Water utilities and developers

Desalination

Membrane desalination

Membrane desalination is a major technology which has seen considerable development over recent years.

Electrodialysis

In Electrodialysis (ED), developed in the early sixties by Ionics of Watertown in the USA, the ions responsible for salinity are caused to migrate through membranes under the influence of an electric charge, with positively charged cations such as sodium and calcium migrating to the cathode, and negatively charged anions such as chloride and sulphate migrating to the anode.

The ED cell is made up of alternating layers of membranes, cation selective and anion selective,which are separated by permeable spacers. The application of an electrical charge across the cell will result in the flow through adjacent streams being concentrated and diluted respectively. In the early seventies, Ionics further refined the process to minimise the effects of scale and foulant build up on the membrane surfaces. In this process (polarity reversal Electrodialysis – EDR), the polarity of the electrodes is reversed periodically,which therefore reverses the roles of the product and brine streams.

The displaced contaminants are flushed to waste before the product stream is collected. Because of the energy requirements, ED and EDR are seldom used for seawater desalination but do find use in brackish water desalination applications. One advantage of ED is that the energy required is in direct proportion to the amount of salt to be removed.

One example where this technology found early application was at Benina in Libya,where increasing demand from the growing municipality of Benghazi resulted in saline intrusion of the coastal aquifer.

Perhaps the most interesting membrane desalination process and that which has certainly seen the most significant developments in recent years is that of Reverse Osmosis of seawater (SWRO) and of brackish waters (BWRO).

There can be no doubt that the applicability of BWRO to the re-use of polluted and contaminated waters has been a major driver in the rapid progress and increasingly widespread use and acceptance of Reverse Osmosis as a preferred technology.

Reverse osmosis

Osmosis, the term derived from the Greek word meaning ‘push’, is used to describe the spontaneous flow of solvent into a solution, or from a more dilute solution to a more concentrated solution,when the two liquids are separated from each other by a semipermeable membrane.

While this phenomenon had been observed using natural (animal tissue) membranes, it was not until 1864 when M. Traube suggested the use of copper ferrocyanide that artificial semipermeable membranes were produced.

The term Reverse Osmosis was coined (and first patented) in 1931 and as its name implies, describes the process of forcing the flow of solvent from a solution, or from a more concentrated solution to a more dilute solution, through a semi-permeable membrane by the application of a pressure greater than that of the natural osmotic pressure of the solution. Thereafter, the development of Reverse Osmosis has been dramatic. Breton & Reid first demonstrated the semi-permeable characteristics of Cellulose Acetate in the USA in 1959.

This was followed by the development of the first asymmetric cellulose acetate membrane by Loeb & Sourirajan in 1962,which was utilised the spirally-wound element by General Atomics in the following year. Thereafter development continued along two main paths. Hollow fibre membranes were developed by DuPont in 1967 (nylon) and 1969 (polyamide); Dow Chemical in 1971 and Toyoba in 1978 (cellulose triacetate). Thin film composites were developed by North Star / Filmtec from 1964 onwards (cellulose acetate).

The interfacial composite membrane was developed by John Cadotte in 1972 (polyamide) who went on to establish the Filmtec corporation. Research into membrane and support materials and into RO element construction methods has progressed to the stage where all major manufacturers of RO membranes are able to offer a product with high salt rejection at low operating pressures.

For BWRO applications, Ultra Low Pressure (ULP) elements are now available. Simultaneous development of energy recovery systems has now brought down the cost of desalination reverse osmosis to levels below those of comparable thermal installations.

The Pelton turbines fitted to the drive shafts of high pressure pumps during the eighties to recover energy from the high pressure concentrate stream have now given way to ‘work energy exchange’ systems, capable of operating at 97-98% efficiency. Many of the more recent SWRO plants utilise a Dual-Work-Exchange- Energy-Recovery technology developed by DWEER of Bermuda and now licensed to Calder of Switzerland. This enables the high pressure pump to be sized to deliver a feed flow equivalent only to the permeate flow plus the very minor ‘leakage’ of the DWEER unit, with the latter imparting almost the entire energy of the concentrate stream to the remaining feed flow. This will then allow a much lower pressure ‘recycle’ booster pump to bring the feed flow to the design requirement. The DWEER system is a doublechamber reciprocating, hydraulically driven pump and the key to its successful operation is a patented ‘Linear Exchanger’ (LinX) Valve.

An animated representation on Calder’s website shows quite elegantly how the system works.

ERI (Energy Recovery Inc) of San Leandro have developed a similar system which utilises a rotary hydraulically driven pressure exchanger. ERI are a key collaborator in the ‘Affordable Desalination Project 2005’which is currently being established to demonstrate a SWRO system with an energy consumption of 1.7kWh/m3 of permeate produced (for the RO section). The equipment will be installed at the US Navy’s Seawater Desalination Test Facility at Pt. Hueneme, California. Needless to say, the results are eagerly awaited.

One barrier to more widespread acceptability of SWRO as a desalination process of first choice has been the low single ion rejection co-efficient for boron but even here, advances have been made. Toray Membranes America have reported recent single element tests on a new membrane formulation where boron rejection rates as high as 95% were observed. Notwithstanding any perceived limitations, there have been several major SWRO projects initiated recently which provide testament to the maturity of the process.

The first phase (165MLD) of the Ashkelon desalination project (total 718MLD) is to produce water at a guaranteed sale price of US$0.527/m3. Hyflux are currently nearing completion of Singapore’s SWRO plant at Tuas South Road. Making use of Toray High Boron Rejection Membranes and Calder DWEER energy recovery, the 136MLD plant will deliver water at an initial tariff of Sg$0.78/m3. Not all projects are without problems though. Tampa Bay’s beleaguered SWRO plant which has been plagued by successive bankruptcies and failure of the performance test is now scheduled to be fully operational by October 2006. The 95MLD plant will produce water at an average cost of US$0.67/m3 after American Water – Pridesa (part of RWE-Thames Water) complete the design modifications and improvements.

The first phase of the world’s largest waste water reclamation project is currently being commissioned at Sulaibiyah in Kuwait. The plant uses the effluent from a conventional sewage treatment plant and after Ultra- Filtration (UF) and BWRO, will produce 311MLD of fresh water for irrigation at a tariff of US$0.47/m3. Already, 15% of Singapore’s ‘new water’, recovered from Bedok, Seletar and Kranji sewage treatment plants is re-introduced into reservoirs serving the potable water distribution system. What is surprising is that it has already gained widespread public acceptance following a carefully crafted public relations exercise by the Public Utilities Board.

It is even more surprising that almost twenty years have passed since the commissioning of one of the world’s first BWRO Waste Water Re-Use Projects. The 30MLD plant built by Ames Crosta Babcock (now Biwater Europe) at the Al Khumra sewage treatment plant in Jeddah, produced potable water to WHO standards - but for irrigation only. Its intended use did not relieve your reporter of the responsibility for personally demonstrating its wholesome qualities during the performance tests.

Not all have had such an easy ride as Thames. Proposed development of desalination facilities in New South Wales, Australia is being frustrated by the environmental lobby. Strong objections, citing the state government’s commitment to the reduction of greenhouse gasses, have been made together with the assertion that more attention should be paid to simple water conservation initiatives and more effective leakage reduction measures. There is a message here, somewhere!