The dewatering of sludges using a tubular filter press

467

Desalinntion, 67 (1987) 467-479 Elsevier Science Publishers B.V., Amsterdam

THE DNATERING

K.

OF SLUDGES

TREFFRY-GOATLEYl,

-

Printed

USING A TUBULAR

M.I.

BUCHAN2,

in The Netherlands

FILTER PRESS

G.E.

RENCKEN2,

W.J.

VORTMAN'

and

C.A.

BUCKLEY1

IPollution Research Group, Department of Natal, King George V Avenue, Durban, 4001, 2 Umgeni Africa

Water

Board,

P.O.

Box

9,

Chemical Republic

Engineering, University of South Africa

Pietermaritzburg,

3200,

Republic

of

of

South

SUNNARY A tubular filter press process using fabric tubes has been developed to dewater the sludge resulting from the clarification of raw water. to dewater the sludge resulting from the A prototype plant, designed by a water treatment of 30 Ml/day of surface water, has been constructed authority. The slurry is fed under pressure into an array of porous tubes, the liquid in the slurry filters through the tube walls while the slurry solids are deposited as a thin layer on the internal walls of the tubes. The cake is dislodged periodically from the tube walls and transported hydraulically out of the tubes where it is drained and conveyed to a collection hopper. The raw dam water associated with the prototype unit is clarified using a The resultant sludge has a mean solids polyelectrolyte/bentonite mixture. concentration of 23 g/l. The prototype tubular filter press has produced on average a cake with a solids concentration of 32% mass/mass and a filtrate with a suspended solids of 57 mg/l. The process operates with minimum supervision and no chemical addition. The solids dewatering capacity for a feed solids concentration of 23 g/l and operating at a pressure of 400 kPa is 1.5 kg dry The performance of the unit can be described using standard solids/m2h. filtration theory modified for a circular filtration surface. The performance to date indicates that this process compares favourably with other commercially available sludge dewatering methods. INTRODUCTION The

treatment

usually and

involves

filtration.

to the 30

of

plant.

g/l,

relatively volume

on

on

tubes

OOll-9164/87/$03.50

the

the

sludge

chemicals Apart

usable

use

of

prior

to desalination stage

can contain

concentration

concentration

of potentially

Studies ceramic

solids

equipment.

low

either

and flocculation

The resultant The

depending

clarification

raw water

a coagulation

for

sludge

stream,

stream

clarification

from the disposal the

for direct

use

by clarification

up to 10% of the feed water

in the sludge

used

or

followed

is between and

the

problem associated flow

contains

a

3 and

type

of

with the

significant

water. the

cross-flow

(ref. 1) and porous

plastic

microfilter tubes

0 1987 Elsevier Science Publishers

process,

usiny

porous

(ref. 2), for the concentrating

B.V.

468 of

inorganic

woven

slurries

fabric

have

hoses as

been

filter

range of water works sludges

a

high

slurry

velocity

filters

through

is maximised

concept

In

tubes,

processes,

are

very

much

that

filtration

thinner

investigate designed

cake

the

porous

and

tube.

is maintained

press process,

4).

are

velocity

for maximum

cake

press

The liquid

in order

to

plates,

3 mm)

as

in

tubular

than other

correspondingly

described is formed

required

the

zero.

The process

production.

the cake as

Secondly,

the rate of cake build-up

to about

process

Firstly,

(typically

was

to treat water.

with

of

higher

above inside

other

filter

filter

resulting

filter

press

press

has

two

the self press

produces

a

processes

so

in a reduction

of

for filtration.

unit

polyelectrolyte

filter

filter

constructed

the full-scale

surface

is pumped under pressure

bore

A high feed velocity

required.

rates

prototype

the

down the length of the tube as the slurry

filter

(ref.

hence

not

the area required

sludge

tubular

features

supporting

the slurry

into

the cross-flow

and operated

the

cross-flow

rate.

by reducing

outstanding

was

m/s)

in the tubular

is then designed

A

of a cake layer on the inner tube surface and to maximise

filtration

In contrast,

with of a

process, 3

the tube wall.

the build-up

the liquid

of

to

solids are concentrated

minimise

A

(1.5

microfilter,

has been used for the concentrating

(ref. 3).

In the slurry concentrating at

reported.

support,

the waste

The

at

operability

water

(with the occasional and

water

treatment

sludge resulting

surface

a concentration

a

and economics

in order

to

The unit

from the treatment

of 30 Ml/d

treatment

addition

plant

of the process.

comprised

of

coagulation

bentonite)

with

and resulted

flow rate of approximately

a

in a

23 g/l and 50 kl/d

respectively. The trials

unit

was

constructed

took place between

the plant has operated future

when

extended

the

between

November

June

and

with full automation

sludge

storage

October

1986 and January

facility

during

have

to 24 h/d, with a second tube array

been

1986,

1987.

and commissioning

Since February

the day shift. expanded,

operation

being commissioned

1987

In the near

towards

will be the end

of 1987.

TUBULAR

FILTER

The process consists collection cake

of

PRESS DESCRIPTION schematic

a tray

removal

feed and

diagram

tank, sump,

a

is given in Fig. 1.

pressure

a tube

and tube cleaning

pump,

flushing

system.

the

The tubular

filter

and cleaning

tube array,

filter press a filtrate

pump and an automatic

469

Reject] RecyclelValve

Feed Slurry

M

I

I Reject

1

Perforated Conveyor Pressure Pump Filtrate

Tray FlusI, Pump

Fig. 1. Tubular

The

filter

press

tube

array

is

an

inlet

and

between polyester

yarn

mm and length

and

valve

filtrate as a cake increases

are

a

up of

a number

reject

thus

permeates layer

on the

Tube

the

pressure

walls

with time and is monitored

of

tubes

connected

tubes

diameter

are

can

in

parallel

constructed

range

into the tube array with the reject in and

the

the

tube

the slurry

tube (Fig. 2).

by measuring

array solids

increases

The cake thickness

the filtration

rate.

Cake Discharge Valve Closed

tubular

filter press tube showing

deposition

the

are deposited

RejectlRecycleiValveIClosed

Fig. 2. Single

from

from 25 to 50

on the application.

the tube walls

inner

filter The

under pressure

As

through

of

manifold.

flexible.

is pumped

closed.

of Works

schematic

from 10 to 20 m, depending

The feed slurry recycle

made

process

Filtrate Sump

of cake layer

470 When the desired

cake thickness

by

the

action

cleaning

is

cleaning

head

moving

point

The resultant

(2 to 3 mm).

is sufficient The cake

the rollers. collected

system

the

is dislodged

the outlet

has

on

a

also

perforated

been

fabric whenever

to dislodge

is conveyed

along

the

tubes

conveyor

installed

from

the

out of the tubes

on

the

The

3).

and turbulence

tube

surface

at

ahead

of

in the bulk fluid flow and

A

belt. cleaning

(Fig.

create a restriction

high flow velocity

the cake

valve is opened

from the inner tube surface

head houses a pair of rollers which when engaged,

in the tubes this

of

has been achieved

The cake

and the flush pump started.

high

head

pressure

water

for cleaning

the

spray filter

necessary.

u I

Fig. 3. Rollers creating

a restriction

tube and dislodging

The

process

stage

operation

(typically

slurry under

is pumped gravity

trailer and

prior

requires

components

to the to

15

a cake

under

level

to

60

removal control

a minimum

The

minutes

filter

in Table

1.

consisting depending

(typically

into the

entire

of operator

cycle

stage

inlet of the water

dumping.

is provided

each

is cyclic,

lasting and

concentration),

in the tubular

press

the cake

works

feed and

process

attention.

of a cake on

3 to tank,

the

formation

feed

sludge

5 minutes).

Feed

filtrate

the cake

is deposited

is micro-processor A description

is returned in a

controlled

of the prototype

471

TABLE 1 Prototype description

Component

Prototype

description

Size

component

type

Filter tube array

Number of parallel

20

tubes

mm

25

>

15.7 10

Capacity

m3lh

5 to 20

Pressure

kPa

1 000

Capacity

m3/h

70

kPa

100

Tube diameter Tube

length

Filtration

Pressure

pump

Flush pump

area installed

Pressure Frequency

Sway pump

of operation

Every cycle

Capacity

m3/h

8.5

Pressure

MPa

6.0

Frequency

of operation

Cake dislodging

Diameter

rollers

Number of rollers

Weekly

mm

Restriction

12 2

between

rollers

mm

2 to 3

THEORY Filtration theory and performance predictions The

filtration

described

(dt/dV)

rate

through

by the Carman-Kozeny

=

V = filtrate

(~GW

V)/(A2

volume,

layer

of

incompressible

filter

(1)

A = cross sectional

m3

area of filter

cake, m2 viscosity,

w = slurry solids cont.,

Pa s kg/m3

cake

(ref. 5)

P)

t = time, s p = filtrate

a

equation

P = filtration d = specific

pressure,

kPa

cake resistance,

m/kg.

is

472 For

a

plane

resistance

filter

plot of dt/dV against The

=

will

the

equation

If

filter

increase

V will yield

Carman-Kozeny

cake deposited.

M

support

to filtration

cake

a straight

may

thickness

proportionally

and

therefore

with filtrate

the

volume.

A

line.

be written

in

terms

of the mass of

M = mass of filter cake solids dry, then

filter

:-

WV

(2)

so (dt/dV)

At

=

(PaM)/(A2

any

time

P)

the

(3)

inverse

filtration

total mass of filter cake solids The time required

rate

deposited

is directly

proportional

to

the

by integrating

eqn

on the filter support.

to filter a volume of filtrate

is given

(1).

t

=

( N aw

=

(wM

V*)/(2 A2 P)

(4)

or

t

V)/(Z A2 P)

Therefore

the

proportional

A

A

scale

plot

curve. to

filter

cake

given

cake

mass

is

directly

when

of

inverse

unit

with

a tube

filtration

was

as

filtration

the

nature cake

is thin

of the

length

theory

situated

increases

by equation

(dt/dV) straight

filter

i.e. at the

up, the decrease

predicted

rate

The plot is initially

the cake

builds

the conditions

press

filter press process

at

the

from the same source as the prototype

The curved

decrease

negligible

a

to the tubular

filter

laboratory

(V) is shown in Fig. 4.

area

deposit

theory

tubular

The

with sludge

typical

upward

to

of 25 mm, was used to relate

operation.

supplied

taken

of filtration

laboratory diameter

press

time

to the cake mass and the filtrate volume.

The application

tube

(5)

in

in filtration

1.3 m and a

water

filter

works

and

unit.

against

filtrate

but thereafter

surface

causes

thickness.

beginning

of

to tubular

shows an

the filtration This

effect

of the cycle.

area causes

(3) as seen in Fig. 4.

volume

is

As the

a deviation

from

473

6x105 Feed Feed Tube Tube

Cont. 19.7 g/l Pressure 300 kPa Diameter 0.025m Length 1.3m

L 2

4

6

8

’

FILTRATE VOLUME (I)

Fig. 4. Plot of inverse

filtration

for the laboratory

The

specific

laboratory cake

cake

scale

is compressible

A

mass

resistance

unit.

balance

Results

calculation

(Table

during

the cake deposition

filtrate

solids

which

reslurried tank

is

during

have

indicates

measured presented

negligible. not

yet

the cleaning

at a range of pressures

using

data

from

approximately

The formed

30% a cake

process.

process,

and

of

the 70%

laboratory of

solids

These results

that the

0.89).

as cake.

and

using the

in Table 2 and indicate

coefficient

phase are recovered

during the cleaning

concentration.

was are

that

filtrate volume

filter press

(compressiblity

press

the

3)

rate against

tubular

of

not cake

solids

tubular entering

tube

The loss of solids recovered solids

increase

consists

which

solids are recycled in an

filter a

have

in of

been

to the feed

in the feed tank

414 TABLE

2

The effect of operating pressure on the specific cake resistance and cake moisture content

Pressure

Cake solids concentration

Specific

$ mass/mass

kPa

33.0

200

cake resistance m/kg

0.75 x 1013

400

34.9

1.35 x 1013

600

35.2

1.95 x 1013

TABLE

3

Laboratory tubular filter press mass balances

Batch

Feed to tube

Cake collected

Fraction solids

1

Volume

1

Solids

cont.

g/l

Solids

mass

8.13

11.13

12.56

19.96

14.98

g

202.60

162.30

166.70

459.00

339.00

335.00

0.35

0.35

0.35

160.65

118.65

117.25

0.79

0.73

0.70

Wet mass

g

Solids cont.

gfg

Dry mass

g

of feed

recovered

CONTROL

During

cake

controlled.

deposition

(3)

the thickness

An excessive

plugs which are difficult Eqn

3

16.13

as cake

PROCESS

2

shows

that

build-up

of

the cake

of cake,

results

in the tube array must in the formation

to remove with the automatic total

mass

of

cake

deposited

cleaning can

be

be

of solid

head. monitored

and

475 controlled or

by observing

by observing

control

the

methods

the

filtration

feed pressure

assume

rate

(for constant

(for constant

a constant

specific

pressure

operation),

rate operation).

cake resistance

Both these

and are independent

of the feed slurry concentration. The

prototype

the

level

When

unit

The

conditions.

in the

the level

was

designed

filtration

rate

feed tank to drop

reaches

the low

to

be

operated

is monitored

exceeds

a

The This

preset

level

success

of

performed

over

that

proposed

control

the

resistance

TABLE

the

sensor,

indicates

method

has been

indicate

cake

it

and the cake removal cycle

assumption

tests

value,

the

a

3

to

and

week

specific

method

that

the

are

of

given

resistance

successful.

can be accommodated

pressure

is

time for emptying

cake

mass

has

been

initiated. specific

cake

6 specific in

Table

shifts

resistance.

cake resistance

4.

sufficiently

Seasonal

by changing

for

(Fig. 1).

pump is started and the

required

a consistent

the results

period

cake be

feed

If the elapsed

is automatically

requires

tested

constant

the time taken

from a high to a low level sensor

feed tank is filled to the top level sensor.

deposited

under

by measuring

The

stable in

results for

the

the

specific

the timer set point.

4

Specific

cake resistance

measured

Feed slurry concentration

over a 3 week operating

Specific

kg/d

period

cake resistance m/kg

14.85

8.87 x 1012

15.63

11.00 x 1012

17.90

9.05 x 1012

23.40

10.50 x 1012

12.56

9.25 x 1012

23.76

9.25 x 1012

Mean

9.65 x 1012

---_ RESULTS The

OF PROTOTYPE average

operation

are

the

and

cake

concentration

OPERATION

water

works

provided filtrate from

sludge

in Table are

the water

characteristics

5.

Analyses

provided works

in

sludge

over

the 5 month

period

of

of the feed to the filter tubes,

Table to the

6. feed

The

increase

in

solids

to the filter tubes,

is

476 caused by the recycling of thickened sludge during cleaning. conditions

and process performance is provided in Table 7.

three consecutive cake

deposited

tendency

The operating The results of

runs are provided in Table 8 and indicate that the mass of

may

be assessed by the filtration rate and that there is a

for thin cakes to reslurry.

This is indicated by the lower cake

recovery fraction recorded at a cake thickness of 2.1 mm TABLE 5 Water works sludge characteristics

Suspended Solids

(g/l)

23

Ash

(% m/m)

86

less than 10.1 micron

(X m/m)

68

less than 4.8 micron

(X m/m)

24

Particle size

TABLE 6 Solids concentration of tubular filter press process streams

Tube feed susp. solids

(g/l)

Cake solids

(% m/m)

Cake density

(kg dry solids/n? cake)

Filtrate susp. solids

(mg/T)

32 32 400 57

TABLE 7 Prototype tubular filter press operating conditions

Feed pressure Feed suspended Initial

solids concentration

filtrate

Final filtrate

flux

flux

Cake mass loading Dewatering Total

400.00

(lldh)

300.00

32.00

50.00

(l/dh)

(kg/d)

(per cycle dry solids)

capacity

0.76

(kg/dh)

(dry solids)

1.50

(min)

cycle time

Cleaning

(kPa) (9/l 1

cycle time

31.00

(min)

Run time

4.50 83.00

(%)

TABLE 8 The effect of filtration rate on mass of cake deposited

Cycles

1

Pressure

3

(kPa) (9/l 1

400.00

400.00

Feed concentration

51.40

36.40

26.30

Total

(d)

440.00

385.00

455.00

(l/dh)

39.00

61.00

70.00

(kg) (mm)

50.00

33.00

24.00

Cake thickness1

4.00

2.50

2.10

Cake solids concentration

(% mass/mass)

31.00

30.00

30.00

Cake mass dry

(kg)

15.50

9.90

7.20

0.69

0.70

0.60

filtrate volume

Filtration

rate end cycle

Cake mass wet

Cake recovery

1

2

Estimated

Eqn

(5)

directly

from cake density

predicts

that

the

to

the

proportional

concentration filtration

fraction

400.00

changes,

the

time will change

time

taken

volume

volume

to

deposit

filtered.

required

to

a

given

Therefore, deposit

the

cake if cake,

mass the and

is feed the

proportionally.

This effect was demonstrated

on the prototype

unit.

Results are presented

478 in

Table

9 and

show

that

the overall

filtration

rate

was

independent

of

feed

solids concentration.

TABLE 9 The effect of feed solids concentration on filtration rate

2

1

Cycle

3

Pressure

(kPa)

400.0

400.0

400.0

Feed concentration

(g/l)

14.5

25.0

31.0

Filtration

rate end cycle

(l/m2h)

92.0

92.0

92.0

Filtration

time

(mins)

23.0

14.0

10.0

828.0

480.0

350.0

36.0

34.0

35.0

Volume

filtered

Overall

(1)

filtration

rate

(l/min)

The results also show that as the feed concentration taken

to

reach

production

a

given

therefore

rate

concentration maintained

filtration

hence

the

rate

(cake

increases

feed

as high as possible

solids

to maximise

the time

decreases.

proportionally concentration

in order

was increased

mass)

with

to

the

The feed

unit

cake

solids

should

be

cake production.

CONCLUSIONS The

construction

demonstrated of

filter

and

operation

the technical

cake

can

of

feasibility

be consistently

The rates

capital and

automated

the

of

the

unsupported

The performance

tests.

tubular

without

can enable

filter

press

A high production

the addition

the capacity

has rate

of chemicals.

of a water works

by up to 10%. cost

plant required

and performance

prototype

produced

The high water recovery of the process to be increased

the

of the process.

device

is low as a result

nature

of

minimal

of the device

operator follows

the

filtration

of the

high

surfaces.

filtration The

fully

supervision. standard

of large units can therefore

filtration

be predicted

theory,

the design

from laboratory

scale

479 ACKNOWLEDGEHENTS The Water

investigation Research

was

Cross-Flow

Microfiltration

Waters

Waste

was

and

carried

Research Water

entitled and

Waters".

out jointly

Group.

undertaken

Commission

in terms I"The

Technical

The design by the staff

The finance

of a research

Development Performance

of

Evaluation

and construction of

Umgeni

for the prototype

Water

contract

Support

of the Board

from the

Systems on

for

Industrial

prototype

unit

and

the Pollution

unit was provided

by the Umgeni

Board.

REFERENCES A.I. Zhevnovatyi, The thickening of suspensions without cake formation, International Chemical Engineering, 4(l) (Jan 1964) 124-128. K. Schneider and W. Klein, The coiicentration of suspensions by means of cross-flow microfiltration, Desalination, 41(3) (1982) 263-375. K. Treffry-Goatley, G.R. Groves and C.A. Buckley, The application of a cross-flow microfiltration unit to the thickening of water works sludge and sewage works waste activated sludge, Institute of Water Pollution Control (SA Branch) Biennial Conference, Durban, May 1985 K. Treffry-Goatley and C.A. Buckley,South African Patent No. 86/1834, Dewatering Slurries (1987) assigned to the Water Research Commission, Pretoria, Republic of South Africa. A.S. Foust, L.A. Wenzel, C.W. Clump, L. Maus and L.B. Andersen, Principles of unit operations, John Wiley and Sons Inc. New York (1960).