Process and apparatus for the purification of water

Abstract: Process and apparatus for the purification of water, oxygen/ozone-gas mixture being introduced under high pressure in the water to be purified, and the thus obtained oxygen/ozone-gas mixture enriched water being passed to a purification stage which comprises at least one reactor. Not or insufficiently degraded contaminants are retained by a membrane filter unit and recycled to the purification stage. Thus contaminants, such as hydrocarbon compounds and the like, are recirculated until they are converted to carbon dioxide and water.

Description: The invention relates to a process and apparatus for the purification of water. Particularly the invention relates to a process for the purification of water, wherein an oxygen/ozone-gas mixture is introduced under high pressure in the water to be purified, this water being enriched with oxygen and ozone is passed through at least one reactor and the water from the reactor is completely or partially fed to a membrane filter unit and the retained liquid and further water from the reactor are recycled to the purification stage. A process of this type is known from EP 0 433 200. In this process an apparatus is used equipped with means for introducing an oxygen/ozone-gas mixture and a reactor for the conversion of contaminants, in particular material in suspension, organic material and in particular bacteria, viruses and other micro organisms as well as macro molecules. To this end the membrane filter unit is provided with an ultra- or micro filtration membrane, having pores varying from 1 nanometre to 0.1 micron. Ozone is introduced under a pressure of 1 bar with the aim of forming micro bubbles in order to prevent fouling of the membrane. The invention aims to provide a process to degrade in a fast and efficient way pesticides and the like present in the rinse water from the cleaning of agricultural spraying apparatus, the waste water from hydroponic cultivation systems, open field farms, fruit growing and arable farms, etc. To this end the invention is characterized in that the oxygen/ozone-gas mixture, in a purification stage, is dissolved in the water to be purified under a pressure of at least 4 bar, and the water from the reactor is fed to a membrane filter unit having a membrane with a cut-off value ≤100 Daltons. Because of the elevated pressure the oxygen/ozone-gas mixture dissolves better and high concentrations ozone are reached. In this way not only the reaction proceeds fast but the degradation capacity is increased as well. Not or insufficiently degraded contaminants are retained by the membrane filter unit and recycled to the purification stage whereby contaminants are kept in circulation until they are degraded completely. According to a favourable embodiment H 2 O 2 is, in case more than one reactor is used, introduced after the first reactor. Hereby a further improvement of the oxidation of the remaining contaminants, such as hydrocarbon compounds, is realized. Advantageously the water to be purified is passed over a biologically active carbon filter. Here microorganisms cause degradation of already partly degraded contaminants. According to a preferred embodiment the water from the purification stage is subjected to a treatment for the removal of gas before being fed to the purification stage. Removal of CO 2 and HCO 3 - results in improved reaction conditions and consequently accelerated degradation. Furthermore, the present invention relates to an apparatus for effectively carrying out the previously disclosed process, and more specifically to an apparatus for the purification of water, provided with means for the feeding of an oxygen/ozone-gas mixture and a reactor for the conversion of contaminants, such as hydrocarbon compounds and the like, to carbon dioxide and water comprising a purification stage with an inlet side and an outlet side, which purification stage comprises at least one reactor, a pump and means for introducing the oxygen/ozone-gas mixture, and the outlet side of which is connected to a membrane filter unit. To this end the apparatus is characterized in that the membrane filter unit comprises a membrane having a cut-off value ≤100 Daltons, and the outlet side of the purification stage being connected to the inlet side as well. Preferably the apparatus comprises a purification stage provided with two serially connected reactors, means being provided between the reactors for feeding H 2 O 2 to the water. This results in a further improvement of the oxidation of the remaining hydrocarbon compounds. In order to accelerate the degradation of contaminants present in the water preferably at least one reactor is equipped with a catalyst. This reduces the residence time of the water to be purified in the reactor. For the improvement of the oxidation by the oxygen/ozone-gas mixture, a device for the removal of gas is provided at the inlet side of the purification stage. CO 2 and HCO 3 - inhibit the degradation of contaminants and are consequently advantageously removed. The purification stage of the apparatus according to the invention is preferably connected to a biologically functioning active carbon filter via a flow control valve, the outlet side being connected to the inlet side of the purification stage and the inlet side of the membrane filter unit. In this biologically functioning active carbon filter the partially degraded contaminants are further degraded by microorganisms. The invention also relates to a reactor with a design as described above. The invention is hereinafter explained by reference to the drawing, in which an example of an embodiment of the apparatus according to the invention is schematically shown. Fig. 1 is a diagram of an embodiment of the apparatus according to the invention for the purification of water. Fig. 2 shows in more detail the reactor of the apparatus of Fig. 1. In Fig. 1 an apparatus for the purification of water is depicted schematically, which is in particular suitable for use in the purification of rinse water from the cleansing of agricultural spraying apparatus, the waste water from hydroponic cultivation systems, open field farms, fruit growing and arable farms, etc. The apparatus is equipped with a collecting tank 1, from which the water to be purified is fed via a feed line 2 to a stripper 3 which is detailed hereafter. The water to be purified is drawn by a pump 4, which feeds the water, preferably from the bottom up, under increased pressure to a reactor 5. The apparatus is further equipped with a corona generator 6, which is known in itself and which feeds oxygen/ozone-gas mixture to line 7, for example, before pump 4. Pump 4 delivers the water under a pressure of, for example, 4 bar. Part of the oxygen/ozone-gas mixture dissolves during the process in the water. In the described apparatus, a pump 8 which is, for example, a centrifugal pump, is connected to the outlet of the pump 4 and feeds the water to be treated to a purification stage with two serially connected reactors 5 and 9. The pump 8 increases the process pressure in this purification step to a value of preferably 16-100 bar, for example, 20 bar. Between the pump 4 and the pump 8 a gas collecting device 10 is provided in which undissolved oxygen/ozone-gas mixture is collected. A water-jet pump 11 draws via a by-pass tube 12 oxygen/ozone-gas mixture from the gas collecting device 10, the oxygen/ozone-gas mixture being delivered by means of a pump 16 which is suitable for pumping gas with sufficient pressure, for example, a water-ring pump . This causes the oxygen/ozone-gas mixture to dissolve completely at the high pressure in the water. In this way it is prevented that undissolved gas impedes the operation of pump 8. If the pressure of the oxygen/ozone-gas mixture rises too high then the gas mixture can be passed via line 14 to the stripper 3. In practice this measure results in some pre-purification in stripper 3. Preferably the water to be purified in which oxygen/ozone-gas mixture has to be dissolved is cooled in a way known per se to increase the gas concentration in the water and to facilitate its solvation in the water. To this end the water to be purified can be passed to a cooling arrangement 15, which cooling arrangement 15 can be provided before the pump 4 and after the stripper 3. A water-jet pump 16 is provided between the two reactors 5,9 which adds H 2 O 2 to the oxygen/ozone-gas mixture, boosting the oxidation process in the second reactor 9. Both reactor 5 and reactor 9 can contain a catalyst. For reactor 5 it is preferably a noble metal catalyst. The pH of the liquid to be purified is in reactors 5 and 9 preferably 7,5 to 8,5, in particular 8,2. The outlet side of the second reactor 9 is connected to a overflow spill 17, the outlet side of which is connected via a flow control regulator 18 with a biologically functioning active carbon filter 19. Under the anaerobic conditions prevailing in the grains of the biologically active carbon filter 18 chlorinated hydrocarbons are degraded. Active carbon is a suitable carrier material because it catalyzes the reaction between any unreacted hydrogen peroxide and any contaminants still present in the water. In this way the attainment of anaerobic conditions is promoted. Instead of active carbon also other carrier materials such as silica can be applied. The outlet side of the biologically functioning active carbon filter 19 is, in the described embodiment connected via the biologically functioning fluidized bed reactor 21 to a membrane filter unit 22. In the fluidized bed reactor 21, which is at the inlet side provided with means 23 for the feed of oxygen-containing gas, the oxidizable compounds formed are degraded oxidatively in the biologically functioning active 19. The water derived from active carbon filter 19 is, in total or partly, fed to the membrane filter unit 22, the membrane of which retains, in the embodiment described here, all molecules with a molecular weight >100. Also a membrane can be applied with a cut-off value of 60 Daltons. The active carbon filter 19 not only serves for further degradation of contaminants but also provides for the removal of undissolved material which could affect the performance of the membrane in the membrane filter unit 22 negatively. Preferably a gas removal device 24 is provided before the membrane filter unit 22 such as a vacuum gas removal device for the removal of gasses, amongst which in particular carbon dioxide, formed in amongst others the biologically functioning fluidized bed reactor 21. A pump 25 is provided between the membrane filter unit 22 and the gas removal device 24 for drawing degassed liquid from the gas removal device 24. Via a line 26 connected with the gas removal device 24 withdrawn gasses are discharged. In this way the interfering action of gasses on the efficiency of the filtration process is prevented. Also material precipitated by the addition of acid, to be described shortly can be removed in this step. To prevent the formation of scale in the membrane filter unit 22 acid, for example, hydrochloric acid can be introduced before the membrane filter unit 22 and preferably before the gas removal device 24, as a result of which the pH is adjusted to a value from 5 to 6,5. This low pH also promotes the removal of carbon dioxide from the liquid stream transported to the filter unit 22. Moreover, a temperature control device 27 can be provided between the purification stage and the membrane filter unit 22. The cut-off value of the membrane in the membrane filter unit 22 depends on the temperature and can be maintained on the desired cut-off value with the temperature control device 27. For the membrane in the membrane filter unit 22 to perform well a sufficiently large flow is required. This can be attained by recycling part of the retained liquid in the feed to the membrane filter unit 22. To carry out the filtration process a pump, not shown, is included in the membrane filter unit 22. The filtrate from the membrane filter unit 22 is fed to an active carbon filter 28. The treated water derived from the active carbon filter 28 is discharged or reused. The retained liquid from the membrane filter unit 22 contains large, consequently possibly still toxic, not yet or not yet completely degraded molecules. The retained liquid is recycled via line 14 to stripper 3. In this way not or not completely degraded contaminants are kept in recirculation until they are adequately degraded. Between the retained liquid outlet line 30 of the membrane filter unit 22 and the purification stage means are located for increasing the pH. For example, a grain reactor 29 can be mounted after the membrane filter unit in which the pH of a part of the retained liquid is increased to 7,5-10 with a base like caustic soda. To this end the retained liquid outlet line 30 is divided in a first retained liquid sub-line and a second retained liquid sub-line 32, the latter of which is provided with means 33 for introduction of pH-increasing compounds and subsequently the grain reactor 29, and the outlet side of the grain reactor 29 is connected with the first retained liquid sub-line 31. The pH increase promotes the deposition of scale-forming salts on grains in the grain reactor 29. The grains may comprise calcium carbonate or dolomite grains. Under these conditions of high pH ions are captured which have not left the system through the membrane of the membrane filter unit 22. The pH of the retained liquid passed via the first retained liquid sub-line 31 is increased with the water discharge at the outlet side of the grain reactor 29, if the occasion arises, to a value suitable for further purification of the retained liquid, for example, a pH of 7,5-8,5. A blower 34, with a preceding air purification unit 35, passes air through the stripper 3 and provides in this way for the removal of CO 2 and HCO 3 - , which compounds interfere with the oxidation reaction by ozone. It is possible to adjust the air-flow rate in dependence of the carbon dioxide concentration to be removed from the liquid stream fed to the stripper 3. As indicated the air can be partially recirculated. This saves energy. Because the stripper 3 performs better when the flow of the liquid stream is large, a pump 36 can be provided. In the embodiment shown a water-jet pump 37 is used to discharge gasses from the gas removal device 24. In the stripper 3 these gasses are discharged into the atmosphere. The apparatus can of course be provided with means for recording and adjusting process parameters such as temperature and acidity to allow an efficient process control. The disclosed apparatus has the advantage of attaining an almost complete degradation of all hydrocarbon compounds to carbon dioxide and water by functioning under highly increased pressure in the purification stage in the reactors 5 and 9. The addition of H 2 O 2 in the purification stage working under increased pressure leads to an efficient further oxidation in reactor 9. Not or still insufficiently degraded contaminants are retained by the membrane of the membrane filter unit 22 working as a molecular sieve and recycled to the purification stage. From the outlet of each reactor and from the membrane filter unit 22 branching lines extend to the inlet side of the purification stage. This is especially beneficial if the contaminants in the water purified by the respective reactors can be degraded more efficiently by a preceding reactor than a subsequent reactor. There may also be branching lines to other reactors (each device in which contaminants are degraded), the same reactor and there may be lines from more than one reactor to the membrane filter unit 22. An apparatus according to the invention provided with a plurality of such branching lines is very flexible and consequently suitable for the degradation of a large variety of compounds. As shown in Fig. 2 in further detail, both reactors 5,9 can be provided with a porous catalyst tube 38 joining the inlet side of the reactor. The catalyst tube 38 is centrally arranged in the cylindrically shaped reactor 5,9. As a catalyst preferably a noble metal is used, for example, palladium, platinum or iridium. In reactor 9 also vanadium, iron, copper or silver can be applied. The apparatus described here is an apparatus with which a large variety of organic contaminants can be degraded. For applications in which only a limited number of contaminants has to be degraded, a simpler design may suffice.

Claims: Process for the purification of water, wherein an oxygen/ozone-gas mixture is introduced under high pressure in the water to be purified, this water being enriched with oxygen and ozone is passed through at least one reactor and the water from the reactor is completely or partially fed to a membrane filter unit and the retained liquid and further water from the reactor are recycled to the purification stage, characterized in that the oxygen/ozone-gas mixture, in a purification stage, is dissolved in the water to be purified under a pressure of at least 4 bar, and the water from the reactor is fed to a membrane filter unit (22) having a membrane with a cut-off value ≤100 Daltons. Process according to claim 1, characterized in that, if more than one reactor is used, H 2 O 2 is introduced after the first reactor (5). Process according to claim 1 or 2, characterized in that the water from the purification stage is passed over a biologically active carbon filter (19). Process according to any one of the preceding claims, characterized in that the further water from the purification stage is subjected to a treatment for the removal of gas before being fed to the purification stage. Process according to any one of the previous claims, characterized in that acid is introduced in the water passed to the membrane filter unit (22) and the acidified water is subjected to a treatment for the removal of gas before being fed into the membrane filter unit (22). Apparatus for the purification of water, provided with means for the feeding of an oxygen/ozone-gas mixture and a reactor for the conversion of contaminants, such as hydrocarbon compounds and the like, to carbon dioxide and water comprising a purification stage with an inlet side and an outlet side, which purification stage comprises at least one reactor, a pump and means for introducing the oxygen/ozone-gas mixture, and the outlet side of which is connected to a membrane filter unit, characterized in that the membrane filter unit (22) comprises a membrane having a cut-off value ≤100 Daltons, and the outlet side of the purification stage being connected to the inlet side as well. Apparatus according to claim 6, characterized in that the purification stage comprises two serially connected reactors (5,9), wherein means are provided between the reactors to introduce H 2 O 2 to the water. Apparatus according to claim 6 or 7, characterized in that at least one reactor is provided with a catalyst. Apparatus according to any one of the claims 6 to 8, characterized in that at the inlet side of the purification stage a device (3) for the removal of gas is located. Apparatus according to any one of the claims 6 to 9, characterized in that the outlet side of the purification stage is connected via a flow control regulator (18) to a biologically functioning active carbon filter (19) the outlet side of which is connected to the inlet side of the purification stage and the inlet side of the membrane filter unit (22). Apparatus according to any one of the claims 6 to 10, characterized in that a retained liquid outlet line (30) of the membrane filter unit (22) is divided in a first retained liquid sub-line (31) and a second retained liquid sub-line (32), the latter being provided with means (33) for the introduction of pH-increasing compounds followed by a grain reactor (25), and the outlet side of the grain reactor (25) is connected with the first retained liquid sub-line (31). Apparatus according to any one of the claims 6 to 11, characterized in that the outlet side of the purification stage and/or the biologically functioning active carbon filter (19) is connected with the inlet side of a biologically functioning fluidized bed reactor (21) which is provided at the inlet side with means for the feeding of oxygen-containing gas.

Inventors: VAN STAVEREN NICOLAAS ARIE, VAN STAVEREN, NICOLAAS ARIE
Applicants: E M ENGINEERING F T S B V, E.M. ENGINEERING F.T.S. B.V.
Application year: 1993
Main classification: C02F1
IPC codes: C02F1, C02F3, C02F9, B08B3