In spite of conventional gas cleaning steps, flue gases released from a great number of processes may still con- tain gaseous and solid contaminants. In the removal of these contaminants from said gas streams, microbiological filters have proven their value. In microbiological fil- ters, the contaminants contained in the gas streams are contacted with a microbial flora containing a microbe or microbes capable of decomposing the contaminants carried along with the gas stream. A gas purification apparatus based on this principle is described in patent applica- tion publication WO 93/07952, for instance.

The apparatus described in cited publication contains a porous substrate for microbial culture growth, made from a mineral wool sheet, through which the gas stream to be purified is passed. The habitat of the microorganisms living in the substrate is controlled, e.g. by passing an aqueous solution of nutrients required by the microorgan- isms into the growth substrate. According to an embodi- ment, the nutrient solution is sprayed into the gas stream to be purified prior to its entry into the filter material acting as the growth substrate. This type of apparatus has been found an effective means of gas puri- fication, because for example in an application used for purifying the flue gas stream of a mineral wool manufac- turing process containing among other contaminants, the basic monomers of the mineral wool binder, the purifica- tion efficiency with regard to phenols has been reported to be better than 95 % and, with regard to ammonia, bet- ter than 98 %. A mechanical filter developed for the same

task has exhibited purification efficiency figures of about 20 % for both phenols and ammonia. With regard to formaldehyde, this type of filter performs, partially due to the chemical reactions occurring therein, with a slightly better efficiency reaching a purification effi- ciency of about 50 t.

One of the problems hampering both mechanical and biolo- gical filters of the pass-through type is filter plug- ging. This is because the process gases inevitably con- tain, in addition to the gaseous chemical compounds to be removed, solids that eventually plug the filter. One ap- proach to the problem is to subject the gas stream to prepurification steps that remove solids from the gas stream. A satisfactory result cannot, however, be attain- ed in conventional filter constructions of a practicable size. An example of complicated waste gases can still be mentioned the waste gas of a mineral wool manufacturing process which, in spite of prepurification steps such as a slowed-down settling stream, contains binder recidues both in gas and aerosol form at its entry to the filter, as well as fine fibers.

The filter plugging problem is avoided in microbiological purification systems in which the gas stream to be puri- fied is contacted with a microorganism culture by way of making the gas stream sweep over the culture medium sub- strate. Such a purification system is described in patent publication DE-A1-4017384, and a specific implementation of such an apparatus is disclosed in patent publication EP-A1-0575 011. Advantageously, the growth substrate for the microorganism cultures is comprised of an inert mate- rial. Examples of such substrate materials are polymer, steel and glass-fiber fabrics. Additionally, such purifi- cation equipment are complemented with spraying means adapted to increase the moisture content of the gas to be purified, whereby the water spraying means serve both to

improve the mass transition of contaminants in the gas- liquid contact and to maintain a suitable moisture con- tent in the biofilm formed on the culture growth substrate. A purification plant may comprise a plurality of filter units in series each containing different bac- terial species.

However, none of the prior-art embodiments have contem- plated the possibility of providing suitable habitats for the different microbial species in different parts of a single and the same culture growth substrate, whereby the cultural conditions on the substrate surface and its in- terior are adjusted different from each other so as to permit the use of different purifying microorganisms in a single substrate.

Unexpectedly, it has now been found in accordance with the present invention that such a coexistence of differ- ent microbial species is possible, even up to the extent that the surface of the culture growth substrate can suc- cessfully support a culture of aerobic microbes, while an anaerobic culture is supported in the interior parts of the growth substrate. This type of function in the appa- ratus is made possible by the specifications disclosed in appended claim 1. The achievement of desired microbial growth conditions are essentially facilitated by addi- tional specifications of the invention disclosed in de- pendent claims 2-5.

The apparatus according to the invention is primarily intended for removal of organic compounds from air-rich process gases by means of an aerobic microbe or a plural- ity of microbes, complemented with a facility of treating the other chemical compounds possibly also contained in the gases by means of anaerobic bacteria. Such other con- stituents are nitrogen compounds, for instance.

The first functional phase of the apparatus, that is, the contacting of the gas contaminants with the microbes com- prises two mass transfer steps: an absorption step in which a major portion of the contaminants to be removed are transferred from the gas into a water-based liquid phase introduced in fine-dispersed form into the gas stream and an adsorption step during which the conta- minants are transferred into contact with the microbial culture. A factor determining the efficiency of the ab- sorption step (from gas into liquid) is related to the amount of liquid used per amount of gas passed through the purification process, with the obvious provision that the distribution of the liquid into the gas is performed maximally efficiently using droplets atomized to the smallest possible size. The amount of liquid used per treated amount of gas is also an essential factor in terms of properly wetting the growth substrate surface and, particularly, assuring the transport of chemical compounds to be treated from the surface of the growth substrate to the interior of the substrate so as to bring the compounds into contact with the bacteria cultured therein. As a guideline figure, the liquid-to-gas volume ratio may be varied in the range of 0.0005 - 0.03, advan- tageously 0.010 - 0.015, within which range a sufficient- ly effective mass transfer onto the substrate and further transportation into the substrate are attained under con- ventional conditions occurring in the purification plants adapted to clean waste gas compositions most commonly released from processes.

Furthermore, the thickness of the growth substrate must be properly dimensioned with regard to its porosity in order to achieve a desired growth habitat for a sustained microbial culture both on the substrate surface and in its interior. With the volumetric densities specified for the growth substrate used in the apparatus, the growth substrate thickness can be selected to be in the range

20 - 50 mm, advantageously about 30 mm.

Obviously, the microbe strain or strains used in the ap- paratus must be selected to be compliant with the com- pounds occurring in the process and the waste gases re- leased therefrom. As the selection of microbe strains exhibiting a capability of breaking down chemical com- pounds is well known in the art and fully familiar to a person versed in the art, these factors will be omitted from the detailed description of the present invention.

The apparatus makes it possible to create conditions in which the polluting constituents of waste gases are ef- fectively brought into contact with suitable microbes and, further, conditions in which the growth of microbes can take place in an advantageous manner so that their metabolism in the decomposition and utilization of the polluting substances occurs with maximized efficiency.

As to the purification efficiency of the apparatus, the adsorption step involving the material transfer from the liquid into contact with the microbes is frequently a factor of greater significance than the mass transfer rate from the gaseous phase into the liquid phase. Com- plete adsorption can take place only if the polluting constituents are allowed to remain for a sufficiently long time in contact with the active microbes, which con- dition can be met only by designing the apparatus for a retention time of adequately long duration. This require- ment sets tight constraints for the shape and design of the microbe culture growth substrate in order to attain a sufficiently long retention time without the need for using an apparatus of an uneconomically massive size due to the large dimensions of the growth substrate.

In the apparatus according to the invention, the essen- tially aqueous liquid which is separated from the gas

stream and serves as a carrier for the contaminants to be removed is arranged to flow over the culture growth sub- strate, whereby with the provision that a sufficiently large amount of the aqueous liquid is used, a portion thereof can enter the interior of growth substrate. At the start-up of the apparatus, however, it may be neces- sary to speed the wetting of the substrate interior by means of impregnating the mineral wool fabric material of the substrate with a hydrophilicity-improving agent.

This arrangement assures maximized utilization of the growth substrate surface. During the flow-down of the aqueous liquid on the surface of the growth substrate and within the interior thereof, the microbes inhabiting the growth substrate will be contacted with the substances carried by the aqueous liquid thus being able to utilize the substances up to their maximum capacity. Obviously, the concentration of contaminants has its maximum value at the top edge of the downstream-sloping substrate sur- face and the concentration falls to a smaller value as the liquid reaches the lower points of the inclined sur- face. Thus, the microbial culture located at the top edge of the growth surface also receives the full dose of nut- rients which is formed by both those added to the aqueous liquid and contained therein in the form of the contami- nants. Hence, the nutritional well-being of the microbial flora in this part of the substrate is maximally good. By contrast, the supply of trace nutrients and nourishment is available in a varying manner to the microbial culture inhabiting the lower part of the inclined substrate, which is a limiting factor to the growth of microbial flora situated lower along the inclined substrate. Resul- tingly, this lower part of the substrate surface forms a reserve growth substrate capable of compensating for pos- sible load peaks of entering contaminants and the aging of the microbial culture in the upper part of the incli- ned substrate. The vitality of the microbial metabolic

functions is further affected by the nutrient conditions prevailing in the growth substrate. In the case of aero- bic microbes suited for the purification of waste gases, a coarse rule of thumb of 100:10:1 can be applied for the ratios of the carbon/nitrogen/phosphorus contents in the basic nutrients. If these mutual ratios of basic nut- rients cannot be maintained simply by the materials car- ried in the gas stream to be purified, restoration of the ratios to a constant value should be performed through introducing complementing amounts of nutrients, e.g., along with the aqueous liquid which is sprayed into the gas stream. Moreover, it is essential that an adequate supply of oxygen is available in the cultural habitat provided on the growth substrate. While the waste gases to be purified normally contain a sufficient amount of oxygen, an additional supply of oxygen to the area of the growth substrate may also be contemplated in the form of, e.g. blowing ambient air therein. Analogously, the mois- ture content and temperature of the habitat are important factors. With regard to the moisture content, a rule of thumb is that the atmosphere surrounding the growth sub- strate should have a humidity of at least 50 %RH. The temperature should be kept in the range 15 - 50 CC, ad- vantageously 30 - 35 CC. If no other measures are availa- ble for keeping the temperature within these limits, the process gas streams must be cooled or heated, as re- quired. Also the pH of the habitat is important to the growth of the microorganisms, whereby a pH control faci- lity must be included if necessary. The optimum pH is dependent on the composition of the gas to be purified and the microbial flora used in the purification process.

Generally, the pH is controlled to an approximately neut- ral value.

In the following, the invention will be described in greater detail by making reference to the appended dra- wings in which

Figure 1 shows a diagrammatic, vertically sectioned view of an embodiment of a filter unit suitable for use in the implementation of the invention; Figure 2 shows a diagrammatic, vertically sectioned view of another embodiment of a filter unit suitable for use in the implementation of the invention; Figure 3 shows schematically a flow diagram illus- trating an application of the apparatus according to the invention; Figure 4 shows a diagrammatic, vertically sectioned view of still another embodiment of a filter unit suitable for use in the implementation of the inven- tion; and Figure 5 shows a diagrammatic, vertically sectioned view of still further another embodiment of a filter unit suitable for use in the implementation of the invention.

With reference to Fig. 1, the filter unit shown therein comprises a plurality of mineral wool sheets inclined at an angle of about 450 and placed in a parallel fashion spaced at a distance from each other. The gas to be puri- fied enters the unit via a connection 2. The aqueous liquid sprayed into the gas stream to be purified is in- troduced via a pipe 4. Thus, the gas stream entering via the connection 2 is contacted with the aqueous solution in a cocurrent manner, whereby both streams are made to travel downstream in the apparatus until reaching the upper edge of the downward inclined growth substrates 1.

At this point, the liquid component precipitates on the surface of the substrates, while the gaseous component proceeds to flow in the intersheet spaces as parallel streams along the surfaces of the substrate sheets thus

becoming contacted with the microbial cultures growing on the sheet surfaces until exiting via a discharge connec- tion 3.

The aqueous liquid introduced via spray nozzles of the inlet pipe 4 into the gas stream to be purified precipi- tates on the upper edge of the growth substrate sheets 1, thus wetting the sheet and starting to flow downward along the upper surface of the sheet until reaching the lower edge of the sheet, whereby the sheet is gradually brought to a suitable moisture content. With the inclined positioning of the sheets 1 shown in the diagram, each sheet is arranged to appropriately shadow the sheet just next below so that the sprayed liquid can precipitate on the upper edge areas of the sheets only. Finally, the aqueous liquid falls into a collecting space adapted un- der the sheets, wherefrom it can be removed via a connec- tion 5, advantageously for recycling.

Figs. 4 and 5 show alternative arrangements of the growth substrate sheets.

The apparatus shown in Fig. 2 represents an alternative type of embodiment having the growth substrate sheets 1 of the embodiment of Fig.l replaced by open-ended hollow cylindrical filter elements 7 of a suitable mineral wool grade that are aligned upright in the chamber 9 of the apparatus. Analogously to the apparatus of Fig. 1, the gas to be purified enters via a connection 2 and is dis- charged via another connection 3. The aqueous liquid to be sprayed into the gas to be purified is introduced via the spray nozzles of the line 4 in a similar manner as shown in Fig. 1. The upper ends of the cylindrical filter elements may be equipped with a flow equalizer formed by a wire mesh 8 or the like. In this embodiment, the aque- ous liquid sprayed into the gas falls downward along the inner and outer surfaces of the cylindrical elements thus

establishing a liquid film on these surfaces after some time of the filter apparatus use. Along with the liquid introduced into the filter unit will be transferred nu- trients and contaminants that can be utilized by the mic- robes inhabiting the pores of the substrate material, whereby also the anaerobic flora in the interior of the mineral wool is supplied with supplementary nutrients.

The aqueous liquid, which has dribbled down from top to bottom in the cylindrical filter elements 7 thereby deli- vering its substances to the microbes, is removed in a similar manner as in Fig. 1 via a connection 5 to be re- cycled or discarded.

In order to increase the overall area of the culture growth surface, it is possible to use cylindrical ele- ments of different diameter and wall thickness, whereby the packing density of the cylindrical elements in the cross-sectional area of the chamber 9 can be made larger.

Obviously, the gas flow direction may be made reverse to that used in all the above-described applications.

In Fig. 3 is shown a system suitable for utilizing the apparatuses according to the invention. The gas to be purified is a process gas discharged, e.g., as the waste gas of the above-mentioned mineral wool manufacturing process, along a line 2 into filter units L, K, which are connected partially in series, partially in parallel.

Obviously, a larger or smaller number of filter units can be used as required.

The aqueous liquid 4 sprayed into the gas stream to be purified is passed in an essentially closed circulation to the filter units L and K, and therefrom, back to a container 9. The amount of liquid stored in the container 9 is kept essentially constant, wherein the water lost by

evaporation along with the purified gas is compensated for by means of fresh water addition. Besides that, fresh water addition may be needed to compensate for the dis- charge of contaminated water. The nutrients required by the microorganisms are added to the aqueous liquid lea- ving the container 7, advantageously regulating the com- position of the liquid by means of a control system. The pH of the liquid can be monitored simultaneously. In the exemplifying embodiment, the pH control is implemented by a phosphoric acid dosing system, whereby also the phos- phorus nutrient level can be controlled. In general, the nutrient solution may contain, among other constituents, K2HPO4, MgSO4, Caul2 and FeSO4. Additionally, the microor- ganisms used in the process can be replenished along with the sprayed liquid.

Example The apparatus shown in Fig. 3 was tested in a pilot-scale practical application serving in the purification of the waste gas discharged from a mineral wool manufacturing plant. The gas to be purified was taken from the actual waste gas stream as a bypass flow of suitable volume rate so that maximum load of the purification apparatus was controlled to 1000 m3/h per each filter unit. The cross- sectional area of a filter unit was 1 m3 with a construc- tion similar to that of Fig. 1 having the culture growth substrate formed by mineral wool sheets with a thickness of 30 mm and a density of 100 kg/m3. The sheets were adap- ted to rest in an inclined position on a metallic grate at a distance of 30 mm from each other, and the overall surface area of the sheets was 40 m3. The test period co- vered 17 weeks during which the purification efficiency of the microbiological filter units was investigated with regard to phenol, formaldehyde, ammonia and total hydro- carbon content. The contaminant concentrations and other waste gas properties varied within the following limits:

Temperature 20-47 °C Fiber content 0-20 mg/m3 Resin droplets 50-100 mg/m3 Phenol 10-30 mg/m3 Formaldehyde 10-30 mg/m3 Ammonia 20-90 mg/m3 With regard to the different chemical compounds, the pu- rification efficiency reached the values given in Table 1 below. No sign of filter plugging was detectable at the end of the test period.

Table 1 Filter efficiency in the removal of chemical compounds. Test Load Temper- Liquid/ Phenol Form- Ammonia no. [m3/h#m²] ature gas removal aldehyde removal [°Cl ratio [%j removal [%] [%] la 500 42 0.014 >95 72-100 13-59 ib 800 38 0.013 96-99 - 52-66 2a 500 28 0.015 97-98 - 96-98 2b 760 33 0.012 96-98 82-92 86-92 2c 970 34 0.010 87 79 90 Also the contents of binder carried in aerosol form in the waste gas were determined in the tests. After puri- fication, no aerosol-form binder was detectable in the filter exhaust gas stream. The conditions of test no. 2b represent those of a normal load in a process gas purifi- cation plant. Numerically, the achieved results at this filter load level are essentially equal to those attained in flow-through-type microbiological filters. However, such a microbiological filter with a flow-through struc- ture must have a filter surface area of about 1000 m2,

while the apparatus according to the present invention offers an equivalent efficiency with a filter surface area as small as about 300 m2.

The apparatus according to the invention can be used in the purification of waste gases released from a plurality of different processes. In the above-described example, the waste gas was taken from a mineral wool manufacturing process. Other applications can be found in the foodstuff industry, agriculture, paper and pulp industries and, in general, the chemical industry.