The invention relates to the production of filter cakes and the like and more particularly to a convenient and effective method of producing filter cakes of substantially predetermined dry solids content.

The invention is seen to good effect in the production of filter cakes from sewage sludge, which cakes are burned in an incinerator. It has been observed that if the proportion of dry solids content varies, the rate of incineration is adversely affected and there is a need for a method of producing filter cakes of a predetermined dry solids content so that this disadvantage is avoided.

It is known to form filter cakes by mixing a sewage slurry with a flocculating or other bonding agent and pumping the mixture at pressure into a filter press to remove the water leaving a filter cake. It is an advantage if the amount of flocculating agent added can be minimised while also producing a cake having a fixed dry solids content and in a fixed time.

It is known to measure the rate of solid mass being transferred into the press as a product of the density and flow rate of sewage sludge entering the press. This measurement is then used to directly control the rate of addition of a flocculating agent. The flocculating agent is added in direct proportion to the rate of solid mass transferred into the press. It has been observed that with this method, the formed filter cake is of variable composition.

It is one object of the invention to provide a method of producing a filter cake, which has a predetermined substantially constant proportion of dry solid matter irrespective of the nature of the supplied material. It is another object to provide a method in which a substantially uniform filter cake is produced in a substantially fixed time period and which needs substantially no intervention by an expert operator.

According to one aspect of the invention there is provided a method of producing filter cake or the like, the method comprising the steps of supplying a suspension of solids in a carrier liquid to a pressure compartment having a perforate wall, adding an agent, eg, a bonding or flocculation agent from a reservoir therecf, applying

pressure to urge liquid out of the compartment through the perforate wall, the bonding agent serving to bond the solids, measuring the physical properties of the suspension and adjusting the rate of addition of the agent according to the measured data, characterised in that the rate of addition of the agent is adjusted to cause the production of a filter cake having a substantially predetermined proportion of dry solids.

Preferably, predetermined reference values indicative of the amount of bonding agent needed to produce a filter cake of substantially fixed dry solids content in a fixed press cycle time are compared to values derived from measurements, and the rate of addition of the bonding agent is adjusted according to any difference between the measured and reference values.

Preferably, the flow rate and density of the suspension are measured and a figure for the mass of solids transferred is derived from these measurements and this figure is multiplied by the flow rate to provide a value indicative of the specific resistance to filtration of the suspension. This value is compared with a predetermined reference value and the rate of addition of bonding agent is adjusted accordinσ to the difference between the two values .

In order that the invention may be well understood it will be described by way of example with reference to the accompanying diagramatic drawings which show schematically one apparatus of the invention.

The apparatus comprises a two part press 1 having a front and back portions 2 and 3 defining a chamber 10 to form a filter cake. The chamber houses a filter cloth 11 and a porous wall leading to an outlet 12, through which filtrate can escape. An inlet pipe 4 conveys a suspension of solids, eg, a sewage slurry to the press and adjacent to the press there is an inlet 5 in the pipe 4 for the supply of a flocculating agent such as polyacrylamide or the like from a reservoir 6 thereof. A pump 13 supplies the slurry through the pipe 4 at pressure to the press.

A microwave Doppler effect device 7 to determine the flow rate of the slurry is located on opposite sides of the pipe 4 before the inlet 5. A density determining device S is located between the flow rate measurement device 7 and the inlet pipe 5. The density determining device 8 comprises a radio emitter and receiver on opposite sides of the pipe. Information form these two devices is supplied to an electronic control unit 9 which in turn controls the rate of supply of the flocculating agent from its reservoir 6 and is adjusted according to the sensed information.

We have found that it is possible to mathematically model the rate of formation of the filter cake in the press upon specific resistance theory. It can be shown that to achieve a standard cake quality in a fixed pressing time with sludges having differing dry solids contents, the specific resistance to filtration must be changed. The specific resistance is related to the concentration of the bonding agent, so that by altering the concentration, the resistance can be altered. In this way, by measuring various physical values of the sludge and comparing them with theoretically or experimentally derived predetermined values, it is possible to determine instantaneously how much flocculating agent needs to be added to produce a filter cake of a predetermined dry solids content in a fixed press time and without the need for an expert operator.

We have found that after the press has been initially filled and the pressure has reached the operating pressure, filtration data showed a correlation to specific resistance theory, as shown below.

dv •= 2PA' dt Urc (Vt + Vo)

Where t = Time

Vt = Filtrate Volume at time t

Vo = Volume after press has just been filled r = Specific resistance c = Concentration of sludge

P = Pressure

A = Area of filter

U = Filtrate viscosity

It can be shown that at a constant pressure

dv = Z

₉ dt Mt where Z = 2PA = constant, and

U dv is equivalent dt Mt = Mass of dry solids to the flow rate at a time t. of the sludge into the press.

Therefore, for example, a value for Z is determined either theoretically or experimentally for a certain pressure. The value for Z is indicative of the filterability of the suspension and is inversely proportional to the specific resistance of the sludge to filtration. The electronic control unit 9, comprising, eg, a computer, calculates a

value for Mt from the measured density and flow rate and then multiplies together Mt and the measured flow rate to give a value for Z. This is then compared with the predetermined desired value Z and the rate of addition of flocculating agent is then adjusted until the values are equal.