WO2006077282A1

WO2006077282A1 - Process and device for cleaning waste water

Info

Publication number: WO2006077282A1
Application number: PCT/FI2006/050029
Authority: WO
Inventors: Olai Lagus; Stefan Sandbacka
Original assignee: Oy Kwh Pipe Ab
Priority date: 2005-01-24
Filing date: 2006-01-17
Publication date: 2006-07-27
Also published as: FI117093B; FI20050073A0

Abstract

The invention concerns a process and a device for cleaning waste water, whereby waste water is collected in a collection tank (1) and pumped to a batchwise operating process tank (2) containing active sludge. The waste water fed to the process tank (2) is aerated, a chemical coagulant is supplied to the waste water in the process tank (2), followed by an anaerobe/anoxic phase and clarification of the batch of waste water, after which the share of purified waste water is tapped or pumped out of the process tank (2). The invention is characterized in that pumping of new waste water to the process tank (2) is carried out in small portions in intervals defined by the rate of waste water flow to the collection tank (1) until a volume of waste water required for a new purifying cycle has been charged to the process tank (2), and that from the end of a purifying cycle and during the whole charging stage a preliminary aeration of the mixture of active sludge and waste water is carried out in the process tank (2).

Description

PROCESS AND DEVICE FOR CLEANING WASTE WATER

This invention refers to a process for cleaning waste water, comprising collection of waste water in a collection tank, pumping collected waste water to a batchwise operating process tank containing active sludge, aeration of the waste water fed to the process tank, supplying a chemical coagulant to the batch of waste water in the process tank, followed by an anaerobe/anoxic phase and clarifying of the batch of waste water, whereby sludge and precipitated material are allowed to sink down to the bottom of the process tank, after which the share of purified waste water is tapped or pumped out of the process tank.

Such a process is known from the Finnish patent application no. 20021557. In this previously known process the whole amount of waste water for a batch is pumped in one go to the process tank. This means that no waste water is pumped to the process tank before the collection tank has received an amount of waste water necessary for a batch. The above mentioned process may cause problems if production of waste water varies widely from one period to another. This is the case for instance at an area where both houses with permanently living inhabitants and holiday houses are connected to a purifying plant operating according to the above mentioned principle. In such a case it may take a long time e.g. during the winter period before enough waste water has been produced to enable starting of the pumping of a new batch of waste water. Hereby there is a risk that the bacterial population in the active sludge does not get sufficient food, resulting in a remarkable reduction or even total death of various bacterial strains.

The object of the present invention is to remove this problem and to obtain a flexible proc- ess suitable to a varying production and composition of waste water. This is achieved with a process characterized in that pumping of new waste water to the process tank is carried out in small portions in intervals defined by the rate of waste water flow to the collecting tank until a volume of waste water required for a new purifying cycle has been charged to the process tank, and that from the end of a purifying cycle and during the whole charging stage a preliminary aeration of the mixture of active sludge and waste water is carried out in the process tank. In this way a continuous supply of nourishment to the bacterial population in the active sludge is ensured resulting in an intensified reduction of BOD and an increased nitrification of ammonium nitrogen in the waste water. In general no pumping of waste water to the process tank is carried out during a purifying cycle proper, especially not during the clarification stage or during discharging of purified waste water from the process tank, because the whole purifying process would then be seriously disturbed, resulting in that more or less unpurified waste water leaves the process tank.

The process according to the present invention will guarantee that the volume of waste water present in each purification cycle is constant, also resulting in that aeration, which is carried out by blowing air in an even flow through the mixture of active sludge and waste water in the process tank, can be carried out in a period of time equal to each purifying cycle.

At the end of the aeration a small exact amount of waster water can be added to the mixture of waste water and active sludge in the process tank, if necessary, in order to intensify the denitrifϊcation and thus the degree of nitrogen treatment. Equally the amount of a chemical coagulant, which should be supplied to the process mixture in order to precipitate phosphor being present in the waste water, can be kept constant in each purification cycle. A salt of three-valued iron or of aluminium, preferably ferrisulphate or aluminium sulphate can be used as chemical coagulant. Also ferrosulphate (two-valued iron) can be used, but then the chemical must be added at the beginning of the aeration phase in order to have the two-valued iron to be oxidized to three-valued iron.

According to a preferred embodiment the purification process proper is started by a first anaerobe/anoxic phase, whereby a second anaerobe/anoxic phase is carried out subsequent to the supply of a chemical coagulant.

Pumping of new waste water to the process tank is not started until purified waste water from the previous purifying cycle has been pumped out of the process tank.

During long intervals between the pumping steps into the process tank, i.e. when the amount of waste water flowing to the collection tank is so small that the interval between two pumping steps exceeds a predetermined period of time, e.g. 24 hours, the preliminary aeration is preferably carried out pulse-wise. Hereby the mixture of sludge and waste water in the process tank can be aerated for instance for 10 - 20 minutes, followed by a period of rest of 1 - 4 hours. When a pumping takes place the preliminary aeration turns over in a constant phase, which continues until a new purification process starts or for an above mentioned predetermined period of time of e.g. 24 hours during which no further pumping step has been carried out.

In order to intensify the anoxic purification process a mixing of the process batch can be carried out by means of a short impulse aeration resulting in mixing of active sludge from the bottom of the process tank with the whole volume of the process batch.

The invention also concerns a device for purifying waste water comprising a collection tank for receiving waste water from one or several households and a batchwise operating process tank containing active sludge with a bacteria population for degrading organic material and nitrogen compounds in the waste water. At the bottom of the collection tank a cutting pump is mounted for pumping waste water from the collection tank to the process tank before a new purification cycle is started. An aeration device fed by a compressor is mounted at the bottom of the process tank for aerating the mixture of waste water and active sludge. At a suitable position in the device a chemical tank provided with a dosage pump is mounted for supplying a chemical coagulant to the mixture of waste water and active sludge in the process tank, in order to precipitate phosphor present in the waste water. A pump for pumping purified waste water at the end of a purification cycle is sub- merged in the process tank to a dept corresponding to 10 - 70 % of the total amount of process liquid. Further the device includes a control unit for controlling all operations of the device.

The device according to the invention is characterized in that the collection tank somewhat above the inlet opening of the cutting pump is provided with a first floating body arranged to send a signal to the control unit to start the cutting pump each time the waste water level in the collection tank has reached an upper limit value, provided a purification cycle is not running in the process tank. Each time the waste water level in the collection tank falls to a lower limit value said first floating body is arranged to send a signal to the control unit to stop the cutting pump. The control unit is arranged to start the compressor after a finished purifying cycle for preliminary aeration of the active sludge and waste water being supplied to the process tank. In the upper portion of the process tank a second floating body is arranged, to sense when a volume of waste water necessary for a new purification cycle has been pumped to the process tank and thus to send a signal to the control unit to disconnect the cutting pump in the collection tank and to start the purification cycle proper comprising anoxic phases, aeration, chemical dosage, clarification and final pumping of purified waste water out of the process tank.

The device according to the invention can either be built in such a way that the collection tank and the process tank are arranged in two separate vessels being connected by means of a connecting channel in the upper part of the vessels, or in such a way that the collection tank and the process tank are included in one and the same vessel and separated from each other by means of a partition wall, whereby a dry technical space can be integrated in the vessel above the process and the collection tanks or as a separate unit outside the vessel, dug below ground level or mounted above the ground level. According to a third alternative the device is built as a horizontally laying tube with closed ends, the interior of which being divided into at least a collection tank, a process tank and possibly into a dry technical space by means on vertical partition walls.

In the construction with collection tank and process tank in separate vessels the collection tank can preferably be provided with a hanging floor dividing the vessel in a dry upper space for i.e. control unit, compressor and chemical tank provided with a dosage pump and in a lower liquid space for incoming waste water.

According to a preferred embodiment a further pump is mounted in the bottom part of the process tank, which through a hose or a tube is arranged to pump a part of the sludge in the process tank, if necessary, to a filter bag detachably attached above the liquid level in the upper portion of the collection tank, from which bag the liquid is allowed to runoff to the waste water in the collection tank and the solid sludge collected in the filter bag can be poured in a compost at equal intervals or together with the filter bag be placed in a compost, if the filter bag is made of a compostable material.

In the following the invention will be disclosed in more detail with reference to the drawings, wherein

Fig. 1 shows schematically an example of a device according to the invention,

Fig. 2 shows schematically a side view of an embodiment of the device with integrated spaces for collection, process and technical equipment (the technical equipment is not shown) and Fig. 3 shows an upper view of the device according to Fig. 2.

In the shown example the device for purifying waste water comprises two separate tanks, i.e. a collection tank 1 and a batchwise operating process tank 2, which tanks in their upper portions are connected to each other through a connection channel 3. Waste water from one or several households are arranged to flow to the collection tank 1 through an inlet 4. A certain amount of active sludge provided with a bacteria population for degrading organic material and nitrogen compounds in the waste water is maintained in the process tank 2. At the bottom of the collection tank a cutting pump 5 is mounted, which pump via a hose or tube 6 is arranged to pump waste water from the collection tank 1 to the process tank 2 through the connection channel 3 before a new purification cycle is started. The operation of the cutting pump 5 is controlled by a control unit 7, which by means of a first floating body 8, mounted somewhat above the inlet of the cutting pump 5, is arranged to get information about the level of waste water in the collection tank 1. If no purification cycle is running in the process tank 2 the control unit 7 is arranged to start the cutting pump 5 at a signal from the first floating body 8 each time the level of waste water in the collection tank has reached an upper limit value, and to stop the cutting pump 5 each time the level of waste water has fallen to a lower limit value. Because the upper limit value for the first floating body 8 is positioned at a relatively low level, waste water is pumped to the process tank 2 relatively quickly although the production of waste water in the households connected to the device would be low at that moment.

At the bottom of the process tank 2 an aeration device 9 is mounted, which via a tube 10 is arranged to be fed by a compressor 11 in order to blow finely divided air through the mix- ture of waste water and active sludge in the process tank 2. According to the invention the control unit 7 is arranged to send a starting signal to the compressor 11 when a purification cycle is finished, in order to preliminary aerate the mixture of active sludge and waste water in the process tank 2. The preliminary aeration is allowed to continue until the process tank 2 by means of batchwise pumping has received a full volume for a new purification cycle. The preliminary aeration gives an intensified reduction of the BOD of the waste water and an increased nitrification of ammonium nitrogen, resulting in denitrifϊcation of a greater part of nitrogen leading to an increased total reduction of nitrogen. If the interval between two pumping steps exceeds a predetermined time, for instance 24 hours, the control unit will send impulses to the compressor 11 to switch to a pulswise preliminary aera- tion until next pumping step. In the upper part of the process tank 2 a second floating body 12 is mounted being arranged to sense when the level of waste water in the process tank 2 has reached a level corresponding to full volume, and simultaneously to start the compressor for preliminary aeration of the active sludge and supplied waste water in the process tank. Hereby said second floating body 12 is arranged to send a signal to the control unit 7 to disconnect the cutting pump 5 and to start the purification cycle proper comprising anoxic phases, aeration, chemical dosage, clarification and final pumping of purified waste water out of the process tank.

The purification cycle proper can be started with an anoxic phase with short aeration pulses, an aeration phase, whereby the control unit 7 keeps the compressor in operation during a predetermined period of time adapted to the full volume of waste water determined by the second floating body 12.

At the end of the aeration a small exactly levelled amount of waste water can be supplied to the process tank 2, if necessary, when starting the anaerobe or oxygen poor (anoxic) purification phase by adjusting the control unit to start the cutting pump 5 for a moment, which will intensify the denitrification of the whole batch of waste water. The device also includes a chemical tank 14 provided with a dosage pump 13 and filled with chemical coagulant in the form of a salt of three-valued iron or three-valued aluminium, preferably ferrisulphate or aluminium sulphate, intended to precipitate phosphor compounds present in the waste water. Also ferrosulphate (two-valued iron) can be used, but then the chemical coagulant must be supplied at the beginning of the aeration phase, in order to have the two- valued iron oxidized to three-valued iron. The dosage pump 13 receives control signals from the control unit to supply a predetermined amount of chemical coagulant to the mix- ture of waste water and active sludge in the process tank 2 through a pipe 15 during the final stage of the aeration, followed by an anoxic phase and clarification, whereby solid material and sludge is allowed to fall down to the lower portion of the process tank 2.

A pump 16 is immersed in the process tank 2 to a dept corresponding to 10 - 70 % of the total amount process liquid, which pump 16 through a pipe 17 is arranged to pump purified waste water from the process tank 2, when the clarification phase is finished, according to signals from the control unit. After this a new batch of waste water can be pumped batch- wise to the process tank 2 in proportion as waste water flows to the collection tank 1. In order to facilitate removing of excess sludge from the process tank 2 said tank can preferably be provided with a third pump 18 immersed to the lower portion of the process tank 2, which pump via a pipe 19 is arranged to pump excess sludge at equal intervals to a filter bag 20 detachably attached in the upper part of the collection tank 1, from which bag Hq- uid is allowed to runoff to the waste water in the collection tank 1, after which the filter bag 20 together with the content can be brought to a compost, if the filter bag 20 is made of a compostable material, and a replacing filter bag can be mounted into the upper portion of the collection tank 1. Alternatively the content of the filter bag 20 is poured in a compost. A suitable time for removing excess sludge from the process tank 2 is immediately after pumping of purified waste water from said process tank 2.

According to a preferred embodiment the collection tank 1 is divided by means of a hanging floor 21 into an upper dry space Ia for i.e. the control unit 7, the compressor 11, the chemical tank 14 with the dosage pump 13, and possibly the upper part of the filter bag 20, and into a lower liquid space Ib for inflowing waste water.

The device shown in Figs. 2 and 3 having the collection tank 1, the process tank 2 and the a dry technical space 21 for technical equipment integrated in a common vessel can preferably be manufactured of plastics by rotary moulding. In the shown embodiment the ves- sel is shaped in principle as a parallelepiped having a central partition wall separating the collection tank 1 from the process tank 2, whereby the technical space 21 is arranged in the upper portion of the vessel above said partition wall extending halfway over the collection tank 1 and the process tank 2. All spaces 1, 2 and 21 of the vessel have the same width.

The process according to the present invention also pays attention in principle to the degree of dilution of the waste water entering the collection tank 1. If the hydraulic flow is large, for instance due to washing or dishing, the collection tank 1 and consequently also the process tank 2 are filled rapidly, resulting in a shorter pre-treatment i.e. preliminary aeration of the mixture in the process tank 2, but due to the fact that the incoming waste water then in principle is more clean, a shorter purification time is sufficient.

Claims

Claims:

1. Process for cleaning waste water, comprising collection of waste water in a collection tank (1), pumping collected waste water to a batchwise operating process tank (2) contain- ing active sludge, aeration of the waste water fed to the process tank (2), supplying a chemical coagulant to the batch of waste water in the process tank (2) followed by an an- aerobe/anoxic phase and clarifying of the batch of waste water, whereby sludge and precipitated material are allowed to sink down to the bottom of the process tank (2), after which the share of purified waste water is tapped or pumped out of the process tank (2), characterized in that pumping of new waste water to the process tank (2) is carried out in small portions in intervals defined by the rate of waste water flow to the collection tank (1) until a volume of waste water required for a new purifying cycle has been charged to the process tank (2), and that from the end of a purifying cycle and during the whole charging stage a preliminary aeration of the mixture of active sludge and waste water is carried out in the process tank (2) .

2. Process according to claim 1, characterized in that between two purifying cycles the pumping of waste water into the process tank (2) takes place as soon as the level in the collection tank (1) has risen to a level being only a fraction of the total volume of the col- lection tank (1) while no pumping takes place during a purification cycle proper.

3. Process according to claim 1 or 2, characterized in that the volume of waste water included in each purifying cycle is constant.

4. Process according to claim 3, characterized in that during a purification cycle proper the mixture of active sludge and waste water is aerated by blowing air from the bottom of the process tank (2) into said mixture as an even flow for a period of time being constant for each purification cycle.

5. Process according to claim 4, characterized in that at the end of the aeration a small exact amount of waste water is possibly added in order to intensify denitrifϊcation of the whole batch of waste water and that a constant amount of chemical coagulant is added to the mixture in order to precipitate phosphor from the waste water.

6. Process according to claim 5, characterized in that a salt of three- valued iron or aluminium, preferably ferrisulphate or aluminium sulphate is used as chemical coagulant.

7. Process according to claim 5, characterized in that ferrosulphate is used as chemical coagulant, which is added to the process mixture already at the beginning of the aeration phase.

8. Process according to any of the claims 1 to 6, characterized in that the purifying process proper is started with a first anaerobe/anoxic phase and a second anaerobe/anoxic phase takes place subsequent to the addition of chemical coagulant.

9. Process according to claim 1, characterized in that the pumping of new waste water into the process tank (2) is not started until the purified waste water from the previous purifying cycle has been pumped out from the process tank (2).

10. Process according to any of the preceding claims, characterized in that during long intervals between the pumping steps the preliminary aeration is carried out pulse-wise.

11. Process according to claim 8, characterized in that during one or both anoxic purifica- tion phases the process batch is mixed by means of short impulse aerations.

12. Device for purifying waste water comprising a collection tank (1) for receiving waste water from one or several households and a batchwise operating process tank (2) containing active sludge with a bacteria population for degrading organic material and nitrogen compounds, a cutting pump (5) mounted at the bottom of the collection tank (1) for pumping waste water from the collection tank (1) to the process tank (2) before a new purification cycle is started, an aeration device (9) fed by a compressor (11) being mounted at the bottom of the process tank (2) for aerating the mixture of waste water and active sludge, a chemical tank (14) provided with a dosage pump (13) for supplying a chemical coagulant to the mixture of waste water and active sludge in the process tank (2) in order to precipitate phosphor present in the waste water, a pump (16) for pumping out a share of purified waste water at the end of a purification cycle is submerged in the process tank (2) to a dept corresponding to 10 - 70 % of the total amount of process liquid, and a control unit (17) for controlling all operations of the device, characterized in that the collection tank (1) somewhat above the inlet opening of the cutting pump (5) is provided with a first floating body (8) arranged to send a signal to the control unit (7) to start the cutting pump (5) each time the waste water level in the collection tank (1) has reached an upper limit value, provided a purification cycle is not running in the process tank (2), and that said first floating body (8) is arranged to send a signal to the control unit (7) to stop the cutting pump (5) each time the waste water level in the collection tank (1) falls to a lower limit value, that the control unit is arranged to start the compressor (11) after a finished purification cycle for preliminary aeration of the active sludge and waste water being supplied to the process tank (2), and that a second floating body (12) is arranged in the upper portion of the process tank (2) arranged to sense when a volume of waste water necessary for a new purifica- tion cycle has been pumped to the process tank (2) and thus to send a signal to the control unit (7) to disconnect the cutting pump (5) in the collection tank (1) and to start the purification cycle proper comprising anoxic phases, aeration, chemical dosage, clarification and final pumping of purified waste water out of the process tank (2).

13. Device according to claim 12, characterized in that the device is built in such a way that the collection tank (1) and the process tank (2) are arranged in two separate vessels connected by means of a connecting channel (3) in the upper part of the vessels.

14. Device according to claim 12, characterized in that the device is built with the collec- tion tank (1) and the process tank (2) included in one and the same vessel separated from each other by means of a partition wall.

15. Device according to claim 12, characterized in that the device is built as a laying tube with closed ends, the interior of which being divided by means of vertical partition walls into at least a collection tank (1) and a process tank (2) and possibly into a dry technical space.

16. Device according to claim 13 or 14, characterized in that the collection tank (1) is divided by means of a hanging floor (21) into an upper dry space (Ia) for i.e. control unit (7), compressor (11) and chemical tank (14) provided with a dosage pump (13), and into a lower liquid space (Ib) for incoming waste water.

17. Device according to claim 14, characterized in that a dry technical space is integrated in the vessel above the process tank and the collection tank or arranged as a separate unit outside the vessel dug below ground level or mounted above the ground level.

18. Device according to any of the claims 12 - 17, characterized in that a pump (18) is mounted in the bottom portion of the process tank (2) for pumping a part of the sludge in the process tank (2) to a detachable and replaceable filter bag (20) arranged above the liquid surface in the upper portion of the collection tank (1).