METHOD OF MIXING AND TREATING LIQUID WITH AIR AND MEANS FOR PERFORMING THE METHOD

Abstract: A method and means for stirring and treating liquids such as pure water and polluted water, particularly sewage, with air, the treatment and stirring being performed with a compressor immersed in the liquid bath, said compressor consisting of a rotating cylinder (6) with blades, located in and beneath a hood (5) which constitutes a pocket for the air utilized for the treatment. Fresh air is supplied to the pocket as required via a compressed air pipe (4) and consumed air is permitted to rise through the liquid to be released at the surface of the tank. The blades (7) are immersed in the liquid bath so that kinetic energy for the stirring process is transmitted mechanically to the liquid in the tank.

Description: METHOD OF MIXING AND TREATING LIQUID WITH AIR AND MEANS FOR PERFORMING THE METHOD The present invention relates to a method of treating liquids with gas while stirring, or stirring the liquid without treating it with gas, in which method a liquid is fed into a treating tank and is removed from the treating tank after remaining there for some time. The method is particularly useful for treating water. The water may be clean water to which oxygen is to be added in order to oxidize iron and/or manganese therein into compounds that can be separated out. It is also possible to use natural pools of water, lakes needing the addition of oxygen in order to conserve the natural stock of plants and animals. However, the most usual application is likely to be the treatment of polluted water, particularly microbiological treatment of sewage in accordance with the active sludge method, and this in combination with nitrogen-reduction processes. The invention will therefore be described in the following with reference to the latter combined treatment Several methods of the type defined in the preamble to the claim are already generally known, both with respect to adding oxygen to clean water and especially to treating polluted water and the like. The methods are utilized when the demand for purification efficiency is high. Biological purification of sewage is based on oxidation and biochemical flocculation of the organic substance while air is added in the treating process. With biological purification methods a sludge is always obtained which constitutes the active substance and contains large quantities of bacteria, Protozoa and other micro-organisms. During the prevailing aerobic conditions organic substance, which for the most part is oxidized, is withdrawn from the sewage. Since a considerable portion of the organic substance is converted to sludge, the tanks in which treatment is performed must be connected to post-sedimentation tanks in which the sludge formed is separated off in the form of a voluminous sludge containing much water. The substance flocculated out during the treatment with air in the tank is conveyed to the post-sedimentation tank, where it is allowed to settle in the form of a sludge. The feedback of such already produced sludge - feedback sludge - acts as catalyst in the flocculation and the biochemical oxidation of the impurities in the sewage. The sludge content in the treating tank is kept relatively high and the sludge is kept floating by means of stirring. The active micro-organisms thus develop and contribute to effective oxidation of the impurities. The sludge returned to the post-sedimentation tanks, which is not returned to the tanks for biochemical purification, is withdrawn from the process in the form of an excess sludge that can be treated separately in thickeners and processed further to compost soil, for instance. According to a method frequently used, additional aeration of the tanks occurs in a subsequent step for nitrogen reduction to which the sewage is conveyed after the initial oxidation process. This is to achieve nitrification and, in subsequent tanks, oxygen depletion causing the nitrogen bound in the nitrification process to depart in the form of gas since, for want of free oxygen, the bacteria present consume the oxygen contained in the nitrate ions. This latter phase is termed denitrification. In the nitrification part aeration is performed using equipment corresponding to that used in the initial process for oxidation of impurities. In the denitrification part stirring is performed using mechanical equipment, without the addition of air. Nitrogen reduction is also performed at the same time as the normal active sludge treatment by forming a zone with stirring but no aeration at the inlet end of the tanks, where denitrification occurs of the sewage containing sludge which is pumped back again. This method necessitates longer durations of stay in the following aeration part of the tank in order to achieve nitrification. Sprayers of various types, with distributing means, are used to supply air to an aeration tank for biochemical purification of sewage. These sprayers are located close to the bottom of the tank. Usually the location is such that when air is supplied the contents of the tank are caused to circulate, thus preventing oxygen saturation of separate parts of the contents, and the air supplied is distributed more uniformly throughout the liquid in the tank so that the oxygen gas in the air will have the intended effect of keeping the active sludge floating, thereby preventing it from settling at the bottom of the tank. The distributing means in the sprayers are usually porous ceramic bodies having a pore size of 0.3 mm or less. Excellent bubbles were obtained in practical operation in aeration tanks having a water depth of 3-5 meter where such distributing means had been selected, enabling utilization of 12-13% of the oxygen from the air. Other forms of distributing means with mechanical methods of forcing oxygen into the water, also designed for location at the bottom of the tank, are already known through patent DE-A-1 800315, for instance, which can be descπbed as a surface aerating means placed under a submerged hood, and DE-B-2 216 917, a brush aeraung means placed in a submerged hood. An invention according to BE-B-457349 operates by means of a fan system which recirculates air at increased pressure under a submerged hood. Common to these three examples is that a condiϋon for function is that the submerged hood must be filled with gas. Furthermore a mimmum quantity of air must be supplied to stir the tank sufficiently, this quantity being substantial and constituting the limit for how much oxygen can be utilized m practical operation. Another requirement for aeration equipment in nitrification tanks is that the oxygen excess in the liquid leaving the tank for transfer to the denitrification step should be extremely low so as not to disturb and extend the denitrification process. In this respect, however, conventional aeration equipment has an obvious disadvantage in that a minimum quantity of air required for stirring and to avoid sludge deposits on the bottom, does not permit the downward control of the oxygemzation desirable. Separate mechanical stirrers of the type forming eddy currents must therefore be installed to stir the liquid in the denitrification zones. The liquid content in the tanks must be given sufficient velocity to prevent sludge deposits on the bottom, and must also be sufficiently mixed to prevent parts of the liquid from retaining injurious remnants of oxygen. The denitrification tanks may be shaped like aeration tanks and tanks for nitrification but must contain mechanical stirrers instead of aerating means. This constitutes a considerable inconvenience with specific volumes being tied up for the various treatment steps where greatly varying flows must be dealt with depending on the day of the week and the time of the year. Temperature differences in the water entering, depending on the time of year, have considerable influence on the treatment. Colder water requires longer duration of stay. To bπdge over these problems both types of equipment, i.e. both aerating means and mechanical stnrers have in some plants been installed in about 50% of the tank space. In other plants compensation is made by adding chemicals or extraneous substances. The object of the present invention is to obtain a method of the type descπbed in the introduction, in which the utilization of oxygen from the air is at least as good as in known embodiments of the aeration process, and which also enables a stirπng effect with no, or only very slight aeration, while at the same time to a great extent avoiding inherent drawbacks. The method described in the introduction is characterised in that forcing air into the liquid and the stirring effect are achieved with the aid of a compressor unit which is entirely submerged in the liquid and which may be permanently installed in the tank or fitted on guides permitting the unit to be raised to a service level above the surface of the tank. The unit can be run either as eddy former or as combined oxyginator and eddy former. The unit includes a hood functioning as an air pocket above the mechanical part. The oxygenization work is characterised in that the air supplied directly to the unit or trapped from below is introduced into a circuit with so many passages through the liquid that the oxygen content of the air is utilized to the full. The air here is under the pressure prevailing in the surrounding liquid level, thus giving a higher oxygen content per unit volume which is dependent on the degree of compression of the air. This effect is particularly favourable when the unit is placed in deep tanks since the power required for forcing in air is not dependent on depth, thanks to the fact that deplacement of the liquid is independent of the depth. The present invention also comprises a device, preferably designed for use with the method according to the invention. It thus comprises a means for treating liquids, particularly for adding oxygen to clean water, or for biologically treating polluted water, such as sewage, by means of the active sludge and biological nitrogen reduction methods, in a treating tank in which the liquid is detained in the form of a bath having an inlet for the liquid to be treated and an outlet for the treated liquid. The device is characterised in that it comprises a compressor immersed in the bath with a cylinder provided with blades or pushers located under a hood that may act as air pocket for primary air supplied from a compressed air source and for secondary air trapped in the process of being treated, and which finally releases the treated air. The device is also characterised in that the rotating cylinder provided with blades or pushers acts both as eddy former and as aeration means when oxygen-containing air is added. Pushers may be used instead of blades, said pushers being made of brushes with closely spaced bristles of sufficient stiffness. This may be suitable if larger particles in the liquid are likely to become wedged against the blades, thereby hindering rotation. The method according to the invention will be described in the following with reference to an accompanying drawing showing in cross-section an embodiment for biochemical treatment of waste water. In the drawing 1 denotes a tank for treating sewage with active sludge with the addition of air. The sewage to be treated is supplied to the tank 1 through an inlet 2 and is removed from the tank via an outlet 3. The sewage introduced into the tank through the inlet 2 has been subjected in the normal way to mechanical pre-cleaning, e.g. screening to remove larger items such as rags, paper, fruit peel, etc. After spending some time in tank 1 the treated sewage is conveyed with flocculated sludge to a post-sedimentation tank, not shown, where the sludge particles are allowed to settle on the bottom. A part of the sludge removed is withdrawn from the process as excess sludge while another part, or the remainder, is returned to the tank 1 as feedback sludge. The returned sludge causes a relatively high concentration of sludge in the tank 1. The feedback sludge contains large quantities of micro-organisms such as bacteria and Protozoa which contribute to oxidizing the polluting constituents in the sewage and then causing flocculation thereof in the water phase. In order to saturate the sewage with oxygen in the treating tank 1, air is supplied through a supply pipe 4 and collected under a hood 5 after having risen as bubbles 9 through the inner water surface created on a level with the lower edge of the hood 5. The cylinder 6, rotated by means of a drive unit not shown, is provided with blades 7 which, upon passing the plate 8, stationarily secured by means of attachments 15, push aside the liquid in the direction of rotation, the air located under the hood 5 accompanying it and then returning through the liquid to the hood 5 in the form of bubbles 10 as shown in the drawing. The quantity of air supplied to the treatment zone is regulated by a valve 11 in the supply pipe 4 and a corresponding quantity of used air rises to the upper surface 13 of the tank 1 in the form of bubbles 12. Since the blades 7 are dipped into the liquid, the liquid is forced to move in the direction of the arrow 14. An equivalent transfer of kinetic energy occurs even if the hood 5 is emptied of air and filled by the liquid in the tank. In order to maintain optimal efficiency in the liquid treatment the air valve 11 can be program-controlled against measured values of, for instance, the oxygen content, using a probe 16 in the air under the hood 5 and a probe 17 in the water in the tank 1. The drawing illustrates assembly of the aeration unit on a conventional guide arrangement to permit the unit to be lifted for inspection, for instance. A guide pipe 22 secured in a base 23 and upper wall attachment 24 functions both as stand for the aerating means in operating position and also as a track when hoisting and lowering the unit by means of a wire 19 secured in an attachment 20 which holds the guide runner 21 and hood 5 together. Thanks to the arrangement in the tank 1 very little energy is required to force the air collected in the hood into the tank liquid. The peripheral speed of the blades is in the vicinity of 1 meter/second and the rotary resistance to be overcome consists substantially of torque from the deplacement of downwardly pressed air caused by the blades. Since the force of this deplacement remains practically the same with increasing installation depth in a tank, but the amount of oxygen treated increases the deeper the hood is located, due to compression of the air, the efficiency of the oxygenization increases with increased installation depth. With the method according to the invention up to 100% of the available oxygen in the air supplied is utilized for the biological purification process, thereby enabling a reduction of the air supply to approximately 15-20% of that required for air treatment in accordance with conventional technology. The method also satisfies the need for stirring the liquid, thanks to the second function of the blades which is to act as mechanical eddy formers. The method also allows the air supply to be throttled to approaching zero without the problem of deposits on the bottom of the tank occurring. This is particularly significant in connection with nitrogen reduction since the invention eliminates the need to install separate eddy formers.

Claims: C L A I M S 1. A method of treating liquids with gas in which a liquid is fed into a treating tank and is removed from the treating tank after remaining there for some time, characterised in that gas is supplied and collected under a hood (5) immersed in the tank ( 1), said hood being provided with a wheel (6) with blades (7) to stir the liquid and also compress the gas, compressed gas being utilized for the treatment and the wheel being caused to rotate even in the absence of gas and in that the gas under the hood (5) is preferably controlled and recorded. 2. A method as claimed in claim 1, characterised in that the gas may be so chosen as to create oxic, anoxic and anaerobic environments for treating the liquid. 3. A device for adding gas such as air, oxygen gas or some other gas to a pool of water consisting of a lake, a tank in a purifying plant, characterised in that a compressor is immersed in the pool of water, with supply openings directed downwards and with outlet openings directed downwards and in that a supply member for gas, the outlet of which supplies the inlet to the compressor with gas from below, and in that the outlet of the compressor is arranged so that gas from the outlet is returned to the inlet of the compressor. 4. A device as claimed in claim 3, for stirring and treating liquid in a treating tank (1) to which the liquid is introduced via inlets (2) and removed via outlets (3) after treatment, characterised in that the device is immersed in the liquid and comprises a hood (5) constituting a pocket for gas which is collected and utilized for treating the liquid, a rotating cylinder (6) provided with blades (7), and a plate (8) secured so as to leave a small gap between itself and the periphery of the blades, together forming a pressure chamber to force down air from the hood to the liquid beneath the device, and in that the rotation of the blades forces the liquid along. 5. A device as claimed claims 3 and 4, characterised in that fresh gas is supplied directly to the hood via a pipe directly connected via a connection (18). 6. A device as claimed claims 3-5,. characterised in that the blades (7) consist of brushes. 7. A device as claimed in claims 3-6, characterised in that the plate (8) can be turned about the centre of the cylinder in order to regulate the immersion depth in the liquid. 8. A device as claimed in claims 3-7, characterised in that the plate (8) is not fitted. 9. A device as claimed in claims 3-7, characterised by an arrangement to optimize efficiency when treating liquid, said arrangement comprising a valve (11) near the air pocket cooperating with a probe (16) near the air pocket and with a probe (17) in the tank (1).

Inventors: LJUNGKVIST LARS, NORDLUND TORE, LJUNGKVIST, LARS, NORDLUND, TORE
Applicants: LJUNGKVIST LARS, NORDLUND TORE, LJUNGKVIST, LARS, NORDLUND, TORE
Application year: 1993
Main classification: B01F3
IPC codes: B01F3, B01F7, C02F1, C02F3