In order to enhance biological oxidation, the organic mass can be further provided with a recyclable oxidation surface.

The invention relates also to an apparatus for cleaning the exhaust gases of a bio-oxidizer, said apparatus comprising a bio-oxidation reactor, means for supplying organic mass to an inlet end of the oxidation reactor, means for supplying dry warm air to the inlet and/or outlet end of the oxidation reactor, a fan for vacuuming a hot wet exhaust gas from the outlet and/or inlet end of the oxidation reactor, means for cleaning the exhaust gas, and a heat recovery unit for the transfer of heat from the exhaust gas to the supply air.

As a novelty to be described hereinafter, the apparatus is provided with ionizing particle separators for cleaning the exhaust gas.

It is prior known to use a bulky, slowly rotating composter drum for the continuous composting of organic masses. Organic mass is supplied to an inlet end, and a valuable end product, useful e. g. as a soil improver, is obtained from an outlet end. In the composter, the organic mass is consumed by high temperature micro-organisms to feed themselves. In the process of feeding and growing, the microbes require plenty of oxygen. At the same time, they produce thermal energy, water, carbon dioxide, and useful biomass.

A problem is how to clean exhaust gases of particulate and gaseous impurities. Particles can be e. g. microbial germs or other very small organic or inorganic particles. Harmful gaseous substances may include e. g.

ammonia, methane, or hydrogen sulphide. The elimination of both particles and harmful gases from a gas mixture to be cleaned is particularly problematic. Especially, the removal of small particles (< 0, 3 um) from air is difficult with conventional air cleaners. Filters become blocked quickly, especially in the process of cleaning moist air, and the cleaning and replacement thereof requires a lot of work. Cleaning of gases is prior known by means of gas scrubbers, but e. g. methane, hydrogen sulphide, and ammonia are often present in such large amounts that the process requires powerful and expensive gas scrubbers, which nevertheless do not provide a sufficient degree of cleaning.

It is an object of the invention to provide an improved method and apparatus, which are capable of effectively removing both particles and harmful gases from gases to be cleaned. Thus, gases can be discharged to the environment in an particle-free and odourless state.

This object of the invention is achieved by a method of the invention, the characterizing features of which are set forth in the appended claim 1. This object is also achieved by means of an apparatus of the invention, the characterizing features of which are set forth in the appended claim 8.

Preferred embodiments of the invention are disclosed in the dependent claims.

When organic mass has a low dry content, e. g. 2%-25% of organic sludges, another problem is how to provide oxidation sufficiently powerful for microbial activity. This problem has been addressed by supplying the sludge mass with various builders, such as chips, peat, etc. , but an optimal oxidation rate is not obtained by the shape and amount of these builders.

The inventive method and apparatus can be additionally used for improving the oxidation rate of the process or the oxygen supply to microbes, such that the entire oxidation process becomes faster and a substantial increase in production capacity is obtained.

This additional object of the invention is achieved by a method as set forth in the appended claim 4 and by an apparatus as set forth in the appended claim 10.

One exemplary embodiment of the invention will now be described in more detail with reference to the accompanying drawing, which shows in a schematic view an apparatus of the invention for implementing a method of the invention.

A tank 1 is supplied with organic mass to be oxidized, which may especially comprise sludge from a sewage treatment plant or liquid manure from farming. The tank 1 is also supplied with a continuous flow of spheres, which comprises spheres 12 of a biologically non-oxidizing material, whose diameter is within the range of 1-5 cm, most conveniently about 2-4 cm, and whose surface has been roughened for enhancing the adherence of organic mass to spherical surfaces.

The flow, constituted by the spheres 12, is admixed e. g. by means an elevating twin-screw conveyor 2 with an organic mass before feeding the flow of spheres, and the organic mass admixed therewith, from a supply tank 3 by means of a feed conveyor 4 to the inlet end of a biological oxidation reactor 5.

The reactor drum 5 is driven in a slow rotating motion, e. g. 4 r/h, while resting on wheels 6. The reactor 5 has its interior provided with adjustable paddles 7, having an extent and design appropriate for mixing spheres and

mass, while advancing the same at a suitable speed. The residence time of mass in the reactor 5 can be two or three days, as opposed to 8-10 days it used to be prior to the enhancement of oxidation according to the invention.

The enhancement of oxidation is based on the fact that the organic mass is primarily present as thin layers 23 adhered or bonded to the surfaces of the spheres 12, resulting in a substantially increased oxidation area. Another contributing factor is the spherical form of the spheres 12, by virtue of which the inter-sphere air space is larger than the volume of mass to be oxidized.

The mass layer present on spherical surfaces has a thickness which is typically only about 2 mm or less. While, in the prior art, the question was about providing more oxidation surface within the mass, the inventive concept is about admixing the organic mass within a flow of spheres. The spheres must be composed of a high melting material, such as polyethylene or nylon. The surface is provided with bonding formations, such as recesses, which further increase the surface area and ensure the adherence of mass to the surface. The reactor 5 should contain as little as possible of such mass which is not included in the layers 23 bonded to the surface of the spheres 12.

The mass, oxidized in the oxidation reactor 5, and the spheres 12 are delivered into a drying drum 9, which is driven rotating at a speed, e. g.

200 r/h, which is multiple with respect to the reactor's 5 rotation speed. The drying drum 9 is provided with internal paddles 9a for removing oxidized mass from the surfaces of the spheres 12.

From the drying drum 9a, the oxidized mass and the spheres are delivered to a drum screen 10 for separating the spheres 12 and a finished mass 11 from each other. The screen 10 has a screwthread on its inner surface for discharging the spheres out of the screen's outlet end, and the mass flows through the screen jacket.

The spheres 12 are returned by means of a return conveyor 13 back into the tank 1, from which the twin-screw conveyor 2 carries a flow of spheres and organic mass, while mixing the same together, up into the supply tank 3. By virtue of the inclination of the twin-screw conveyor 2, there will occur certain reflowing of mass and spheres, which enhances mixing for supplying the reactor 5 with spheres 12 having a special coating of sludge 23. In the reactor 5, the spheres 12 must have a volume which exceeds that of the organic mass 23. The spheres 12 may be hollow, but the specific gravity thereof must be adapted to be as close as possible to that of the organic mass.

A hot wet gas to be discharged from the reactor 5 is cleaned by means of ionizing particle separators 14,24 and 16. The operating principle of ionizing particle separators is such that the particles, present in a gas flowing through a separating chamber, are charged and the charged particles are collected by means of an electric field on energized collector surfaces, from which the particles are flushed with water and the water is discharged to receivers and/or further treatment processes, which do not constitute objects for this invention.

Between two ionizing particle separators 14 and 16 is fitted a fan 15 for establishing a negative pressure in the first particle separator 14, in the reactor 5, in a supply air inlet duct 22, and in the enclosed assembly 1-4 at the inlet end. By virtue of this vacuum system, the oxidation plant and its environment can be rendered odourless. The exhaust gases are cleaned by the ionizing particle separators 14 and 24 sufficiently for fans 15 and 25. A singlet oxygen generator 17, downstream of the fans 15 and 25, and a second ionizing particle separator 16 are capable of cleaning the exhaust air effectively of the remainder of small particles, odours, and bacteria. The singlet oxygen generator is operationally based on the formula °2 + E > °'+O'; °1+02 > °3 (wherein E represents energy).

The formation and amount of ozone in exhaust air is monitored continuously by means of a monitor.

The exhaust gas is monitored for its moisture and temperature by means of hygro-and thermometers 19, which enable controlling the process, such as the fan's 15 rotation speed and the reactor's 5 rotation speed and direction.

The process equipment is subjected to vacuuming air from its odour- generating parts by means of a pipe 29, which connects with a duct between the ionic cleaners upstream of the singlet oxygen generator 17.

Heat is transferred from the cleaned exhaust gas to the reactor's 5 supply air by means of a heat recovery unit 18. Fresh air is drawn from outside by way of a pipe 20. The dry fresh air, warmed up in the heat recovery unit 18, is carried by way of a duct 22 to the composter's 5 inlet end.