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. There is a water deficit in Malta. It occurs especially in summer when there is a great demand from the farmers for their irrigation and from the tourism sector. 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 all 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. 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.
Due to increasing water demand on the island and stringent regulations for drinking water, including low TDS and boron maximum concentration not exceeding 0.9 mg/L, 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 thin film nanocomposite (TFN) SWRO membranes that provide
s the best finished water quality and optimal fit in the actual plant configuration. Overall, 8 trains in the Pembroke facility, 5 trains in Ghar Lapsi, and 2 trains in Cirkewwa are scheduled for replacement with LG Chem SWRO TFN that started in 2016 and will be finished in the upcoming months.
In March 2016, LG replaced the existing 630 membranes in two trains in Pembroke RO Plant followed by four more trains later that year and in 2017 bringing the total number of LG elements to 1,890. These trains have 45 pressure vessels each, with 7 elements per vessel. Membrane replacement also started in Ghar Lapsi and Cirkewwa facilities in 2017. A hybrid configuration with LG membranes is employed in Pembroke: five (5) LG SW 440 SR and two (2) LG SW 440 GR elements. The hybrid design lowers the feed pressure and energy consumption and maintains excellent water quality. Table 1 summarizes the specifications of the RO membranes installed at Pembroke plant.
Table 1: LG Chem TFN SWRO specifications installed in Pembroke RO Plant
Figure 1 shows the operating conditions and performance of all trains with LG membranes installed in Pembroke plant. Conductivity and temperature of the feed water remain stable throughout the year, as does the feed pressure. All trains demonstrate a very stable performance and high quality permeate water.
Figure 1. Operating Parameters for LG Membranes
Figure 2 shows normalized salt passage performance of the LG trains. The system salt passage is the ratio of the product TDS over the RO feed TDS. The value is normalized with respect to the initial stabilized performance at startup.
Figure 2: Evolution of normalized salt passage in LG trains for the first year of operation
Overall, the salt passage is very low and translates to a system salt rejection of about 99.8% on average between all the trains. The product TDS fluctuates around 80 ppm. 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.
One of the most challenging tasks in operating these plants is to meet stringent regulations for boron concentration in the final product. The LG TFN SW membranes performance is outstanding with that respect. The boron concentration varied from in the range of 0.5 – 0.6 ppm throughout the first year of operation and is significantly lower than the maximum allowed 0.9 ppm which was a challenge to meet by other competitors.
The Pembroke RO desalination plant installation showcases the benefits of using LG Chem TFN SWRO membranes: excellent product quality and low energy consumption.