Effects of the Chemical Injection Points in Pre-treatment

Desalination 167 (2004) 209–216

0011-9164/04/$– See front matter © 2004 Elsevier B.V. All rights reserved

Presented at the EuroMed 2004 conference on Desalination Strategies in South Mediterranean Countries: Cooperationbetween Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the EuropeanDesalination Society and Office National de l’Eau Potable, Marrakech, Morocco, 30 May–2 June, 2004.

*Corresponding author.

Effects of the chemical injection points in pre-treatmenton reverse osmosis (RO) plant performance

M. Hafsi*, A. Khaoua, S. Ben Abdellah, M. El Mghari TabibDirection Contrôle de la Qualité des Eaux, Station de Traitement, avenue Oued Akrach,

BP Rabat Chellah,10002 Rabat, MarocTel. +212 (7) 759600; Fax +212 (7) 752377; email: [email protected]

Received 5 March 2004; accepted 15 March 2004

Abstract

Good characterization of raw water quality and subsequent identification of the most appropriate pre-treatmentare not sufficient to ensure successful functioning of a reverse osmosis plant. The Tan Tan demineralization plantwas set up in July 2003 for 20 l/s production capacity. The brackish water with total dissolved salt (TDS) of 3 ppmis supplied from Khang Lahmam underground water located 70 km from the plant. Since it started working, themain problem encountered in the Tan Tan demineralization plant was the continuous presence of a membranebiofouling problem which provokes high pressure drop values and requires an increased washing frequency, asoften as twice a month. The diagnosis undertaken to remedy this anomaly has concerned all the pre-treatment stepsincluding the raw water, quality of the applied chemical reagent types and concentrations, and the composition ofthe biofilm. Collected data confirm the good raw water quality specially the absence of heterotrophic bacterium(aerobic-anaerobic analyzed at 37°C+1°C and to 22°C+1°C) and low oxydation values (<3 ppm). Chlorine-rawwater contact time, about 17 h, is largely sufficient to insure its efficiency. Analyses of the biofilm have shown theabundance of heterotrophic germs Pseudomonas sp. and ferobacter absence despite the reddish color of the biofilm.The hypothesis retained to interpret the origin of the biofouling is the sodium bisulfate injection point, upstream ofthe microfilter. The microfilter then plays the role of a bioreactor favoring a bacterial proliferation upstream of themembranes. The simple displacement of the injection point of that of sodium bisulfate will allow remedyingdefinitively the disorder caused by frequent appearance of biofouling. The present article, through the analysis ofoperational data, and particularly of results and actions undertaken, focuses on the crucial role of reagent injectionpoint in optimization of reverse osmosis plant operation.

Keywords: Demineralization; Biofouling; Biofilm; Appropriate pre-treatment

210 M. Hafsi et al. / Desalination 167 (2004) 209–216

1. Introduction

The reverse osmosis Tan Tan demineralizationplant began operations in July 2003, to reinforcethe drinking water supply of Tan Tan and neigh-boring communities. Endowed with productioncapacity of 20 l/s, the plant is supplied with brackishwater from Khang Lahmam under ground waterhaving a salinity of about 3 g/l. Since its start-up,the operation of this plant was confronted with asemi-permanent presence of the biofouling pheno-menon which increases membrane pressure drop.The first consequence was the very high frequencyof cleaning, as often as twice per month.

2. Description of the plant

2.1. Raw water characterization

The raw water to treat is characterized by atotal dissolved solids (TDS) of about 3300 ppm,good bacteriological quality and a very loworganic content (<3 ppm). The mixture and pre-

Treated water reservoir

Prechlorination

Raw water reservoir

Sand filterMicrofilter

Line 1

Brine

SBS

Antiscalan

Borehole

By-passed brackish water

NaOH

H2SO4

CaOCl2

Borehole

Borehole

Fig. 1. Schematic presentation of the Tan Tan demineralization plant.

chlorination of water of three boreholes are madein a reservoir located at Zaouia to approximately80 km from the plant.

2.2. Presentation of pre-treatment

The pre-treatment was designed to improve theraw water quality, and to conform the pre-treatedwater with the reverse osmosis membranesrequirements. It includes the steps shown in Fig. 1.

Chlorine injection in the raw water is done80 km upstream of the demineralization plant.About 17 h is approximately the contact timeduration between chlorine and raw water. It isdesigned to inhibit any bacterial proliferation, ableto generate the formation of a biofilm, and ifnecessary to oxidize any manganese and iron tracein raw water.

2.2.1. Acidification

Sulphuric acid was injected to prevent calciumor carbonate scaling and to adjust pH to pH 6.9.

M. Hafsi et al. / Desalination 167 (2004) 209–216 211

2.2.2. Sand filtration

Sand filtration is done by two high pressurevertical filters (one filter per line) filled with sandwith a diameter of about 1 mm, to retain thesuspended solids that might be present. The speedof filtration is 10 m3/m2.h.

2.2.3. Antiscalant injection

Injection of antiscalant agent (Flocon 260)upstream of the microfilters ensures the inhibitionof sulphate salt and iron scaling at the membranes.

Sodium bisulfate and microfiltration: pre-treated water dechlorination is attained by Sodiumbisulfate (SBS) injection at the entry of themicrofilters, which are the ultimate protection ofthe membranes against any suspended matter.

2.3. Reverse osmosis stack

The reverse osmosis plant production capacityis 1740 m3/d with a TDS <500 ppm. The totalproduction is assured by thirty-six membranes oftype Filmtec BWLE 440 DOW. There aredisposed in brine-staging configuration. The con-version ratio is about 67–69%. Both lines 1 and 2are laid out in parallel and operate under the sameconditions.

2.4. Post-treatment

The post-treatment of produced water to re-establish its calco-carbonic equilibrium is ensuredin two steps:• Mixture of the membrane produced water with

an equivalent quantity of brackish water• pH adjustment by sodium hydroxide injection

The treated water quality has to respect thefollowing conditions:• Saturation ratio of 1.2–1.5• Saturation index of 0.1–0.3• Ca2

+ of 8°F (<32 ppm)• pH<8.2

This water was disinfected by calcium hypo-chlorite injection.

3. Analysis of working conditions

In order to optimize the working of the reverseosmosis Tan Tan plant, checks of the water quality,pre-treatment and physical parameters of opera-tion (flow, pressure) were established. Waterconductivity, water flow, and membranes pressuredrop was continuously controlled.

Initially, the measured values for these threeparameters were respectively 39 m3/h, 70 µ.s/cm,2.0 bar in the first stage and 1.2 bar in the second.

After approximately one month of working, itwas noticed:• The pressure drop increased to 2.2 bar at the

level of the first stage (line 1).• The pressure drop at the level of the second

stage, quality and total quantity remainedstable.

One month later the continuous increase ofpressure reached the threshold limit value (3.1 bar)which causes shut down of the plant. Inspectionof the equipment upstream of the reverse osmosismembranes showed the presence of gelatinoussubstance, specific to biofilm development.

To overcome this abnormality, it was decidedto examine how to remove the biofilm and howto remedy definitely to the origin of this disorder.Two actions were undertaken:• Chemical cleaning using sodium hydroxide at

0.2% by recirculation during 5 h, then rinsingwith produced water

• To re-establish the prechlorination, first byapplying the chlorination shock at 3 ppm todisinfect the system from the chlorine injectionpoint to the high pressure pump, about 80 km.Then applying the raw water chlorine demand,0.6 ppm, was adopted. The residual chlorineobtained at the entry of the plant was about0.4 ppm, after about 17 h water–chlorinecontact time.


3.1. Observation of disorder

Figs. 2 and 3 give the pressure drop evolutionover four months.

Line 1: The evolution of pressure drop at thefirst stage showed that after approximately amonth, an abrupt increase was measured asindicated before. Approximately a month ofexploitation after this first fouling a secondincrease was noted to reach the automaticthreshold lately pressure drop evolution regulatedat 3.5 bar, thus indicating the second fouling. Indetail examination of the profile shows an expo-nential tendency.

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Fig. 2.Line 1: profile of the pressure drop evolution.

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Fig. 3. Line 1: detail of pressure drop evolution showing the exponential profile.

Line 2: A continuous increase of the pressuredrop was noted in the first stage without reachingthe maximum automatic threshold limit value.However, during all this period, the pressure dropremained practically constant (1.4 bar) in thesecond stage.

Having regarded the evolution of the pressuredrops and visual observations it was possible toconfirm that the origin of the observed disorder isthe membrane biofouling:• Inspection of the equipment upstream of the

membranes revealed abundant gelatinousmatter.


• Only the first stage was affected by the pressuredrop increasing, with a typical exponentialform, specific to the consequences of thebacterial proliferation.

• The presence of the salt crystals in the secondstage argues in favor of mineral fouling.

4. Actions undertaken

To remedy the disorder, some actions wereundertaken to identify the biofilm origin andcomposition, to remove it, thereafter to adopt theappropriate modification in the pre-treatment.

4.1. Analysis of biofilm composition

The bacteriological analysis of the biofilm(Table 1), carried out respectively by the CentralWater Quality Control Laboratory (ONEP) andthe Brest Ocean Laboratory (France), revealed anabsence of heterotrophic bacterium (aerobic–anaerobic analyzed at 37°C+1°C and to 22°C

+1°C), an abundance of common heterotrophicgerms: Pseudomonas sp., and an absence offerruginous bacteria, in spite of the reddish colorof the biofilm.

4.2. Optimization of the pre-treatment

The development of the biofilm in spite of thepresence of prechlorination can be explained onlyby bacterial proliferation after the chlorineneutralization by sodium bisulfate injection. Thisis perfectly in concordance with former studieswhich attronite the phenomenon to a bacterialresistance to chlorine. Indeed, the rare bacteria,initially present naturally in water, could resist theremnant effect of chlorine, then after neutrali-zation of the chlorine by SBS, a very importantdevelopment happened on the microfilters whichwere used as support for the biological increaseactivity.

To overcome this abnormal situation, the SBSinjection point was moved from upstream todownstream of the microfilters.

An immediate effect on the pressure drop onthe level of microfilters was measured as given inFig. 4.

To explain these results other chemical analysisconsisting of total organic carbon measure wereconducted. Results are given in Table 2.

Fig. 4. Microfilters pressure drop profile evolution.

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10H0022H0010H0022H0010H0022H0010H0022H0010H0022H0010H0022H0010H0022H00

3/12 4/12 5/12 6/12 7/12 8/12 9/12

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)

SKID I SKID II

Displacement of SBS injection point

Table 1Bacteriological characterization of the biofilm

Microorganisms at 22°C, 68 h >3 e+007 n/ml Microorganisms at 36°C, 44 h >3 e+007 n/ml


Examination of these results lets us make thetwo following remarks:• When chlorine neutralization by sodium bi-

sulfate was carried out upstream of the micro-filters, the concentration of the total organiccarbon (TOC) in water increased after passagein the microfilters, because of increased biolo-gical activity in the microfilter itself.

• When chlorine neutralization by sodium bi-sulfate was carried out downstream of the themicrofilters, the concentration in TOC fell afterpassage in the microfilters. This indicates theefficiency of the microfilters in stopping sus-pended matter in general and organic matterin particular, that is present in the pre-treatedwater.

4.3. Optimization of the conversion ratio

In the second stage of the operations period,the researchers noted the presence of salt crystals.Analysis of the operating condition of the plantrevealed in fact that changes of rates of raw waterwere operating. Indeed during the startup of theplant, researchers increased the conversion ratioby decreasing the rate of raw water, this variationwas about 2–6%, while keeping the membraneproduction flow at the level of the first stageconstant. The raw water supplying the secondstage then had an abnormally high salt concen-tration relative to design conditions, while the anti-scalant amounts were kept constant. To remedy

Table 2Total organic carbon evolution at different pre-treatment step

Total organic carbon, ppm Sampling point Before the modification of the injection point* After the modification of the injection point**

Raw water 0.4 0.89 Filtered water, SF 0.3 1.33 Filtered water, µF 0.5 0.75 Produced water 0.49 Produced water 0.54

*Prechlorinated raw water; **Not prechlorinated raw water

this situation, dose of antiscalant and conversionratio were adjusted.

4.4. Cleaning of membranes

After the identification of the origin of thedysfunction, it was appropriate to establish acleaning protocol of the membranes, to removethe biofilm and to inhibit the bacteriologicalactivity upstream the membranes.

The membrane cleaning protocol consisted ofwashing the membranes with several chemicalsolutions:• Citric acid. Two percent recirculation during

50 min with velocity of 18 m3/h under 3.7 barpressure assumed to remove mineral cloggingin general.

• Na EDTA (0.1%) recirculation to dissolvecalcium and magnesium salts.

• NaOH (2%) recirculation to remove thebiofilm.

• Formol (0.5%) recirculation during 50 minassumed to disinfect the membranes surface.

In spite of the pre-treatment modification andthe chemical cleaning, the pressure drop evolutionremained above normal values, and tended toincrease. Fig. 5 gives the pressure drop evolutionmeasured for each stage.

These pressure drops remained constant in thefirst stage around 2.75 and 1.5 bar in the secondstage for line 1, and 2.6 bar on the first stage and


1.5 bar on the second stage, for line 2. It appearsthat the chemical cleaning did not remove thebiofouling problem, however it inhibited the bac-terial proliferation in contact with membranes. Wecan simply conclude that the chemical cleaningprotocol must be reviewed in terms of kind ofreagent, amount, and application condition.

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Fig. 5a. Line 1: pressure drop evolution profile.

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Fig. 5b. Line 2: pressure drop evolution profile.

5. Conclusion

Even if the undertaken actions did not permitoperators to re-establish the start-up pressure dropvalues in each level of the plant, it allowedfocusing on the role of the SBS injection pointchoice. Then it is important to retain in particular:


• The choice of SBS injection point, meant toneutralize the residual chlorine, is very im-portant for inhibition of the biofouling appear-ance. In Tan Tan demineralization plant theSBS neutralization upstream the microfiltersgenerated a high biological activity in thesefilters which played a bioreactor role. Thedirect consequence was then membranes bio-fouling.

• The chemical cleaning applied did not allowremoving the biofouling, the formal is not theappropriate reagent to reach this target.

• The use of the residual chlorine, after havingchanged the SBS injection point, gave imme-diate and efficient results. The chemical clean-ing protocol to be optimized have probably touse the sodium hypochlorite or an equivalentreagent to re-establish the start-up working

conditions in Tan Tan reverse osmosis demine-ralization plant.

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Effects of the Chemical Injection Points in Pre-treatment

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