The present invention relates to a water treatment plant comprising at least one bioreactor, the said at least one bioreactor comprising a tank section provided with inlet means for the water to be purified and outlet means for the purified water, inside which tank is carrier material onto which a biofilm may be formed, in which tank are in addition arranged means for supplying a fluid containing a reaction gas required by the purification process, the tank section being essentially circular or elliptical in cross-section, whereupon the water outlet means are provided in the tank section in such a way that the tank section is essentially full of water during the purification process, and which reactor comprises control means for operating the fluid supply means in such a way that a spinning motion of the carrier, the water and the fluid containing reaction gas is effected thereby around a rotation centreline passing essentially through the tank's cross-sectional centre.

In the biological purification of water, such as, for example, waste water, the water is passed through a reactor, wherein micro-organisms are utilised for converting water-borne impurities into harmless end products such as carbon dioxide, minerals and water. In biological water purification, the micro-organisms may also bind to themselves, that is, to the biomass, non-biodegradable products, for example heavy metals. Purification may be carried out aerobically or anaerobically. Prior known are various bioreactors for purifying waste water such as trickling filters, biorotors (rotating biological contactors), fluidized bed reactors, fixed bed reactors and moving bed reactors. One prior art bioreactor comprises conveying a carrier material to the process, on the surface of which carrier material micro-organisms may grow in the form of a biofilm.

In an earlier international published application WO2007077298A1 of the present application is disclosed an improved bioreactor by means of which the

disadvantages of both the fixed process and the moving process are eliminated, thus achieving a higher degree of purification of impurities per unit of volume than with other known reactors and, therefore, lower purification costs. In such rotating type of bioreactor, the cross-section of the tank section is essentially circular or elliptical, the water outlet means being arranged in the tank section in such a way that the tank section is essentially full of water during the purification process. The fluid supply means are disposed on the tank wall and the reactor comprises control means for operating the fluid supply means in such a way that a spinning motion of the carrier, the water and the fluid containing reaction gas is effected thereby around a rotation centreline passing essentially through the tank's cross-sectional centre, whereby the control means are adapted to optionally effect a deactivation of the fluid supply means at desired times.

For example, plants treating municipal waste waters require substantial amounts of energy for transferring the treated waters from one basin to another.

Similarly, for example, in fish farms quite considerable pumping energies are required for changing the water in the fish farming tanks completely sufficiently often, typically about once an hour. The aim of the present invention is to provide a solution by means of which the pumping energy required in the various water treatment plants, where water is purified biologically, can be produced economically and efficiently.

To achieve this aim, the water treatment plant according to the invention is characterised in that in the tank section is arranged a downwards directed passage, which passage is connected to the inlet of the water to be treated, and in which passage are arranged supply means for air or other reaction gas at a distance from the level of the lower edge of the passage in such a way that the reaction gas rising upwards in the passage transfers the water in the passage to the reactor, thus bringing about a pumping effect in the water in the bioreactor and transferring it to the next stage at the plant. The bioreactor is preferably the rotating-bed bioreactor disclosed in international published application WO2007077298A1.

The invention is described in greater detail in the following, with reference to the accompanying drawings, in which:

Figure 1 shows a diagrammatic view in principle of a bioreactor applicable for use in a plant according to the invention, Figure 2 shows a diagrammatic view in principle of an embodiment of a fish farm implemented according to the invention, and

Figure 3 shows an algae-growing arrangement provided in connection with the fish farm.

The bioreactor 1 shown in Figure 1 comprises an elongated, tubular tank section 2 which is preferably circular or elliptical in cross-section. The tank section 2 is provided with inlet means 7 for the water to be purified and outlet means 6 for the purified water. In the embodiment shown in Figure 1, in the lower part of the tank 2 is arranged a downwards directed passage 8 to which the inlet 7 of the water to be treated is connected. The water inlet means and outlet means 6 are disposed in such a way that the supply of the water to be purified and the discharge of the purified water can be carried out so that the tank section 2 is essentially full of water during the purification process. In Figure 1, the water level is designated by the letter W.

Inside the tank section is provided carrier material 3 onto which micro-organisms may be layered as a biofilm. The carrier material may consist of, for example, a single carrier element or several carrier elements joined permanently with one another, or of several separate carrier material elements, in which case, when using several carrier material elements, they may be identical to or differ from one another, for example, in terms of their size, shape, density and/or other properties. In the lower part of the tank 2 is arranged a downwards directed passage 8 which is connected to the inlet 7 of the water to be treated. In the passage are provided supply means 4 for air or other reaction gas at a distance from the level of the lower edge of the passage in such a way that the reaction gas rising upwards in the passage transfers the water in the passage to the reactor, thus bringing about a pumping effect in the water in the bioreactor and transferring it to the next stage at the plant. The means 4 operate both to supply the reaction gas required by the purification process and at the same time to produce the buoyancy bringing about the pumping effect in the water to be treated. The supply means may be, for example, about 1/3 of the vertical dimension of the passage extending from the lower edge of the passage 8. By means of the length of the passage 8 can be determined the lifting height of the water and by locating the water outlet means 6 of the bioreactor close to the water level, the effect of buoyancy is converted into a pumping effect which transfers water. In a rotating-bed bioreactor, the rotating motion of the carrier material elements intensifies the pumping effect considerably compared with the pumping effect brought about by buoyancy alone. One or more bioreactors operating as a mass pump can be arranged to carry out the recycling of the water to be treated essentially completely, thereby avoiding the acquisition of separate pumps and the acquisition and operating costs incurred by them. Figure 2 shows diagrammatically a fish farm implemented according to the invention. The farm comprises a fish farming tank 21 and at least one bioreactor 20 in flow connection with the tank, in which the water from the tank is purified biologically before supplying it back to the farming tank 21. Reference numeral 23 denotes a drum screen and reference numeral 24 an oxidation column. The aeration system is denoted by reference numeral 25.

There are preferably several bioreactors 20 arranged in succession in such a way that aerobic purification takes place in the first bioreactor or group of bioreactors and oxygenous nitrification (NH ₃ -> N0 ₂ ^" -> N0 ₃ ^" ) takes place in the next bioreactor or group of bioreactors, after which the water is conveyed to an oxygen-free denitrification stage taking place in the next bioreactor or group of bioreactors, where the nitrogen in the form of a nitrate is reduced to nitrogen gas (N0 ₃ ^" -> N0 ₂ ^" -> NO -> N ₂ 0 -> N ₂ ). Finally, the purified water can be delivered, for example, to a phosphorus precipitation section.

Figure 3 shows an algae-growing arrangement provided in connection with a fish farm, which comprises at least one algae-growing bioreactor 26, in which, for example, the removal of phosphorus from the water discharged from the tank takes place. Algae may be grown, for example, for foodstuff, nutrient or energy purposes. From the reactor or reactor group 26 the water is conveyed to a flotation device 27 from which a part of the water is conveyed through a pipe 32, for example, to the bioreactor 20 of Figure 2 and a part to the clarifier 28, at the bottom of which is a pipe connection for the recovery 31 of algae and for sampling 30. The clarifier is connected through a balancing reservoir 29 in flow connection with the algae- growing bioreactor 26. The solution according to the invention makes it possible to recycle the water to be treated with essentially the same reaction gas supply which is already required for the biological purification process; in other words, without separate pumps and the additional costs incurred by them. Furthermore, for example, a fish farm can be made into a completely closed system which will not burden the environment. In addition to a fish farm, the plant can be implemented as a farm for other aquatic species, such as crabs and shellfish.