A METHOD FOR BIOLOGICALLY TREATING LIQUID WITH OXIDIZING MEDIUM, AND APPARATUS FOR CARRYING OUT THE METHOD

Abstract: A method and apparatus for treating liquids, such as pure water and contaminated water, particularly sewage water, with oxidizing medium, in which there is used a contact hood (6) for contact between the oxidizing medium and the liquid. Part of the oxidizing medium is circulated in a conduit system (4, 7) with the aid of a fan or blower (8). A further, consumed part of the oxidizing medium is allowed to depart from the system and is replaced with fresh medium through a compressed-oxidizing medium source (12) connected to the conduit system (4, 7).

Description: A METHOD FOR BIOLOGICALLY TREATING LIQUID WITH OXIDIZING MEDIUM, AND APPARATUS FOR CARRYING OUT THE METHOD The present invention relates to a method for treating liquids with oxidizing medium, in which the liquid is fed into a treatment basin or tank and removed there¬ from subsequent to a given residence time therein. The method is particularly useful for treating water. The water concerned may be pure water and the treatment may be one in which oxidizing medium is added to the water with the intention of oxidizing the iron and/or the manganese present therein, so as to form compounds which can be readily separated from the water. The method, however, is probably most commonly used in the treatment of contaminated water, and in particular in the micro¬ biological treatment of sewage water in accordance with the activated sludge method. Consequently, the following description of the method according to the present inven- tion will be made with reference to this latter kind of treatment. Methods of the kind set-forth in the pre-characterizing clause of Claim 1 are known to the art in various forms with regard to both the oxygenation of pure water, and above everything with regard to the treatment of conta¬ minated water and the like. The methods are applied when high demands are placed on the cleansing effect achieved. The biological purification or cleansing of sewage is based on the biochemical oxidation and flocculation of organic substances present in the sewage, while supplying air there¬ to. Hitherto, flocculation is normally the most significant of these cleansing processes from the quantitative aspect. When practicing biological purification methods there is always obtained a sludge which constitutes the active substance and which contains large quantities of bacteria, protozoa and other microorganisms. Organic substance which is partially oxidized, is taken up from the sewage under the aerobic conditions that prevail. Since a large part of the organic substance present is converted to sludge, it is necessary to connect the aerobic treatment tanks to downstream settling tanks, in which the resultant sludge is isolated in the form of a voluminous sludge that contains very high proportions of water. The flocculated substance obtained when treating the sewage in the treatment with air is passed to the downstream settling tank or basin, in which it is allowed to settle to form a sludge. This sludge can be recycled to the treatment tank - as return sludge - and there used to catalyse the flocculation and biochemical oxidation of the organic compounds present in the sewage water. The proportion of sludge in the treatment tank is held at a relatively high level, about 10% by volume, and the tank contents are agitated in order to keep the sludge in suspension. This results in the development of active microorganisms and contributes to the effective oxidation of the contaminants. The part of the sludge produced in the downstream settling tank and not cycled back to the treatment tanks for biochemical cleansing of the water is removed from the process as surplus sludge and, e.g., can be processed separately and converted to soil compost. The air is supplied to an aerating tank intended for the biochemical purification of sewage water, through various kinds of sprays or injectors that are equipped with distributing devices. These sprays or injectors are located in the close proximity of the bottom of the tank. The sprays or injectors are normally positioned in a manner such that when air is supplied therethrough the contents of the tank are caused to move in a circulatory path, i.e. are imparted a so-called rolling motion, which renders it impossible for individual parts of the tank contents to be saturated with oxygen gas and causes the air to be distributed substantially uniformly through the whole of the liquid bath in the tank, so that the oxygen contained in the air will have the effect desired, and which also holds the active sludge in suspension in the tank liquid, so as to prevent the active sludge from settling on the tank bottom. The distributing devices used in conjunction with the air injectors or sprays normally comprise porous ceramic bodies having a pore size of 0.3 mmor therebelow. When using aerating tanks which have a sewage depth of 3 - 5 meters and which incorporate such distributor devices, there is obtained extremely fine bubbles which enable up to 12 - 13% of the oxygen contained in the air to be utilized. The results obtained in other respects, however, are not so favourable, because the pores in the ceramic material become quickly blocked. The higher resistance caused by this blockaging of the pores results in higher energy requirements, while the costs involved in unblocking the pores of the ceramic bodies is unacceptably high. Conse¬ quently, sprays or injectors that incorporate porous distributing devices have gradually been replaced with sprays or injectors provided with distributing devices which incorporate relatively large apertures, the dia¬ meter of which may be as large as 25 mm, even though it is necessary to utilize with such devices large quantities of air, in order to ensure that the desired degree of oxygen saturation is achieved and effectively maintained, therewith resulting in higher energy require¬ ments. In the case of such so-called "large bubble systems" it is possible to utilize from 8 - 9% of the oxygen contained in the air supplied. A further disadvantage encountered when using spreaders which require large quantities of air to be supplied is that voluminous aerosols are formed above the surface of the liquid in the aerating tank. The object of the present invention is to provide a method of the kind described in the introduction which will enable the oxygen contant of the air supplied to be utilized to the same good extent as that achieved with embodiments of the method while at the same time substantially eliminating the intrinsic drawbacks of such embodiments. To this end it is proposed that when carrying out the method set forth in the introduction the air is introduced into a pressurized chamber defined in a contact hood or the like which is held submersed in the liquid; that the liquid is vigorously whipped and/or agitated in a manner to create a wide specific reaction-surface area between liquid and air present within the hood, so as to deplete the air of oxygen? and that the depleted air is permitted to depart from the contact hood and is replaced with fresh air. According to one embodiment of the invention the air is suitably supplied to the liquid located immediately beneath the contact hood, while vigourously agitating the liquid, optionally with the aid of mechanical agitating devices, so that a wide transfer surface between air and liquid is obtained in air bubbles beneath the surface of the water and on upwardly splashing water droplets above the liquid surface located beneath the contact hood. According to a further embodiment of the invention the air is supplied to the liquid located immediately beneath the liquid surface defined in the pressurized contact hood with an intensity such that the liquid is circulated through an ejector effect. In accordance with a further embodiment of the invention the air depleted of oxygen is captured beneath a collec¬ ting hood and is conducted to a cleansing system or is passed to atmosphere. The particular application of the invention in the bio- logical treatment of contaminated liquids, e.g. sewage water, will afford the combined advantages of greatly reducing a) the high costs previously entailed in cleansing blocked distributing devices, b) the high energy require¬ ment for finely dividing the treatment air to be introduced into the contaminated water and the active sludge in the treatment tank, and c) the disadvantage manifested by the poor use of the oxygen content of the air supplied to the system. Furthermore, the exaggerated formation of aerosols above the surface of the liquid in the treatment tank is also greatly reduced, particularly in the embodi¬ ment incorporating a collecting hood. The invention also relates to an apparatus preferably intended for use when carrying out the method according to the invention. Accordingly, the invention relates to apparatus for treating liquids, such as pure water, for oxygenating the pure water, or contaminated water, e.g. sewage water, for biologically treating said water in accordance with the activated sludge method in a treatment tank, in which the liquid is held in the form of a bath and which has an inlet through which the liquid to be treated is introduced into the tank and an outlet for removing the treated liquid therefrom. The apparatus is characterized in that it includes a contact hood or the like which is immersed in the liquid bath and which is connected to a compressed-air source in a manner which enables fresh air to be introduced into the tank as a replacement for air utilized in the treatment process. According to one embodiment of the invention the arrange¬ ment includes a delivery conduit system which includes a pipe having provided therealong one or more ejection nozzles through which air is introduced into the liquid located beneath a pressurized chamber in the contact hood, and further comprising a return conduit system for recyc¬ ling air from said chamber, and a circulation fan or blower which terminates the circuit formed by the two conduit systems and which is effective for circulating air through the closed circuit. This arrangement affords the advantage that the fan. or blower is able to operate at a low power- input, since the only force which need be overcome is the force presented by the water cσlum present between the location at which air is introduced into the tank and the water-surface beneath the contact hood and the air resistance in the air delivery and air return conduits. According to a further embodiment of the invention the arrangement may include a collecting hood which terminates sealingly with the surface of the liquid bath on a part of said surface where air, which has been depleted of oxygen during the treatment process, from the contact hood reaches the upper surface of the bath, and a vent pipe connected to the collecting hood. The method according to the invention will now be described in more detail with reference to the accompanying drawing, which schematically illustrates an embodiment of an arrange¬ ment for the biochemical treatment of sewage water, in which arrangement a circulation fan or blower is used for delivering the kinetic energy required to create a large contact surface between air and sewage water. With reference to the drawing, the reference numeral 1 designates a tank in which sewage water is treated with active sludge while supplying air to the tank. The sewage to be treated is introduced into the tank 1 through an inlet 2 and is discharged from the tank through an inlet 3. The sewage entering the tank through the inlet 2 has been prε-cleansed mechanically in a conventional manner, e.g. passed through screens in order to remove from the sewage coarse, slurried contaminants such as rags, paper, fruit peel, etc. Subsequent to holding the sewage in the tank 1 for a given period of time, the sewage together with flocculated sludge is removed from the tank and passed to a downstream or secondary settling tank (not shown) in which the sludge particles settle onto the bottom of the tank. Part of the sludge sediment is removed from the process as surplus sludge, while a further part, or the remainder of the sludge, is recycled to the tank 1 as return sludge. As a result of the sludge recycled to the tank, the tank 1 will contain a relatively high proportion of sludge. The return sludge contains large quantities of microorganisms, such as bacteria and protozoa, which contribute to oxidizing the contaminated constituents of the sewage water and subsequently to flocculate these constituents in the water phase. For the purpose of saturating the sewage water in the treatment tank 1 with oxygen, air is supplied through an air delivery conduit 4 and is distributed with the aid of an ejector nozzle 5 provided at the end of the conduit. The nozzle presents one or more apertures having a diameter of 10 mm, in which the air is throttled and converted to bubbles which disperse in the mixture of sewage water and active sludge. The kinetic energy of the bubbles thus formed is transmitted to the mixture and causes the mixture to circulate vigourously in the tank 1 , in a closed path which includes both the upper part and the lower part of the tank 1. The bubbles formed will relatively quickly enter a zone located at some distance from the nozzle exit orifices and will rise upwardly through the active sludge suspension. The majority of these bubbles are captured by a contact hood 6, the open part of which faces downwardly towards the bottom of the tank in spaced relationship therewith. The air deriving from the bubbles captured by the contact hood is passed from the hood 6 through a return-air conduit 7, which forms part of a circulation conduit system in which air can be circulated with the aid of a circulation fan or blower 8, and which also incorporates the delivery conduit 4. The circulation system incorporating the conduits 4 and 7, the nozzle 5, and the contact hood 6, is connected to a delivery line 11 which in turn is connected to a compressed-airsource in the form of a compressor 12. The air required to replace the oxygen- depleted air is supplied through the delivery conduit 11 with the aid of the compressor 12. This air is permitted to depart from the contact hood 6 through the sewage water to the surface thereof. The bubbles which rise upwardly from the contact hood and which contain consumed air are permitted to rise to the surface of the liquid in the tank 1 , where the bubbles are captured by collec¬ ting hood 9 and permitted to depart to atmosphere through a discharge conduit 10. The air pressure lost in the circulation system due to liquid pressure is extremely small, as a result of the arrangement in the tank 1 , resulting in a commensurately small power consumption for circulating the air and the sewage water in the tank system. The fine air bubbles required to saturate the circulating sewage water rapidly with oxygen can be achieved with a nozzle 5 which presents but a relatively small resistance and which has but relatively small blockage tendencies. Because the amount of consumed air leaving the tank is reduced and can be collected and departs through the collecting hood 9, the load on the environment is commensurately small. When practicing the method according to the invention up to 70% of the oxygen content of the air supplied to the system is utilized in the biological cleansing process, which implies a reduction in the air supply of about 15-20% of that required when performing known air treatment processes. The saturated air which leaves the surface of the treatment tank 1 in the form of an aerosol is reduced to a corresponding degree, which greatly relieves the measures required to protect the environment. Although the oxidant used in the aforedescribed embodiment for treating the contaminated liquid comprises atmospharic oxygen, it will be understood that the gaseous oxygen used may be taken from air enriched in oxygen or may com¬ prise pure oxygen. Other oxidants may also be used, e.g. ozone-containing media or optionally chlorine gas.

Claims: C L I M S 1. A method for treating liquids with oxidizing medium such as air, in which the liquid is introduced into a treatment tank and discharged therefrom subsequent to a given residence time therein, c h a r a c t e r ¬ i z e d by introducing the oxidizing medium into the liquid in a manner to form a large quantity of fine air-bubbles, introducing the bubbles thus formed into a pressurized, inverted contact hood immersed in the liquid and located in the path of the bubbles; whipping and/or vigourously splashing the liquid in a manner to cause the bubbles to entrain liquid into the hood chamber so as to provide an enlarged contact surface between the liquid and the oxidizing medium present within the hood (6) , such as to deplete the oxidizing medium of oxidizing agent, and permitting depleted medium to pass from the contact hood (6) and replacing said medium with fresh medium. 2. A method according to Claim 1, c h a r a c t e r ¬ i z e d by introducing the oxidizing medium into the liquid at a location immediately beneath the contact hood (6) while vigourously agitating, optionally mechanically, the liquid in a manner to form a wide transfer surface between medium and liquid in the medium bubbles beneath the surface of the water and on upwardly splashing water-droplets above the surface of the liquid located within the contact hood (6) . 3. A method according to Claim 1 or 2, c h a r a c t e r i z e d by introducing the oxidizing medium into the liquid located immediately beneath the surface of the liquid enclosed by the contact hood (6) with an intensity such as to cause circulation of the liquid in the tank (1) through an ejector effect. 4. A method according to any of Claims 1 - 3, c h a r a c ¬ t e r i z e d by capturing the depleted oxidizing medium beneath a collecting hood (9) and conducting the thus capture medium to a purification plant or discharging said medium to atmosphere. 5. A method according to any of Claims 1 - 4 when applied to oxygenate pure water. 6. A method according to any of Claims 1 - 4 applied in the treatment of contaminated aqueous liquids with the aid of microorganisms. 7. A method according to any of Claims 1 - 6, wherein the oxidizing medium is selected from the group consisting of air, oxygen-enriched air, pure oxygen, ozone-containing air and chlorine. 8. An apparatus for treating liquid with oxidizing medium in a treatment tank (1) in which the liquid is held for a given residence time in the form of a liquid bath, and comprising an inlet (2) for introducing the liquid to be treated into the tank, and an outlet (3) for treated liquid, c h a r a c t e r i z e d in that the apparatus further comprises a contact hood (6) or corresponding device which is immersed in the bath and which is connected to a compressed-oxidizing medium source (12) for introducing into the tank fresh oxidizing medium in replacement of oxidizing medium consumed during the treatment process. 9. An apparatus according to Claim 8, c h a r a c t e r ¬ i z e d in that the apparatus further comprises a delivery conduit (4) having a pipe which incorporates one or more ejection nozzles (5) for introducing oxidizing medium into the liquid present within a pressurized chamber in the con¬ tact hood (β) , and a return conduit (7) for oxidizing medium which is to be recycled from the chamber, and a circulation fan or blower (8) which is connected to the circuit formed . by the two conduits (4, 7) for circulating oxidizing medium through said closed circuit. 10. An apparatus according to Claims 8 or 9, c h a r a c t e r i z e d in that it further comprises a collecting hood (9) which is positioned in gastight relationsship with a part of the surface of the liquid bath in which oxidizing medium depleted of oxidizing agent during the treatment process and departing from the contact hood (6) reaches the upper surface of the bath; and an evacuating conduit (10) connected to the collecting hood (9) .

Inventors: LJUNGKVIST LARS, NORDLUND TORE, LJUNGKVIST, LARS, NORDLUND, TORE
Applicants: PURAC AB
Application year: 1987
Main classification: C02F3
IPC codes: C02F3