WO2006116825A1

WO2006116825A1 - Method and device for the purification of waste water

Info

Publication number: WO2006116825A1
Application number: PCT/BE2003/000058
Authority: WO
Inventors: Hendrik Seghers
Original assignee: Seghers Keppel Technology Group
Priority date: 2003-03-31
Filing date: 2003-03-31
Publication date: 2006-11-09
Also published as: AU2003249791A1; AU2003218538A1; WO2004087582A1

Abstract

The invention relates to a device (1) for the biological purification of a waste fluid (17), in particular waste water, the device (1) comprising a first, second and third compartment (10, 20, 30) for receiving the waste fluid (17), each compartment (10, 20, 30) containing an aerating device (2, 2', 2') for aerating the waste fluid (17), adjacent compartments being connected to each other to allow the liquid to flow from one compartment to the other, the first and third compartment (10, 30) each containing an inlet (13, 33) for influent waste liquor and an outlet (14, 34) for effluent purified liquid (18), the first and third compartment (10, 30) being provided to alternately function as aeration compartment for aerating the waste fluid and settling compartment to allow activated sludge (16) to settle and separate from the purified liquid (18), whereby the first, second and third compartment (10, 20, 30) are sequentially arranged in a circular configuration, the first and third compartment (10, 30) comprising a plurality of gravitational settling devices (3, 4) for increasing the settling surface for the activated sludge (16).

Description

Method and device for the purification of waste water.

The present invention relates to a device for a biological purification of waste fluids from impurities present therein in the presence of activated sludge, as is described in the preamble of the first claim. In particular, the present invention relates to a device for biodegradation of impurities contained in waste water in the presence of activated sludge.

State of the art.

In an activated sludge system mostly three separate steps are to be distinguished: (1) conversion of organic material present in the waste water by suspended activated sludge biomass into CO₂ and H₂O in aerobic conditions; (2) physical separation of activated sludge and purified water and (3) removal of purified water, removal of excess activated sludge. To guarantee an optimum separation of the activated sludge from the purified water it is of importance that the development of flocculating micro-organisms is favoured and the proliferation of filamentous micro-organisms is suppressed as much as possible as they show bad sedimentation characteristics and thus involve a too poor separation of the activated sludge from the purified water.

From BE-A-899.757 a device for the purification of waste water is known which consists of an essentially rectangular reactor divided into at least two sequentially arranged, hydraulically connected multifunctional compartments of approximately the same dimensions. The outer compartments have a dual function and in the course of the purification process, alternately function as (1) aeration compartment in the exogenous phase where conversion of the organic material into CO₂ and water by the micro-organisms of the activated sludge takes place and where reserve-materials are accumulated in the activated sludge ; and (2) settling compartment where sedimentation and separation of the activated sludge from the purified water takes place. The UNITANK® reactor is operated in cyclic manner wherein two alternating main phases are separated from each other by two alternating intermediate phases. In a first main phase, the fluid or liquor to be purified is supplied to the first outer compartment where it is actively mixed with the activated sludge present therein and aerated. This is the so-called accumulation ofwhere organic materials out of the liquor to be purified are absorbed into the cells. As influent is supplied to the first compartment, mixed liquor flows through an opening in the separation wall, from the first to the second central compartment following the compartment. principles of communicating vessels. In the second compartment the mixed liquor is further aerated and actively mixed in a so-called regeneration or stabilisation phase, in which the absorbed and accumulated organic materials in the cells are further converted into CO₂ and water, so called dissimilation of accumulated. From the second compartment the mixed liquor flows further to an opening in the bottom region of the separation wall to the third, outer compartment to allow sedimentation of the activated sludge to take place. In this stage the third compartment is not aerated, nor mixed. The activated sludge accumulates by sedimentation and separates from the purified liquor or water above it. The purified water is continuously discharged from the third compartment through an overflow system. ^" As soon as activated sludge has accumulated in the third compartment to a predetermined level, influent of waste water into and aeration of the first outer compartment is cut off and the first intermediate phase is started. The activated sludge present in the first compartment is allowed to settle to a sufficient extent. Thereafter the liquid flow is reversed by supplying waste water to the third outer compartment.

After the first intermediate phase has terminated, a second main phase is started. Influent waste water is supplied to the third outer compartment where it is aerated and mixed with the activated sludge present therein. The first compartment now functions as sedimentation compartment. The second main phase is followed by a second intermediate phase in which influent of waste water into and aeration of the third outer compartment is cut off again. The activated sludge present in the third compartment is allowed to settle.settle in order to create the conditions for restarting the first main phase. Activated sludge may be evacuated from the first and third outer compartment at the end of each main phase.

The device disclosed in BE-A-899.757 however has the disadvantage that it is rather voluminous and occupies a large surface area. The reason is that a large settling surface is required if an efficient purification process is aimed at, the required settling surface area varying with the nature and origin of the waste liquid. As a consequence the device disclosed in BE-A-899.757 can only be implanted in regions where sufficient area is available.

It is the aim of the present invention to provide a device, which is more compact than the device known from the state of the art.

Brief description of the invention.

A more compact device for the purification of waste water is achieved according to the present invention with the technical features of the characterising part of the first claim.

In the device of this invention, the first, second and third compartment are arranged in a circular configuration.

Due to the circular configuration a reactor wall with a minor thickness and higher water levels and/or higher reactor walls may be installed as compared to the rectangular configuration without this going at the expense of the strength of the wall. The presence of a higher reactor wall involves that the total reactor surface area may be decreased without touching the total reactor volume. The increased reactor height also involves that the amount of activated sludge that may be accumulated in the reactor, in particular the amount of activated sludge that may be accumulated in the settling compartment, can be increased so that the frequency with which the process needs to be reversed and the number of intermediate phases can be decreased accordingly. It is preferred to minimise the occurrence of intermediate phases as much as possible for the following reasons: (1 ) in the course of an intermediate phase the active volume of the purification reactor is reduced and limited to the central compartment, as the first and third compartment are not aerated; (2) in the intermediate phase influent of waste liquor takes place in the central compartment, activated sludge is pushed to the first or third compartment - depending on the flow direction of the liquor - thus further increasing loss of activated sludge in the compartment involved. The improved over-all efficiency of the purification process that may be obtained with the circular configuration is of particular importance with diluted waters (often municipal waters) and high influent water flow rates where often a significant movement and accumulation of activated sludge in the settling area has been observed.

To increase the settling surface for the activated sludge, the first and third compartment comprise a plurality of gravitational settling devices, in particular counterflow or crossflow separators. The use of gravitational settling devices is known from the art of water filtration facilities, where flocculated" solids have to be separated from an influent flow of liquid. An influent flow of mixed liquor is distributed over the width of the plates, the liquor flows upwardly under laminar flow conditions thus allowing solids present in the water/activated sludge liquor to settle on the plates while clarified effluent exits from the top of the plates. As the clarified liquid is removed through effluent launders mounted above the plates, the solids slide down towards the bottom of the first/third- settling compartment.

The presence of the gravitational settling devices involves that the volume and the surface area occupied by the first and third settling compartments may be decreased without this going at the expense of the available settling surface area and thus the hydraulic capacity of the reactor

Decreasing the volume of the first and third compartment involves that the surface area occupied by the second, central compartment may be increased by the extent the surface area of one of the first and third compartment has been decreased. In that way the active reactor volume that is aerated in a particular stage remains at least constant even with a lower total reactor volume. In general it can be said that the dimensions of and surface area occupied by the first and third compartment can be decreased with approximately 25% or even further without this going at the expense of the settling capacity of the compartment concerned. Transfer of active aeration from the first and third compartment to the central compartment and concentration thereof in the central compartment involves that the installed aeration and mixing capacity and thus the size thereof may be decreased without touching the performance of the device. In that way a cheaper device and process can be provided. Transfer of active aeration from the first and third compartment to the central compartment and concentration thereof in the central compartment also involves the possibility of increasing the relative contribution of the active reactor volume in which the waste liquor is aerated, the relative contribution of the passive reactor volume in which settling of the activated sludge takes place and decomposition of waste components is minimal, being minimised. The result is an improved over-all process efficiency and a process in which the influent waste liquor flow rate may be increased without this going at the expense of the quality of the purification process.

Conventional type clarification devices utilising tube and plate settlers to increase the hydraulic loading are often faced with the problem of fouling and bio-solid accumulation, so that regularly a cleaning action is required to unclog the settlers. This is mostly done manually with pressurised water. This invention combines the use of the settling devices with air injection. Oxygen containing gas for the biological oxidation process is preferably supplied in a lower region of each compartment below the gravitational settling devices to create turbulence in the reactor and an upstream flow of liquor over the plate separators to achieve that the latter are automatically cleaned and may remain unclogged in the course of the process. Conventional type clarification devices utilising tube and plate settlers to increase the hydraulic loading are often faced with the problem of fouling and bio-solid accumulation, so that regularly a cleaning action is required to unclogg the settlers. This is mostly donemanually with pressurised water.

The device of this invention preferably also comprises an activated sludge re-circulation system to re-circulate activated sludge between the first and third compartment. It has namely been found that activated sludge is transported from the compartment in which the influent flow of waste liquor takes place, via the central aeration compartment, to the settling compartment where an accumulation of activated sludge takes place. With diluted waters and high flow rate of the influent waste water, the rate at which this accumulation of activated sludge occurs has been found to be too high, particularly when operating with low water levels or first and third compartments with a smaller volume.

Re-circulation of the activated sludge appears to be an efficient means to counteract washout thereof from the influent compartment towards the effluent settling compartment where the activated sludge has been found to accumulate. Activated sludge re-circulation between the first and third compartment further entails the advantage that the duration of the main phase in which two of the three compartments are aerated may be prolonged and that the frequency with which the liquid flow needs to be reversed may be decreased, thus involving a serious save of time. To re-circulate the activated sludge, preferably use is made of an air lift system comprising a transport pipe mounted in the first and third compartment. The transport pipe extends in height direction of the reactor and contains in a lower part of the pipe an inlet for the activated sludge. The inlet of the transport pipe is preferably located below a position at which the amount of activated sludge in that compartment reaches a maximum. At a position above the activated sludge inlet, an air inlet valve, or more in general a gas inlet valve, is provided which is connected to an air supply system. As air is supplied to the air inlet and blown through the transport pipe, activated sludge is sucked from the reactor into the pipe and transported with the air flow towards the top of the pipe and the pipe outlet in the third, respectively first compartment.

To facilitate construction of the parts of the purification reactor in which the use of air is involved, and to reduce building and operation costs as much as possible, the air inlet for the activated sludge re-circulation system is preferably connected to the air supply system used for the aeration of the first and third compartment.

However other sludge re-circulation systems known to the man skilled in the art may also be used.

Along the outer wall of the reactor, preferably along the outer wall of the first and third compartment an effluent trough for removing purified water is mounted at a position in the vicinity of the liquor surface, the effluent launder being closed off by a cover, to prevent splashing of mixed liquor into the launder. The lower edges of the effluent launder extend to a position below the liquid surface, the top surface of the cover extends above the liquor surface. The launder comprises an air inlet valve, which is connected to an air supply system for providing an air over pressure, to prevent an influent of mixed liquor from the aerated compartment concerned. The launder further comprises a liquid outlet valve, which can be opened and closed to allow or prevent effluent of purified water. By having this valve closed the launder may be kept filled with clear water. With these technical features the necessity of installing a discharge well for rinsing water for the launders can be overcome. Also as no non purified water or mixed liquor can enter the effluent launder there is no need anymore to interrupt discharge of purified water, thus permitting that the purification reactor is operated in a continuous manner in contrast with the former invention described in BE-A- 899.757.

To facilitate construction and keep installation and operation costs to a minimum, the air supply for the launder can be connected to the air supply of the aeration device.

The device of this invention preferably also comprises a compartment for conditioning and thickening the activated sludge. In the course of the sedimentation step, accumulation of activated sludge in the compartment involved takes place. It is an object of this invention to keep the amount or concentration of the activated sludge in in each compartment or in the entire reactor virtually constant, or to have it varied within limited ranges only to guarantee an optimal functioning of the aeration system. The reason for this is that the aeration system, in particular the air supply system and air flow, is usually designed based on an amount of activated sludge to be present in each compartment or reactor and the type of water that needs to be purified and thus the organic load of the reactor. To provide optimum functioning of the aeration device there is a need to remove the activated sludge that has accumulated in the course of the purification reaction and thus to dump excess of activated sludge

However when removing activated sludge from the reactor, one is confronted with the problem that the concentration of the activated sludge in the mixture is rather .low, e.g. 1 wt. % of activated sludge vs. 99 wt. % of water. To overcome the need of providing large storage volumes, a sludge stabilising- thickening compartment can be built in the purification reactor of this invention in which gravitational compaction of the activated sludge as well as decomposition of the organic material present in the activated sludge may take place. The presence of remaining organic material is unwanted as it may produce an undesired smell when recycling or dumping the activated sludge. Further details of the activated sludge conditioning and thickening compartment are comprised in the description of the preferred embodiments given below.

The present invention also relates to a method for the purification of waste liquids, in particular waste water. The present invention also relates to a method for the purification of waste liquids, in particular waste water.

Detailed description of the invention. The invention is further elucidated in the attached figures and the description of the figures.

Figure 1 is a schematic presentation of the prior art water purification process.

Figure 2 is a schematic comparison of the prior art process and the process of this invention.

Figure 3 is a cross section of the purification reactor of this invention along the line Ill-Ill of figure 5.

Figure 4 is a cross section of the purification reactor along the line IV-IV of figure 5, showing the activated sludge thickening compartment of this invention.

Figure 5 is a view to the top of a water purification reactor of the present invention.

The device of this invention shown in figure 3 comprises a purification reactor 1 with a concentric outer and inner wall 5, 6, a bottom slab 7 and a top face 8 which may be open or closed. The circular construction of the purification reactor 1 allows to work with thinner walls as compared to the rectangular construction or arrangement of the compartments, and thus to reduce construction costs.

The purification reactor 1 shown in figure 3 contains at least three sequential compartments, a first 10, second 20 and third 30 compartment, mounted in circular configuration. Within the scope of this invention however the number of compartments may be increased to four, five or more compartments.

The first, second and third compartments 10, 20, 30 are separated from each other by respectively a first, second and third separation wall 11 , 21 , 12. As there is no need that the separation walls 11 , 21 , 12 are watertight, they can be made of light, cheap construction material and need not be sealed. The first, second and third compartment 10, 20, 30 are hydraulically connected to each other, through a flow system provided in the separation walls. This can be achieved in various ways, e.g. the flow system may for example comprise an opening 15, 15', 15" provided respectively in separation walls 11, 21 , 12, by positioning the separation walls at a distance from the bottom slab 7, through pipes or any other way known to the man skilled in the art.

To assist in obtaining a laminar flow of the liquor in the settling compartments and to slow down the flow rate of the liquor when flowing from one compartment to the other, the opening 15, 15', 15" may be protected by a baffle or a tranquilliser. It is also possible to guide the liquid flow from one compartment to the other through a distribution box, which guarantees a quiet inflow of the liquor.

Baffles may be placed diagonal with respect to a wall in the vicinity of the opening 15, 15', 15" to improve and tranquillise the laminar flow of the liquid.

As can be seen from figure 5, the reactor 1 is built up of two concentric circles, the outer circle comprising the first, second and third compartment 10, 20, 30, the inner circle comprising a fourth compartment 40 namely the activated sludge conditioning and thickening compartment 40 with on top the process control and technical centre. If so desired, for example in case the composition of the waste water so required, additional compartments may be added to the outer circle.

The first and third compartment 10, 30 comprise an inlet with an inlet valve 13, 33 for influent waste water which is fed via an influent delivery pipe 25. The second compartment preferably also comprises an inlet with an inlet valve 23 for the influent. The inlet valves may be opened and closed to control influent of waste water.

In the vicinity of the bottom slab 7 of each compartment 10, 20, 30, an aerating device 2, 2', 2" is submerged. The aeration device is provided for supplying the air needed for the biological decomposition of the organic impurities present in the waste water. The aeration device also ensures an intimate mixing of the mixed liquor with air. The aerating device 2, 2', 2" may for example comprise a plurality of air diffusers or jet aerators, or a perforated membrane or a perforated plate through which pressurised air is flown. The aerating device 2, 2", 2" is preferably mounted at a level below the gravitational settling devices 3, 4.

The aeration devices 2, 2.', 2" are arranged to operate intermittently in the compartments 1 0, 30, depending upon the direction of the liquid flow, whereas the second compartment 20 is virtually always aerated. In case influent takes place in the first compartment 10, this compartment is aerated while the third compartment 30 is not aerated. Preferably the air flow rate of the aeration device 2, 2', 2" is adjustable to provide the optimum compromise between the rate of the decomposition reaction and agitation of the mixed liquor. In another embodiment, the air flow rate of each of the aeration devices 2, 2', 2" is individually adjustable. Each of the aeration devices 2, 2', 2" is dimensioned and designed taking into account the composition of the water that needs to be purified, the reactor type, the amount of activated sludge present in the reactor. The dimensions of a.o. air supply pipes, valves, air flow are selected in such a way that they are capable of providing a sufficient flow rate to keep activated sludge into a state of suspension in the compartments that are aerated, and to keep the gravitational settling devices 3, 4 clean.

To facilitate air provision, the device of this invention preferably comprises a central air supply system with a supply valve, which may be opened or closed to provide or prevent an air flow towards aerating device 2, 2', 2".

To increase the surface area over which the activated sludge can settle preferably a plurality of gravitational settling devices 3, 4 are mounted in the first and third compartment 10, 30 which alternately function as aeration and settling compartment. The second compartment 20 will usually not contain gravitational settling devices 3, 4 as it is not intended to function as sedimentation compartment. The presence of the gravitational settling devices involves that the surface area occupied by the first and third compartment 10, 30 may be diminished without this going at the expense of the settling characteristics. Simultaneously, the volume of the central compartment 20 may be increased by the amount the first or third compartment has been decreased, thus promoting the amount of waste liquid that may be aerated and increasing the purification rate without adversely affecting sedimentation characteristics of the reactor 1.

As gravitational settling devices 3, 4 preferably use is made of a plurality of gravity tube settlers or gravity plate separators. Gravity plate settlers 3, 4 are well known for use in water filtration facilities for separating flocculated solids from an influent flow of liquid. In accordance with well known prior art systems, a plurality of parallel plates or tubes are connected to each other in series. The series of plates are mounted onto a support structure, at a distance from the bottom slab 7 of the reactor 1 , above the aeration device 2, 2', 2". The plates or tubes 3, 4 are usually inclined with respect to the bottom slab 7 of the reactor to facilitate settling of the activated sludge.

When a compartment is in the sedimentation phase mixed liquor flows upwardly along the tube settlers 3, 4. The solids settle on the surface thereof due to gravitational forces while clarified effluent exits from the space between the plates 3, 4 at the top.

The number and surface area of the gravitational settling devices 3, 4 may vary within wide ranges. However when assessing these parameters preferably account is taken of the so called surface load, which is the ratio of the influent waste liquid flow to the available settling surface. The surface load should not be too high to prevent washout of activated sludge with the purified effluent.

The aerating device 2, 2', 2" is preferably mounted in the vicinity of the bottom 7 of the reactor, in particular at a level below the gravitational settling devices 3, 4. The upward directed flow of air/mixed liquor through the gravitational settling devices 3, 4 ensure a constant cleaning so that clogging thereof is counter-acted. As the first and third compartment 10, 30 have smaller dimensions than the second compartment 20, the aeration capacity may be decreased with respect to the known art without this going at the expense of the efficiency of the purification process. The first 10 and third 30 compartment alternately function as sedimentation and aeration compartment, and are mostly only aerated in case they function as aeration compartment. The second compartment 20 mainly functions as aeration compartment and is virtually permanently aerated. The first and third compartment 10, 30 comprise a discharge system for discharging purified water. In an embodiment of this invention, the discharge system comprises an effluent launder 9 in the first compartment 10 and an effluent launder 39 in the third compartment, each effluent launder 9, 39 being mounted in such a way that its upper rim extends above the water level. Mostly the effluent launder 9 will be mounted above the gravitational settling devices 3, 4 and will be partly countersunk within the water. The effluent launder may be mounted along the separation wall between adjacent compartments or at any other suitable position. If it is desirable to increase the outflow of purified water two or more effluent launders may be provided in each or either one of the first and third compartment 10, 30. The effluent launder 9, 39 is preferably virtually always kept filled with purified water.

The effluent launder 9, 39 comprises an inlet which may be open or closed and through which purified water is sucked into the launder. The effluent launder 9, 39 comprises an outlet 14, 34 which may be opened and closed to allow or prevent discharge of purified water. The effluent launders 9, 39 may be fixed at a pre-determined position, or may be displaceably mounted in height direction of the reactor, in particular if it is desired to work with varying liquid levels. To prevent an inflow of mixed liquor, the effluent launder 9, 39 is preferably closed off by a cover 24, the edges of which extend below the water surface as is illustrated in figure 3. With this provision the necessity of spilling or re-circulating water from the launder 9, 39 to the second compartment 20 can be overcome. When aerating the first or third compartment 10, 30, agitation of the mixed liquor is involved. Mixed liquor may end up in the effluent launder 9, 39, and activated sludge may settle down therein. As a consequence, before purified water can be discharged from the launder 9, 39, the latter needs to be rinsed or else a first portion of purified water discharged through the launder must be spilled or re-circulated to the second compartment 20. A rinse phase typically takes 5-10 minutes, which is an unwanted waste of time.

Inflow of mixed liquor into the effluent launder 9, 39 is further counteracted by building an air over pressure above the purified water level in the launder. The over pressure in a particular effluent launder 9, 39 will in general only be maintained when the compartment 10, 30 involved is aerated. Usually the over pressure is released when the compartment involved is in the sedimentation phase as in that phase clear water must be capable of entering the effluent launder and the mixed liquor may be agitated to a limited extent only. To provide the over pressure, the effluent launder 9, 39 comprises an air inlet valve which is connected to an air supply system, preferably the air supply system of the aeration devices 2, 272".

The dimensions of the first, second and third compartment 10, 20, 30 may be varied within wide ranges and will mostly be determined taking into account the biological and hydraulic rules governing the biological purification of waste water as described hereafter. However, as the gravitational settling devices ^' 3, 4 are mounted within the first and third compartment 10, 30, the volume thereof needs to be adapted to the dimensions of the gravitational settling devices 14, 15. The device of this invention further preferably comprises a device 22 for recycling activated sludge between the first and third compartment 10, 30 as can be seen from figure 3 and 5. Thereto, use can be made of any sludge transport system known to the man skilled in the art.

In an embodiment of this invention, use is made of an airlift system which comprises a vertically extending pipe installed either in the first or third compartment or both, against the separation wall 12 between the first and third compartment 10, 30. In a lower part of the pipe 22, at least one hole 27 is provided through which activated sludge present in the lower part of the compartment 10, 30 may enter the pipe 22. At a position below the hole 27 pipe 22 is preferably connected to an air inlet valve 29 for supplying an air flow from the bottom to the top of the pipe 22. Upon opening of valve 29 air is flown from the bottom to the top of pipe 27, as a consequence of which activated sludge is sucked from compartment 10 or 30 through hole 27 into the pipe 22, transported through pipe 22 to the top thereof and further to compartment 10 or 30. Preferably the air inlet valve 29 is coupled to the air supply system 28 for the aeration system 2, 2', 2" of the compartments 10, 20, 30 through a bypass system. If so desired, in case the composition of the waste water so requires, the process can be optimised in order to have an optimal nitrogen removal by the well known principles of nitrification and denitrification. The nitrification process takes places in the presence of oxygen. The denitrification process requires whichthe strict absent of oxygen. Thus in order to ensure nitrogen removal, alternative aerobic and anoxic conditions are applied within the basins. This can be done by the control in time or by the control in place concept. Within the control in time concept the nitrification and denitrification takes place thein the same basins by water.switching the aeration alternatively on and off. Within the control in place concept the denitrification takes place in specifically separated compartment which are never aerated.

In another embodiment of this invention, a device 40 for conditioning, stabilising and thickening activated sludge excess present in the purification reactor 1 is provided. I n the course of the purification process activated sludge may accumulate in the second compartment and in the compartment functioning as a sedimentation compartment, often to such an extent that insufficient volume remains to allow an optimal separation of purified liquid and activated sludge to take place. In such cases there is a need to dump excess of activated sludge. However one is then confronted with the problem that the concentration of the activated sludge in the activated sludge-purified water mixture is rather low, e.g. 1 wt. % of activated sludge vs. 99 wt. % of water, so that large volumes are required for storing minor amounts of sludge.

As can be seen from figure 4, the activated sludge conditioning compartment 40 comprises a transport system for transporting excess activated sludge - purified water liquor from the first or third compartment 10, 30 to a fourth compartment 40, where the activated sludge may be conditioned and stabilised. This transport system from the first and third compartment 10, 30 may for example be comparable to the air lift system 22, 27, 29 used for transporting activated sludge between the first and third compartment 10, 30. Excess activated sludge is preferably fed through the top of the fourth compartment 40, through a cylinder 43 mounted centrally of the fourth compartment.

To achieve a gravitational thickening of the activated sludge a rotatable scraping device 32 is mounted in the fourth compartment 40 to force the activated sludge to flow from the sides of the fourth compartment towards the centre thereof and further towards an outlet 41 in a bottom 42 of the fourth compartment. The bottom 42 is preferably conical to facilitate flow of the activated sludge towards the outlet 41. The scraping device 32 comprises a rotational axis 38 which is preferably hollow, mounted within cylinder 43, to which axis 38 two or more arms 36 are mounted which preferably run parallel to the bottom of the fourth compartment and extend in the direction of the side wall 37 of the fourth compartment 40. To the arms preferably a plurality of scrapers 35 which are inclined with respect to the bottom 42, is mounted. As the scrapers 35 are displaced over the bottom 42 of the fourth compartment, the activated sludge layer 19 is cut to allow gases contained in the activated sludge to be released, and to force the activated sludge to flow from the side wall 37 towards the outlet 41 of the fourth compartment. Excess activated sludge is allowed to settle during at least one, preferably several days sometimes 10-40 days. In that way a 10 fold or even higher concentration of the activated sludge may be obtained, for example from 5-8 kg/m³ to 40-50 kg/m³, depending on the water content and composition of the activated sludge.

Usually the rotation speed of the axis 38 is chosen rather low, for example 2 - 5 or 3 rounds per hour. At a position somewhat above the bottom 42 of the fourth compartment 40, at least two sideways extending arms 36 are mounted to the hollow rotational axis 38. Onto the arms 36 an aeration system 26 is mounted. This aeration system is preferably connected to the air supply 28 of the compartments 10, 20, 30, and comprises a plurality of air diffusing devices 31. Air supplied through the hollow axis 38 is distributed within the activated sludge as a consequence of which decomposition of organic material present in the activated sludge 16 is enhanced to avoid production of bad odour by the activated sludge As in the fourth compartment 40 the activated sludge is no longer fed with carbon containing material, self-digestion of the micro-organisms is promoted, thus reducing the amount of activated sludge present in the fourth compartment 40

Air is preferably evenly distributed over the air diffusing devices in 31. The air flow is preferably adjusted in such a manner that a laminar upward flow of air bubbles through the upper zone of the activated sludge is created. In that way an efficient activated sludge/oxygen mixing can be achieved and a sufficient concentration of the activated sludge in the upper stabilisation zone (to above 10 kg/m³) by self-digestion of the micro-organisms. Simultaneously, agitation of the activated sludge phase in the vicinity of the aerators 31 and the bottom 42 and disturbance of the settling and compaction of the activated sludge in the lower region of the fourth compartment 40 is counter-acted. As air supply below the air diffusers 31 is limited, this zone remains sufficiently calm to allow settling and intensive compacting of the activated sludge to be performed.

The air flow rate from the air diffusers 31 is preferably adjustable to the type of activated sludge and the concentration thereof. Preferably each air diffuser 31 is provided with a valve which can be opened or closed when operating the device, to for example limit air flow from one arm and increase and concentrate air flow from the other one. By doing so extra air will be released from the latter, involving increased agitation and mixing and lifting of the activated sludge. It is also possible to virtually completely shut off the air flow. The air flow rate from the air diffusers 31 and the rotation speed of the axis 38 are preferably adjusted in such a manner that an optimum compromise is achieved between chemical stabilisation of the activated sludge and gravitational compacting.

So, in the device of this invention, stabilisation and compacting of the excess activated sludge is combined in one single compartment. By such combination fouling in the course of the thickening process, denitrification and phosphate release and the production of bad odours may be minimised. In state of the art devices, both steps were usually carried out in separate devices, entailing the above enumerated adverse effects.

The operation of the device of this invention can be described as follows.

In aerobic waste water treatment processes, use is made of aerated activated sludge or activated sludge which is capable of oxidising the unwanted organic compounds present in the waste water to harmless CO₂ and H₂O. By selecting the appropriate activated sludge and reaction conditions, also advanced removal of reduced nitrogen can be obtained through nitrification or de- nitrification reactions. Examples of such reduced nitrogen compounds are ammonia, amino acids. Furthermore, also an advanced removal of phosphor compounds mainly phosphates can be obtained.

The aerobic waste water treatment of the present invention is characterised by its own set of principles, which distinguishes it from conventional activated sludge systems. In the device of this invention, the first and third compartment 10, 30 may have two functions, namely an aeration/decomposition function and a sedimentation function, to which they are alternately subjected.

In a first main phase, waste liquid is fed to the first compartment 10 which functions as an aeration compartment. The influent waste liquid to be purified is mixed with the activated sludge present in the first compartment and aerated by supplying air through the aeration device 2. The intake and conversion of the organic compounds by the micro-organisms of the activated sludge involves a growth of the activated sludge phase in the first compartment 10. In the first main phase the third compartment 30 functions as sedimentation compartment. To allow that purified liquid/water separates from the activated sludge the third compartment is not aerated. In a second main phase, influent waste liquid is fed to the third compartment 30 as a consequence of which the liquor flow is reversed. The first compartment 10 now functions as a sedimentation compartment.

The second central compartment 20 functions as aeration tank in any phase.

In general two sets of rules apply to the first and the third compartment 10, 30:

1. When functioning as aeration compartment, the volume of the compartment is determined by the activated sludge loading or the ratio of the amount of waste liquor supplied to the compartment with respect to the amount of activated sludge present.

2. To allow activated sludge and purified water to separate, the hydraulic pressure which is the flow rate of the waste liquor-activated sludge mixture should remain below a maximum value, to take account of the type of waste water and the sludge settling properties. It is also advised that the flow rate of the waste liquor- activated sludge mixture be adapted to so called "maximum surface load". The "surface load" is the amount of purified liquid divided by the available settling surface. This means that the higher the flow rate of the mixture, the larger the settling surface needs to be. In a first main phase influent waste liquid 17 is fed to the first compartment 10 through inlet valve 13 of the influent supply 25. Inlet valve 23 and 33 of respectively the second and third compartment 20, 30 are closed. Air is supplied from the central air supply system 28, through air inlet valve 29, to the aeration device 2 of the first compartment 10. The air flow rate will in general be adapted to the type of water that needs to be purified, the type and amount of activated sludge present. The air flow rate will mostly be adapted in such a way that it is neither too high nor too low, to provide a virtually laminar flow of the mixed liquor. As the air is forced to flow through the gravitational settling devices 3 of the first compartment 10, that they are kept clean. In the course of the aeration step, influent waste liquid 17 is mixed with the activated sludge 16 and aerated and biological decomposition of organic impurities present in the waste liquid takes place. This is the so-called exogenous or accumulation phase in which activated sludge is mixed with a high concentration of carbon substrate. As additional waste liquid is supplied to the first compartment 10, the mixed liquor of waste liquid/activated sludge flows through opening 15 in the first separation wall 1 1 to the second compartment 20.

An air over pressure is maintained within the effluent launder 9 to prevent mixed liquor from entering and contaminating the launder 9. The inlet valve to the effluent launder 9 is closed to prevent mixed liquor from entering it the outlet valve of the effluent launder 9 is closed off to keep the launder filled with a volume of purified water.

In the second compartment 20 the mixed liquor is further mixed and aerated by supplying air through the aeration device 2'. The mixed liquor is subject to the so-called regeneration or endogenous phase in which the activated sludge has to survive on accumulated substrate. The alternation of an exogenous and endogenous zone/phase favours the formation of flocculating micro_¬ organisms which show an improved sedimentation behaviour as compared to filamentous micro-organisms. With the device and method of this invention because of the increased dimension of the second compartment, the exogenous phase may be prolonged, thus favouring the growth of flocculating micro_¬ organisms.

As a consequence of mixed liquor flow from the first 10 to the second compartment 20, the mixed liquor is forced to flow through the hole 15' in the second separation wall 12 to the third compartment 30 which is not aerated in this phase and functions as a sedimentation compartment. In the third compartment 30, the liquid 18 is allowed to separate from the activated sludge 16 and to settle. Inlet valve to effluent launder 39 is open to allow purified liquid to enter it and to be discharged. Air inlet valve for providing an air over pressure in the effluent launder is closed. Activated sludge is allowed to sink over the surface of the gravitational settlers whereas the purified liquid 18 leaves the gravitational settling devices 4 through the top towards the effluent launder 39.

By the flow of the mixed liquor from the second 20 to the third compartment 30 activated sludge 16 is displaced from the first to the second and further to the third compartment and accumulates in the third compartment. Depending on the volume of the third compartment 30, it will take a few to several hours before the amount of sludge 16 accumulated therein becomes excessive To prolong the time of the first main phase, excess sludge 16 is re-circulated to the first compartment 10 through the sludge re-circulation system. After the first main phase has been terminated, a first intermediate phase is carried out in which the first and third compartment 10 30 are not aerated. This means that the micro-organisms of the activated sludge present in these compartments have to survive on the accumulated surface area Influent inlet valve 13 and 33 of respectively the first and third compartment are closed. The first and third compartrrϊent 10, 30 are not aerated and the activated sludge present in these compartments is allowed to settle. Influent waste liquid is supplied to the second compartment 20. The second compartment 20 remains aerated. As a consequence of the influent inflow in the second compartment activated sludge is transported from the second compartment 20 to the first and third compartment 10, 30 through holes 15 and 15'. An over-pressure is maintained in the effluent launders 9, 39. Inlet valve to effluent launder 39 is closed to prevent inflow of liquid from the third compartment. As soon as the activated sludge in the first compartment 10 has settled to a sufficient extent to allow purified water to be discharged from it, influent inlet valve 23 to the second compartment is closed, influent inlet valve 33 to the third compartment is opened and influent waste liquid is supplied to the third compartment 30. Air inlet valve to effluent launder 39 is opened to build an over-pressure in the effluent launder of the third compartment and prevent inflow of non purified liquid. The air inlet valve to the aeration device 4 of the third compartment 30 is opened. In the first compartment, the air over-pressure in the effluent launder 9 is cut off, the inlet valve to the effluent launder is opened to allow discharging of purified water

The second main phase is started, in which influent waste liquid is supplied to the third compartment 30 and allowed to flow from the third to the first compartment where it is discharged. The course of the second main phase is similar to the first main phase which has been described above.

The growth of the filamentuous micro-organism during the process creates often an operational problem in waste water treatment plants This filamentuous bacteria deteriorate the settling characteristics of the biomass and cause foaming. The operational conditions of a waste water treatment should be adjusted in order to avoid the growht of filamentuous bacteria and to stimulate the flock-forming bacteria. This can be done by the alternating application of an accumulation phase and a regeneration phase. During the accumulation phase substrate.fresh carbon substrate is added to the micro-organisms, which is immediately absorbed by these rnicro-organism. Regeneration of the micro_¬ organisms phase takes place in the regeneration zone where the activated sludge is aerated without the addition of fresh carbon substrate, thus where the micro_¬ organisms have to survive on substrate that has accumulated in the micro- organisms during the accumulation phase. Because the flock-forming bacteria are not able to accumulate a large amount of substrate during the accumulation phase, the growth of these bacteria is limited. The accumulation phase takes place in one of the outer compartments, whereas the regeneration phase takes place in the middle compartment. This invention makes it possible to reduce the outer compartments. The middle compartment, where the regeneration phase takes place, is increased. The regeneration is thus further promoted and the growth of filamentuous bacteria prevented.

Claims

CLAIMS.

1. A device (1) for the biological purification of a waste fluid (17), in particular waste water, the device (1) comprising a first, second and third compartment (10, 20, 30) for receiving the waste fluid (17), each compartment- (10, 20, 30) containing an aerating device (2,2',2") for aerating the waste fluid (17), adjacent compartments being connected to each other to allow the liquid to flow from one compartment to the other, the first and third compartment (10, 30) each containing an inlet (13,33) for influent waste liquor and an outlet (14,34) for effluent purified liquid (18), the first and third compartment (10, 30) being provided to alternately function as aeration compartment for aerating the waste fluid and settling compartment to allow activated sludge (16) to settle and separate from the purified liquid (18), characterised in that the first, second and third compartment (10, 20, 30) are sequentially arranged in a circular configuration, the first and third compartment (10, 30) comprising a plurality of gravitational settling devices (14, 15) for increasing the settling surface for the activated sludge (16).

2. A device as claimed in claim 1 , characterised in that means (2, 2', 2") are provided for supplying an oxygen containing gas in a lower region of each compartment below the gravitational settling devices (3, 4).

3. A device as claimed in claim 1 or 2, characterised in that the first and third compartment (10, 30) have a smaller volume as compared to the second compartment (20).

4. A device as claimed in any one of claims 1 -3, characterised in that the device comprises a sludge re-circulation system (22) for re-circulating sludge from the settling compartment (10, 30) towards the influent compartment (10, 30).

5. A device as claimed in claim 4, characterised in that the sludge re-circulation system (22) comprises a first pipe (27) in the first compartment (10), the first pipe comprising an inlet (28) for the activated sludge (16) in a lower part of the pipe (27) and an air inlet (29) at a position below the activated sludge inlet (28), the air inlet being connected to an air supply (28), and an outlet for the activated sludge in the third compartment (30).

6. A device as claimed in claim 4 or 5, characterised in that the sludge re-circulation system further comprises a third pipe (27') in the third compartment (10), the third pipe comprising an inlet (28') for the activated sludge (16) in a lower part of the pipe (27') and an air inlet (28') at a position below the activated sludge inlet (28'), the air inlet being connected to an air supply (28), and an outlet for the activated sludge in the first compartment (10).

7. A device as claimed in any one of claims 5-6, characterised in that the air inlet (28, 28') is connected to the air supply (28) of the aerating device

⁽D-

8. A device as claimed in any one of claims 1-7, characterised in that the reactor (1) comprises an outer wall (5) along which in the first and third compartment (10, 30) an effluent launder (25) for purified water is mounted at a position in the vicinity of the liquor surface, the effluent launder (25) being closed off by a cover (24) which at least partly extends above the liquor surface and being connected to an air supply system (28, 38) for building air over pressure in the launder (25), the air supply system being operated by a controllable inlet valve, the effluent launder comprising a controllable valve (34) so as to allow for or prevent outflow of purified water.

9. A device as claimed in any one of claims 1-8, characterised in that the device comprises a transport device for transporting excess sludge containing purified fluid from the settling compartment (10, 30) to a fourth compartment (40) for stabilising and reducing the sludge (16), the fourth compartment comprising a rotatable scraping device (32) for forcing the sludge to flow towards an outlet (41) in a bottom (42) of the fourth compartment.

10. A device as claimed in claim 9, characterised in that to the rotatable scraping device (32) an aeration device (31) is mounted which extends in cross direction of the fourth compartment (40) for aerating the sludge containing liquid.