Ultrafiltration Pretreatment in a Large Seawater Desalination Plant in the Arabic Gulf
The water in the Arabic Gulf is known to be challenging for reverse osmosis (RO) seawater desalination.
By Ralf Krüger
The water in the Arabic Gulf is known to be challenging for reverse osmosis (RO) seawater desalination. The water is relatively shallow and the temperatures are high. Many small islands off the coast reduce the water exchange with the open sea. This results in seawater with high salinity, high organic content and microbiological activity posing a very high fouling risk for the RO membranes. Carefully designed pretreatment is absolutely essential to ensure economical plant operation.
In difficult waters, ultrafiltration (UF) has proven to be the best technology as the final pretreatment step prior to RO. UF delivers superior water quality compared with conventional treatment due to the defined, very fine pore structure. It delivers a continuously good filtrate quality independent of feed water quality variability caused by, for instance, seasonal changes.
The largest iron and steel factory in the United Arab Emirates, the ‘Emirates Steel Industries’ (ESI), based in Abu Dhabi, has undergone a significant expansion process, increasing steel production capacity from 0.7 to 1.8 Mio tons. The expansion includes a new seawater desalination plant located in Mussafah, the industrial city of Abu Dhabi. The water treatment plant primarily covers the steel mill’s demand for desalinated water for the steel production process. Secondly, it produces make-up water for the cooling water cycle.
Figure 1: Schematic flow diagram of the SWRO process
The seawater desalination plant was designed and supplied by Bernardinello Engineering S.p.A. based in Padova, Italy. The plant was successfully commissioned in January/February 2009.
Water Treatment Plant Process Design
The feed water to the seawater desalination plant is a blend of two different sources. The majority — about ninety percent of the total feed water flow — is seawater coming from an open intake in the Arabic Gulf. The remaining ten percent is low-salinity water coming from the rolling mill cooling water cycle. The water treatment plant can work with only seawater; however, in this case to reach the required water quality the total water output would be lower due to the increased feed water salinity.
After the open intake, hypochlorite and polyaluminiumchloride (PAC) are dosed to the seawater. The PAC is used as a coagulant for the dissolved air flotation (DAF) unit. The main task of the DAF unit is to remove oil and grease, which might clog the membrane surface. Another target, especially in summer when the raw water quality is potentially more difficult, is the reduction of turbidity, organics and suspended matter to improve the feed water quality to the subsequent UF system. The UF system purifies the water of biological, organic and particulate contaminants before it enters the two-pass RO system where 600 m³/h of desalinated water with a TDS of approximately 20 ppm are produced. The first-pass concentrate is discharged to the sea; the second-pass concentrate is recirculated to the feed of the first pass.
UF System Design
The decision to use UF as a pretreatment system to RO was made based on the fact that the Arabic Gulf is known for its very difficult nature. It is very shallow and has very little water movement due to its many small islands. As a result, the water can contain very high levels of turbidity, organics and bacteriological activity, especially in summer. The raw water silt density index (SDI) after 15 minutes exceeds 15 on a frequent basis.
The UF system is designed for a total net output capacity of 40,000 m³/d. It consists of a total of seven trains, each accommodating 82 dizzer® 5000plus UF-modules totalling 574 modules. The modules are mounted in a vertical position in 4 rows per rack with central collectors in the middle.
Upstream of the UF, an inline coagulation unit is installed for optional use depending on feed water conditions. By dosing ferric chloride, the inline coagulation creates micro-flocs, which proves to be especially beneficial in water with a high organic content. The flocs reduce compaction of the fouling layer improving the backwash efficiency and overall performance of the UF membranes. At the same time, organics are embedded in the flocs, increasing the organics rejection and reducing biological fouling not only on the UF membrane but on the RO membranes as well.
Of the seven trains, five are equipped with two 50% feed pumps plus one stand-by pump. For the other two trains, one pump serves as a permanent feed pump, another is in stand-by. In both plant sections, the flow is controlled by one flow transmitter in the central feed collector to all trains of the respective section. Modulating valves are installed in the feed line to each train for individual flow control.
After the feed pump, a self-cleaning filter with a nominal filtration rate of 200 microns is installed in each line. These filters protect the UF fibers from potentially harmful objects like swarf. For the backwash, two pumps plus one stand-by are installed in the section with five trains and one pump plus one stand-by is installed in the section with two trains. Feed and backwash pumps are equipped with a variable frequency controlled motor.
The UF is operated in dead-end mode with a design flux of slightly more than 72 L/(m2·h) in filtration mode. The flux rate is relatively low for seawater but due to the special circumstances in the Arabic Gulf a fairly low flux was chosen. In earlier pilots carried out in the area, the UF dizzer modules could be operated in stable conditions at this flux.
The filtration takes place from inside to outside. The design filtrate cycle time is 25 minutes until the fibers are backwashed. The feed water enters the modules from the top or from the bottom in alternating mode. This means that the modules are operated during a filtration cycle with feed water entering through the bottom port before being backwashed. In the following filtration cycle the feed water then enters from the top port. With the alternating flow directions, the total fiber length can be used more efficiently compared to feed entering the module from only one end. This way, the build-up of a fouling layer on the membrane surface can be avoided or delayed.
The difference between feed and filtrate pressure, the transmembrane pressure (TMP), is the main indicator of how clean the membrane is. In order to keep fouling or TMP, respectively, at a continuously low level close to the initial 0.1 to 0.2 bar of a new module, chemically enhanced backwashes (CEB) are carried out. A CEB is a regular backwash where a chemical is dosed into the backwash water. The type of chemical and the frequency depend on the water quality. In this case, the plant is designed for a CEB using sodium hypochlorite with approximately 20 ppm residual chlorine and a soak time of approximately 5-10 minutes up to six times a day to control biological fouling. Provisions are made for adding an additional CEB unit should this become necessary.
|Figure 2: Complete operating cycle with dizzer 5000 elements|
UF Modules and Membrane Technology
The 574 dizzer 5000plus modules installed in the UF system are manufactured by inge watertechnologies AG, a German company manufacturing UF fibers, modules and racks.
|Figure 3: Cross-section of a Multibore fiber|
The dizzer modules are equipped with the patented Multibore® UF hollow fibers operating in in-to-out mode. The fiber combines seven capillaries into one fiber (see Fig. 3), which results in an exceptionally high mechanical fiber strength. This overcomes a major obstacle of UF hollow fiber technology: a rather high degree of fiber breakages during operation. In municipal applications fiber breaks must be repaired immediately due to bacteria and viruses entering the filtrate water. In industrial applications, fiber breaks will result in quicker fouling of the RO membranes and UF will not provide its full benefits. With Multibore fibers, no fiber breakages have been reported in more than seven years of operation in hundreds of plants. The benefits for the OEM and particularly for the end user include:
- reduced maintenance and repair cost
- reduced plant down-time
- increased protection for treatment steps downstream
- increased operating safety
|Figure 4: Filtrate and backwash flow in the annular gap design of the dizzer module|
The Multibore fibers are made from modified polyethersulphone with a high pH tolerance from 1 to 13, which allows efficient cleanings even in extreme conditions. The inner diameter of each individual capillary is 0.9 mm, which is larger than the widespread 0.7 – 0.8 mm individual fibers. Furthermore, a larger diameter reduces the pressure drop and supports an even flow through the membrane along the fiber length, which distributes contaminants more evenly and increases backwash efficiency. The nominal pore size of the membranes is approximately 0.020 mm, which provides for an effective rejection of bacteria and viruses.
The dizzer module design is characterized by an annular gap between the outer shell and the inner tube to collect the filtrate and to backwash from. This is in contrast to the commonly used central core tube, which originates from the RO design without considering the specific hydrodynamical operating conditions of the UF.
During backwash, water flows from the annular gap towards the middle of the module. On its way, the total backwash water flow is reduced as some of the water enters the fibers. Since the number of fibers also decreases towards the middle of the module, the backwash flux will remain at a similar level over the complete cross-section of the module. This results in a more efficient cleaning and subsequently in higher permeability and less chemical consumption (see Fig. 4).
The dizzer modules are mounted vertically to enable proper de-aeration of the system during operation and after integrity tests. Proper de-aeration eliminates water hammers, particularly in large systems. The vertical arrangement allows simple access to each module in case of maintenance. The modules are delivered with integrated shell, end caps and transparent connecting ports.
The decision to utilize these modules was based to a large extent on the additional operational safety benefits along with commercial benefits to the OEM and the end user. Furthermore, inge could demonstrate a convincing reference situation with dizzer modules operating in a number of seawater plants, including pilot plants.
Plant Operational Data
The UF system was commissioned in January 2009 and took fifteen days. Due to ongoing construction work, the actual water demand is not yet at design level. For this reason, only three racks were initially started up. This enabled careful control of the membranes performance, resulting in an easier set-up of the UF system.
Initially, the only feed water available was raw seawater because the steel mill, which would supply the low-salinity water from the cooling water system, was not yet fully operational.
The UF feed water treated by the DAF units exhibited consistent quality during the commissioning period. The SDI 15 was in the range of 4.6 and the turbidity was about 5.3 NTU. Chemical analysis of the water coming from the DAF provided no detectable values for organic compounds, oil, or grease. The winter season, with its reduced biological activity, most likely accounted for the relatively good water quality. This is expected to change significantly in summer.
The raw seawater was treated with polyaluminiumchloride as a coagulant upstream of the DAF system. The inline coagulation system immediately in front of the UF was not in operation.
For initial disinfection prior to start-up, a sodium hypochlorite solution with a concentration of 100 ppm residual chlorine was introduced into each rack and left there to soak overnight. The initial working flow rate was set at 250 m3/h per rack, resulting in a flux rate of approximately 61 L/(m2·h). For the first three days, each row was continuously operated for only 10 hours and then left in sodium hypochlorite solution (100 ppm) overnight for further disinfection.
|Figure 5: Turbidity and TMP during the first month of operations|
The intensified disinfection procedure was instituted because the modules had been on site pre-installed in the racks for almost a year without any special preservation treatment. Due to the extreme ambient conditions, it was likely that some biological fouling had occurred. This is supported by the fact that the SDI 15 values detected at the UF outlet in the first days of operation were much higher than later on during operation. After three days the SDI 15 quality of the ultrafiltrated water improved dramatically from 2.4 to 0.8. It continued to gradually decrease and stabilized at 0.4. At this point the flux rate was set to the design value of 72 L/(m2·h).
Figure 6: SDI 15 values before and after UF
After the commissioning period, the UF system continued to operate at design parameters. Currently, due to low demand, three racks are operated at a time. Every four weeks, one rack is shut down and another rack is started up to prevent fouling, which could happen if some racks were permanently left in stand-by mode. Right before a UF rack is shut down, a CEB is carried out and the modules are left with a concentration of approximately 20 ppm residual chlorine.
At the time of publication, the data available cover a period of 27 days of continuous operation. Figures 5 and 6 show some of the more significant UF operating parameters. The feed water temperature was between 26 and 29°C.
The TMP shows a very stable trend, staying below 0.2 bar even during a short-term increase of the feed water turbidity caused by a leaking underground pipe upstream of the water treatment process. The pipe was repaired on February 6.
At day 19, the SDI 15 of the feed water increased to an immeasurable value, probably due to a change in the feed water mixture resulting from the steel works. What is remarkable is that the quality of the ultrafiltered water remained at an SDI 15 of less than 0.4.
About the Author:
Ralf Krüger began his career in 1992 as a proposal and project engineer in the Engineering and Contracting Division of Linde AG. In 2000, he joined Hydranautics as Sales Manager - Central Europe for RO and UF membranes. In 2005, he became head of international sales at inge watertechnologies where his focus is on market development in the EMEA countries.