, while the purifying action of the filter is maintained. In an embodiment of the invention the sub­ stantially inactive constituent consists of the coarser particles of the initial compost. Thereby, the advantage is obtained that as filter material only initial compost is used and it is not necessary to add additional material. The invention will be elucidated here below by reference of the embodiments. Biological filters comprise a bed of filter material formed by a medium containing micro-­ organisms. Usually, compost is used as medium. Such filters are used for odour removal in among other sewage water purification and destruction plants. In the so called compost filter the compost bed has a height of approximately 1 m. The gases to be purified, in particular stinking gases, are conveyed vertically through the bed and disinte­ grated by the micro-organisms in the compost. For conveying the malodourous gases an installation is necessary having a certain energy consumption. In view of said energy consumption the pressure drop in the filter bed is of great importance. If the height of the compost bed is chosen too high, the pressure drop is too high. In order to maintain the energy consumption of the installation as low as possible, the height of the filter bed is kept as low as possible. However, a too low height of the filter bed will easily induce continuous cracks resulting in leakage in the filter. A height of approximately 1 m seems to satisfy well in practise. An example of the fraction composition of the initial compost gives the following numbers: Fractions having a particle size 45% smaller than 4 mm: Fractions having a particle size 20% between 4 and 6 mm: Fractions having a particle size 25% between 6 and 10 mm: Fractions having a particle size 10% of more than 10 mm: The two first mentioned fractions seem to consist of active material and the two last men­ tioned of substantially inactive material. Thus, the conversion by the micro-organisms substantially takes place in the fractions of the smallest particles. While by screening the fraction of the smallest particles,for example smaller than 4 mm, from the initial compost, the pressure drop of the filter bed may be reduced maximally by a factor 3, however, the activity of the filter is not sufficient by the lower content of active constituents. Surprisingly it is found that with respect to a filter from initial compost the pressure drop may be reduced extremely, at least by a factor 10, because for the filter bed the bigger particles of the active fraction of the initial compost are used, to which the substantially inactive material having considerably bigger particles than the active fraction is added, while the operation of the filter with respect to the disintegration by micro-­ organisms is maintained. Thereby, for conveying malodorous air through the filter an installation may be sufficient having a very reduced energy consumption, or a smaller filter area with an equal energy consumption. It is found, that for the substantially inactive material the coarser particles may be taken from the initial compost. Thereby, additional material to be added is not necessary. The active fraction contains particles having a size between approximately 4 and approximately 6 mm, whereas the coarser compost particles are of a size bigger thai approximately 10 mm. By reference of the followimg example the invention will be elucidated in detail. Example To a filter having non-screened compost a gas having an area load of 35 m³/m²h was supplied. Thereby, a pressure drop of 80 mm water (785 Pa) column was measured over the filter. By area load of a filter is understood the volume of conveyed gas per hour per unit of area. For a filter filled up with the compost composi­ tion according to the invention a pressure drop of 100 mm water column (981 Pa) was found at an area load of 500 m³/m²h. Thus, with the same yield the area of the filter may be reduced extremely. Besides the above mentioned reduction of pressure drop and decreasing of the filter area found, the compost filter according to the inven­ tion has yet the advantage that the problem of settling of the filter is reduced extremely. By the more stable stacking the pressure drop remains more constant seen in the time. An important condition for the disintegration activity of the micro-organisms is the humidity of the compost bed. Therefore, frequently compost filters are moisturized artificially. However, an excess of water induces a high pressure drop. In a filter implemented according to the invention this is not a problem because excess of water is discharged effectively by the looser stacking of compost particles.

Claims: 1. Biological filter for purifying gases, in particular malodorous gases, in which a bed of compost screened form initial compost is provided, characterized in that the bed consists of the bigger particles of the active fraction of the initial compost mixed with a substantially inactive constituent that consists of considerably bigger particles than the active fraction, and that the sizes of the particles of the active fraction amount between approximately 4 and approximately 6 mm and of the substantially inactive constituent more than approximately 10 mm. 2. Filter according to claim 1, characterized in that the substantially inactive constituent consists of the coarser particles of the initial compost. 3. Filter according to one of the preceding claims, characterized in that the mixing ratio of the two fractions amounts to approximately 1: 1.

Inventors: DON, JOHANNES ALBERTUS, FEENSTRA, LOURENS
Applicants: NEDERLANDSE ORGANISATIE VOOR TOEGEPAST-NATUURWETENSCHAPPELIJK ONDERZOEK TNO
Application year: 1982
Main classification: B01D53
IPC codes: B01D53