GB1602601A - Solid bowl decanter centrifuges - Google Patents

Description

(54) IMPROVEMENTS IN SOLID BOWL DECANTER CENTRIFUGES (71) We, THOMAS BROADBENT & SONS LIMITED, a British company, of Huddersfield, HD1 3EA, West Yorkshire, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This application relates to solid bowl decanter centrifuges of the scroll discharge type and more specifically to their use in applications entailing sewage sludge dewatering assisted by the addition of polymeric flocculants.

The final dewatering of combined primary, secondary or digested sewage sludges or mixtures of these may be advantageously carried out using solid bowl decanter centrifuges of the scroll discharge type. The process invariably entails conditioning of the sludge by the addition of a polyelectrolyte to promote the aggregation of small primary particles to larger aggregates capable of settling under the action of centrifugal force at a greater rate than the primary particles. One of the major problems associated with the use of flocculants in decanter centrifuges concerns the high local shear forces generated in the zone where the solid/liquid suspension enters the rotating bowl and is accelerated to bowl speed.With some flocculants, if the flocculant is added to the suspension prior to its introduction into the machine, then in the presence of velocity gradients and turbulence in the feed zone any flocs which may have formed will experience shear forces causing them to disrupt. With polymerically bridged flocs this disruption is irreversible.

To overcome this problem the introduction of flocculant directly into the centrifuge bowl pond via a separate feed pipe has been suggested. This effectively overcomes the problem of floc disruption but often mixing of suspension and flocculant and the residence time within the bowl after mixing, is inadequate to promote effective conditioning.

Other methods have been proposed where the feed chamber within the conveyor consists of a conical section to provide gradual acceleration of the feed suspension and flocculant. As a further solution, in an effort to overcome the turbulence associated with the addition of flocculant to the suspension in or prior to the feed zone, special high shear resistant flocculants have been developed. With these the turbulence occuring in the region of the feed zone effectively aids mixing and can be beneficial to the subsequent flocculation process.

In accordance with the present invention there is provided a solid bowl decanter centrifuge of the scroll discharge type comprising a solid bowl which is adapted to be rotated at a first speed, a scroll conveyor which is adapted to be rotated within the bowl at a second, slightly different speed for conveying separated solids to a solids discharge end of the bowl, a first stationary feed pipe for introducing to the interior of the bowl a suspension which is to be centrifuged, the first feed pipe exiting into a feed compartment formed in a hub portion of the scroll conveyor and communicating with the interior of the bowl via one or more apertures in the conveyor hub portion, and a second stationary feed pipe for introducing to said feed compartment a polyelectrolytic additive, said second feed pipe exiting either directly into the feed compartment or into said first feed pipe at or adjacent the discharge end of the latter feed pipe, and the feed compartment, coupling the first and second feed pipes to the bowl interior, being constituted by one or more substantially radially extending passages which receive suspension and polyelectrolytic additive from said feed pipes and expell same into the bowl with substantially zero residence time in the feed compartment.

The polyelectrolytic additive may be introduced into the feed compartment either directly or via said first feed pipe for the suspension.

It has been found that the aforegoing arrangement is advantageous compared with the conical type feed chamber accelerators and other known types previously employed in that a reduction in the quantity of polyelectrolytic flocculant necessary for conditioning a given quantity of liquid/solid suspension is obtained.

Obviously, this is advantageous in the operation of decanter centrifuges in sewage sludge dewatering and can yield a substantial reduction in processing costs.

The method of expressing the amount of flocculant required for conditioning a particular type of sludge is normally expressed in terms of flocculant quantity per unit weight of dry solids processed. The amount of flocculant added is controlled to give a satisfactory effluent discharge from the centrifuge under given conditions of suspension feed rate, centrifuge rotational speed, etc. On this basis a feed compartment in accordance with the invention has been found to yield on average savings in polyelectrolyte consumption of 20% compared with known conventional feed compartment arrangements when assessed under identical operating conditions.

The invention is described further hereinafter, by way of example, with reference to the drawings accompanying the provisional specification in which: Fig. Ia is a longitudinal cross-section through the conveyor hub and feed zone section of a solid bowl decanter centrifuge employing a conventional feed compartment arrangement; Fig. Ib is a cross-section on the line II of Fig. la; Fig. 2a is a view similar to Fig. la but showing an embodiment which employs a known conical accelerating section designed to reduce turbulence in the feed zone; Fig. 2b is a section on the line Il-Il in Fig. 2a; Fig. 3a is a view similar to Fig. 2a but showing an embodiment in accordance with the present invention yielding a reduction in flocculant dosage requirements;; Fig. 3b is a section on the line Ill-Ill in Fig, 3a; and Fig. 4 is a diagram giving best results to illustrate the improvement provided by the present arrangement.

The known arrangement of Figs. la and lb comprises a solid bowl 10 which is rotatable about a horizontal axis by a drive means (not shown). Coaxially mounted within the bowl 10, for rotation at a slightly different speed from the bowl, is a conveyor comprising a tubular hub portion 12 having a helical scroll flight 14 on its outer surface whose tips lie close to but slightly spaced from the inner surface of the bowl 10. The arrangement is such that, when the bowl and conveyor are rotated at high speed with a small differential speed therebetween a solids/liquid pond 16 is formed by centrifugal action, and solids present being scrolled axially along the bowl (in this case to the left as viewed in Fig. la) to a solids discharge port (not shown). The liquid is discharged via a liquid discharge port (not shown) at the opposite end of the bowl.

Suspension is introduced into a feed compartment 18 through a stationary feed pipe 20 coaxially mounted within the tubular hub portion 12 of the conveyor. The suspension in the compartment 18 is accelerated and discharged through one or more ports 22 in the conveyor hub wall 24 into the pool 16 rotating with and within the centrifuge bowl wall 28 where flocculation and subsequent separation under the action of centrifugal force occur. Provision is made for the introduction of flocculant into the feed pipe 20 at a point 30 along its axial length, or alternatively at its discharge end, by a smaller second stationary pipe 32 housed within the main feed pipe 20.

A second known feed compartment arrangement is shown in Fig. 2a where a conical section 34 is incorporated within the conveyor hub section 36 to aid gradual acceleration of the feed suspension and flocculant within the feed chamber 38.

It may be seen that both the arrangements illustrated in Figs. la and 2a are characterised by a feed chamber in which the feed and flocculant mixture is held for a finite time prior to being discharged into the centrifuge bowl. This delay in discharging into the bowl is increased further in the arrangement illustrated in Fig. 2a where the suspension is detained initially in the conical accelerator arrangement.

In the evaluation of alternative feed chamber arrangements it has been found in the case of cationic polyelectrolytes having high shear resistance and normal primary, secondary and digested sewage sludges and mixtures of these that improved performance in terms of flocculant dosage requirement is achieved by arranging for the residence time within the feed chamber to be extremely short, but for the turbulence and shear forces to be high.This condition is satisfied by the preferred arrangement in accordance with the present invention shown in Figs. 3a and 3b wherein a substantially radial passage 40 approximately at right angles to the rotational axis is provided which is arranged to expel the suspension and flocculant mixture into the bowl immediately it leaves a stationary feed pipe 42, unlike the known arrangements, typified by Figs. la and 2a, where the feed and flocculant mixture is deliberately held for a relatively long period in the feed chamber prior to being discharged into the bowl interior. The flocculant is introduced bv means of a second stationary feed pipe 3i which exits either directly into the feed compartment 40 or (as shown in Fig. 3a) into the first feed pipe 42 at or adjacent the discharge end of the latter pipe.

The arrangement shown is for a single flight conveyor in which the two discharge openings 46, 48 communicating with the bowl interior are necessarily staggered axially to permit incorporation within the space between adjacent conveyor flights. In the case of two start flight conveyors the openings 46, 48 may be diametrically opposite one another.

The results of comparison tests between two identicals, solid bowl scroll discharge decanter centrifuges are represented graphically in Fig. 4 where the necessary flocculant dosage to achieve an acceptable effluent, expressed in terms of dry solids processed, is shown against feed suspension throughput. The machines differ solely in the construction of their feed compartments. Test results B apply to a conventional feed compartment arrangement and test results A to the preferred arrangement of the type disclosed. To minimise the effects of variation in processed sludge characteristics, tests were conducted simultaneously on the two machines, the results obtained being denoted by an identical reference numeral.

From Fig. 4 it may be observed that the decanter centrifuge having the preferred feed compartment arrangement required on average 20% less flocculant to yield an acceptable effluent clarity compared with the machine employing a conventional feed compartment arrangement. The improved performance of the new feed compartment disclosed in terms of reduced flocculant dosage is attributed to the rapid acceleration and mixing of the feed suspension and flocculant. Since the time over which the feed suspension and flocculant is subjected to high shear velocities is short, the formation of polymerically bridged flocs during this initial mixing period is reduced, subsequently reducing the disruption of floc bonds.The disruption of polymerically bridged flocs is an irreversible process and its prevention permits an effective improvement in flocculation within the centrifuge bowl yielding a reduction in the dosage required.

WHAT WE CLAIM IS: 1. A solid bowl decanter centrifuge of the scroll discharge type comprising a solid bowl which is adapted to be rotated at a first speed, a scroll conveyor which is adapted to be rotated within the bowl at a second, slightly different speed for conveying separated solids to a solids discharge end of the bowl, a first stationary feed pipe for introducing to the interior of the bowl a suspension which is to be centrifuged, the first feed pipe exiting into a feed compartment formed in a hub portion of the scroll conveyor and communicating with the interior of the bowl via one or more apertures in the conveyor hub portion, and a second stationary feed pipe for introducing to said feed compartment a polyelectrolytic additive, said second feed pipe exiting either directly into the feed compartment or into said first feed pipe at or adjacent the discharge end of the latter feed pipe, and the feed compartment, coupling the first and second feed pipes to the bowl interior, being constituted by one or more substantially radially extending passages which receive suspension and polyelectrolytic additive from said feed pipes and expell same into the bowl with substantially zero residence time in the feed compartment.

2. A centrifuge as claimed in claim 1, in which the or each radially extending passage is formed in the interior of the hub so as to rotate therewith but communicates with the stationary feed pipes such that the passage acts as a transversely directed extension of the feed pipes leading directly to the bowl interior.

3. A centrifuge as claimed in claim 1 or 2, in which the feed compartment is constituted by two said radial passages having respective exit ports communicating with the bowl interior and located at substantially diametrically opposed positions on the conveyor hub.

4. A centrifuge as claimed in claim 3, in which the outlet end of the first feed pipe extends into the feed compartment through an aperture in a wall of the feed compartment which extends substantially perpendicularly to the first feed pipe.

5. A solid bowl decanter centrifuge

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. rotational axis is provided which is arranged to expel the suspension and flocculant mixture into the bowl immediately it leaves a stationary feed pipe 42, unlike the known arrangements, typified by Figs. la and 2a, where the feed and flocculant mixture is deliberately held for a relatively long period in the feed chamber prior to being discharged into the bowl interior. The flocculant is introduced bv means of a second stationary feed pipe 3i which exits either directly into the feed compartment 40 or (as shown in Fig. 3a) into the first feed pipe 42 at or adjacent the discharge end of the latter pipe. The arrangement shown is for a single flight conveyor in which the two discharge openings 46, 48 communicating with the bowl interior are necessarily staggered axially to permit incorporation within the space between adjacent conveyor flights. In the case of two start flight conveyors the openings 46, 48 may be diametrically opposite one another. The results of comparison tests between two identicals, solid bowl scroll discharge decanter centrifuges are represented graphically in Fig. 4 where the necessary flocculant dosage to achieve an acceptable effluent, expressed in terms of dry solids processed, is shown against feed suspension throughput. The machines differ solely in the construction of their feed compartments. Test results B apply to a conventional feed compartment arrangement and test results A to the preferred arrangement of the type disclosed. To minimise the effects of variation in processed sludge characteristics, tests were conducted simultaneously on the two machines, the results obtained being denoted by an identical reference numeral. From Fig. 4 it may be observed that the decanter centrifuge having the preferred feed compartment arrangement required on average 20% less flocculant to yield an acceptable effluent clarity compared with the machine employing a conventional feed compartment arrangement. The improved performance of the new feed compartment disclosed in terms of reduced flocculant dosage is attributed to the rapid acceleration and mixing of the feed suspension and flocculant. Since the time over which the feed suspension and flocculant is subjected to high shear velocities is short, the formation of polymerically bridged flocs during this initial mixing period is reduced, subsequently reducing the disruption of floc bonds.The disruption of polymerically bridged flocs is an irreversible process and its prevention permits an effective improvement in flocculation within the centrifuge bowl yielding a reduction in the dosage required. WHAT WE CLAIM IS:

1. A solid bowl decanter centrifuge of the scroll discharge type comprising a solid bowl which is adapted to be rotated at a first speed, a scroll conveyor which is adapted to be rotated within the bowl at a second, slightly different speed for conveying separated solids to a solids discharge end of the bowl, a first stationary feed pipe for introducing to the interior of the bowl a suspension which is to be centrifuged, the first feed pipe exiting into a feed compartment formed in a hub portion of the scroll conveyor and communicating with the interior of the bowl via one or more apertures in the conveyor hub portion, and a second stationary feed pipe for introducing to said feed compartment a polyelectrolytic additive, said second feed pipe exiting either directly into the feed compartment or into said first feed pipe at or adjacent the discharge end of the latter feed pipe, and the feed compartment, coupling the first and second feed pipes to the bowl interior, being constituted by one or more substantially radially extending passages which receive suspension and polyelectrolytic additive from said feed pipes and expell same into the bowl with substantially zero residence time in the feed compartment.

2. A centrifuge as claimed in claim 1, in which the or each radially extending passage is formed in the interior of the hub so as to rotate therewith but communicates with the stationary feed pipes such that the passage acts as a transversely directed extension of the feed pipes leading directly to the bowl interior.

3. A centrifuge as claimed in claim 1 or 2, in which the feed compartment is constituted by two said radial passages having respective exit ports communicating with the bowl interior and located at substantially diametrically opposed positions on the conveyor hub.

4. A centrifuge as claimed in claim 3, in which the outlet end of the first feed pipe extends into the feed compartment through an aperture in a wall of the feed compartment which extends substantially perpendicularly to the first feed pipe.

5. A solid bowl decanter centrifuge

substantially as hereinbefore particularly described with reference to and as illustrated in Figs. 3a, 3b and 4 of the drawings accompanying the Provisional Specification.