Sludge dewatering process assisted by flocculating reactant and plant for the implementation of such a process

Abstract

Sludge dewatering process assisted by flocculating reactant, said process comprising an injection of flocculating reactant into the sludge and a step of dewatering said sludge, characterized in that it comprises a preliminary step that consists in mixing said sludge in a mixer (4) comprising a cylindrical chamber (4a) equipped with blades (4c) rotatably mounted on a shaft (4b) rotating at a speed of rotation of between 500 rpm and 4000 rpm, so as to destructure the sludge and reduce the viscosity thereof, and in discharging the sludge from said mixer (4) via a network (11) to said dewatering step, and in that it comprises a step of depressurizing said mixer (4) and said network giving rise to the lysis, by cavitation, of said sludge, said depressurizing step being carried out over a period of at least 0.1 second. Corresponding plant.

Claims

1. A method of dewatering sludge with assistance of a flocculating reagent and increasing efficiency of the flocculating reagent, the method comprising: injecting the flocculating reagent into the sludge; directing the sludge into a mixer having a cylindrical chamber provided with rotating blades; destructuring the sludge and reducing the viscosity of sludge in the mixer by rotating the blades at a speed of 500-4,000 revolutions per minute; discharging the destructured sludge from the mixer into a sludge transportation network that directs the sludge from the mixer; lysising the sludge in the mixer and in the network by depressurising said mixer and network for at least 0.1 seconds which results in the lysising of the sludge in the mixer and the network by cavitation; after lysising the sludge, directing the sludge to a dewatering unit and dewatering the sludge; and wherein the flocculating reagent is mixed with the sludge in the mixer or at a point downstream of the mixer.

2. The method of claim 1 wherein there is a valve disposed upstream of the mixer and a pump disposed downstream of the mixer, and wherein the method includes causing cavitation of the sludge in the mixer and in the network by partially closing the valve disposed upstream of the mixer and operating the pump.

3. The method of claim 2 wherein a second valve is disposed downstream of the pump, and wherein, after a cavitation of the pump, the second valve returns the pump to a pump performance curve.

4. The method of claim 1 wherein the dewatering unit includes a centrifuge including a spout; and wherein the method includes injecting the flocculating reagent into the spout of the centrifuge where the flocculation reagent is mixed with the sludge.

5. The method of claim 1 wherein depressurising the mixer and the network comprises reducing the pressure in the mixer and network to approximately 0.1-0.3 bar.

6. The method of claim 1 wherein the flocculating reagent is mixed with the sludge in a collector located upstream of the mixer or injected into the mixer.

7. The method of claim 1 wherein the sludge, before treatment, includes a sludge dryness of less than 15 wt. % and wherein the method produces a dewatered sludge having a sludge dryness of 32 wt. % or greater.

8. The method of claim 1 wherein depressurising the mixer and the network causes the release of bound water from the sludge and furthermore depressurising the mixer and the network reduces the size of particles contained in the sludge.

9. The method of claim 1 wherein depressurising the mixer and the network has a duration of between 0.1 seconds and 30 seconds.

10. The method of claim 1 wherein depressurising the mixer and network gives rise to cavitation that in turn mechanically lysises the sludge.

11. The method of claim 1 wherein before destructuring the sludge, the method includes preheating the sludge by injecting hot water or steam into the sludge.

12. The method of claim 1 wherein prior to lysising the sludge, injecting dilution water into the sludge or oxygenating the sludge.

13. A system for dewatering sludge that employs a flocculating reagent to assist in dewatering the sludge, the system comprising: a mixer comprising a cylindrical chamber having rotating blades and configured to receive the sludge, destructure the sludge, reduce the viscosity of the sludge and to mix the sludge; a flocculating reagent line configured to inject a flocculating reagent into the sludge at or upstream of the mixer or downstream of the mixer; a dewatering unit disposed downstream of the mixer and configured to dewater the sludge; a transport network operatively interconnected between the mixer and the dewatering unit for directing sludge from the mixer to the dewatering unit; and means upstream of the dewatering unit for depressurising said mixer and network and lysising the sludge in the mixer and network by cavitation.

14. The system of claim 13 wherein the means for depressurising the mixer and network comprises a pump located in the network between the mixer and the dewatering unit and a valve located upstream of the mixer.

15. The system of claim 14 wherein there is provided a second valve in the network disposed between the pump and the dewatering unit.

16. The system of claim 13 wherein the dewatering unit is a centrifuge having a spout, and wherein the flocculation reagent line extends from a flocculation reagent source to the spout of the centrifuge and wherein the centrifuge and the flocculation reagent line are configured to mix the flocculation reagent with the sludge in the spout.

17. The system of claim 13 including a collector disposed upstream of the mixer; and wherein the flocculation reagent line extends from a flocculation reagent source to the collector and wherein the collector and flocculation reagent line are configured to mix the flocculation reagent with the sludge in the collector.

18. The system of claim 13 wherein the flocculation reagent line is operatively connected between a flocculation source and the mixer and wherein the flocculation reagent line and the mixer are configured to mix the flocculation reagent with the sludge in the mixer.

19. A method of dewatering sludge comprising: directing the sludge to a sludge collector; directing the sludge from the sludge collector through a first control valve and into a mixer having a cylindrical chamber provided with rotating blades; directing a flocculation reagent into the sludge collector or into the mixer; mixing the flocculation reagent with the sludge in the mixer; destructuring the sludge and reducing the viscosity of the sludge in the mixer by rotating the blades in the cylindrical chamber at a speed of 500-1,000 revolutions per minute; after destructuring the sludge, transferring the sludge from the mixer to a transport network operatively interconnected between the mixer and a dewatering unit and wherein the transport network includes a pump; with the assistance of the pump, pumping the sludge from the mixer through the network and to the dewatering unit; lysising the sludge in the mixer and in at least a portion of the network by depressurising the mixer and at least a portion of the network for at least 0.1 seconds which results in lysising the sludge by cavitation; and after lysising the sludge, dewatering the sludge in the dewatering unit.

20. The method of claim 19 wherein depressurising the mixer and at least a portion of the network includes partially closing said first valve and causing cavitation of the sludge in the mixer and in at least a portion of the network.

21. The method of claim 20 wherein there is a second control valve downstream of the pump and wherein the second control valve creates a loss of load downstream of the pump which, after the depressurising step, returns the pump to a non-cavitation state of operation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention, in addition to the different advantages thereof, will be more easily understood after reading the following description of an embodiment of the invention, given for illustrative purposes and in no way limiting the scope of the invention, with reference to the figures, in which:

(2) FIG. 1 is a diagrammatic representation of a facility according to this invention;

(3) FIG. 2 is a graph showing the levels of flocculating reagent (polymer) consumption during the implementation of the facility according to FIG. 1 using the method according to the invention on the one hand, and a conventional method of the prior art on the other hand.

DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION FACILITY

(4) With reference to FIG. 1, the facility comprises a sludge dewatering equipment item 1 constituted by a centrifuge. Said centrifuge is connected to sludge supply means 2 and to polymer injection means 3.

(5) In accordance with this invention, the facility further comprises a mixer 4 provided upstream of said dewatering equipment, provided with water supply means 6 and, where necessary, means for injecting ferric chloride 6a in the event of chemical conditioning of the sludge. The optional addition of ferric chloride is used to reduce the colloidal stability of the sludge.

(6) The sludge supply means 2, the polymer injection means 3, the water supply means 6 and the optional ferric chloride injection means 6a are connected by pipes, respectively 12, 13, 16 to a collector 7. Valves 22, 23, 26 enable the distribution therein of the sludge, polymer, and water respectively, potentially mixed with ferric chloride.

(7) The sludge supply means 2, the polymer injection means 3 and the water supply means 6 are connected by pipes, respectively 32, 33, 36 to the centrifuge 1. Valves 42, 43, 46 enable the distribution of the sludge, polymer and water respectively, directly to the spout thereof.

(8) The pipes 16 and 36 for conveying water respectively to a mixing tank 7 and to the centrifuge are each equipped with a common flow meter 56.

(9) In accordance with this invention, the mixer 4 comprises a cylindrical chamber 4a equipped with a rotating shaft 4b on which blades 4c are mounted. The rotating shaft is moved by a motor (not shown in FIG. 1) which enables the blades to be driven at a high rotational speed in the range 500 revolutions per minute to 4000 revolutions per minute.

(10) The mixer 4 receives the sludge mixed with polymer, and where relevant with ferric chloride, and where relevant with water originating from the mixing tank 7, via a common pipe equipped with a valve 10. The mixed, lysed sludge is conveyed towards the centrifuge by a pipe 11 equipped with a pump 12 and a valve 13.

(11) The facility described herein enables the sludge, water and polymer to be conveyed to the collector 7 and/or directly to the centrifuge 1.

METHOD

(12) The facility shown in FIG. 1 was implemented to dewater mixed sludge, digested according to the prior art on the one hand and according to the invention on the other hand. Said sludge had an initial dryness of 28%.

(13) Within the scope of these experiments, the centrifuge was always used at maximum capacity (2000 G).

(14) In a first experimental phase, the valves 22, 23, 26, 46 were closed and only the valves 42 and 43 were open so as to direct the sludge and the polymer originating from the supply means 2 and 3 of said components directly to the spout of the centrifuge 1, without travelling through the mixer, according to the prior art.

(15) In a second experimental phase, according to the invention, the valves 23, 26, 46 were kept closed. The valve 22 was opened to allow the distribution of the sludge in the mixer 4 via the tank 7 and the valve 42 was closed. The valve 43 was kept open to continue to convey the polymer to the spout of the centrifuge 1.

(16) In a third experimental phase, the valves 26 and 46 were kept closed. The valve 22 was kept open, the valve 43 was closed and the valve 23 was opened to allow, according to the invention, the conveyance of the sludge and polymer to the mixer 4.

(17) During said third experimental phase, the mixture originating from the collector 7 was pumped using the pump 12 in the mixer 4 and the valve 10 was partially closed, so as to cause a cavitation of said mixture by the depressurisation of the chamber of the mixer 4 in addition to the network between the valve 10 and the pump 12 for 1 to 5 seconds. In practice, the pressure in said chamber and in the network is lowered to 0.1 to 0.3 bar below atmospheric pressure.

(18) The depressurisation of the network between the closed valve 10 and the pump 12 causes the cavitation of the pump 12 resulting in the operation thereof outside of the pump performance curve thereof. The valve 13 creates a loss of load downstream of the pump 12 so as to return the pump 12 to a pump performance curve (the TDH of the pump is corrected) and such that the latter is always under load and does not become bound.

(19) During each of said three experimental phases, the polymer was used at three different doses, namely 5 kg/TDM (tonnes of dry matter), 7.5 kg/TDM and 11 kg/TDM.

(20) The mixer was used for the second and third experimental phases with a blade speed of 2000 revolutions per minute, enabling the sludge to be de-structured before being conveyed to the centrifuge 1.

(21) Since the sludge did not need it, no ferric chloride was added.

(22) The dryness results for the sludge at the output of the centrifuge 1 are summarised in the graph shown in FIG. 2.

(23) Said results show that, with the same polymer dose, it is possible, by means of the invention, to obtain a sludge dryness that is far better with the invention, in particular when the polymer is injected into the collector provided upstream of the dynamic mixer.

(24) Thus, for a polymer dose of 11.3 kilograms per tonne of dry matter (TDM), by means of the invention, a sludge dryness of 32% was obtained, and a sludge dryness exceeding 33% was obtained by injecting the polymer upstream of the dynamic mixer, whereas the dryness obtained according to the prior art was only 28.5%. These results were produced without the addition of ferric chloride and compressed air, because this was not needed by the sludge. A comparable dryness of 29% was obtained by implementing the polymer at a rate of only 5 kg/TDM, resulting in savings of nearly 50% in the quantity of polymer.