Malta is an archipelago of three islands situated in the Mediterranean Sea, around fifty miles south of Sicily. There are no rivers of any significance on the islands, and the sparse annual rainfall is only about 500 mm. In order to bridge the gap between supply and demand, Malta has long ago started desalination of seawater. The technologies were initially based on evaporation. In 1981 the Government decided to invest in RO desalination capacity. In 1983, after a construction period of only 11 months, the first seawater RO facility in Ghar Lapsi became operational. This was followed by a second plant at Cirkewwa in 1988 and third plant at Pembroke in 1994. In 1992 a government owned company, Water Services Corporation (WSC) was formed to supervise the water production and to manage the distribution system throughout the country.
Today WSC operates three desalination plants in Malta. For all three plants seawater is extracted from deep shore wells sunk in Coralline Limestone. This water is of very good quality, free from silt and organic material (SDI<1). The plant in Ghar Lapsi consists of 12 single-pass trains operating at 33% recovery to produce 24,000 m3/day of potable water. The second plant in Cirkewwa operates 5 single-pass trains at 42% recovery to produce 18,600 m3/day. Finally, the third plant in Pembroke produces 54,000 m3/day of water on 12 single-pass trains at 45% recovery. The water produced by the plants is used for drinking and irrigation. The latter implies stringent requirements on the boron concentration in the final product. The local regulations require the boron concentration in the permeate stream to remain below 0.9 mg/l after 5 years of operation.
To address these challenges, WSC decided to invest in retrofitting the RO plants and replace the installed RO membranes with new ones that improve product water quality. After comparing RO membranes available on the market, WSC selected LG Chem SWRO membranes. These membranes are manufactured with the patented Thin-Film Nanocomposite (TFN) technology that provides higher productivity and best-in-class salt rejection. Overall, 8 trains in the Pembroke facility, 5 trains in Ghar Lapsi, and 2 trains in Cirkewwa were scheduled for replacement with LG Chem membranes.
Started in March 2016, the replacement is largely completed by now. A hybrid configuration with LG membranes is employed in Pembroke plant while the other two plants are retrofitted with the conventional single-type element configuration. Table 1 summarizes the specifications of the RO membranes installed in all three plants.
Table 1: Specification of LG Chem SWRO membranes installed in Malta plants.
The results obtained in this study during more than two years of operation clearly demonstrate successful and reliable long-term performance of LG seawater membranes. Overall, the salt passage is very low and translates to a system salt rejection of about 99.8% on average between all the trains (Figure 1). The consistency of the product quality delivered by LG trains is explained by the stability of the nanocomposite element performance (salt passage) and the constancy of the operating conditions (feed salinity and temperature) throughout the year.
Figure 1. Normalized salt passage of LG membranes in Pembroke plant.
One of the most challenging tasks in operating these plants is to meet stringent regulations for boron concentration in the final product. During the past two years LG membranes demonstrated an outstanding performance with respect to boron rejection. The permeate boron concentration varied in the range of 0.40 – 0.73 mg/l in the first two years of operation and is significantly lower than the maximum allowed 0.9 mg/l. This translates to the normalized boron rejection (back calculated using the proprietary projection software Q+) close to 95% (Figure 2). The latter results become even more impressive considering the fact that the RO feed water is dosed with sulfuric acid which makes it even more challenging to maintain low boron levels in the permeate stream (feed water pH is about 6.7).
Figure 2: Normalized boron rejection at standard SWRO test conditions using LG NanoH2O projection software Q+.
This case study demonstrates a successful implementation of LG membranes to maintain excellent water quality and low boron concentration without use of additional passes or pH adjustment bringing potential OPEX benefits to desalination facilities