WO2000069780A1

WO2000069780A1 - Process control of the dewatering of sludge

Info

Publication number: WO2000069780A1
Application number: PCT/SE2000/000955
Authority: WO
Inventors: Sven Bernhardsson
Original assignee: Sven Bernhardsson
Priority date: 1999-05-14
Filing date: 2000-05-14
Publication date: 2000-11-23
Also published as: EP1196355A1; AU4965700A; SE9901772D0

Abstract

This invention relates to a method of controlling a sludge thickening process, in which one or more flocculation agents (F1, F2) and/or coagulants (K) are fed into the incoming sludge (IS) prior to feeding said sludge into a thickener (1) and filtrate (RF) and thickened sludge (FS) is discharged separately from the thickener (1). According to the invention the amount of solid substance (SS content) in the filtrate (RF) is measured with an adjustable frequency, data from the measurement of the amount of solid substance (SS content) in the filtrate is fed to a control unit (2), a feeding unit (3) for providing a metered feed of a flocculant (F1) is connected to the control unit (2), whereby the metering of the flocculant (F1) is sensed, an upper (SSmax) limit value of the amount of solid substance (SS content) in the filtrate is set in the control unit (2), and the measured amount of solid substance is compared to the limit value, and, when exceeding the upper (SSmax) limit value for the measured amount of solid substance (SS content), the control unit (2) increased the feeding of the flocculant (F1) until the measured amount of dry substance falls below the limit value again, an upper limit value (F1max) for the feeding of flocculant (F1) is set in the control unit (2) and is compared with the sensed value, and, when exceeding the upper limit value (F1max) for the feeding of flocculant (F1) the feeding of flocculant (F1) is decreased through the control unit (2) until it falls below the limit value (F1max) for the feeding of flocculant (F1) again and the control unit (2) gives an alarm signal to the operator or alternatively the control unit switches over to feeding another flocculant (F2).

Description

TITLE: PROCESS CONTROL OF THE DEWATERING OF SLUDGE

TECHNICAL FIELD

This invention relates generally to sludge thickening processes and in particular concerns a method and a system of controlling such a process.

BACKGROUND

Sludge thickening using e.g. drum thickeners or belt thickeners is commonly used in the industry and in municipal sewage treatment and water treatment works. Often a flocculation agent of polymer type is added using a polymer feeding or dosing unit. In other cases inorganic additions such as metallic salts and calcium hydroxide are used.

The selection of the flocculant, which is to be added, is often made through manual short time testing, for a certain type of sludge, sludge composition and sludge temperature and with certain other specific process parameters. The amount of flocculant/polymer to be added is selected by visually controlling if the filtrate (the water separated at filtration) is reasonably clear or not. A filtrate test can be made, however when the analysis of that test is ready the process conditions may have changed and an adjustment of the polymer addition based on the test result is no longer relevant.

The amount of dry substance coming out of a thickener, ⁰/oDS_out, is set in a similar way by visually studying thickened sludge. This is often done after the metering or the feeding rate of the polymer has been selected. A sludge test can be taken and corrections can be made, but the test takes a certain time to perform, also here causing the test result to lag behind the changes in process conditions.

However, important process parameters can change relatively fast. This is valid for the type of sludge, % DS in incoming sludge, the amount of sludge pumped in per unit of time, how the sludge has been pretreated e.g. by chemical sedimentation, the sludge temperature etc. This means that the selection of polymer, the polymer feeding rate and the setting of the DS content of outgoing sludge is no longer optimal, which also goes for other process parameters. The result is that too much expensive polymer is consumed, that the DS content is wrong, that the filtrate contains too much particles (SS) or that the filter cloth becomes clogged due to a too high polymer feeding rate and at the same time the flocculation is ineffective.

Direct measurement of the outgoing DS content and control of said DS content is difficult to 5 perform at larger DS contents. This means that the control of the sludge thickening process is often inaccurate and inefficient. The sludge thickening performed today is therefore often associated with process problems costing both time and money for the user.

SUMMARY OF THE INVENTION

10 The basic object of the present invention is to eliminate or at least largely reduce the above discussed deficiencies associated with the known technique within the field of sludge thickening. In particular, the object is to provide a method of improving the efficiency of processes of the kind described above, by optimizing the use of flocculants in the process and thereby minimizing the amount of added flocculant and/or coagulant, while at the same time

15 the DS content of the thickened sludge lies within a desired interval.

In accordance with the invention, the above stated object is achieved by employing, in the process control, different ways of measuring process parameters on-line, of processing the measured data and, preferably through an iterative control strategy, of changing the process 20 parameters in such a way that the DS-content in the thickened sludge and the flow rate thereof become as desired, so that the consumption of polymers or metallic salts is kept low.

In particular, this object is achieved by controlling the feed rate of the polymer through measurement of the SS content in the filtrate, in order to thereby lower the consumption of the 25 polymer, but above all govern the thickening process in such a way that the DS content of the thickened sludge lies between a highest and a lowest value.

According to another aspect of the invention, a system is provided for controlling a process of the kind described above, in accordance with the basic principles of the invention.

J 0

A further aspect of the invention relates to the use of the method and/or system according to the invention for thickening before a digester. Further objects, characteristics and advantages of the invention are clear from the independent patent claims as well as from the following description of exemplifying embodiments.

BRIEF DESCRIPTION OF THE DRAWING The invention will now be described in closer detail in connection with the enclosed drawing that in the form of a flow sheet illustrates one example of a sludge thickening process with a control according to the invention.

DESCRIPTION OF EMBODIMENTS With reference to the drawing figure, the basic principles of the invention will now be described using a number of presently preferred embodiments of the invention. At the same time the differences in relation to conventional techniques will be described.

Example No. 1 concerning controlling the addition of flocculant with the purpose of minimizing the polvmer consumption at drum thickening or of warning for a flocculant mixture that is no longer active, while at the same time obtaining a DS content in the thickened sludge that lies within a desired range.

As indicated in the introduction, the main object of the invention is to improve the efficiency of thickening processes by optimizing the utilization of flocculants and/or coagulants in the process, thereby minimizing the added amount of flocculants and/or coagulants and at the same time achieving a DS content in the thickened sludge that lies within a desired interval.

According to the first embodiment of the present invention, as shown in the enclosed figure, this is achieved in connection with a thickener (1), by providing an optical sensor 14, preferably of infrared type, either applying direct measurement and provided with a cleaning device, or configured in such a way that staining of the sensor units is compensated for, or that changes of the color of the filtrate RF is compensated for. The sensor 14 is provided in a specific compartment (not shown) in connection with the pipe 7 carrying the filtrate out from the thickener 1. The sensor 14 as well as the opening and closing functions of the compartment and possible associated flushing units are connected to a control unit 2 that is also connected to a feeding pump 3 at units 3, 4 for the addition of flocculants Fi and F₂, i.e. often polymers. The sensor 14 is calibrated considering the specific application and is working in the range of monotonic increased addition of polymer-monotonic decreasing DS content. Two to three limiting conditions are selected, depending upon the measuring conditions. The first measuring limit SS_max is set equal to the SS content (amount of solids in the filtrate) above which the amount of SS is considered as unacceptable. If the SS content exceeds this value, the control unit 2 increases the feed rate of flocculant Fi until the SS content goes below this value, which is say 400 mg SS/1.

If the feed rate increases above the second limit value Fι_maχ, say 5 kg polymer per ton DS, this is an indication that either the wrong polymer is used, or that the actual polymer mixture has been stocked too long time and has lost its flocculation properties, or that the performed flocculation is not complete, due to a too low temperature or due to a too short dwell time before entrance into the thickener 1, or that the coagulation treatment of incoming sludge IS, before the addition of flocculant/polymer, has been insufficient or incorrect. In this situation the control unit 2 sees to it that the polymer addition does not exceed the limit value Fι_max for maximum allowed polymer feeding, and gives an alarm signal so that the operator notices the situation and can take measures.

Alternatively, the control unit 2 switches over to the addition of another flocculant F₂ with other flocculation properties.

If the SS content in the filtrate is too low, say under the third limit value SS_mιn 100 mg SS/1, this is an indication that the rate of polymer feed is too high. In this case the filter cloth will become clogged and the thickening process does not function in an optimal way. Then the control unit decreases the rate of polymer feed until an SS-value above 100 mg SS/1 has been exceeded.

If the measuring conditions and the properties of the sensor 14 are such that it is difficult to receive reliable measuring values below say 100 mg SS/1, too large polymer additions are instead avoided in the following way: The control unit 2 regularly senses the SS content in the filtrate RF and feeds or meters, through an iterative process, in such a way that when the limit value 400 mg SS/1 is reached through a certain polymer feeding, the polymer addition is adjusted down one unit, where the unit size depends upon the actual process conditions. The control unit 2 is programmed to sense the SS content at a frequency that is set by the operator. The control unit 2 can control the addition of flocculant/polymer based on single measuring points, but preferably changes of the polymer addition are based on mean value calculations based on a number of measuring points.

Since the temperature of the sludge does influence the flocculation process, it is advantageous to have a temperature sensor for following-up the sludge temperature. To compensate for a low sludge temperature, hot water can be used as mixing water for the dilution of the concentrated polymer, and said hot water can be controlled to a certain temperature, and can be injected into a flocculant-water mixing vessel 4 through a hot water mixer 5. This increases the normalized time for maturing of the diluted polymer mixture and the resulting effect is lowered polymer consumption.

By using the now described method, the main object of the invention is accomplished, i.e. by measuring the SS content in the filtrate the polymer feed rate can be controlled in such a way that the SS content is kept between a lowest value SS_mιn and a highest value SS_max. In this way the polymer consumption decreases but above all the thickening process is controlled in such a way that the DS content of the thickened sludge is between a highest and a lowest value. It has in fact been shown that there exists a proportionality between the polymer metering or feed rate, the SS content in the filtrate - increased polymer feed rate means a lower SS content- and the DS content of the thickened sludge - everything else constant, an increased polymer feed rate means a higher DS content. This means that by using this invention it is possible to indirectly control the DS content of the thickened sludge, i.e. so that the outgoing DS content, DS_0Ut, can be controlled within a certain interval.

This last mentioned property is particularly advantageous and desirable in an application that is presently regarded to be of particular importance, i.e. the thickening of sludge before a digester. In such an application the DS content of the thickened sludge should not be higher than 7 %, in some cases not higher than 6 %, since thicker sludge can cause problems in the subsequent process. By using this automated way of controlling the polymer feed rate, manual adjustments of parameters which affect the SS content in the filtrate, e.g. the speed of drum rotation, can then be made to minimize the addition of flocculant/polymer.

Example 2 concerning controlling the polvmer addition to minimize the polvmer consumption, avoiding clogging of the filter cloth and warning for the use of the wrong polvmer or a polymer mixture which is no longer active, in connection with drum thickening, while at the same time other process parameters are automatically changed to minimize the polymer consumption and a changed DS content of the thickened sludge is obtained.

In this case the automatic control of the polymer addition, as described above, is supplemented by a continuous iterative search for minima in polymer addition as a function of different process parameters, using the same sensor 14 as in example 1 above. Minimization of the flocculant/polymer consumption is made in the same way as in example 1, but is in this case supplemented with an iterative search for polymer addition minima also as a function of other parameters, like e.g. the speed of rotation of the drum. During the dynamic process of minimization of the polymer addition, the speed V of drum rotation is stepwise lowered from a preselected value, say 10 rpm, with alternating adjustment of the feed rate of polymer addition, and at the same time the SS content in the filtrate RF is measured. At a decreasing rate V of drum rotation, the SS content of the filtrate will decrease, everything else constant. In this way, the polymer addition can be further reduced. This goes on until the filter cloth is clogged, which in turn can be compensated for with an increased frequency of spraying the filter cloth.

At wintertime the incoming sludge IS is often cold. It may therefore be advantageous to mix the concentrated polymer with hot water to a temperature of e.g. 30 degrees C. The control unit 2 can be equipped with a function that governs the temperature of the dilution water in such a way that the polymer addition can be decreased.

By using a simple sensor 14 in the filtrate and a simple control unit 2 programmed for an iterative search technique related to the process conditions, an efficient process control may be performed for minimizing the polymer addition. This results in large economical and process advantages.

Example 3 concerning control of the DS content within a desired interval while at the same time minimizing the consumption of the flocculant/polymer.

Results show that even with only a sensor 14 in the filtrate it is possible to control the DS content of the outgoing sludge, DS_0Ut, with reasonable accuracy, so that it is possible to obtain a desired DS content of the thickened sludge FS that is discharged from a thickener 1 through an outlet 8.

The accuracy of this process increases if the pump for the incoming sludge IS is such that the flow into the thickener 1 is constant, and if the DS content, DS_ιn, of incoming sludge is also reasonably constant.

In particular, this is valid if the pump 9 is controlled by using a sensor 10 that starts the pump 9, and thereby the thickening process, when the DS content of the sedimentation basin or the gravitational thickener is within a certain interval. Under these conditions, a desired DS content, DS_out, can be set in an indirect way, by choosing a certain SS content of the filtrate as set value for the control unit 2, in combination with the selection of other process parameters.

Example 4 concerning a wav of indirectly, through a multi sensor system, measuring and controlling the outgoing DS content, and at the same time minimizing the polymer addition during drum thickening.

While the simple technique described above provides many advantages from process and economical points of view, it is of interest by larger process plants to achieve complete control of the thickening process, i.e. also of the DS content of the outgoing sludge, DS_out. in a more exact way. Even though more and more efficient sensors appear on the market, it is difficult to measure, with high precision, the outgoing DS content at the level of 5-7 %, and this is due to the consistency of the sludge FS at these levels. We have therefore developed an indirect technique for a more exact measuring and control of high DS contents out from a thickener 1 and for a control of this DS content while at the same time minimizing the polymer addition, se figure 1.

The optical sensor 14 in the filtrate RF is supplemented with a flow meter 15. In the pipe for incoming sludge two sensors are also inserted, a sensor 10 for the incoming DS content and a sensor 11 for the flow.

The control unit 2 described above can also be connected to the following sensors and functions: i. The flow meter 15 for the filtrate RF ii. The flow meter 11 for incoming sludge IS iii. The DS sensor 10 for incoming sludge IS

and, in the case of a drum thickener, to the following functions: a) Pump 9 for incoming sludge IS b) Lift for the control of the slope of the drum c) The control of the speed of drum rotation d) The control of the frequencies of spraying, can be one or several e) To other functions as alarm functions, level sensors and DS sensors in sludge basins before digesters etc.

After setting the desired content and flow of DS_0Ut on the control unit 2, the control unit controls and measures according to the following:

I. Standard values are selected for the slope of the drum, the speed of drum rotation and the frequency of spraying. II. The combination of the effect of the sludge pump and DS,_π is adjusted by the control unit in such a way that the desired DS_0Ut and flow out are obtained and in such a way that the sludge pump works in an optimal way.

III. The polymer addition is made according to example 1 above. IV. In an iterative process, the DS content DS_out is then controlled by stepwise changes of the slope of the drum, the speed of drum rotation and the frequency of spraying. The polymer addition is continuously optimized in a sub-iterative process as described above. V. Through an efficient mixing and a continuous measurement of the thickened sludge in the sludge basin before the digester, the DS content in this sludge basin can be used as the main control parameter instead of DS_0Ut, and then an adjustment of DS_out is performed considering the difference between the set value and the actual DS value in the sludge basin. This procedure gives a maximum of efficiency for the following processes like pumping, heat exchanging and digesting in a digester, still minimizing the polymer addition as an important result.

VI. If DS_out and the flow thereof are not reached, the operator is warned.

In this way the thickening process can be controlled in an optimal way, completely auto- matically and without any inspection as well as providing a process economy at its maximum.

If the process conditions are drastically changed, e.g. through the addition of sludge of another type, a changed temperature of the incoming sludge, flocculation agents with other properties or any other change, the control unit 2 senses that and sets new optimal process parameters.

In this way, the following advantages can be obtained:

The thickening process can be completely automated a) The cost for the addition of polymers can be minimized, which means considerable savings every year and also provides advantages for the environment. b) The cost for the water used for spraying is minimized, which also means large savings. c) The filtrate will on the average become cleaner, which decreases the load on the sewage treatment plant. d) DS_out and the flow will be as desired, which gives large advantages for the following process. In the case of a biogas plant this means an increased biogas production and a smaller amount of heated water, and both these two factors considerably improve the economy. In the case that the thickened sludge will be transported, significantly smaller volumes are obtained by using the present invention.

In this example focus has been on drum thickeners. However similar advantages are obtained also for e.g. the following equipment for the thickening of sludge:

a) Thickeners of belt type that function like a drum thickener, but have another geometry. b) Belt presses in combination with a drum thickener or a belt thickener arranged ahead thereof. c) Centrifuges in combination with a drum thickener or a belt thickener arranged ahead thereof.

The above applies both to municipal and industrial treatment plants with different kind of sludge.

Example 5 concerning the design of a sludge sensor and the control thereof.

The sludge content sensor 14 is preferably of the optical type, preferably working in the infrared range, but it is also possible to use e.g. UV-light. The sludge sensor can also be of another type, e.g. density sensors or stirrers for sludge basins, sensors for the electrode potential in the case of addition of oxidizing salts, e.g. FeCl₃ or sensors for the conductivity in the case of addition of other salts.

An infrared sludge sensor and the method of controlling such a sensor is described in the following:

The infrared sensor consists of two senders/receivers or several pairs thereof, between which the infrared light moves. To avoid staining the senders/receivers are flushed or wiped clean just before the measuring moment, as a part of the measuring process. In the case that more than two senders/receivers are used the color of the water or stains on the interfaces of the senders/receivers can also be compensated for. When measuring the SS content of the filtrate RF, the sensor shall preferably be placed in a specific compartment arranged in such a way, that this compartment can be cleaned by flushing or mechanically, just before the measurement, followed by letting in new filtrate and performing measurements. In this way representative measurement results are obtained when making DS measurements.

Example 6 concerning a combination of controlling additions of coagulants and of flocculants by sludge thickening.

In certain cases the sludge IS can be difficult to thicken also at high polymer additions. One reason can be that the addition of coagulants in the sedimentation basin has not been made in the correct manner. In such a case, a sensor 13 can be connected to the control unit for measuring of the charge of the sludge particles before the polymer addition, as well as equipment 12 for the addition of coagulants, in most cases a metallic salt. In this case, the addition of coagulants is controlled in such a way that either the charge of the sludge particles enters a certain level or the need for polymer addition decreases below a certain level. In this way, the addition of polymer can be decreased.

The man skilled in the art realizes that different modifications and changes can be made to the present invention, without departing from the scope of the invention as defined by the enclosed patent claims.

Claims

1. A method of controlling a sludge thickening process, in which one or more flocculation agents (Fi, F₂) and/or coagulants (K) are fed into the incoming sludge (IS) prior to feeding said sludge into a thickener (1), and in which filtrate (RF) and thickened sludge (FS) is discharged separately from the thickener (1), characterized in that:

a) the amount of solid substance (SS content) in the filtrate (RF) is measured with an adjustable frequency;

b) data from the measurement of the amount of solid substance (SS content) in the filtrate is fed to a control unit (2);

c) a feeding unit (3) for providing a metered feed of a flocculant (FI) is connected to the control unit (2), whereby the feeding of flocculant (Fj) is sensed;

d) an upper (SS_max) limit value of the amount of solid substance (SS content) in the filtrate is set in the control unit (2), and the measured amount of solid substance is compared to the limit value;

e) when exceeding the upper (SS_max) limit value for the measured amount of solid substance (SS content), the control unit (2) increases the feeding of the flocculant (Fi) until the measured amount of dry substance falls below the limit value again;

f) an upper limit value (Fι_max) for the feeding of flocculant (Fj) is set in the control unit (2) and is compared with the sensed value;

g) when exceeding the upper limit value (Fι_max) for the feeding of flocculant (Fi) the feeding of flocculant (Fi) is decreased through the control unit (2) until it falls below the limit value (Fι_max) for the feeding of flocculant (Fi) and the control unit (2) gives an alarm signal to the operator or alternatively the control unit switches over to feeding another flocculant (F₂); h) in the control unit (2) a lower limit value (SS_mi_n) is set for the amount of solid substance (SS content) in the filtrate (R-F), and the measured amount of solid substance is compared to the lower limit value, and when the measured amount of solid substance (SS content) falls below the lower limit value (SS_mιn), the feeding of flocculant is reduced through the control unit (2) until the lower limit value of the measured amount of solid substance is exceeded again, and

i) by means of the upper (SS_raax) and lower (SS_mm) limit values a DS content of the thickened sludge (FS) is obtained that lies within a desired interval.

2. A method of controlling a sludge thickening process according to claim 1 , characterized in that the dry substance content (DS_0Ut) of the discharged thickened sludge (FS) is controlled to lie between 5-12 %, preferably below 7 % and in particular below 6 %.

3. A method of controlling a sludge thickening process according to claims 1 or 2, characterized in that in the control unit (2) mean value calculations are performed based on a number of measuring points concerning the measurement of the amount of solid substance (SS content) in the filtrate (RF) and in that the feeding of flocculant (Fi, F₂) is controlled based on such calculations.

4. A method of controlling a sludge thickening process according to one or more of the preceding claims, characterized in that through the control unit (2) is performed a continuous iterative search of the minima for the consumption of flocculants (Fi, F₂) as a function of process parameters, such as the rotation rate (V) of the drum and/or an increased frequency for spraying the filter cloth clean, and/or the addition of hot water to the flocculants (Fi, F₂) and controlling the temperature of said hot water respectively, in the case of a drum thickener (1).

5. A method of controlling a sludge thickening process according to one or more of the preceding claims,, characterized in that the amount of solid substance (SS content) in the filtrate is measured using an optical sensor (14), in that this sensor (14) is placed separately in a compartment in connection with the filtrate pipe (7) from the thickener (1), and in that the opening and closing of the compartment is controlled by the control unit (2).

6. A method of controlling a sludge thickening process according to claim 5, characterized in that the sensor (14) for measuring the amount of solid substance (SS content) in the filtrate (RF) and the compartment in which this sensor is placed are flushed before every measurement, and in that this flushing is likewise controlled by the control unit (2).

7. A method of controlling a sludge thickening process according to one or more of the preceding claims, characterized in that the flow of incoming sludge (IS) to the thickener (1) is kept constant, e.g. by the selection of a suitable pump (9), and in that the dry substance content (DS,_n) in incoming sludge (IS) to the thickener is kept substantially constant, whereby the desired outgoing dry substance content (DS_0Ut) from the thickener (1) is controlled by setting a corresponding amount of solid substance (SS content) in the filtrate (RF) as a set value for the control unit (2), in combination with other process parameters.

8. A method of controlling a sludge thickening process according to one or more of claims 1-6, characterized by measuring the flow of filtrate (RF) out from the thickener (1), the flow of incoming sludge (IS) into the thickener (1) and the dry substance content (DS,_n) in the incoming sludge (IS) to the thickener, and in that data from these measurements is fed to the control unit (2) and in that the desired outgoing dry substance content (DS_out) as well as the flow out from the thickener (1) is set in the control unit (2) as a function of these measuring data.

9. A method of controlling a sludge thickening process according to claim 8, when employing a drum thickener (1), characterized in that the control unit (2) is connected to and controls a pump (9) for incoming sludge (IS), a lift for controlling the slope of the drum, the speed (V) of drum rotation and the frequency for flushing the filter cloth clean, in that a set of standard values for the slope of the drum, the drum rotation and the flushing frequency is selected and in that in an iterative process the outgoing dry substance content (DS_0Ut) from the thickener (1) is controlled through stepwise changes of the slope of the drum, the drum rotation and the flushing frequency.

10. A method of controlling a sludge thickening process according to one or more of the preceding claims, characterized by measuring the charge of the particles in the incoming sludge (IS) before the addition of flocculants (FI, F2), in that data from these measurements of the charge of the sludge particles is fed to the control unit (2), in that a feeding unit (12) is connected to the control unit for the addition of coagulants (K) and in that the addition of coagulants (K) is controlled, depending upon the sensed actual value of the charge of the sludge particles, so that the charge of the sludge particles will be close to a set value in the control unit or so that the need for flocculant addition falls below a preselected level.

11. A method of controlling a sludge thickening process according to one or more of claims 1-6, 9 or 10, where the outgoing sludge (FS) from the thickener (1) is conducted to a sludge basin before a digester, characterized in that the DS content of the thickened sludge (FS) in the sludge basin is measured and is used as a main control parameter, in such a way that the adjustment of the dry substance content (DS_out) from the thickener (1) is performed depending on the difference between a set value entered in the control unit (2) and the actual measured value of the DS content in the sludge basin.

12. A system for controlling a sludge thickening process, in which equipment (3,4,5,12) is used for feeding one or more flocculants (FI, F2) and/or coagulants (K) into the incoming sludge (IS) before it is fed into a thickener (1) and in which filtrate (RF) and thickened sludge is separately discharged from the thickener (1), characterized by a first, preferably optically working sensor (14) arranged in connection with a filtrate pipe (7) and intended for the measurement of the amount of solid substance (SS content) in the filtrate (RF) and by a control unit (2) for receiving and processing measuring data from the first sensor (14) and from a sensing of a metered feeding of flocculant (FI) through a feeding unit (3), and for controlling the feeding of flocculant (FI) as a function of the received and processed measuring data.

13. A system for controlling a sludge thickening process according to claim 12, characterized by a flow meter (15) provided in connection with the filtrate pipe (7) for measuring the flow of filtrate (RF) out from the thickener (1), by a flow meter (11) provided in connection with a sludge supply pipe (6) for measuring the flow of incoming sludge (IS) to the thickener (1) and by a dry substance sensor (10) for measuring the dry substance content (DS_ιn) in the incoming sludge (IS) to the thickener and in that these measuring devices and sensors, respectively, are connected to the control unit (2), whereby data from these measurements are fed to the control unit (2).

14. A system for controlling a sludge thickening process according to claim 13, characterized in that the control unit (2) is connected to and controls a pump (9) for incoming sludge (IS), a lift for controlling the slope of the drum, the rate (V) of rotation of the drum and the frequency for the flushing of the filter cloth, to control the outgoing dry substance content (DS_om) from the thickener (1) through stepwise changes of the slope of the drum, the rate of drum rotation and the frequency for flushing.

15. A system for controlling a sludge thickening process according to one or more of claims 12-14, where the thickened sludge (FS) discharged from the thickener (1) is conducted to a sludge basin before a digester, characterized by a sensor, preferably mechanical, intended for measuring density or viscosity and provided in the sludge basin for measuring the dry substance content of the thickened sludge in the sludge basin and in that this sensor is connected to the control unit (2) such that data from said measurement is fed to the control unit (2).

16. A system for controlling a sludge thickening process according to one or more of claims 12-15, characterized in that the thickener is a drum thickener or a belt thickener.

17. The use of a method or a system according to one or more of claims 1-16 for thickening before a digester.