Water & effluent water treatment using reverse osmosis

507

DesaEnation,67 (1987) 507-521 Elsevier Science PublisbersB.V.,Amsterdam-PPrintedinThe Netherlands

WATER & EFFLUENT WATER TREATMENT USING REVERSE OSMOSIS R.N. PATRA, S. PRABHAKAR, B.M.MISRA & M.P.S. RAMANI Desalination Division Bhabha Atomic Research Centre Bombay 400085, India SUMMARY Reverse osmosis modules in tubular and plate configurations have been developed by the Bhabha Atomic Research Centre, Bombay, India for desalination of brackish ground waters, containing dissolved salts up to a maximum of 10,000 ppm. The modules use cellulose acetate membranes, developed and prepared in our Based on a few fie,ld trials, the membranes have been laboratory. found useful for treatment of low level radioactive liquid waste, effluent water treatment from selected process industries and with production of boiler feed quality water in combination conventional demineralisers. This paper discusses the results obtained from the abdve studies alongwith the salient design features and technoeconomics. 1.

INTRODUCTION Reverse separation

osmosis

process

has

been

chemical

in

established industry

as

in

a

the

proven

past

two

Since the discovery of the semipermeable properties (1) cellulose acetate membranes by Loeb-Sourirajan ,who

decades. of

first demonstrated and sea

water,

production

of

industrial

its potential for desalination of brackish

the

wastes

concentration

of

has

process

boiler

feed

for recycle fruit

as

other

many

complex

separation

by

development

in

semipermeable process

the

cost

applications.

years in

effective

in USA

for

a

view

reasonable

for of

reuse

of

of

water,

valuable

and for processing

considered

difficult

methods.

for

Significant

the

synthesis

and

Japan

most

of

of superior

have the

made

the

industrial

In India, the research and development work on

reverse osmosis was undertaken with

adopted treatment

separation

mixtures,

recent

as

industries

conventional

membranes,

widely water,

well

juices,

products in the pharmaceutical of

been

quality

to

supply

cost to the

good

by the National quality

villages

drinking

situated

in

OOll-9164/87/$03.500 1987ElsevierSciencePublishersB.V.

Laboratories water the

arid

at

a

and

508

semiarid zones. The

reverse

osmosis

Research

Centre,

Bombay

configuration

having

is

developed

based

on

from brackish

of 10,000 ppm total

in Bhabha Atomic

tubular

acetate

cellulose

produce potable water maximum

system

water

dissolved

and

plate

membranes,

which

feed containing

solids.

a

The reverse

osmosis plants have been operating in the rural areas for the last two years and producing from brackish total

dissolved

capacity The

solids

of the plants

operational

reported

as per

IS-10 500

under

a

National

Programme.

The

are in the range of 25 to 30 m3/day. of these plants, al (2), Treatment of

experinece

Prabhakar

recycle,

using

and

indigenous

workers.

et

have

been

level radioactive liquid wastes('), separation of isotopicmixtures(4) for

by

potable water

ground water containing 4,500 ppm to 7,500 ppm

concentration RO

systems

of

food

low

productstreams(5)

have been reported

In the present paper,

by various

an attempt has been made

to

demonstrate the optimal design criteria for a reverse osmosis plant using

cellulose

acetate

membrane,

developed

at Bhabha

Atomic Research Centre, Trombay for production of boiler feed quality water, treatment of industrial waste

for water

reuse

and recycle, treatment of radioactive waste and concentration of

food

and

beverage

streams.

The

inference drawn from the field trials,

study

comprises

laboratory

the

test runs

and theoretical analysis. 2.

MEMBRANE/MODULE Cellulose

SYSTEM acetate membranes

have been developed in both

sheet and tubular forms at the Desalination Atomic

Research

Centre,

Loeb-Manjikian(6) polymer.

Their

Trombay.

technique

from

semipermeable

cellulose

properties

rejection, and permeate flux at various feed

salinities,

temperature

and

the

Division,

The membranes

effect

of

2.5

such

operating membrane

on the above have been discussed

Bhabha

are cast by

at

acetate

as

%

salt

pressures, annealing lenath

bv

Thomas (7). The

tubular

membranes

are

supported

by

porous

fibre

glass reinforced plastic tubes of 19 mm internal diameter and 125 ems which

length.

A

is considered

single

tubular

the building

reverse

osmosis

module

block of a 30 to 50 m'/day

509

osmosis

reverse

support

tubes

consists

plant,

along with

of

22

numbers

the membranes,

which

of

are

porous

suitably

arranged in a triangular pitch and connected to each other by series

For higher

flow end adaptors.

the tubes in a module

capacity

can be connected

plants,

in parallel

all

and the

series flow adaptors can be dispensed with. In stacked

the

case

of plate modules

in between

glass

the sheet membranes

reinforced

plastic

are

which

plates,

hold them in position and allow the saline water to flow over 2 them under the high operational pressure of 40 Kg/cm . A single plate module, which osmosis

units

membrane

of

sheets placed

reinforced

can be repeated

any capacity,

plastic

to build

comprises

on either

side

plates mentioned

of

each

above.

of

of 198 the

The details

the two modules are presented by Prabhakar et al (2) 3.

reverse

a maximum

100 of

.

RO-DM SYSTEM FOR PRODUCTION OF BOILER GRADE WATER A demonstration tried

On

Station,

round

the

Dhuvaran,

unit of one clock Gujarat

lit/min

operation for

one

at

capacity was field the

month.

Thermal The

Power

test

was

followed by another field trial at the Thermal Power Station, Ennore, Tamil Nadu feed waters

for three months.The

analysis

of saline

at Dhuvaran and at Ennore are presented in Table

1. Table 1 Feed water analysis of Dhuvaran and Ennore thermal power station Dhuvaran

*Ennore

1. Conductivlly at 25aC @nhos cm )

2513.0

850 - 2000

2. Turbidity

Traces

nil

(silica scale)

3. pH unit 4. Phenolphthalene as CaC03(ppm)

alkalinity

5. Methyl orange alkalinity as CaCC3(ppm) 6. Total hardness as CaC03(ppm)

7.86

7.14 - 7.2

Traces

nil

609

220 - 265

129.67

205 - 560

7.

Calcium hardness as CaC03(ppm) 41.5

140 - 370

8. 9.

Chloride as Cl-(ppm) Sulphate as S04" (ppm)

120 - 460 62.4 - 67.2

10. Silica as Si02(ppm) 11. Total dissolved solids (ppm) 12. Dissolved oxygen (ppm 02)

335 43.08 36.04 1295 7.917

12 - 20 545 - 1280

510

13. Total iron as Fe (ppm)

0.0275

nil

14. Nitrate as N03' (ppm)

9.286

nil

* The feed water quality at Ennore thermal power station iS subjected to wide seasonal variations The feed water was filtered through sand gravity filter followed by filtration through a fine cartridge filter having cut off size for 5 micron suspended particles.

The water was A high pressure

later acidified to bring down the pH to 5.5. triplex plunger water

through

pump circulated

the acidified

filtered

feed

two numbers of tubular reverse osmosis modules

at 40 Kg/cm2 and the flow rate was maintained The system pressure

was maintained

at

10 lit/min.

at the desired

level by

means of spring loaded self actuated back pressure regulator. The highlights

of Dhuvaran

field

trials

is

summarised

in

Table 2. Table 2 Highlights of Dhuvaran field trials Date

operating pressuse (kg/cm 1

Feed flow rate (lit/min)

Product flow rate (lit/min)

%SR

Product flow (lit/m2/day)

15.3.84

32.0

10

1.34

91

20.3.84

32.0

10

1.31

92

993

25.3.84

32.0

10

1.30

95

985

30.3.84

32.0

10

1.33

91

1008

4.4.84

32.0

10

1.32

93

1000

9.4.84

30.0

10

1.28

90

970

15.4.84

30.5

10

1.25

91

947

The tests rejection silica

indicated

1016

little decline of the membrane salt

over the period

of one month.

In

addition,

the

load on the anion exchange columns is also minimised.

Lastly organic and possible iron fluling of the ion exchange resins

are

completely

ruled

out

as

they

are

effectively

removed in the reverse osmosis section. Based on the above to introduce

reverse

demineraliser columns make up water

field

osmosis

trials,

a scheme was proposed

desalination

for production

for power plant

units

of boiler

followed by feed quality

boilers in areas of high feed

o PRODUCT FLUX

.SALT

z

h/day

)

REJECTION tW

%

,

(

FIG.2, ARRANGEMENTS OF MODULES AND ION EXCHANGES COLUMNS IN RO-DM UNIT

RECOVERY

= 60%

RECOVERY

= 26%

0.5

mhoa cm”

513 salinity

such as those encountered

As is evident cellulose water

from fig.1,

2.5

acetate

conforming

quality

make

rejection

in

membrane

water

feed

in

of

can

substantially

there

is not only

reduce

of boiler

is

the

feed

feed

The

salt

diminished

500 ppm.

on

deliver

However, salinity

with the RO

so

that

saving in the capital cost after the

RO

plant

but

for alkalies and acids for regeneration

can be brought

down.

anion

columns

completly

based

operation.

membrane

below

plant,placed

also the requirement

and possible

step

a considerable

of a demineralisation

and Ennore.

plant

cannot economically

one

the

salinity

plant

exchange

osmosis

to the rigid specification

up

capability

decrease

at Dhuvaran

a reverse

In addition,

the silica

is also minimised.

iron fluling

of

the

ion

load

Lastly,

exchange

ruled out as they are effectively

on

the

organic

resins

removed

are

in the

reverse osmosis section. While

designing

a

RO-DM

system for boiler

feed

water

treatment, it is obviously necessary to determine the optimum cut

off

point

between

RO

and DM.

In other words

parameters such as water flux, salt rejection, for 'a given downstream optimisation are

membrane

temperature

feed

is

also

product

demineralisation

also

of the RO plant. but generally reduces

important

finite

quantity.

But

reject

concentration

recovery

salinity

become

and

etc.

cost

parameters by

the

annealing

in conflict with each other. feed handling

when high

the

and treatment

avaliable

recoveries

adding

of for

Water flux and salt rejection

characteristics. controlled

product recovery It

salinity,

besides

to

the

raw

result

in

operation

handling highly saline water and adversely

water

High costs. is of

increased

problems

affecting

of

product

salinity. The. analytical expression, deduced by Gupta (8) from the three basic

transport

equations of reverse osmosis presented

by Sourirajan(') was used to calculate total number of tubular modules required and the permeate purity product

recoveries

and

three

different

for three different membrane

temperatures using the saline water available

annealing

at Dhuvaran

as

feed. The analysis was done for a plant capacity of 20 m'/hr. which was considered the maximum size of water treatment Unit at

Dhuvaran

Thermal

Power

Statibfnig/%e * -

costs

of

the

514

pre-treatment market recoveries operating quality

section and DM section were estimated

costi for and

each

membrane

make

investments

up

for

membrane

above

case

the

based

of

temperatures.

one cubic meter

including

water

was computed

a

the

annealing

cost of producing

on

product

The

total

of boiler

amortized

feed

capital

for each of the above combinations

of parameters. The optimum 70%

of

of

product

recovery

was found

92% salt rejection

(79OC

to be

annealing

temperature). The net savings in the operating cost per mJ of product RO-DM

paid back the entire

additional

unit over that of a conventional

investments DM unit

in

on

a

0.2

years. Similar analysis Power

Station,

was

also done for the Ennore

feed

where

salinity

was

only

Thermal

600

ppm

as

against 1300ppm at Dhuvaran. A recovery of 90% was determined case for membrane of 92% salt (10) have arrived at a break even rejection. Prabhakar et al to

be

value

optimum

of

this

in

400 ppm feed salinity

beyond which

RO-DM

is more

economical than DM alone for boiler feed water treatment. 4.

TREATMENT OF EFFLUENT WATER IN PROCESS INDUSTRY In

keeping

effluents

with

the

requirements

and

considerations

for a reverse

effluent water

treatment

above.

treatment

The aim was

of

the

philosophy process,

osmosis pilot

are different

to maintain

the

of

the

the

design

plant meant

for

from those discussed

operation

cost

of

the

plant to a minimum and recover as much water as possible from the

effluents

by the reverse

osmosis

process.

The effluent

comprises backwash stream from sand gravity filter,regenerant and

column

columns

wash

streams

of the DM unit.

from

cation

and

Hence considerable

the suspended solid contents of the effluents 100 ppm to

anion

exchange

fluctuations

1000 ppm) along with wide pH variations

of

stream (2 to 12) are observed. As the life of the membrane considered

maximum addition

The

or

acid

the

varying

is

to the desired pH value either by acid

depending

alkali

neutralisation

the

inthe pH range of 5 to 6, the effluents

require neutralisation or alkali

in

(it varies from

on

consumption

their was

incipient reduced

by

pH

value.

providing

tanks of about 8 hrs. hold up capacity so that

pH

of

the

stream

will

self

neutralise

the

515

516

contents

of the hold up tank over the time. In addition, the

hold up unit also serves as a settling tank and removes most of the suspended solids. Marshall and Slusher equations (11)developed on the basis of

Debye-H';ckel extended

law,

were

used

to

evaluate

the

concentration factor of the effluent samples at which CaSO4 would exceed the thermodynamic

solubility limit under ambient

temperature operation. It was estimated that seven out of the ten

effluent

gypsum

remaining gypsum

samples

solubility three

scaling

collected

limit

at random would

if treated

samples, at product

upto

60%

however,

indicated

recovery

less than

not exceed

recovery. possibility

The of

50%. Thus the

product recovery of the plant was fixed at 50% without taking recourse

to

complicated

and

expensive

methods

of removing

Ca++ or SO --ions from the feed solution in order to realise 4 a higher product recovery. Instead, dosing of a powe$z&ntisealant such as FLOCON-100 marketed by M/s. Pfizer India Ltd. was recommended in order to increase the threshold

value

of

precipitation of gypsum. The analysis of the effluent samples along

with

the

range

of

variations

of

their

different

constituents and the basic design parameters of the RO plants are presented in Table3 to Table4 and fig. 3. Table 3 Analysis of effluent water from process industry

1.

2.

PH Total dissolved solids (ppm)

2-

12

2700 - 10000

3.

Suspended solids

4.

Sulphate (ppm)

100 - 2400

5.

Chloride (ppm)

6.

P. alkalinity as CaC03 (ppm)

7.

MO alkalinity as CaCO3(ppm)

8.

Total hardness as CaCO3(ppm)

28 - 450

9.

Calcium hardness as CaC03 (ppm)

15 - 300

10.

Magnesium hardness as CaC03 (ppm)

11.

Sodium (ppm)

1500 - 3300 30 - 125 o-

800

o-

200

8-

180

1860 - 1900

s &

3

p

+

i I----* I i

tl1’

IL

! ________

i3 _

j

!P

$I 1: II giTT_________

3

[’

n? ii

518

Table 4 50 ma/day effluent water treatment plant-operating parameters ----~--~--~----~---------------~--_--_---------_---------__-_____ Capacity

35 lit/min

2.

Feed flow

58 lit/min

3.

% feed recovery

60

4.

Feed water TDS

6,000 ppm (max)

5.

Suspended load in feed water

1,000 ppm (max)

6.

Operation temperature

35OC (max)

7.

Operational pressure

40 kg/cm2

1.

8.

Product water TDS

700 ppm (max)

9.

Module type

Plate module with CA membrane

and No.of modules

9 NOS.

10.

Membrane area per module

7.65 m2

5.

RADIOACTIVE LIQUID EFFLUENT TREATMENT PLANT Unlike

the

reverse

unit

osmosis

for

effluent

water

treatment, the 30m3/day reverse osmosis low level radioactive liquid would

effluent

treatment

plant

proposed

at

BARC,

Bombay

be an intermediate processing unit, which would result

in volume reduction of large volume of the existing lo .level r 1g.4 radioactive effluents in the treatment unit at Bombay. /An analysis of the effluent is presented in Table 5. Table 5 Analysis of low level radioactive wastes 1.

Specific radioactivty

: lob2 bci/ml (The radioactivity is mostly due to Cs and Sr isotopes)

2.

Activity associated with particulates

:4x

10-3r_lci/ml

3.

Activity associated with ionic form

:6x

1o-3 pci/ml

4.

Cs ionic activity

5.

Sr ionic activity

6.

Dissolved solid concentration

: 5.4 x 10-' yci/ml : 0.6 x 10-' pci/ml : 350 ppm

7.

Suspended solid (particulate concentration range) Particulate above 40 microns Particulates in the range of 20 to 40 microns Particulafes in the range of 5 to 20 microns

: 50 to 150 ppm : 5 to 20% : 5 to 30% : 25 to 75%

519 Table 5 continued

: 15 to 25%

5 microns and below

Particulates 8.

Feed pH range

: 7to9

9.

Feed temperature range

: 25 to 306 C

The effluent

contains

about 330 ppm of dissolved solids and

can be treated to a very high percentage such

as

recovery

90%

about

by

reverse

of the effluent

the permeate

beyond

would

the

of product

raise the activity

limit prescribed

disposal to the environment. sea.

The

concentrated

higher level of

by ICRP for direct

Hence the recovery is limited to

about 80% so that the permeate can be directly the

recovery

However,

osmosis.

stream

original volume is sent back

which

is

to the chemical

discharged about

l/5

treatment

to the

unit

for removal of activity by chemical precipitation method. The total radioactive around

dose

the RO units

calculated

from the activity balance

is 100 Krad and the

cellulose

acetate

membrane can withstand them for 500 days before deterioration. Thus

there

would

be no significant

life due to radioactivity

in

the

loss of usual membrane

above

effluent

treatment

unit. 6.

CONCENTRATION

OF FOOD AND BEVERAGE EFFLUENT STREAM

Laboratory

scale studies

have been conducted

the suitability of concentrating molasses using

BARC

RO

modules.

The

trials

to assess

by reverse

were

conducted

osmosis with

6%

sucrose solutions and attempts were made to concentrate it to about 20% by using tubular module. It was seen that there was significant above

rise

in the viscosity

10% concentration

of

the

sucrose

solution

with concomitant rise in its osmotic

pressure. Thus the water

flux was significantly

reduced

and

higher flow rates were waranted for maintaining the concentration polarisation

at the optimum

surcrose concentration.

level for achieving higher

The results of the test are presented

in Table 6. Table 6 Results of sucrose concentration tests

520

Table 6 Results of sucrose concentration tests

_Ilit/m

(kg/cm2)

Loose

% Salt rejection

Elux

Pressure

Membrane type

8.6%

Feed 4.9%

Feed 4.9%

8.6%

40

891.3

594.2

96.3

96.7

30

679.1

445.6

96.3

96.4

20 445.6 275.9 95.1 95.2 ---___---___----_-----~---~---------------___---~___---_____-~___

Tight

40

331.00

305.5

99.2

30

264.00

203.7

99.2

99.1

20

152.8

112.0

99.0

98.8

Molasses

will

require

all suspended matters suspended system

in

plate

in tubular

membranes sucrose

elaborate pretreatment for removal of organic

and colloidal

processing

may in

or inorganic

form before

module.

An

configuration

be

considered

molasses

98.9

by

ultrafiltration

in both for

membrane

based on cellulose acetate prior

plate

present

it is recommended

to

modules

concentration to obtain

of

the best

result. 7.

USE OF INDIGENOUS C.A. MEMBRANES

IN EFFLUENT TREATMENT

Cellulose acetate membranes suffer from the problems of their vulnerability and

to bacterial

range

low

processing

of

electroplating

of

pH

some

attack,

tolerance

of

the

poor chemical

i.e.

streams

4

to

resistance Thus

the

such as effluents

8.

from

industries, paper mill wastes which are either

highly acidic or alkaline in nature are rendered difficult by cellulose acetate membranes.

Their poor bacterial resistance

calls for chlorine dosing for a residual

chlorine

content

of

Property of cellulose acetate to hydrolyse in both 2 ppm. acidic and alkaline media warants the pH of the feed streams to be maintained continue

to

availability membranes module

in between 5 and 6, with consequent rise in

Inspite of all the above drawbacks

chemical cost. be

in

replacement

in

RO The

and low cost.

is merely

or

use

plate cost

systems

due

replacement

to

they will their

cost

of

easy these

6 to 8% of the total cost of the tubular module

as

encountered

against in imported

the

60%

membrane

spiral wound

and

521 hollow

The membranes

fibre modules.

can be easily replaced at

the site thus avoiding the replacement and transportation entire

module.

Judicious

selection

of operating

definetly

ensure

membranes

even

choice

parameters

of

and membrane

a longer membrane for

the

most

pretreatment,

properties will

life for cellulose

challenging

of the optimal

application

acetate in

the

process industries. REFERENCES (1)

S.Loeb and S.Sourirajan:Adv.Chem, Sec., 38, 117 (1962)

(2)

S.Prabhakar et al: 3rd World congress on Desalination and Water Reuse, Cannes, France, 1987

(3)

V.Ramachandhran and B.M.Mishra, 1641 (1983) SCi . ,28,

(4)

S.Prabhakar et al: Radio chemica Acta 39, 93-96 (1986)

(5)

V.J.Shah et al :Proc.Third National conference on Water Desalination, C.S.M.C.R.1, India, 152-155,(1984)

(6)

S.Manjiktan, S.Loeb and J.W.McCutchan:Proc.First International Desalination Symp. Paper SWD/12, Washington D.C.Oct 3-9 (1965)

(7)

K.C.Thomas: "Membrane Separation Reverse Osmosis", M.Sc Thesis, Bombay University, India (1981)

(8)

S.K.Gupta: 1.E & C (Process Design and Development), 1240 (1985)

(9)

S.Sourirajan: 'Reverse Osmosis' Logos Press Ltd., London (1970)

(10)

S.Prabhakar et al: Indian Journal of Technology, Communicated

(11)

W.L.Marshall and R.Slusher: J.Chem.Engg.Data 68.

J.Appl.Rolym.

24,

13 (1) Jan.