15

CHAPTER 2

EXPERIMENTAL STUDIES OF THE RO

DESALINATION PROCESS

The scenario for the demand of portable water in the world and in

India, the process of reverse osmosis desalination, objective of the present

study and the scope of the present work have been described in previous

chapter. Based on the outcome, as described in chapter 1, it is proposed to

carryout experimental work for setting up scaled up plants of higher capacity

to meet the demand for water. The experimental facility is described in this

chapter.

2.1 INTRODUCTION

Potable water is produced from sea / brackish water, using

desalination with the RO method. Domestic and industrial waste waters are

also treated by this technique. RO has the potential to remove TDS, organic

compounds and all classes of pathogens. The raw feed (sea/brackish water)

enters the mixing tank and is pumped through the RO membranes. The

resulting product is potable water, part of which is stored in the permeate

tank. The other by product is brine, part of which is recycled, and the rest is

concentrated for reuse and generation of value added salt-products. The

entire system consists of the integration of the mixing tank, pump, RO

membranes and permeates / brine tanks. The main objective of this chapter is

to report the experiments conducted to collect data from the different units

16

that comprise the RO desalination system, and to develop simple

experimental models which may be used to predict the performance of each

unit of the RO desalination system. The purpose of the modelling is to scale

up/down the process for the design, the calculation of the optimal operating

conditions, output predictions and for the fabrication of commercial units. An

experimental facility (3.8 MLD Desalination plant at Ramanathapuram, TN)

consisting of the mixing tank, dual media filter, polished dual media filter,

disc filter, cartridge filters, high pressure pump, RO brine tank and the

permeate tank, has been chosen to carry out the experiments to characterize

the system. Figure 2.1 shows the 3.8 MLD Reverse Osmosis Plant set up in

Ramanathapuram.

Figure 2.1 Experimental desalination plant set up with reverse osmosis

in Ramanathapuram (3.8MLD)

It is required to build two types of models for this study: steady

state and transient. The experimental set-up / plant operates at optimal steady

state conditions. The transient characteristics of the output can be obtained by

introducing external excitation / perturbations in the input variables. During

17

the operation of the RO system, the conditions, such as pressure, temperature

and feed water quality, can cause variations in the product water quality and

productivity. The dynamic and steady state performance operation of the RO

was obtained from the experimental work. The experimental runs were

performed on different units of the RO plant, such as a mixing tank, RO

membrane, brine tank and permeate tank. The inlet and outlet parameters of

each unit of desalination process, such as the flow rate, pH and total dissolved

solids (TDS) were recorded. The inlet and outlet flows are measured by the

Paddle wheel flow meter. At the end of each test, samples were taken from

each unit, and the TDS was measured using the conductivity meter and the

pH by using the pH meter. The main aim of the experiment is to enhance

higher diffusion of the solute away from the membrane, to reduce the

magnitude of the salt concentration at the membrane wall, cw , relative to the

concentration in the bulk of the feed cF .This leads to a higher permeate flux

(Jw ) , decrease in the salinity of the product water, increased recovery, and

an acceptable rejection rates.

2.2 DESCRIPTION OF EXPERIMENTAL SET UP

A sea water based desalination plant has been installed in

Narippaiyur village of Ramanathapuram district, Tamil Nadu, India, with a

capacity of 3.80 MLD of drinking water. This is the first Desalination plant

for the production of potable water in South Asia. The arrangement of the

experimental set up is shown in Figure 2.2.

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Figure 2.2 Schematic of the experimental set-up for the desalination of

sea water

2.2.1 Filters and Pumps

A well with an inner diameter of 1000 mm is dug in the sea and is

covered with a basket with small holes and mesh (100x15 mm GRP material)

so that fish and algae are not allowed to enter the well, which is connected to

an equalisation tank of 70sq m x 7m by two pipes (GRP material) of an inner

diameter of 450 mm and a length of 450 meters. GRP is a composite

consisting of two different materials such as glass fibre-reinforced plastic.

The pipe is connected to the equalisation tank with a certain slope, so that the

height of water in the equalization tank is equal to the height of the sea. The

tank is equipped with three vertical turbines and two horizontal centrifugal

pumps (capacity 450 500 m3/hr) that deliver water to the filters section with

a pressure of 3 - 3.5 kg/cm2. Three rough dual media filters (RDMF) of 3m

diameter and 7m height, with a capacity of 120 170 m3/hr filled with sand

and gravel, are employed in a series as the primary filters to remove coarse

particles and suspended solids from the water. The dirt and turbidity are also

reduced to some extent at the first and second stages of filtering. Water then

19

enters the third stage of filtering, viz, the polished dual media filters (PDMF)

where suspended solids, dirt and turbidity are reduced to yield colourless and

odourless water. Figure 2.3 shows the different layers of the PDMF

Figure 2.3 Schematic diagram of the typical PDMF filter

After PDMF, water enters the disc filter to eliminate particles larger

than 5 microns. Figure 2.4 shows the cross sectional view of the disc filter.

Figure 2.4 Cross sectional view of a disc filter

Water enters through the inlet pipe at the bottom of the disc filter

housing through the hydraulic valve way 1&2 and way 3 is closed. As the

water enters at the base of the filter chamber it passes through fin plate blades

20

which force the water to enter the filter chamber with a vortex effect. As the

water swirls within the filter chamber, some of the dirt particles are forced

outwards and upward and collect on the wall of the upper part of the filter

chamber, where they remain until the next flushing cycle. The water flows

from outside of the disc stack to the inside, passing along the specially

designed grooves on the surface of the disc which catch the suspended

material. The stack of discs is held compresses by the piston at its top. Clean

filtered water flows out of the filter from the centre outlet through the

hydraulic valve way 2 & 3 and way 1 is closed.

Then the feed water enters the cartridge filter, which will remove

particles sized more than 3 microns. Fig 2.5 shows the front view of the

cartridge filter.

Figure 2.5 Front view of a cartridge filter

The feed water is pumped (discharge pressure > 50 Kg/cm2) to the

RO sections, using a high pressure centrifugal pump.

21

2.2.2 RO Section

RO operates over a pH range of 2 to 11 with excellent performance,

in terms of flux, salt, organic rejection, and microbiological resistance, with a

free chlorine tolerance of less than 0.1 ppm. The water and solute

permeability characteristics depend on the performance of the membrane. The

feed water is allowed to enter the innermost radius of the spiral RO section.

The permeate comes out through the outermost layer of the RO. The ions are

attracted by the polyamide material of the membrane. The TDS reduces from

40000 ppm to 500 ppm for a running (operation) time of 12 hours through

168 ROs (spiral bound) of 1m length each. One RO consists of 30 membrane

leaves. Each leaf is made up of two membrane sheets glued together back to

back with a permeate spacer between them. The consistent glue line of about

1.5 inches wide seals the inner (permeate) side of the leaf against the outer

(feed/concentrated) side. The leaves are rolled up with a sheet of feed spacer

between each of them (shown in Figure 3), which provide the channel for the

feed and concentration to flow. The permeate (p) and brine (b) from all the

ROs are collected and passed to the opposite direction of the feed water

entering section of the RO. The brine from the RO section is collected, that

amounts to approx 50% of the feed. A schematic process flow sheet

(Figure 2.6) describes the flow rates in m3/hr, pressure in kgf/cm2 and the

TDS in ppm of different streams in the entire plant.

22

Figure 2.6 Process flow scheme of the desalination plant with streams

under operating conditions in (m3/hr, kgf/cm2, ppm)

About 70% of the brine is recirculated to the feed-mixing tank.

The rest of the brine can be used for the recovery of salts. 10% of the feed

precipitates or forms scales that get adhered to membrane. About 35-40% of

the feed goes to the permeate tank, and can be used for potable purposes.

MIXING TANK

HP PUMP

BRINE TANKENERGY

RECOVERYDEVICE

RO UNIT PERMEATETANK

SEA WATER

PRODUCTTANK

(80, 1, 850)

(80, 0.75, 850)CAKE

(65, 8, 60000)

(180,60, 41350)

(115, 3, 40000)(180, 11, 41350)

(10, 60000)

( 90, 59, 60000)

( 90, 59, 60000)

(25, 51, 60000)

SEA

(25, 0.25, 850)

23

To boost the feed water pressure from 3 kg/cm2 to 60 kg/cm2,

(centrifugal type of energy recovery devices, electromechanically operated

butterfly dump valve and a flow control valve) are used by taking (energy

conservation /recovered) the energy from the brine stream.. The percentage of

salt rejection is found to be 97.5%. The designed recovery is calculated as 50

%. Figure 2.7a and 2.7b show the cross sectional view of one RO vessel and

one RO (from Filmtech) . One RO vessel consists of six RO modules. The

plant consists of 28 numbers of RO vessel for one bed. The plant has two

such beds.

The total feed flow rate of one RO bed is 180 m3/hr. Feed

flow/vessel=180/28=6m3/hr; Permeate flow/vessel= 4m3/hr; Brine

flow/vessel= 2m3/hr

Figure 2.7a. Schematic flow of feed, brine, permeate in one RO

membrane module (TWAD board, Ramanathapuram)

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Figure 2.7b. Cross sectional view of a typical RO membrane

Table 2.1 shows the technical details of the RO Plant. From

Figure 2.7a it is seen that the permeate (due to the radial component of

velocity) comes out along the surface of the membrane while the

concentrated brine (due to the axial component of velocity) flows axially

through the membrane. The RO process can be thought of as comprising a

grey box model (Figure 2.8) with one input or manipulated variable, several

disturbance and design variables, and output or control variables. Due to the

transverse diffusion across the walls of the RO tubes (cylindrical), the

permeate comes out and gets accumulated in the product tank. The axial

stream flows out as rejection or brine. Part of the brine flow is recycled with

raw feed that enters to the mixing tank.

25

Pre-treated Feed Water Permeate FP, CP PP

FF,CF,PF

Brine

Fb,Cb,Pb Brine

Figure 2.8 Representation of the RO process as a grey box model

Table 2.1 Important technical details of the RO plant (TWAD Board)

SerialNo Components Description Specification

1. Sea water intakemouth

Intake mouth covering meshsize

10015 mm slots

2. Connecting pipefrom intake mouth toEqualization tank

Inner diameter and outsidediameter

450mm, 490 mm

3. Feed Pump motorSquirrel Gaugeinduction type

SpeedRating

2900 rpm30KW,415 V, 3 phase

4 Feed Pump(Vertical CentrifugalType)

Head 49/39 m of watercolumn (wc)

5 RDMF(Horizontal) i. Sand mesh size 14 heightii. Gravel 1/81/16 heightiii. Gravel1/41/8 heightiv. Gravel 3/81/4v. Gravel 3/43/8vii Diameterviii Length

400 mm150mm150mm150mm600mm3000 mm7000 mm

6 PDMF(Horizontal) i. Anthracite heightii. Gravel 1/81/16 heightiii. Gravel1/41/8 heightiv. Gravel 3/81/4v. Gravel 3/43/8vi. Refilling freqvii. Diameterviii Length

300 mm100 mm100 mm100 mm600 mm5 year once3000 mm7000 mm

Kw

Ks

26

Table 2.1 (Continued)

SerialNo

Components Description Specification

7. Disc Filter:2ADF i. Disc element: Polyethyleneii. Flushing cycle timeiii.Max flowrateiv. Max working pressure

15-25 secs25 m3/hr10 bar

8. Cartridge Filter i. Material: Polypropyleneii. Filter element sizeiii. No of Catridge elements/Filter

2.6OD1.1ID40Length

30 nos

9. Energy RecoveryDevice

i. Material:Duplex stainless steelii. Feed flowiii. Brine flow

158 m3/hr79 m3 /hr

10. HP Pump i. Centrifugal pumpii. Total discharge headiii. Suction pressure Min

634 m of WC5 m of WC

11. HP pump motorrating

Sq. cage motor 450KW, 2980rpm,50 HZ, 6.6 KV,3 phase,

12. RO Membrane i. Type:Filmtech,SW30HR380 Outer material:Polyamide Center material:Polysulphoneii. Lengthiii. Outer diameteriv. One vessel length(6 ROS)v. Vessel inner diavi. Top most vessel pressurevii Feed flow/vesselviii Permeate flow/ vesselix. Brine flow /vessel

1 meter86.6 meter210 mm2 kgf/cm2

6 m3/hr5m3/hr1.5m3/hr

27

2.3 UNIT OPERATIONS IN THE DESALINATION PROCESS

2.3.1 Pre- treatment and Post- treatment (or) Demineralization

As the feed water comes from the sea / brackish source, it contains

clay, dirt and suspended materials that need to be separated . Hence pre and

post treatment using clarifiers are done in the factory. Chlorination and de-

chlorination are done to remove bacteria & pathogens; turbidity is removed

during pre-treatment. Calcium and bi-carbonate ions are removed from the

feed during post treatment when the pH is raised slightly above 7. The details

of these methods are not under the scope of this research.

2.3.2 Data Collected across Each Unit of the Experimental Setup

The experiments were performed on the RO desalination process

equipped with spiral membranes. First, the data for the feed flow rate, TDS,

pH and the HP (high pressure) pump were collected across the mixing tank,

for the verification of the dynamic and steady state operation of the mixing

tank during the starting of the process, after the power shut down of the plant

for nearly15 minutes. The data are listed in Table 2.2. The corresponding

graphs are shown in Figure 2.9. It can be seen from Figure 2.9(a) that after

1000 sec of startup time, the feed flow rate attains steady value of 0.05

m3/sec. Figure 2.9(b) reveals that after 1000 sec of startup time, the

concentration in mixing tank attains steady value of 4.176104 ppm, pH

becomes 12.83 (Figure 2.9c) and the pressure of HP pump after 150 sec of

startup time changes to 6105 pascals (Figure 2.9d)

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Table 2.2 Data to describe the dynamic characteristics of the feed in

mixing tank

SerialNo Time in sec

HP pump pressurein Pascal*10000

Feed flowratein m3/sec

Feed TDSin ppm

Feed pH

1. 1 3 0 41400 12.9382. 10 3.3 0 41444 12.9343. 20 3.5 0.0055 41500 12.904. 30 3.7 0.00833 41580 12.895. 40 4 0.0111 41600 12.896. 50 4.2 0.01388 41700 12.877. 60 4.5 0.01944 41760 12.8658. 70 4.8 0.0222 41765 12.859. 80 5 0.025 41780 12.84

10. 90 5.2 0.02666 41800 12.8311. 100 5.4 0.029166 41820 12.82512. 150 5.6 0.031944 41840 12.8213. 200 5.8 0.0388 41845 12.8114. 250 5.9 0.04388 41825 12.80515. 300 6 0.04666 41805 12.81216. 350 6 0.04833 41790 12.81517. 400 6 0.04888 41780 12.8218. 450 6 0.04944 41770 12.8319. 500 6 0.049722 41767 12.83220. 550 6 0.05 41767 12.83521. 600 6 0.050277 41760 12.83622. 650 6 0.0501388 41760 12.8423. 700 6 0.05 41760 12.8424. 750 6 0.05 41760 12.83725. 800 6 0.05 41762 12.83726. 850 6 0.05 41761 12.83627. 900 6 0.05 41762 12.83528. 950 6 0.05 41473 12.83529. 1000 6 0.05 41761 12.83530. 1050 6 0.05 41760 12.83431. 1100 6 0.05 41760 12.83432. 1150 6 0.05 41761 12.83433. 1200 6 0.05 41760 12.834

29

(a) Feed flowrate vs time (b)Feed TDS vs time

(c).Feed pH vs time (d) Feed Pump pressure vs time

Figure 2.9 Dynamic characteristics of the feed in mixing tank

Then, the data are collected for the brine flow rate, pressure, TDS

and pH of the brine tank, for studying the dynamic performance of the brine

tank during the start-up of the process, after the power shut down of the plant

for nearly15 minutes. The data are listed in Table 2.3. The graphs are shown

in Figure 2.10. It can be observed from Figure 2.10 that, after 1200 sec of

start uptime, the brine flow rate becomes 0.0258 m3/sec, the brine TDS

settles at 6.366 104 ppm and the brine pH attains a pH of 12.65.

30

Table 2.3 Data to describe the dynamic characteristics of the feed

stream in the brine tank

SerialNo

Timein secs

Brine flowratem3/secs

Brine TDSin ppm

Brine pH

1. 1 0 61000 12.68242. 10 0 62000 12.66653. 20 0.0001 62500 12.6664. 30 0.0006 62700 12.66595. 40 0.0011 62900 12.66586. 50 0.0017 63000 12.66567. 60 0.0022 63050 12.6658. 70 0.0044 63100 12.66499. 80 0.0069 63125 12.6648

10. 90 0.0097 63200 12.664511. 100 0.0181 63250 12.66412. 150 0.0250 63300 12.665813. 200 0.0256 63340 12.665714. 250 0.0256 63355 12.65615. 300 0.0258 63365 12.653516. 350 0.0261 63374 12.65117. 400 0.0256 63420 12.6518. 450 0.0258 63450 12.64919. 500 0.0256 63470 12.64820. 550 0.0253 63480 12.64721. 600 0.0253 63490 12.64622. 650 0.0258 63520 12.64723. 700 0.0258 63550 12.64824. 750 0.0258 63530 12.64925. 800 0.0258 63600 12.6526. 850 0.0258 63600 12.65127. 900 0.0258 63610 12.65228. 950 0.0258 63620 12.65129. 1000 0.0258 63650 12.65130. 1050 0.0258 63650 12.6531. 1100 0.0258 63660 12.6532. 1150 0.0258 63660 12.6533. 1200 0.0258 63660 12.65

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(a)Variation of the brine flowrate with time (b) Variation of the brine TDS with time

(c) Variation of the brine pH with time

Figure 2.10 Dynamic characteristics of the feed stream in the brine tank

The data are collected for the permeate flow rate, pressure, TDS

and pH in the permeate tank, for studying the dynamic performance of the

permeate tank during the starting of the process after the power shut down of

the plant for nearly15 minutes. The data are listed in Table 2.4.The graphs

are shown in Figure 2.11. It can be observed from Figure 2.11 that, after 650

sec of start up period, the permeate flow rate reached a steady value of

0.0229 m3/sec; after 270 min, the permeate TDS drops down to 819.75 ppm

and the pH settled at 7.5163.

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Table 2.4 Data to describe the dynamic characteristics of the feed

stream in the permeate tank

Serial No Time insecs

Permeateflowratem3/sec

Permeate TDS PermeatepHTime in

secs*60Permeate

TDS in ppm1. 1 0 0 860 7.51602. 10 0.0002 5 862.65 7.51603. 20 0.0005 10 883 7.51604. 30 0.0012 20 881.35 7.51605. 40 0.0014 30 850.00 7.51616. 50 0.0022 40 870.70 7.51617. 60 0.0026 50 867.60 7.51618. 70 0.0042 60 864.30 7.51619. 80 0.0069 75 852.75 7.516210. 90 0.0133 90 863.75 7.516211. 100 0.0194 105 871.45 7.516212. 150 0.0217 120 847.80 7.516213. 200 0.0225 135 839.00 7.516214. 250 0.0228 150 846.70 7.516215. 300 0.0233 165 847.80 7.516316. 350 0.0236 180 845.05 7.516317. 400 0.0228 195 839.00 7.516318. 450 0.0233 210 840.00 7.516319. 500 0.0228 225 830.75 7.516320. 550 0.0231 240 828.00 7.516321. 600 0.0231 255 824.70 7.516322. 650 0.0228 270 819.75 7.516323. 700 0.0230 7.516324. 750 0.0230 7.516325. 800 0.0229 7.516326. 850 0.0229 7.516327. 900 0.0229 7.516328. 950 0.0229 7.516329. 1000 0.0229 7.516330. 1050 0.0229 7.516331. 1100 0.0229 7.516332. 1150 0.0229 7.516333. 1200 0.0229 7.5163

33

(a)Permeate flow rate with time (b) Permeate TDS with time

(c) Permeate pH with time

Figure 2.11 Dynamic characteristics of the feed stream in the permeate tank

The data are collected for potable water flow rate, pressure, TDS

and pH of the potable tank for the dynamic performance of the potable tank

during the start-up of the process, after the power shut down of the plant for

nearly15 minutes. The data are listed in Table 2. 5. The graphs are shown in

Figure 2.12. It can be seen from the graph ( Figure 2.12) that after 15 mins of

start up/shunt down, the potable water flow rate became 2510 l/min, the

TDS is fluctuating between 1000-1100 ppm, the pH fluctuates around 9.2;

34

the recovery comes down to 38% in 240 mins and the rejection becomes 97.4

% after about 200 mins.

Table 2.5 Data to describe the dynamic characteristics of the permeate in

the potable water tank

SerialNo

Timein

mins

Potablewater flow

rateLiters/min

PotablewaterTDSppm

Potablewater

pH

Timein

mins

%Rejection

%Recovery

1. 1 10 1000 9.13 5 97.3 46.65

2. 2 200 950 9.15 10 97 45.933. 3 500 900 9.20 20 97 47.264. 4 750 1050 9.23 30 97.35 46.555. 5 960 1000 9.32 40 97 45.796. 6 1450 850 9.15 50 97.238 45.617. 7 1750 900 9.17 60 97.23 44.898. 8 2000 950 9.23 75 97.32 44.579. 9 2225 1050 9.24 90 97.25 43.85

10. 10 2530 1100 9.12 115 97.22 43.4011. 11 2490 1050 9.14 130 97.34 42.8112. 12 2500 1100 9.15 145 97.4 42.8113. 13 2520 1000 9.12 160 97.36 42.9414. 14 2545 1050 9.16 175 97.37 41.7615. 15 2530 1100 9.21 190 97.36 39.8816 16 2510 1000 9.22 205 97.37 39.17

220 97.40 38.46235 97.44 38250 97.44 37.27265 97.45 36.79280 97.5 36

35

(a) Potable water flow rate (b)Potable water TDS

(c)Potable water pH (d) RO bed recovery

(e)RO bed rejection

Figure 2.12 Dynamic characteristics of the permeate in the potable tank

36

2.4 SUMMARY

A large scale SWRO Desalination plant has been studied to

evaluate the process development. The preliminary results obtained from the

experimental set up lead to some observations on the individual units, such as

the mixing tank, brine tank, permeate tank and potable tank to predict the

performance of each system. In this study, the flow rate of the feed water in

and out of the mixing tank is 0.05m3/sec, the feed TDS value is around

41,000, the feed pH value is 12.8 and the operating pressure of the HP pump

pressure is 600000 Pascal. During this study the steady operating flow rate of

the brine water in and out of the brine tank is 0.0258 m3/sec, the brine TDS

value is around 61,000, and the brine pH value is 12.6 . The steady flow rate

of the permeate water in and out of the permeate tank is 0.0229 m3/sec. The

steady permeate TDS value is around 800 ppm and the steady permeate pH

value is 7.5163. The results show that the steady flow rate of the potable

water out of the potable tank is 0.04167m3/ sec (2500 litter/min). The potable

TDS steady value is around 1,000 and the potable pH steady value is 9.22.

These experimental results will be helpful in developing step response models

across different units for step changes in inlet feed flow rate, concentration

and pressure exerted by HP pumps using reaction curve method. Based on the

experiment, the following conclusion can be drawn in the present study, viz,

the percentage rejection of salt by the RO is 97.5% and the percentage

recovery is around 40%. With the above results on system investigation and

the quality of the feed water, the modelling, design and simulation of large

scale Seawater Reverse Osmosis process (SWRO) are developed and is

described in the next Chapter.