Desalination 203 (2007) 327–330

Presented at EuroMed 2006 conference on Desalination Strategies in South Mediterranean Countries: Cooperationbetween Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by theEuropean Desalination Society and the University of Montpellier II, Montpellier, France, 21–25 May 2006.

Separation steps inspection for improving the efficiency ofreverse osmosis desalination plants

Alaa’ Abdulrazaq JassimUniversity of Basrah, Engineering College, P.O. Box 1458, Al-Ashaar mail, Basrah, Iraq

Tel. +964 7801392172;email: alaahade88@yahoo.com

Received 30 January 2006; accepted 4 April 2006

Abstract

This paper inspects the performance of two separation units in one of the experimental desalination plants. Inthe first step, the efficiency of the de-aerator system will be specified, and then the capability of improving the rateof dissolved gas removal by using professional types of packing will be evaluated. In the second step, the examinationof the sedimentation tank performance will be tested, by using the jar test, and then the best chemical additivesconcentration will be specified. This work is done at the water desalination plant at Basrah University, in southernIraq. The plant has a capacity of 9 m3/h and uses Koch membranes, model 8822-HR. The results show that the rateof dissolved gas removal can be improved when the area of contact between the gas and liquid streams is increasedby using the professional types of packing. Also, the accuracy of chemical dosing for polyelectrolyte and ferricchloride in the sedimentation section improves the performance of the pretreatment section and reduces the chemicaladditives consumption. The backwash processes for sand filters and the period of replacement for fine filters hasbeen improved after regulating the methods of separations.

Keywords: Reverse osmosis; De-carbonator; Sedimentation; Performance

1. Introduction

The quality of water production from waterdesalination systems depends on the efficiency ofseparation methods. Various separation methods

applied, such as sedimentation, adsorption, filtra-tion and de-aeration, in the pretreatment sectionbefore desalination processes. Most of these me-thods depend on mass transfer phenomena. How-ever, water quality varies from place to place and,

0011-9164/07/$– See front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.desal.2006.04.015

328 A.A. Jassim / Desalination 203 (2007) 327–330

in any one place, from season to season, and sub-sequently the resources for construction and ope-ration of a water treatment plant from place toplace. So, the selected treatment must be basedon a particular situation.

Varying amounts of free carbon dioxide (CO2)are present in natural waters. The amount of CO2absorbed by rain water from the atmosphere isvery small, ranging from 0.5 ppm to 2 ppm, where-as most surface water contains 0–5 ppm of CO2and groundwater usually contains 1–50 ppm ofCO2 [1]. The concentration of carbon dioxide isdone by titration the sample of water with standardsodium carbonate solution until the color changesto pink where phenolphthalein is used as an indi-cator. The following reactions will be carried out:

2 2 2 3H O CO H CO+ → (1)

2 3 2 3 2 3 2 2Na CO H CO Na HCO H O CO+ → + +(2)

The concentration of CO2 in the sample is cal-culated from the formula as follows:

2MgCO / latter 22,000 / ml sampleN v= ⋅ ∆ ⋅ (3)

where N is the normality of sodium carbonate and∆v is the volume of the titration sample (ml).

Shortage of water in Iraq has caused the needfor the application of desalination processes to har-ness other types of natural water for human use;however, these produce some negative exter-nalities. The effects of gases dissolved in distilledwater produced by desalination systems include,for example, corrosion problems due to increasedconcentrations of these gases, and likewise, thepresence of the gases influences the pH values ofthe water. A separation process, called de-aerationor degasification, is therefore used to remove con-centrations of corrosive gases such as oxygen, car-bon dioxide, hydrogen sulfide or nitrogen thathave been dissolved in distilled water.

Carbon dioxide (CO2) in water may be remov-ed or reduced by means of an aerator, de-gasifier

or de-carbonator and may be neutralized by theaddition of lime or alkali such as caustic soda,but these procedures are limited to raw or treatedwaters containing relatively small amounts ofCO2. The performance of the de-carbonator great-ly depends on the rate of water flowing through itand the temperature of the water undergoing de-aeration [2]. The packed column is useful in carry-ing out mass transfer between gas and liquid whenthe fluids pass counter current flow [3]. A packedcolumn consisting of a vertical vessel and fillingwith a packed medium has been used to strip thedissolved CO2 gas via an air stream. The transport-ation process is achieved due to the difference inpartial pressure between the two streams. Themass transfer operation between the two streamsis based on the absorption theory, the concept ofresistance, and the heat transfer theory [4].

Full treatments of water are important, butnone more than settlement, which removes up to90% of the suspended solids and can affect theperformance of the whole treatments probablymore than any other single process. Also, not allwaters require full treatment and not all treatmentplants require settling basins. In any given casethe amount of treatment required has first to bedecided before considerations have given to thebest way of providing it. The aim of the sedimenta-tion process is to reduce the turbidity of water.This can be accomplished with the aid of two otherprocesses. These two processes include coagula-tion and flocculation. Different materials can beused as coagulants such as alum, ferric chloride,ferric sulfate, sodium aluminates and lime. Theinfluence of chemical additives with different con-centrations on the turbidity removal efficiency canbe examined by using a jar test.

2. Plant description

The plant has a capacity of about 9 m3/h. Theraw water is first treated by two chemical additivesin the precipitator tank, alum and polyelectrolyte,and the treated water then passes through sand,

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active carbon and cartridge filters before it ispumped to the membranes. The fresh water passesthrough the membranes while most of the dis-solved salts are rejected. Lastly, a de-carbonatorvessel is used to remove dissolved carbon dioxidein the permeate water.

The permeate water is fed from the top sectionof the vessel and passes through the distributor.The feed water stream comes into contact withthe air flow which is pumped by an air blower atthe bottom of the de-carbonator vessel. The count-er current flow is carried out through a vessel 0.75 mwide and 3 m long. The vessel is full with packingfor increasing the area of contact between the twophases (liquid and gas).

3. Results and discussion

3.1. De-carbonator system

To specify the effect of packing height on therate of CO2 removal, a comparative study betweentwo types of packing, conventional and profess-ional, was realized.

The first type of conventional packing, namelyInteox Siddele, is used to decrease the concentra-tion of CO2 dissolved in water by pumping airfrom the bottom section of the column. The CO2concentration in the feed stream is about 2.99 ppm.After the de-carbonator process is carried out, theconcentration of CO2 is reduced to about 0.799ppm for a packing height of approximately 2 m.

To increase the rate of CO2 removal, a secondtype of professional packing, plastic pall rings, isused and its comparative efficiency gauged. Atitration process using a sodium carbonate solutionto specify the concentration of CO2 in the effluentstream is conducted. The results proved that therate of CO2 removal increased by about 20%.

For specifying the best height of packing,various heights of packing are examined. Table 1depicts the effect of packing height on the carbondioxide concentration in the product stream.

The results demonstrate that the removal effici-ency of the professional pall ring type is around

Table 1Effect of packing height on the CO2 removal for bothtypes of packing

CO2 concentration (ppm) Height of packing (m) Conventional type Professional type 1.2 1.947 1.558 1.4 1.548 1.254 1.6 1.298 1.025 1.8 0.998 0.818 2.0 0.799 0.65

20% greater than the conventional type. The re-sults proved that the increased area of contactbetween the two phases, obtained when the plasticpall rings are used, is considered to be the deter-mining factor in improving the rate of CO2 transferfrom the liquid to gas phase.

3.2. Sedimentation process

The aim of this part is to establish the appro-priate dosage of two types of coagulants (alumand polyelectrolyte) for removing the turbidity ofraw water. The raw water represents the TigersRiver water with TDS equal to 750 ppm and tur-bidity of about 18 NTU. Alum and polyelectrolyteare mixed with distilled water inside two mixingtanks separately, the two tanks are provided withone vertical shaft mixer which is connected to adriving motor which has a variable speed control.The alum and polymer solutions are injected alsoseparately (at the same time) to the sedimentationtank by using two dosing pumps. The flow rate ofinjection of both pumps is 6 l/h, but the concentra-tion of each solution is different. However, theconcentration of alum and polymer solutions ischanged from 15 to 28 ppm and 0.58 respectively.The percentage of turbidity removal was calcu-lated by the following equation:

turbidity (1) turbidity (2)% removal = 100turbidity (1)

− ×

(4)

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where turbidity (1) and turbidity (2) represent theturbidity of raw and treated water respectively.

The chemical structure of alum is Al2(SO4)3.18H2O and when alum is added to the raw water,small floc particles are formed inside the treatedwater. This leads to an increase in the settling velo-city of particles and subsequently a decrease inthe turbidity values for the water to about 5 NTUwhen the concentration of alum about 28.6 ppm.

For improving the rate of sedimentation andthe quality of treated water, alum and polymerare added alternatively. Different concentrationsof coagulants have been chisen for specifying theinfluence of coagulant on the turbidity values oftreated water. When alum is added to raw waterat the initial speed of mixer of 100 rpm, and afterthe small floc particles are formed the polymer isadded at the speed of mixer of 20 rpm. The resultsshow that the values of turbidity are improved byusing this method. The value of turbidity is reduc-ed from 18 NTU to 1 NTU by using 22.9 ppm ofalum and 4.57 ppm of polymer. This is due to theeffectiveness of the polymer that contains long-chain molecules with a large number of charged(negative) groups, where the small floc particlesare aggregated by the polyelectrolyte to yield thelarge floc by chemical bridging, so that the largefloc leads to increasin the settling velocity, andincreasing the effectiveness of the coagulationprocess. Also, alum and polyelectrolyte are added

together, but at the same time and at the sameinitial mixer speed — 100 rpm. The results showthat the values of turbidity for treated water aregreater compared with the previous method. Usingthis method of addition, the floc particles insidethe raw water are formed, but the results showthat the size of floc is less than that in the previousmethod.

4. Conclusions

The efficiency of the de-carbonator columnwith the Intal Saddle type of packing is about 67%,while the efficiency is improved to about 83%when the Pall Rings packing is used. The resultsshow that injecting alum and polymer to raw waterseparately at different times is better than additionof chemical additives at the same period.

References[1] S.S. Dara, A Text Book in Engineering Chemistry:

Water Treatment. Chand Company, Ltd., 1988, p.23.

[2] F.I. Belan, Water Treatment. Mir Publishers, Mos-cow, 1981.

[3] E.D. Howe, Fundamentals of Water Desalination.Marcel Dekker Inc., 1974.

[4] L. Elmer and F. Charles, Vacuum degasification ina packed column, Chem. Eng. Prog., 49(8) (1953)527.