A method and a device for the purification of waste water

Abstract: The present invention relates to a method for the treatment of waste water. The invention relates in particular to a method whereby polluted water (2) is fed to a water purification plant (1), which water is treated in the water purification plant (1), whereby an effluent (3) is obtained that is partly contacted with combustion gasses from a combustion process (10) in order to raise the temperature by means of heat transfer, after which the heated effluent (4) is again fed to the water purification plant. In this way the water purification plant can be operated at an elevated temperature, which results in the acceleration of the biological processes in the plant. More in particular, the effluent can be fed to the combustion gasses of a waste incineration plant.

Description: The present invention relates to a method and an apparatus for the treatment of waste water, comprising the supply of polluted water to an inlet of a water purification plant, the treatment of the water in the water purification plant, resulting in treated effluent, and the discharge of the effluent. Such methods are generally known in practice. The water may come, for example, from urban sewage systems, or from another source, and is treated biologically in the water purification plant. Generally speaking, the processes taking place in the plant are biological processes. In addition, sedimentation processes and optional filtration steps are also provided. A general draw back of such methods is that they are temperature-dependent. The temperature depends on the time of year. This means that because of the low temperatures prevailing in the winter, more time is required than in the summer. Due to the fact that the water purification plant has to be dimensioned on the basis of the lowest reaction rate, the winter temperature has to be taken as point of departure. Derwent Abstract AN-1995-129744 (SU-A-1,834,864) describes a method for breaking down so-called secondary sludge, by mixing incineration gases with said sludge. This mixing takes place in an aeration tank of a sewage water purification plant. A disadvantage of the method is that odorous components are emitted from the aeration tank into the atmosphere. Document DE-4420111 describes a method for the biological treatment of wastewater in which the temperature of the water is increased directly by the introduction of heat energy such as combustion grasses. Smaller treatment bassins are required to achieve the required treatment result. The object of the invention is to improve the method such that it can be carried out more quickly. The object of the invention is in particular to increase the reaction rate at all times. The object of the invention is especially to increase the reaction rate by raising the temperature of the water to be treated. The object of the invention is also to provide an apparatus with which the above-described objectives can be realized. In order to realize the above-mentioned objectives, the invention provides a method as mentioned in the preamble, which is characterized in that the temperature of the effluent is raised by means of heat transfer, by contacting at least a portion of the effluent with combustion gasses from a combustion process, and the heated effluent is fed to the inlet of the water purification plant. The term "contacted" refers to both direct and indirect contact. According to a preferred embodiment, the effluent is brought into direct contact with the combustion gasses in a counterflow gas washer. If direct contact is not desired, the heat transfer can take place, for example, with the aid of a plate heat exchanger or a tubular heat exchanger. According to another preferred embodiment the heated effluent is first mixed with polluted water subsequent to which the mixture obtained is fed to the inlet of the water purification plant. In this way the mixture to be treated that is fed to the water purification plant has the raised temperature right from the beginning. According to yet another preference the effluent is contacted with the combustion gasses from a waste incineration plant. It is particular preferable that the effluent is contacted with the combustion gasses in a final step prior to discharging the combustion gasses via a flue. The advantage of this is that the rest heat still present in the combustion gasses and which otherwise would be discharged via the flue, is now utilized for elevating the temperature of the effluent. An unexpected advantage is that due to the effluent directly contacted with the combustion gasses, pollutants present in the combustion gasses of the waste incineration plant are absorbed. These substances are then degraded in the water purification plant and converted into substantially harmless compounds. Possible non-degradable pollutants that are absorbed by the effluent are adsorbed by the sludge in the water purification plant. After dewatering, this sludge may be fed to, for example, the waste incineration plant. In this way said pollutants are converted into harmless residue. According to a preferred embodiment, sewage water is fed to a water purification plant. In the winter, sewage water has a temperature of approximately 11°C and in the summer a temperature of approximately 22°C. The water purification plant will work better at a higher temperature due to the fact that the biological processes are accelerated. This applies in particular to nitrification. This means that at a stable effluent quality, a smaller reaction volume will suffice in the summer or that at a same system volume a higher rate is created. An elevation in the sewage water temperature from 11°C to approximately 25°C will mean that the original system volume may be reduced up to 25%. A rise in temperature from 11°C to approximately 18°C will allow the system volume to be halved. Preferably therefore, the temperature of the water to be fed to the water purification plant is as high as possible, while still allowing the necessary processes to carry on. According to the invention a portion of the effluent is fed to the combustion gasses of an incineration plant. This portion of the effluent may, for example, be fed to the combustion gasses of a waste incineration plant. To this end the effluent may be contacted with the combustion gasses in a gas washer. In the washer the effluent will absorb a part of the heat of the combustion gasses (assuming that the temperature of the combustion gasses is higher than the temperature of the effluent), by means of condensation it will absorb a part of the gaseous water in the combustion gasses, and will also absorb a part of the pollutants in the combustion gasses. The heated effluent may subsequently be returned to the inlet of the water purification plant. The figure shows a schematic illustration of a water purification plant in combination with a waste incineration plant. Of the waste incineration plant only the latter part of the combustion gasses' path is schematically illustrated. The invention will now be further elucidated with reference to an example of the method according to the invention. In the present example the temperature of the combustion gasses entering the gas washer is 61°C. The relative humidity of the combustion gasses is 100%. Having passed through the gas washer, the temperature of the combustion gasses will be, for example, 30°C, while the relative humidity remains at 100%. Thus part of the water in the combustion gasses will have been condensed in the effluent. The water purification plant 1 has an inlet 2 and an outlet 3. As shown in the figure, the water purification plant also has a second inlet 4. The inlet 2 feeds the purification plant with, for example, sewage water, while the second inlet 4 feeds heated effluent to the water purification plant. The second inlet 4 may optionally be positioned as indicated by the dotted line, which means that in fact only one inlet 2 remains. In the water purification plant the processes that are common practice are carried out for the purification of the water. At 5, part of the effluent is branched off and fed to a gas washer 6, in which the effluent is sprayed in a manner generally known in the art. At the bottom of the gas washer at a position 8, combustion gasses 7 are fed to the gas washer 6 and in the top of the gas washer at a position 9, they are discharged. At a position 10 in the top of the gas washer, the effluent is fed into the gas washer and is discharged at a position 11 in the bottom of the gas washer. The effluent must always enter at a position such that it comes into contact with the combustion gasses. Due to the fact that the gas washer is operated in counterflow, contacting between effluent and combustion gasses is satisfactory. When being fed to the gas washer 6, the combustion gasses 7 usually have a temperature of 61°C. In a steady state condition, the effluent fed to the gas washer will have a temperature of approximately 18.2°C. In an exemplary case as commonly occurs in practice, approximately 600 tons per hour (dry base) combustion gas will be fed to the gas washer. An amount of approximately 5000 m 3 of effluent per hour will be fed to the gas washer. The effluent will then be heated to 30°C, abstracting 85 m 3 of water per hour from the flue gasses. On exiting from the gas washer, the combustion gasses will have a temperature of approximately 30°C. At an influent flow rate to the water purification plant consisting, for example, of sewage water having a temperature of 11°C, of 7800 m 3 per hour, the mixture being fed to the water purification plant will have the total resulting temperature of 18.2°C. This means that the water purification plant can be operated in the winter at a temperature of 18.2°C (instead of 11°C), whereas the temperature in the summer may be 29.2°C (instead of 22°C). At an influent flow rate of 7800 m 3 per hour as mentioned above, this means that the system volume may decrease by 50% from 125,000 m 3 to 62,500 m 3 because of the elevated system temperature and the consequential acceleration of the biological processes. The pollutants that are removed by the gas washer from the combustion gas are absorbed by the effluent. Due to the fact that this effluent is again led through the water purification plant, the components absorbed from the combustion gasses, such as organic components (C x H y and PCDD/F), as well as metals and the corresponding metal sulphides, fluoride in the form of calcium fluoride and dust present in the gasses will to a large extent settle in the water purification plant and be adsorbed by the sludge. This sludge is separated in the usual manner from the water phase. After dewatering, the sludge may be fed to the waste incineration plant. Said organic components will then be incinerated. As the flue gasses in a waste incineration plant are already being thoroughly cleaned, in addition to which simultaneous residual separation takes place, virtually all the residue fed via the sludge to the waste incineration plant will be removed. Subsequently, in the manner described above, any remaining material in the gas washer is absorbed from the flue gasses by means of the effluent. In addition, the ammonia that was present in the flue gasses and that was absorbed by the effluent in the gas washer, will in the water purification plant be converted into N 2 . The remaining components such as fluoride and sulphate compounds will be discharged with the effluent. An example of the extent to which pollutants are abstracted from the combustion gasses is as follows. Component Before gas washer (µg/Nm 3 ) After gas washer (µg/Nm 3 ) HF 4 < 1 HCL 190 < 20 SO 2 2800 < 300 NH 3 83 < 10 C x H y 380 < 250 PCDD/F 46·10 -6 < 10·10 -6 Dust 550 < 100 Cd 0.32 < 0.05 Hg 1.79 < 0.5 Zn 23.4 < 5 Remaining heavy metals 23.5 <5 According to a further preferred embodiment it is possible via the gas washer to also remove NO x or CO respectively, from flue gasses by means of oxidation into nitrate or CO 2 , respectively. In the water purification plant the nitrate may subsequently undergo biological conversion into nitrogen. In that case, the combustion gasses, which are subsequently discharged from the gas washer contain approximately 10% by volume of CO 2 . These combustion gasses are very useful for the production of biomass. For example, it may be fed to green houses for the production of plants. In that case, discharge of the combustion gasses via a flue can be omitted altogether. The invention is not limited to the embodiment described in the above example. For example, it is possible to feed combustion gasses having a temperature that differs from the one mentioned above to the gas washer or to abstract them therefrom. Also, the temperature of the effluent may differ from that given above. Other adaptations will be obvious to a person skilled in the art.

Claims: Method for the biological treatment of waste water comprising the supply of polluted water to an inlet (2; 4) of a water purification plant (1), the treatment of the water in the water purification plant (1) resulting in treated effluent, and the discharge of the effluent, characterized in that the temperature of the effluent is raised by means of heat transfer, by contacting at least a portion of the effluent with combustion gasses (7) from a combustion process, and the heated effluent is fed to the inlet (2; 4) of the water purification plant (1). Method according to claim 1, characterized in that the heated effluent is first mixed with polluted water subsequent to which the mixture obtained is fed to the inlet (2) of the water purification plant (1). Method according to claim 2, characterized in that the effluent is contacted with the combustion gasses (7) in a counterflow gas washer (6). Method according to claims 1-3, characterized in that the polluted water is sewage water. Method according to claims 1-4, characterized in that the combustion gasses (7) are derived from a waste incineration plant. Method according to claims 1-5, characterized in that in a final step, the effluent is contacted with the combustion gasses (7) prior to discharging the combustion gasses via a flue (9). Method according to claims 1-5, characterized in that in a final step, the effluent is contacted with the combustion gasses (7) before discharging the combustion gasses (7) to a biomass production unit. Method according to claims 1-7, characterized in that in the gas washer (6) water-vapour from the combustion gasses (7) is condensed in the effluent. Method according to claims 1-8, characterized in that in the gas washer (6) water-soluble compounds are dissolved in the effluent. Method according to claims 1-9, characterized in that sludge from the water purification plant (1) is supplied to the combustion process. A water purification plant (1), comprising an inlet (2) for water to be purified and an outlet (3) for treated effluent, as well as a treatment section for the biological purification of waste water, characterized in that the water purification plant (1) also comprises a second outlet (5) for supplying a portion of the effluent to a contacting plant (6) in order to bring the portion of the effluent into contact with combustion gasses (7) of a combustion process in order to heat the portion of the effluent, as well as a second inlet (4), which is located at a position upstream from the second outlet, for the supply of the heated effluent to the water purification plant (1). Water purification plant (1) according to claim 11, characterized in that the second inlet (4) is located at the beginning of the water purification plant (1), and the second outlet (5) at the end of the water purification plant (1).

Inventors: DE VRIES CORNELIS, M A J VAN BERLO, DE VRIES, CORNELIS, M.A.J. VAN BERLO
Applicants: AMSTERDAM GEM DIENST AFVALVERW, GEMEENTE AMSTERDAM, GEMEENTELIJKE DIENST AFVALVERWERKING
Application year: 2001
Main classification: C02F3
IPC codes: C02F1, C02F3