CA1140278A - Method and apparatus for purifying sewage by means of activated sludge - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An apparatus and method for purifying sewage by means of activated sludge, whereby the raw sewage to be purified is passed consecutively to a reaction tank for aeration, an aeration tank for further aeration, and a sedimentation tank. Part of the raw sewage is passed directly to the aeration tank. The settled and concen-trated activated sludge is passed from the sedimentation tank, partly to the reaction tank and partly to the aera-tion tank. The mixture is aerated in the reaction tank and/or the aeration tank and, dependent upon the 02 content, is circulated, rotated and/or concentrated. At least two treatment units of the apparatus, each containing a re-action tank and an aeration tank are arranged in parallel, the units being aerated intermittently while otherwise operating in parallel.

Description

The present invention relates to a method and ap-paratus for purifying sewaye by means of activated sludge.
In co-pending Canadian application serial number 318,258, filed December 20, 1978, now Canadian patent 1,099,037, issued April 7, 1981, sewage to be purified is treated in a reaction tank, an activation or aeration tank, a sedimen-tation tank and a sludge-return unit which picks up the returning activated sludge from the sedimentation tank.
Sewage-treatment installations of this kind are intended to considerably reduce the chemical and biological 2 demand, and to eliminate nitrogen.
It is known that when sewage containing nitrates is mixed with activated sludge the 2 is consumed after a short time, while the released nitrogen from the resulting nitrates escapes into the atmosphere. In the above noted co-pending application a large part of the nitrogen is elimina-ted, however, this cannot be controlled by a particular de-sign or method of operation of the installation.
According to an aspect of the invention there is provided a process for sewage purification by means of ac-tivated sludge, wherein raw sewage to be purified is fed successively to a reaction tank for aeration, to an aeration tank, also for aeration, and to a sedimentation tank, a portion of the raw sewage being fed directly to the aeration tank, wherein settled and concentrated activated sludge is returned from the sedimentation tank partly to the reaction tank and partly to the aeration tank, and wherein, dependent upon the oxygen content, the mixture in the reaction tank and the mixture in the aeration tank are caused to rotate, and de-pendent upon the oxygen content, are aerated and circulated, mab/~

~ - 114l)Z78 and wherein the activated sludge in the lcwer parts of the reaction tank is circulated and thickened, characterized in that there are pro-vided in parallel at least two treatment units, each comprising a reaction tank and an aeration tank, and that Lhe treatment units are aerated intermittently in an operation which, in other respects, is in the parallel mcde.
According to a f~rther aspect of the invention there is pro-vided a sewage purification apparatus for sewage purification which includes a raw-sewage influent pipe, a reaction tank provided with means for aeration dependent upon the oxygen content of the reaction tank, an aeration tank provided with means for aeration dependent upon the oxygen content of the aeration tank, a sedimentation tank, an effluent pipe for re~oving clarified sewage from the sedimentation tank, means including branch conduits for conveying settled and concen-trated activated sludge in the sedimentation tank to the reaction and aeration tanks, means including branch conduits for supplying raw sewage to the reaction and aeration tanks and conveyor means located between the reaction tank and the aeration tank, characterized in that there are provided in parallel at least two treatment units, each comprising a reaction tank and an aeration tank, including associated raw-sewage supply means, activated-sludge-return means and aeration means, and that an aeration-air distributor means feeds aeration-air intermit-tently to the treatment units in a controllable ratio between periods of aeration/non-aeration which periods are particularly dependent upon the oxygen content of the aeration tanks.
Thus, according to the invention, nitrogen separation is assured; and since nitrification of nitrogen-containing contaminants in the sewage is dependent upon concentration, temperature and other factors, the periods of residence for nitrification and denitrification may differ in length. This improves purification, reduces power and plant _ 2 -mab/C~

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costs and provides increased reliability.
According to the invention, the raw sewage is purified as disclosed in the co-pending Canadian application, but aeration of the respective tanks is not carried out continuously. Instead, the aeration, activated-sludge and reaction tanks are, preferably, in the form of at least two treat-ment units of the same size arranged side by side and paral-lel with each other, the treatment units being aerated con-trollably and intermittently for a predetermined time until nitrification is achieved. The necessary air-supply is switched, by suitable means, after nitrification has been achieved in a first unit from that unit to a second unit, in which further biological purification and nitrification is carried out; during this time, no air is supplied to the first unit, for which reason the nitrates, developed in the mixture of activated sludge and water, are consumed by micro-organisms because the 2 content of the sludge mix-ture is rapidly reduced; thus the released nitrogen can es-cape into the atmosphere. Continuous repetition of this pro-cedure eliminates the nitrogen which is normally unwanted in the discharge.
Before being introduced into the aeration tank, `` the incoming raw sewage is passed into the reaction tank and is mixed with the return-sludge from the sedimentation tank. This removes some of the nitrogen from the nitrate-containing return-sludge, which means that the incoming raw sewage is preliminarily cleaned, to a certain extent, particularly as regards contamination and nitrogen-content and only then passes to the aeration or activation tank.

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1~40Z'~8 Further contaminants are removed from the sewage in the aeration tank and, for this reason, draining the purified sewage into a public sewer has fewer disadvantages than in the case of conventional biological sewage treatment.
The purification process then proceeds as set forth in the noted co-pending application; the necessary supply-lines to the centre of the reaction tanks are provided with branch lines, thus providing a selective supply, as ex-plained in the co-pending application.
Aeration is also carried out, in the manner ex-plained in the co-pending application, by means of a rotating, continuous aeration bridge having aerators extending to the bottom of the tanks. This not only produces aeration but also keeps the activated sludge in suspension, both in the aeration tank and in the reaction tank.
In order to achieve reliable elimination of nitro-gen, 02-poor, 02-less and 02-rich zones are created tempor-arily in different parts of the apparatus, by controlled switching of the aeration air supply. This makes it pos-sible to allow for changes in sewage volume, contamination, temperature and the like, thus achieving optimal nitri-fication and denitrification times and promoting the best possible purification process.
The necessary aeration, the duration of which is controlled, as already explained, by means comprising a timed control-unit, is according to sewage volume; by means of a compressor unit containing either one multi-stage com-presser or a plurality of single-stage compressors adapted to be operated simultaneously. These compressors are acti-.

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vated by the t~ed control-unit, in conjunction with a probe located in the aeration tank. Since the aeration pipes within the tanks are supplied with air during active aera-tion periods only, it is desirable for the air outlet aper-tures in the pipes to be designed in such a manner that they do not fill with water when the air-supply is shut off, but remain filled with air. This may be achieved, with advan-tage, by supplying a small amount of air to the inoperative aerators, in order to prevent them from becoming blocked.
This air may be taken, through a bypass line and a control-lable slide-valve, from a compressed-air equalizing container used to damp-out surges of air.
The invention is explained hereinafter in conjunc-tion with the examplary embodiment illustrated in the accom-panying drawing, in which the single figure is a plan view of a sewage-purification apparatus according to the invention, comprising a two unit apparatus.
The drawing shows two reaction tanks 1 and 1.1, two aeration tanks 2 and 2.1, respectively, associated with the reaction tanks, a sedimentation tank 3 associated with aeration tanks 2 and 2.1 and a return conveyor-device 4 for transporting the activated sludge from sedimentation tank 3.
Reaction tanks 1 and 1.1 have surrounding walls 5 and 5.1, respectively, separating them from aeration tanks 2 and 2.1 associated therewith. Each of the latter has a surrounding wall 6 and 6.1, respectively. Reaction tank 1 and aeration tank 2 constitute one unit 10 of the apparatus, while reaction tank 1.1 and aeration tank 2.1 form the second unit 10.1 thereof. Each of these units comprises an aeration bridge 18 and 18.1, respectively, rotating in the direction of arrow mab/(\b 114~Z ~8 7, the bridges being equipped with aerators 1.6 and 1.61, respectively. Probes 9 and 9.1, are provided in units 10 and 10.1, respectively, of the apparatus, more particularly in aeration tanks 2 and 2.1.
Raw sewage is fed, through an inlet 8, to the sewage-treatment apparatus entering first i.nto a distribution duct 1.9, and then passing, through lines 1.2 and 1.21, to the centre of reaction tanks 1 and 1.1, respectively. The mixture of sludge and sewage passes from the reaction tanks around the periphery of walls 5 and 5.1 surrounding them and into aeration tanks 2 and 2.1, where it is aerated and flows, through outlets (not shown) and lines 10.2 and 10.21, respectively, to the centre of sedimentation tank 3. The purified sewage then passes, through an outlet 10.4 and a drain-line 10.5, into a drainage ditch (not shown).
Concentrated activated sludge flows, through a line 3.1, into return device 4, passing thence, through supply-: lines 2.4 and 2.41, provided with slide-valves (not shown), to the centre of reaction tanks 1 and 1.1, respectively.
As explained in the noted co-pending application, a part of the sewage is also fed to respective aeration tanks

2 and 2.1, through branch-lines 1.3 and 1.31 fitted with slide-valves, while activated sludge is supplied from return device 4 through branch-lines 2.3 and 2.31.
In this connection, raw sewage fed through line 8 may be passed, through a projecting structure 19, to distri-buting duct 1.9. Concentrated activated sludge, taken from the bottom of sedimentation tank 3, is lifted, in return device 4, by means of a worm-pump or the like, so that it may be fed, respectively, to units 10 and 10.1 of the apparatus.

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-- 1141)Z78 Moreover, as described in the co-pending application, and not shown here in detail, part of the mixture emerging from aeration tanks 2 and 2.1, and to be passed to the sedi-mentation tank, is fed directly to return device 4.
Assuming that units 10 and 10.1 of the apparatus are of the same size, it is desirable to feed them with equal amounts of raw sewage through lines 1.2 and 1.21, respectively.
The air required for aeration is drawn in through a compressor unit consisting, in the examplary embodiment illustrated, of a compressor 15 for minimal loading (during the night, for example), a compressor 16 for average loading and a compressor 17 for maximal loading. Obviously it is also possible to use a single compressor adapted to be operated in several modes, or to use a plurality of compres-sors of the same or different capacities. The air drawn in passes to a compressed-air shock-absorber 14. The compres-sed air passes thence, through supply-lines 2.8 and 2.81, to bridges 18 and 18.1, respectively,preferably to a point above the centres of rotation thereof. It then passes, through aerators 1.6 and 1.61, into reaction tanks 1 and 1.1, respectively, and through further aerators 2.5 and 2.51, into aeration tanks 2 and 2.1, respectively. Agitators 1.4 and 1.41 may also be fitted to the bridges, more particularly in reaction tanks 1 and 1.1, respectively.
Also provided is a timed control-unit 11 which, according to signals received from probes 9 and 9.1 in units 10 and 10.1, respectively, of the apparatus, more particul-arly from aeration tanks 2 and 2.1 therein, controls the opening of valves or slides 12 and 12.1 at the outlet from compressed-air shock-absorber 14, thus introducing air either mab/ r~J

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- into line 2.8 or line 2.81. In this connection, slide-valve 12 may be bypassed by a line 13 with a slide-valve (not shown), while slide-valve 12.1 may be bypassed by a line 13.1 with a slide-valve (not shown), in such a manner that, with slide-valves 12 and 12.1 closed, and bypass lines 13 and 13.1 open, a small amount of air may be fed to air-supply lines 2.8 and 2.81.
Finally, as in the co-pending application, radial partitions 2.7 and 2.71 may be provided in each aeration tank 2 and 2.1, respectively, as explained therein and for the purpose set forth therein.
The sewage-treatment apparatus according to the invention is operated in such a manner that the raw sewage, to be purified, is distributed as uniformly as possible to both units 10 and 10.1 of the apparatus. Aeration of tanks 2 and 2.1 is carried out in such a manner that tank 2 is supplied ; with air for a predetermined length of time through open slide-valve 12, and with slide-valve 12.1 closed, after which tank 2.1 is supplied with air through open slide-valve 12.1 and with slide-valve 12 closed. Thus units 10 and 10.1 of the apparatus are aerated consecutively and, whereas slide-valve 12 is opened while air is supplied to unit 10, slide-valve 12.1 is opened while air is supplied to unit 10.1, i.e., tank 2.1 is aerated for a predetermined length of time while air is sup-plied to unit 10, slide-valve 12 being opened and slide-valve 12.1 being closed. Aeration times for units 10 and 10.1 are selected by e~periment.
As already indicated, several compressors 15, 16 and 17, possibly with reserves, are provided for the supply of air, the compressors being switched on and off, according to ~ - 8 -mab/ ~b the volume of incoming sewage, by signals from electrical probes 9 and 9.1. In the case of a multi-stage compressor, suitable change-overs may be carried out. During the night, when the flow of water and contaminants is at its lowest, timed control-unit 11, without altering the aeration times, ensures that the 2 content in the tanks is neither above nor below a predetermined value. Moreover, the aeration times are such that the treated sewage which drains away is sufficiently nitrified or denitrified.
For example, during the night, the time of minimal flow, the minimal-capacity compressor 15 is switched on, and the sewage is aerated thereby for a certain length of time, namely until the selected and necessary 2 content in the tanks and corresponding units 10 and 10.1 of the apparatus is reached. This takes a certain time, as determined empirically when the sewage-treatment apparatus is started up. If the selected 2 content of the mixture of sewage and activated sludge, to be aerated, as a result of uninterrupted input, drops to below 1 mg/l of 2~ signals from probe 9, for example, cause slide-valve 12 for unit 10 to open and, simultaneously, slide-valve 12.1 for unit 10.1 to close and vice-versa. This continues intermittently as long as there is no change in the volume, contamination, temperature or other condition of the incoming sewage. In the event of sewage concentration or tem-perature change, or some other unfavourable condition, a cor-rection is made in the aeration time to allow optimal puri-fication action to be achieved. This is effected by timed control-unit 11. If the 2 content drops below the set level, another compressor, compressor 16 for example, is switched on. This increases the volume of air supplied, restoringthe 2 content _ g _ mab/ ~) 1~4~27F~
of units 10 and 10.1 of the apparatus to the predetermined level, and ensuring nitrification of the mixture of actuated sludge and water in units 10 and 10.1 of the apparatus.
Further increases in the volume of incoming water, contaminants, etc. switches on a third compressor 17. Through slide-valves 12 and 12.1, these three compressors supply units 10 and 10.1 of the apparatus with sufficient air so that, in spite of large volumes and high degrees of contamination, the 2 con-tent of the mixture of activated sludge and water remains at the desired level selected.
However, if there is a drop in volume and/or conta-mination in inlet 8, signals from probes 9 and 9.1 cause one ; of the compressors, compressor 17 for example, to be switched off, thus making it possible to avoid any increase in 2 con-tent. In the examplary embodiment illustrated, comprising three compressors, under normal conditions only two compres-; sors 15 and 16 are in operation until either an increase in - volume causes compressor 17 tc be swit~ed on, or at night only compressor 15 remains in operation. In this way, the installation achieves nitrification and denitrification with the same 2 content.
` According to the invention, therefore, timed control-- unit 11 provides adjustable, predetermined aeration times.
This assures the lowest permissible 2 content in the mixture of activated sludge and water. ~oreover, in spite of the increased elimination of nitrogen, the sewage-treatment apparatus achieves substantial purification at a very low power cost.

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Claims (15)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for sewage purification by means of activated sludge, wherein raw sewage to be purified is fed successively to a reaction tank for aeration, to an aera-tion tank, also for aeration, and to a sedimentation tank, a portion of the raw sewage being fed directly to said aeration tank, wherein settled and concentrated activated sludge is returned from said sedimentation tank partly to said reaction tank and partly to said aeration tank, and wherein, dependent upon the oxygen content, the mixture in said reaction tank and the mixture in said aeration tank are caused to rotate, and dependent upon the oxygen content, are aerated and circulated, and wherein the activated sludge in the lower parts of said reaction tank is cir-culated and thickened, characterized in that there are provided in parallel at least two treatment units, each comprising a reaction tank and an aeration tank, and that said treatment units are aerated intermittently in an opera-tion which, in other respects, is in the parallel mode.

2. The process as set forth in Claim 1, wherein said treatment units are supplied with varying amounts of aeration-air depending upon the raw sewage contamination, the quantity of raw sewage, the raw sewage temperature and the seasonal meteorologic conditions.

3. The process as set forth in Claim 2, wherein the relationship between aeration and non-aeration times is varied.

4. The process as set forth in Claim 1, 2 or 3, wherein the volume of aeration-air supplied per unit time is varied.

5. A sewage purification apparatus for sewage purification which includes a raw-sewage influent pipe, a reaction tank provided with means for aeration dependent upon the oxygen content of said reaction tank, an aeration tank provided with means for aeration dependent upon the oxygen content of said aeration tank, a sedimentation tank, an effluent pipe for removing clarified sewage from said sedimentation tank, means including branch conduits for conveying settled and concentrated activated sludge in said sedimentation tank to said reaction and aeration tanks, means including branch conduits for supplying raw sewage to said reaction and aeration tanks and conveyor means loca-ted between said reaction tank and said aeration tank, characterized in that there are provided in parallel at least two treatment units, each comprising a reaction tank and an aeration tank, including associated raw-sewage sup-ply means, activated-sludge-return means and aeration means, and that an aeration-air distributor means feeds aeration-air intermittently to said treatment units in a control-lable ratio between periods of aeration/non-aeration which periods are particularly dependent upon the oxygen content of said aeration tanks.

6. The sewage purification apparatus as set forth in Claim 5, wherein said aeration tanks surround said re-action tanks in the form of a ring, and said means for aeration as well as those for suspension, rotation, circu-lation and mixing are formed by rotary bridges spanning said reaction and aeration tanks with aerators mounted thereon via air-supply pipes and located near the bottom of associated tanks.

7. The sewage purification apparatus as set forth in Claim 5, wherein said aeration-air distributor means com-prises control means for regulating the aeration/non-aeration periods responsive to oxygen-sensor output signals derived from oxygen sensors in said aeration tanks, and wherein the largest quantities of aeration-air can be controlled with appropriate aeration/non-aeration periods by said control means, depending upon the raw sewage contami-nation, the quantity of raw sewage, the raw sewage tempera-ture, the seasonal meteorlogic conditions and the desired purification, sequentially into either one or the other of said treatment units; said apparatus further comprising a compressor and a compressed-air shock absorber for sup-plying said aeration-air.

8. The sewage purification apparatus as set forth in Claim 6, wherein said aerators are mounted on said bridges a short distance above said tank bottoms and are so designed that consecutive air supply and non-air supply is possible.

9. The sewage purification apparatus as set forth in Claim 6, 7 or 8 wherein for greater operating safety said distributor means contains bypasses through which said aerators are continuously supplied with a small quantity of air in order to prevent them clogging.

10. The sewage purification apparatus as set forth in Claim 7 or 8 wherein said compressor is multistage and in its weakest stage, for light sewage loads, including night sewage flow, only sufficient aeration-air is gener-ated to attain and maintain a predetermined minimum oxygen content in said aeration tanks.

11. The sewage purification apparatus as set forth in Claim 7 or 8, wherein said compressor consists of a plurality of compressors, whereby generation of aeration-air can be controlled by switching on or off one or more compressors until a predetermined oxygen content is attained in said aeration tanks.

12. The sewage purification apparatus as set forth in Claim 7 or 8, wherein said control means regulates said aeration/non-aeration periods in such a manner that the oxygen content in said aeration tanks is kept sufficiently low so that under a given set of operating conditions com-plete nitrification up to the end of the aeration period and, thereafter, complete denitrification up to the end of the next non-aeration period can be accomplished.

13. The sewage purification apparatus as set forth in Claim 5, 6 or 7, wherein said aeration/non-aeration periods are adjusted by probes in said treatment units tes-ting the conditions therein so that complete nitrification and complete denitrification can be accomplished.

14. The sewage purification apparatus as set forth in Claim 5, 6 or 7, further including a preliminary coarse sludge removal means.

15. The sewage purification apparatus as set forth in Claim 5, 6 or 7, wherein said treatment units are of equal size and are fed by substantially equal volumes of raw sewage.