Abstract

Standardization of reverse osmosis (RO) membrane system allows transparency and accountability in performance benchmarking of different membrane elements, especially when a new product is introduced to the market. In the current study, we compared performance of three polyamide composite RO membranes (one from new entrant in the market, the other two are established manufacturers) for seawater desalination. Experimental work was conducted at a desalination plant in Egypt. The new membrane had higher permeate conductivity and lower salt rejection values than the two established products. Similar trend was observed as far as permeate flow was concerned.

INTRODUCTION

Reverse osmosis (RO) plant owners, operators, designers and membrane manufacturers have benefitted greatly due to adoption of standardization in element design, material and supporting equipments (Fritzmann et al. 2007). Standardization allows transparency and accountability in performance benchmarking of different membrane elements, especially when a new product is introduced to the market and a plant owner is willing to yardstick against established players. In the current study, we compared performance of three polyamide composite RO membranes (one new membrane, the other two are established manufacturers) for seawater desalination.

MATERIALS AND METHODS

The study was conducted at a desalination plant in Egypt, from 06th June 2015 to 30th April 2016. The 21,000 m3/day capacity plant receives seawater from Red Sea and the feed water conditions are summarized in Table 1.

Table 1

Feed seawater characteristics at desalination plant in Egypt

Feed water parametersRangeAverage
Conductivity (μS/cm) 55,000–64,100 59112 (±2408) 
Temperature (°C) 23.0–30.2 27.6 (±1.33) 
pH  7.2 
Feed water parametersRangeAverage
Conductivity (μS/cm) 55,000–64,100 59112 (±2408) 
Temperature (°C) 23.0–30.2 27.6 (±1.33) 
pH  7.2 

Data collected between 6th June, 2015 and 30th April, 2016.

The plant operators spared three pressure vessels (PVs) for the study, each containing six elements. The location of the test PVs in membrane train is shown below (Figure 1).

Figure 1

Location of PVs in the train of the desalination plant.

Figure 1

Location of PVs in the train of the desalination plant.

The general information and application data for each membrane is summarized in Table 2. SWRO1 is the new membrane entrant to market while the other two are well established and have numerous references globally. The data in Table 2 indicate that except for larger membrane area in SWRO2 other elements characteristics are comparable.

Table 2

General information and application data of SWRO elements

General InformationSWRO1SWRO2SWRO3
Feed spacer thickness 0.8 mm 0.8 mm 0.8 mm 
Membrane area 37.2 m2 41 m2 37 m2 
*Salt rejection, av. 99.80% 99.80% 99.75% 
*Salt rejection, min 99.50% 99.55% 99.50% 
*Boron rejection, typical 93.00% 91.50% 93.00% 
*Permeate flow rate, av. 24.6 m3/d 37.48 m3/d 24.6 m3/d 
Application DataSWRO1SWRO2SWRO3
Operating pressure, max. 83 bar 83 bar 83 bar 
Operating temperature, max. 45 °C 45 °C 45 °C 
pH range during operating 2 to 11 2 to 11 2 to 11 
pH range during cleaning 1 to 12 1 to 13 1 to 12 
Pressure drop per element, max. 1.0 bar 1.0 bar 1.4 bar 
Pressure drop per vessel, max. 3.5 bar 3.4 bar 4.0 bar 
Chlorine concentration, max. 0.1 ppm <0.1 ppm <0.1 ppm 
General InformationSWRO1SWRO2SWRO3
Feed spacer thickness 0.8 mm 0.8 mm 0.8 mm 
Membrane area 37.2 m2 41 m2 37 m2 
*Salt rejection, av. 99.80% 99.80% 99.75% 
*Salt rejection, min 99.50% 99.55% 99.50% 
*Boron rejection, typical 93.00% 91.50% 93.00% 
*Permeate flow rate, av. 24.6 m3/d 37.48 m3/d 24.6 m3/d 
Application DataSWRO1SWRO2SWRO3
Operating pressure, max. 83 bar 83 bar 83 bar 
Operating temperature, max. 45 °C 45 °C 45 °C 
pH range during operating 2 to 11 2 to 11 2 to 11 
pH range during cleaning 1 to 12 1 to 13 1 to 12 
Pressure drop per element, max. 1.0 bar 1.0 bar 1.4 bar 
Pressure drop per vessel, max. 3.5 bar 3.4 bar 4.0 bar 
Chlorine concentration, max. 0.1 ppm <0.1 ppm <0.1 ppm 

*Test Conditions: applied pressure 55.2 bar, NaCl concentration 32,000 mg/L, boron concentration 5 mg/l, operating temperature 25 °C, pH 8 and recovery 8%.

Source: Manufacturer Product Sheets.

The plant operators maintained train feed pressure at 60 bar for the duration of the study. Feed pressure of the three test PVs could not be measured due to system constraints. Nevertheless, the objective was to profile performance, i.e. permeate conductivity, salt rejection and permeate flow, in varying feed seawater conditions.

RESULTS AND DISCUSSION

Permeate conductivity and flow was measured in the first 24 hours of start-up operation of the three PVs. Their comparative values are shown in Figure 2 and Figure 3. SWRO1, the new entrant, had higher permeate conductivity and lower permeate flow than SWRO2 and SWRO3. The two established membranes had comparable conductivity values while SWRO3 had better permeate flow values.

Figure 2

Permeate conductivity during start-up of three SWRO membranes.

Figure 2

Permeate conductivity during start-up of three SWRO membranes.

Figure 3

Permeate flow during start-up of three SWRO membranes.

Figure 3

Permeate flow during start-up of three SWRO membranes.

Figure 4 and Figure 5 show permeate conductivity and salt rejection results of the three SWRO membranes, respectively, for the whole duration of the study. As expected, it took a couple of days before performance became stabilized as well as approaching manufacturer specification for all the membranes. The average salt rejection in SWRO1, SWRO2 and SWRO3 were 98.95%, 99.64% and 99.62%, respectively. As observed during start-up, performance was very similar in SWRO2 and SWRO3 while SWRO1 had higher permeate conductivity and lower salt rejection values consistently.

Figure 4

Permeate conductivity profile of three SWRO membranes.

Figure 4

Permeate conductivity profile of three SWRO membranes.

Figure 5

Salt rejection profile of three SWRO membranes.

Figure 5

Salt rejection profile of three SWRO membranes.

Permeate flow data, as illustrated in Figure 6, stabilized quickly for SWRO1. SWRO2 stabilized after approximately 50 days while steady decline was observed in SWRO3. The average flow values for SWRO1, SWRO2 and SWRO3 were 3.60 m3/hour, 4.10 m3/hour and 3.82 m3/hour, respectively. SWRO2 permeate flow, while considering higher membrane area, was better than the other two during the whole trial. Flow recovered in all the membranes after chemical cleanings, only to decline immediately.

Figure 6

Permeate flow profile of three SWRO membranes.

Figure 6

Permeate flow profile of three SWRO membranes.

To get better understanding of the performance of the individual elements, conductivity profiling was done periodically during the study. The comparative values for SWRO1, SWRO2 and SWRO3 are summarized in Figure 7. The observation was consistent as above with SWRO1 having consistently higher permeate conductivity than the established SWROs in all six elements. SWRO2 and SWRO3 had similar profile across the elements. Another observation from Figure 7 was that the conductivity increased from lead element (Element 1) to last element (Element 6). This was expected as elements treat progressively higher saline water as seawater passes through the elements in a pressure vessel.

Figure 7

Conductivity profile individual elements of three SWRO membranes.

Figure 7

Conductivity profile individual elements of three SWRO membranes.

CONCLUSIONS

The new membrane (SWRO1) had higher permeate conductivity and lower salt rejection values than the other two established products. Similar trend was observed as far as permeate flow was concerned. However, SWRO3 had the sharpest permeate flow decline in the first 170 days. When all three parameters were considered, SWRO2 performed the best during the pilot trial.

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