Rotary press for sludge dewatering

Abstract

A rotary press for dewatering sludge wherein a polymer is added to the sludge at a rate which is inversely dependent on the sludge flow rate of the press.

Description

ROTARY PRESS FOR SLUDGE DEWATERINGBACKGROUND OF THE INVENTION

[0001] This invention relates to the treatment of sludge.

[0002] “Sludge” is to be interpreted broadly and includes a composition comprising a mixture of a liquid and a solid material, typically of particulate or fibrous form. “Sludge” extends, by way of example only, to products produced in the paper industry, by waste water treatment, in the processing of minerals, agricultural and food products, to compositions produced by fisheries, breweries and wineries, and to products produced through chemical, oil and tar sand processes.

[0003] “Press” relates to a method or apparatus for extracting liquid from a sludge. Without being restrictive “press” includes a filter press with rotatable screens for example of the kind described in the specifications of US7166229, US8146750 and US7946225.

[0004] A press, acting on sludge, separates liquid (filtrate) from solids in the sludge. If the solids are sufficiently dry, and compressed by the action of the press, the product which results is referred to as cake.

[0005] “TSS” refers to the total suspended solid content in a sludge.

[0006] Each of the patent specifications referred to describes a press for treating a sludge, and for producing a filtrate and a cake. The sludge is pumped into a channel in the press which includes walls which are made from a screen material. It is possible that particulates in the sludge can be sufficiently small to pass through mesh apertures in the screen, together with the filtrate. This reduces the efficiency of the process.

[0007] To prevent the loss of fine particulates with the filtrate and so improve the sludge processing operation a suitable polymer material is added to the sludge, with mixing taking place in a flocculator, before the mixture enters the press. The action of the polymer is such as to cause the particulates in the sludge to bind together, effectively becoming larger in size, so that passage of these particulates through the mesh apertures is not possible.

[0008] It is technically difficult and expensive to measure the particulate content in the sludge on a real time basis before the sludge enters the press. Thus the addition of the polymer to the sludge, in a manner which is dependent on the TSS in the sludge, is not easily effectively implemented. Instead, the polymer is added at a fixed rate, which is related to the sludge volume flow, to the sludge. This results in an inefficient exercise in that excessive amounts of polymer can be added without producing a commensurate process improvement, or an inadequate quantity of polymer can be added to the sludge. In the former case the excess polymer is wasted and, moreover, the output of the press may comprise a product which cannot be regarded as comprising cake of an acceptable quality. In the other instance the sludge can cause a blockage in a flow channel in the press, leading to down time and possible damage to equipment.

[0009] The TSS in a sludge can sometimes vary by a factor of 2 or more. Normally the polymers are effective in a TSS band of a given value wherein the width of the band depends on the type of polymer and, possibly, on characteristics of the sludge. An operator may be called upon to make a polymer addition adjustment when a change in TSS is suspected but in the absence of an exact TSS measure any adjustment is speculative. Additionally, downstream processes can be adversely affected with excess polymer dosage causing settling or inappropriate chemical or biological actions.

[0010] Another factor which is not easily addressed is that apart from polymer addition the performance of a press is dependent on the type of sludge treated. For example the results from a press treating a clay-based sludge would normally vary from the results achieved when a waste-paper-based sludge is treated.

[0011] Other variables which can affect the treatment of sludge include pressure levels at a sludge input to the press and at a sludge output from the press. The speed of rotation of screens inside a housing of the press, which screens form walls of a channel between the sludge inlet and the sludge outlet, also plays a role in the sludge treatment process.

[0012] As is the case with the polymer addition rate these variables (inlet and outlet pressure and screen movement) should preferably be responsive to the TSS of the incoming sludge. The TSS cannot however readily be measured in a real time and ongoing basis, at a reasonable cost.

[0013] The performance of a sludge dewatering press can be assessed primarily by measuring the dryness of a cake produced by the press, the sludge capacity throughput and the quality of the filtrate or the solids capture rate.

[0014] The relationship between the polymer dosing value, the speed of rotation of the screens, the sludge inlet pressure and the sludge outlet pressure is complex. A primary factor which affects performance is the variability of the sludge flow rate due to TSS changes. For example when the sludge flow rate drops due to an increase in the sludge TSS the output capacity can be increased by increasing the rotational speed of the screens. The dryness of the cake might then reduce but that can possibly be compensated for by adjusting the sludge inlet pressure or the sludge outlet pressure.

[0015] A typical sludge inlet pressure is of the order of 40kPa while the outlet pressure can be higher by a factor of 10 i.e. of the order of 400kPa. If the output pressure is to be adjusted to optimise press operation only a relatively small increase in the outlet pressure can be sustained for, if the sludge output pressure is increased beyond a particular limit, the effect of the polymer could be negated or the press could malfunction or become damaged. This means that the output pressure should not be increased above a threshold value in an attempt to improve press performance. A similar reservation applies to an adjustment of the inlet pressure and to an adjustment of the rotational speed of the screens.

[0016] The invention aims to address, at least to some extent, the aforementioned situation.SUMMARY OF THE INVENTION

[0017] One aspect of the invention is based on the realisation that the sludge flow rate is an indirect indicator of the solids in the sludge i.e. the TSS thereof. This is based on the observation that an increase in the flow rate of a particulate sludge type, through the press, with other operational parameters non-varying, is due to a decrease in the TSS of the sludge. Conversely a decrease in the flow rate of the sludge is due to an increase in the TSS of the sludge.

[0018] In contrast therefore to the prior art approach of adding polymer at a rate which is dependent on (proportional to) the sludge volume flow rate, the invention proposes that the polymer addition rate is inversely dependent on the sludge flow rate i.e. the polymer addition decreases as the sludge flow rate increases, and the polymer addition increases as the sludge flow rate decreases. The polymer acts on the solids, and not the liquids in the sludge. Also, it is the solids content in the sludge which produces the variation in the sludge flow rate referred to.

[0019] Other variables of operation of the press, including the prevailing pressures at the sludge inlet and at the sludge outlet, and the rotational speed of the screens inside the press, can with benefit be made dependent on an ongoing and real time use of the aforementioned concept i.e. an increase in sludge flow rate is caused by a reduction in TSS, and vice versa. The nature of such dependence in respect of each parameter does however vary.

[0020] For example with the pressure at the sludge inlet initially at a constant value, an increase in the sludge flow rate is indicative of a reduced TSS in the sludge. This results in a lowering of the pressure at the sludge inlet. To maintain a sludge processing rate and to ensure the production of good quality cake, the speed of rotational movement of the screens inside the press can be reduced thereby to cause a pressure increase in the press which manifests as a rise in pressure at the sludge inlet. Similarly, by increasing a pressure applied to sludge leaving the press at the sludge outlet, the pressure between the inlet and the outlet increases, and the pressure at the sludge inlet increases.

[0021] Thus in response to a measurement of the sludge flow rate, into the press, one or more of the following operating parameters of the press are adjusted in real time in the manner which is described hereinafter.(a) the dosing of the polymer into the sludge,(b) a speed of rotation of screens in the press,(c) a target pressure of the sludge at a sludge inlet to the press, and(d) a target pressure of the sludge at a sludge outlet from the press; this is the pressure which, per an output from a controller, is to be applied by an actuator to sludge cake at the outlet.

[0022] Additionally, account is taken of the maximum pressures which can be generated at the sludge inlet and outlet in an attempt to improve press performance but without mechanically compromising the press. Excessive pressure at the inlet or outlet can also adversely affect the flocculation outcome for the flocs can be caused to break up.

[0023] According to the invention, as an initial step to improve press performance, the dosage of the polymer to the sludge is given in a non-limiting and exemplary form by the following expression:where D is a polymer dosing target;A is a base sludge flow rate which in this specification is taken to correlate with a base TSS in the sludge;B is a measure of the instantaneous sludge flow rate to the press;C is a base polymer dosing rate. C is the rate at which the polymer is dosed when the base sludge flow rate is A.D is a proportion (% or a fraction) of C which is a proportion of A. However, for A and B “rate” relates to flow per unit time e g. litres per hour, litres per second, kilograms per minute, etc.

[0024] To give effect to the aforegoing concept, the invention provides apparatus for treating sludge which includes a press with a housing, a channel in the housing, rotatable screens which form walls of the channel, a sludge inlet to the channel, a sludge outlet from the channel, a first pump which is operable to pump sludge into the press through the sludge inlet, a sensor at the inlet which monitors the pressure of the sludge at the inlet and which produces a first signal which is dependent on the sludge pressure at the inlet, a first flow meter which is operable to obtain a measure of the sludge flow rate to the press and which produces a second signal whichis dependent on this sludge flow rate, a polymer source, a flocculator at the sludge inlet, a second pump which is operable to add polymer to the sludge upstream of the flocculator, a second flow meter which is operable to obtain a third signal which is dependent on the polymer flow rate, and a controller which is responsive to the first, second, and third signals to vary in real time the addition of polymer from the source to the sludge, at a rate which is inversely dependent at least on the sludge flow rate i.e. the second signal.

[0025] The invention further provides that the controller, responsive to the first, second and third signals in real time, implements one or more of the following:(a) a variation of a rotational speed of the screens in a manner which is dependent on the second signal, thereby to vary the pressure of the sludge in the channel and thus the pressure of the sludge at the inlet,(b) a variation of a pressure to be applied to cake at the sludge outlet in a manner which is dependent on the second signal thereby to vary the pressure of the sludge in the channel and thus the pressure of the sludge at the inlet, and(c) a variation of the sludge inlet pressure in a manner which is dependent on the second signal.

[0026] As explained hereinafter the aforementioned Expression 1 can, with benefit, be modified to the form:where Eav is a sensitivity factor which may be varied in accordance with operating requirements and characteristics of the sludge, based on an initial evaluation of the working of the press achieved e.g. by means of tests using various operating parameters of the press. The value of Eav, for a given set of operating conditions, is calculated in the manner described hereinafter.

[0027] It is observed that: when Eav = 0, D = C i.e. the base dosing rate, and when Eav = 1, D = A / B x C which is the ratio of the base sludge flow rate to the instantaneous sludge flow rate.

[0028] Values of E are determined for different operating conditions, based on a response of the press to a polymer dosage rate which is dependent on the instantaneous value of the sludge flow rate B and on the type / nature of the sludge which is being treated, and from those values an average E value (i.e. Eav) is selected to match anticipated operating conditions.

[0029] The invention also extends to a method of operating a filter press which includes a housing, a channel inside the housing, rotatable screens which bound the channel, a sludge inlet to the channel, a sludge outlet from the channel, and a polymer source from which a polymer is added to a sludge feed upstream of the sludge inlet, wherein the method includes the step of measuring the flow rate of sludge to the press and is characterised by the step that, in response to said sludge flow rate measurement, the addition of the polymer from the source is varied in real time at a rate which is inversely dependent on the sludge flow rate.BRIEF DESCRIPTION OF THE DRAWING

[0030] The invention is further described by way of example with reference to the accompanying drawing which shows how a rotary filter press is operated in accordance with the invention.DESCRIPTION OF PREFERRED EMBODIMENT

[0031] The drawing relates to a press 10 which, in an exemplary embodiment of the invention, is a press of the kind described in the specification of US7166229.

[0032] The press 10 operates to extract liquid from a humid mass (a sludge) 12. The press includes a housing 14 with an inlet 18, an outlet 20 and a channel 22 which extends inside the housing between the inlet and the outlet. Walls of the channel are formed by screen materials 24 which are mounted for rotation at a controlled, variable speed about an axis 26, relative to the channel, by a drive mechanism 30. A restricting device at the outlet comprises a compressive device 32 which, under the effect of an actuator 34, applies pressure to cake 36 exiting the outlet. This action contributes to the buildup of pressure inside the apparatus which in turn causes additional liquid (filtrate) 38 to be drained through the screens.

[0033] A sludge pump 40 pumps the sludge 12 through a sludge flow meter 44 to a flocculator 46 at the inlet 18.

[0034] A polymer pump 48 is used to pump a suitable polymer from a polymer source 50 through a polymer flow meter 54 to the sludge upstream of the flocculator 46. The polymer 50 and the sludge 12 are mixed in the flocculator 46 and the resulting mixture is directed to the sludge inlet 18. A pressure sensor 56 monitors the pressure of the mixture at the inlet 18.

[0035] A computer-based controller 60 receives a first signal 64 from the sensor 56, that represents the sludge inlet pressure, a second signal 66 which is indicative of the sludge flow rate as measured by the flow meter 44, and a third signal 68 that represents the flow rate of the polymer 50 as measured by the flow meter 54.

[0036] The controller 60 outputs a first control signal 70, which regulates the speed of operation of the sludge pump 40; a second control signal 72 which controls the speed of operation of the polymer pump 48; a third signal 76 to the mechanism 30 to control the rotational speed of the filter screens 24 in the press; and a fourth signal 80 which controls the pressure which is exerted by the actuator 34 on the cake 36 at the outlet 20.

[0037] In the prior art, a polymer pump is operable to deliver polymer at a rate which is proportional to the sludge volume flow to the inlet 18 and which is not dependent on the TSS of the sludge. The addition of excess polymer is expensive for the excess polymer is wasted. If too little polymer is added down time and possible damage to the filter can arise. Also, fines in the sludge can exit from the channel through the screen materials which bound the channel. This adversely affects the quality of the filtrate.

[0038] As a primary goal the invention has an objective of relating the actual polymer dosing rate in real time more accurately to the true polymer requirement. As an increase in TSS reduces the sludge flow rate the first objective can be achieved at least partially by making the polymer dosing inversely dependent on the sludge flow rate.

[0039] A factor which must be taken into account is that the characteristics of the sludge which is being treated, and the characteristics of the press, can influence the polymer dosing rate. For example the type of solids in the sludge e.g. clay particles or waste paper particles or the like, influence the optimum value of the polymer dosing rate.

[0040] The invention proposes as an initial step the use of the expression C todetermine a target polymer dosing rate. In this expression: A is a base sludge flow rate; B is an instantaneous sludge flow rate which varies in operation depending on the TSS of the sludge; Cis a base polymer dosing rate; and D is a target polymer dosing rate i.e. the rate of polymer dosing for the sludge, to be implemented at any given time, in response to the value of B.

[0041] The base values A and C are taken as constants (in respect of polymer dosage for this implementation). The press is worked initially during initial test conditions under constant pressure control conditions during which the sludge is processed in the press.

[0042] During the test phase A is measured by the sludge flow meter 44 i.e. the second signal 66. For this test a polymer addition rate C is accurately determined beforehand using real time measurements made on the TSS of a respective sample of the sludge. Thus C is the correct dosing rate for the press while the sludge inlet pressure and TSS are constant. The parameters B and D are variable. B is an instantaneous value and D is a referred to as the target value. These are:B, which is the instantaneous sludge flow rate, which varies during operation depending, inter alia, on the TSS, as reflected by the signal 66; and D which is a target polymer dosing rate (to be achieved in operation) which is the rate of polymer dosing for the sludge at any given time (the signal 68).

[0043] As a first degree of control, the use of the expression:to determine a target polymer dosing rate results in a substantial increase in the efficiency of polymer dosing. The controller 60 functions to control the polymer pump 48 to ensure that the dosage of the polymer 50, directed into the sludge upstream of the flocculator 46, as measured by the flow meter 54, is controlled to be consistent with Expression I.

[0044] Expression I does not however take the characteristics of the sludge, and the characteristics of the press, into account. For example, the type of solids in the sludge e.g. clay particles, waste paper particles or the like can have a strong influence on a determination of the optimum polymer dosing rate.

[0045] To address the aforementioned variables, Expression I is modified to:where A, B, C and D have the aforementioned values and Eav, referred to as a sensitivity factor, is dependent on the nature of the sludge, the characteristics of the press, the sludge flow rate through the press and on the sludge TSS.

[0046] It is possible with appropriate tests to determine a workable value of Eav, which is in the nature of an average value, which is directly dependent on the anticipated working parameters of the press which are considered to be the most likely mid-range set of conditions to prevail during press operation for the given sludge type.

[0047] For initial test conditions a working range of TSS values, for the particular sludge type to be processed, is established, centred on the sludge TSS related to the base sludge flow rate. Thus in Table 1 the middle TSS value is the value which is expected to be encountered in operation of the press. This may be regarded as the average TSS value during use of the press. The corresponding base sludge flow rate is 3000, and expected upper and lower values of the sludge flow rate are 4000 and 2000 respectively. These figures represent a typical variation in the sludge flow.

[0048] Sludge samples are prepared under controlled conditions with different TSS values as may be expected in a real situation. The solids must be of the same source and nature.

[0049] Table 1 indicates that three TSS values have been chosen namely 2%, 1% and 0,5% - these values are indicative of the range in TSS values which is likely to arise during press operation.TABLE 1

[0050] The rotary press is operated with the sludge sample which has a TSS of 1% i.e. in the middle of the range. The sludge is dosed with the polymer at a mathematically correct calculated rate based on an accurately measured value of TSS. The press is operated to get a state of equilibrium with a constant input pressure, a constant output pressure, and a constant screen rotation speed, and at that time the sludge flow rate is measured. In Table 1 this sludge flow rate is 3000 - this is the base sludge flow rate.

[0051] In a second phase the sludge to the press is changed to the higher TSS value of 2.0% and the polymer dosage rate is changed to a mathematically calculated correct rate for the 2% TSS. The sludge flow rate decreases under these controlled conditions and, once equilibrium has been achieved, the flow rate is measured. In Table 1 the sludge flow rate has dropped from 3000 to 2000.

[0052] The aforementioned exercise is repeated with the sludge being changed to the lower TSS value of 0,5%. The polymer dosing rate is again changed to a mathematically calculated correct rate which is dependent on the TSS of 0,5%. Once equilibrium has been achieved, under these controlled conditions, the sludge flow rate is measured - this is indicated as having increased to 4000.

[0053] From Expression II, E can be written as:

[0054] The value of E is calculated from Expression III for the upper and lower limits of operating conditions. In Table 1 E is recorded as being equal to 1.7 (E1) and 2.4 (E2) at the high TSS value and at the low TSS value respectively. The average value of E i.e. Eav which is 2.1 has been entered into Table 1 for the anticipated (average) TSS value.

[0055] The sensitivity factor Eav is thus determined to be the value to be used in Expression II when the press is operating under equilibrium conditions with a sludge which has an expected TSS (determined from actual measurements on a sludge sample). Eav is the average of a first E value (E1) which is calculated using Expression III from data which is reliant on calculations and measurements at a TSS value which is higher than the expected or average TSS at which the press is to operate, and of a second E (E2) value which is similarly calculated using a TSSvalue which is lower than the average expected TSS of the sludge. Thus is acalculated factor which is based on expected variations of the TSS of the sludge, referenced for the expected TSS (mid-range TSS value) of the sludge.

[0056] If Eav = 0, the polymer dosing rate C is maintained unaltered, for D = C.

[0057] If Eav = 1 , the target polymer dosing rate, D is decreased as the sludge flow rate increases - this is indicative of a TSS decrease. D is thus proportional to A / B i.e. the ratio of the base sludge flow rate to the instantaneous sludge flow rate. However the introduction of an Eav value, determined as described, into the operating regime results in a better control of the polymer addition. This has been demonstrated through tests on a variety of sludge types.

[0058] the polymer addition is inversely dependent on the sludge flow rate,although operational factors affect the nature of that dependence.

[0059] A benefit of the invention lies in the fact that the operation of a press of a known kind can be altered with minimal expenditure to arrive at a functional condition in which polymer usage is more accurately adjusted to the prevailing TSS in the sludge.

[0060] It has also been found that other operational parameters (apart from the polymer addition rate) can be controlled in real time using the sludge flow rate, to the press, as a control element. However the relationships between the sludge flow rate and these parameters are different from that which applies to the polymer dosing.

[0061] Variables which are to be considered are:(1) the rotational speed of the screens which bound the channel, and a factor which expresses the degree to which the screen rotation speed can be increased - there are inherent physical limits to the minimum and maximum screen speeds which are dependent on the construction of the press;(2) the sludge inlet pressure, taking into account physical constraints of the press; and(3) the sludge outlet pressure, taking into account the degree to which the sludge outlet pressure can be increased without compromising operation of the press.

[0062] These variables are also dependent on the expression but as Eav can bepositive or negative the variables can be directly, or inversely, dependent on the sludge flow rate.

[0063] Factors which restrict or limit the adjustment of some operational parameters include physical factors of the press such as construction techniques, types of materials, pump sizes and the like, and the nature of the flocculated sludge subject to the constraint that the benefit which is achieved by flocculation must not be negated by excessive sludge pressure.

[0064] Once the dosing rate for the sludge type has been determined, as aforesaid, the three variables can be adjusted to achieve a satisfactory processing rate. Complexities do however arise. The sludge feed rate can only be increased to a limit determined by the sludge feed pump, and the processing capability of the press. There are upper and lower limits to the screen rotational speed.

[0065] The sludge output pressure is dependent at least on the sludge input pressure and the screen speed. Nonetheless, for a polymer dosing rate determined as aforesaid, counterparts to Expression II can be used for each of the three variables to determine operating values whichare dependent on the sludge feed rate i.e. on the TSS of the sludge and which give improved performance. In each case the calculated value of E can be positive or negative. This means that the variable can be inversely, or directly, dependent on the sludge flow rate. The relationship between the three variables and the polymer dosing rate is complex but it is possible to represent each variable as a function of the operating variables of the press and then to choose values for the variable which allow for a satisfactory sludge processing rate while observing a suitable polymer dosing rate.

[0066] From the aforegoing the TSS of the sludge can be deduced from the input sludge flow rate, with other operational parameters being kept constant. Thus the polymer dosage rate can be adjusted to dose the correct polymer for the solids which are present in the sludge. By adjusting the polymer dosing, as aforesaid, the press can be effectively operational over a wider range of TSS before manual adjustments must be made to secondary parameters by an operator. These secondary parameters include screen rotation speed, the pressure of the sludge at the inlet to the press and the pressure of the sludge at the outlet from the press. Each sludge type has a specific response to these secondary parameters and to variations in these parameters.

[0067] Through the use of a suitably programmed controller the base sludge flow value can be adjusted when changes are made to the aforementioned secondary parameters. It follows that, when taking into account the secondary parameters, the base sludge flow rate is not a constant but is variable. Changes to all the operating parameters of the press can be made automatically through the operation of a suitably programmed controller, and do not call for manual intervention.

[0068] By way of example assume that the original base flow is 3000 litres per hour and that the incoming sludge flow rate drops to 2000 litres per hour due to an increase in the TSS of the sludge. The drop in the incoming sludge flow rate indicates a non-optimal production capacity of the press. To improve the production rate the screen rotational speed can be increased. This variation is detected by the controller which then causes the incoming sludge flow rate to be increased to a rate at which the pressure at the sludge inlet is raised to a correct value. The higher value of the incoming sludge flow rate is related to a higher base flow rate determined by the controller and that rate would be increased, say, to 3300 litres per hour.

[0069] On the other hand if the incoming sludge flow rate increases, due to a TSS decrease, the screen rotation speed is reduced to ensure satisfactory dewatering and cake production, and the base flow rate is altered and will be closer or identical to the original rate of 3000 litres per hour.

[0070] Variations in the values of the secondary parameters affect the performance output of the press in different ways. In the preceding example in addition to the increase in the screen speed of rotation, the inlet pressure might be reduced (this corresponds to a negative value of E) to ensure that the cake has a satisfactory dryness. This change would be processed by the controller to produce a new base flow rate and for adjusting the operating parameters including the polymer dosing rate.

[0071] The values of these three variables are determined as follows:

[0072] 1 - Screen SpeedReferring to the aforementioned operational tests (referred to as initial test conditions) which gave rise to a determination of the value of E, a base operational speed in rpm of the screens during the same test conditions is measured - this is designated F.Other parameters are G and H, where: G is a target operational speed of the screens during normal operation of the press; andH, expressed as a fraction of F, is such that 0<H<1 and is an rpm adjustment limit i.e. H expresses the degree to which the screen rotation speed G can be changed during press operation. H is a value which is mechanically determined and is attributable to the nature of construction of the press and of the screens, and on the sludge type which is to be processed. G, which is the rotational speed of the screens, can be adjusted in real time using the instantaneous sludge flow rate B as a controlling factor in accordance with the following relationship:where A, B and E are the aforementioned parameters. F, H, A and E are constant predetermined values, and B is the only variable. More simply expressed, for the sake of convenience i.e. G is a function of A, B and E.In Expression IV, E is not necessarily equal to Eav i.e. the E value for the polymer dosing rate. E is determined from tests to allow for G to be adjusted, to a satisfactory rotational speed which is subject only to variations in B i.e. the TSS of the sludge.Thus by using B, the instantaneous sludge flow rate, as an indicator of the TSS of the sludge, the rotational speed of the screens G is regulated in real time by the controller 60, to comply with Expression IV.

[0073] 2 - Sludge Inlet PressureA further extension of the invention lies in controlling the sludge inlet pressure in real time using the instantaneous sludge flow rate B as an indicator of the sludge TSS. Expression IV is used unchanged, except that values G, F and H respectively designate:G1= target inlet sludge pressure;F1= base inlet sludge pressure during said initial test condition; andH1= limit of inlet sludge pressure adjustment expressed as a fraction of Fi such that 0<Hi<1.As for the screen speed, the target inlet pressure is a function of A, B and E which is not necessarily equal to Eav. E is adjusted, as described, for screen speed G, to allow for the target inlet pressure to be varied only in response to B i.e. the TSS of the sludge.

[0074] 3 - Sludge Outlet PressureExpression IV is used unchanged except that G, F and H respectively designate:G2= target outlet sludge pressure;F2= base outlet sludge pressure during said initial test condition; andH2= adjustment limit of outlet sludge pressure. As before H2 is a fraction of F2 and 0<H2<1.Again as for the screen speed, and the target inlet pressure, the target outlet pressure is conveniently stated to be a function of A, B and E which is not necessarily equal to Eav (the Evalue for polymer dosing), with the value of E being determined, as described for a screen speed G.

[0075] The following Table II illustrates the effects of the value of E, and of the value of H, for different sludge flow rates, for different screen rotational speeds and inlet and outlet pressures, derived from Expression IV for a typical press. Notably E can be positive or negative, although for the polymer addition E is positive. If E is negative the variable is directly dependent on the sludge flow rate. If E is positive the variable is inversely dependent on the sludge flow rate.

[0076] As stated, H is known and fixed. From the table, or any similar table with a range of E values, screen rotation speeds, and inlet and outlet sludge pressures can be chosen, as input values to a program / algorithm executed by the controller, to give good control of the press andachieve a high production rate, dependent nonetheless on the sludge flow rate i.e. on the TSS of the sludge.

[0077] A fundamental principle of the invention thus lies in a recognition that the instantaneous sludge flow rate (B) is indicative of the TSS of the sludge. It then becomes possible by a suitable configuration of the control unit and by using B as a control parameter to control the dosage rate of the polymer (C) (in an inverse manner) in a cost effective and efficient manner. Further, other operational parameters of the press including the rotational speed of the screens which bound the channel between the sludge inlet and the sludge outlet, and the pressures at the sludge inlet and the sludge outlet can be controlled using B as a control parameter in an inverse, or directly, proportional manner, while observing a determined polymer dosing rate, to optimise press performance.

Claims

CLAIMS1. Apparatus for treating sludge (12) which includes a press (10) with a housing (14), a channel (22) in the housing, rotatable screens (24) which form walls of the channel, a sludge inlet (18) to the channel, a sludge outlet (20) from the channel, a first pump (40) which is operable to pump sludge into the press through the sludge inlet (18), a sensor (56) at the inlet which monitors the pressure of the sludge at the inlet and which produces a first signal (64) which is dependent on the sludge pressure at the sludge inlet (18), a first flow meter (44) which is operable to obtain a measure of the sludge flow rate to the press and which produces a second signal (66) which is dependent on the sludge flow rate, a source of polymer (50), a flocculator (46) at the sludge inlet, a second pump (48) which is operable to add polymer to the sludge upstream of the flocculator, a second flow meter (54) which is operable to obtain a third signal (68) which is dependent on the polymer flow rate, and a controller (60), and which is characterised in that the controller (60), responsive to the first, second, and third signals (64; 66; 68), varies in real time the addition of the polymer (50), to the sludge (12) at a rate which is inversely dependent on the sludge flow rate (68).

2. Apparatus according to claim 1 wherein the controller (60), responsive to the first, second and third signals (64; 66; 68), implements in real time one or more of the following:(a) a variation of a rotational speed of the screens (24) in a manner which is dependent on the second signal (66) thereby to vary the pressure of the sludge in the channel (22) and thus the pressure (64) of the sludge at the sludge inlet (18).(b) a variation of a pressure applied to cake (36) at the sludge outlet (20) in a manner which is dependent on the second signal (66) thereby to vary the pressure of the sludge in the channel (22) and thus the pressure (64) of the sludge in the inlet (18), and(c) a variation of the sludge inlet pressure (64) in a manner which is dependent on the second signal (66).

3. A method of operating a filter press (10) which includes a housing (14), a channel (22) inside the housing, rotatable screens (24) which bound the channel (22), a sludge inlet (18) to the channel (22), a sludge outlet (20) from the channel (22), and a polymer (50) which is added to a sludge (12) feed upstream of the sludge inlet (18), wherein the method includes the step of measuring the flow rate (66) of sludge to the press and is characterised by the step that, in response to said flow rate (66) measurement, the addition of the polymer (50) to the sludge (12) is varied in real time at a rate which is inversely dependent on the sludge flow rate (66).

4. The method of claim 3 which includes at least one of the following steps:(a) varying in real time a rotational speed of the screens (24) in a manner which is dependent on the measured flow rate (66) of the sludge, thereby to vary the pressure of the sludge in the channel and thus the pressure (64) of the sludge at the inlet (18),(b) varying in real time the sludge inlet pressure (64) in a manner which is dependent on the measured flow rate (66) of the sludge, and(c) varying in real time a pressure applied to cake (36) at the sludge outlet in a manner which is dependent on the flow rate (66) of the sludge thereby to vary the pressure of the sludge in the channel (22) and thus the pressure (64) of the sludge at the inlet (18).

5. The method of claim 3 which is characterised in that the polymer (50) is added to the sludge (12) in accordance with the following:where D is a polymer dosing rate for the sludge which is the polymer dosing to be implemented at any time in response to the value of B;A is a base sludge flow rate determined under test conditions with a polymer dosing rate C;B is an instantaneous sludge flow rate to the press (10); andC is a base polymer dosing rate determined for the sludge with a known TSS and is the polymer dosing rate when the base sludge flow rate is A.

6. The method of claim 3 which is characterised in that the polymer (50) is added to the sludge (12) in accordance with the following:where D is a polymer dosing rate for the sludge which is the polymer dosing to be implemented at any time in response to the value of B;A is a base sludge flow rate determined under test conditions with a polymer dosing rate C;B is an instantaneous sludge flow rate to the press (10);C is a base polymer dosing rate determined for the sludge with a known TSS and is the polymer dosing rate when the base sludge flow rate is A, andEav is a calculated average of a first E value (Ei) which is determined using Expression III for a sludge with a TSS value which is higher than the anticipated TSS of the sludge at which the press is to operate, and of a second E value (E2) which is determined using Expression III for the sludge with a TSS value which is lower than said anticipated TSS of the sludge, wherein the said TSS values represent a range of TSS values for the sludge which is to be processed by the press, and and7. The method of claim 4 which is characterised in that in step (a) the rotational speed G of the screens is varied in accordance with the following:where A is a base sludge flow rate determined under test conditions with a polymer dosing rateC;B is an instantaneous sludge flow rate to the press (10);C is a base polymer dosing rate determined for the sludge with a known TSS and is the polymer dosing rate when the base sludge flow rate is A.D is a polymer dosing rate for the sludge which is the polymer dosing to be implemented at any time in response to the value of B;F is a base operational speed in rpm of the screens during said test conditions,H is an rpm adjustment limit, such that 0<H<1 , and8. The method of claim 4 which is characterised in that in step (b) the sludge inlet pressureG1is varied in accordance with the following:where A is a base sludge flow rate determined under test conditions with a polymer dosing rateC;B is an instantaneous sludge flow rate to the press (10);C is a base polymer dosing rate determined for the sludge with a known TSS and is the polymer dosing rate when the base sludge flow rate is A.D is a polymer dosing rate for the sludge which is the polymer dosing to be implemented at any time in response to the value of B;F1is a base inlet sludge pressure during said test conditions,H1is a limit of inlet sludge pressure, such that 0<H1<1 , andE is proportional to9. The method of claim 4 which is characterised in that in step (b) the sludge outlet pressure G2is varied in accordance with the following:where A is a base sludge flow rate determined under test conditions with a polymer dosing rate C;B is an instantaneous sludge flow rate to the press (10);C is a base polymer dosing rate determined for the sludge with a known TSS and is the polymer dosing rate when the base sludge flow rate is A.D is a polymer dosing rate for the sludge which is the polymer dosing to be implemented at any time in response to the value of B;F2is the base outlet pressure during said test conditions,H2is an adjustment limit of outlet sludge pressure , such that 0<H2<1 , and E is proportional to