Water Journal : Water Journal April 2012
asset management 132 APRIL 2012 water technical features Introduction Since first appearing in the 1970s in the Middle East, desalination plants are now found in more than 150 countries around the world. This is due to a combination of population growth, increase of industry and agriculture, and fresh water scarcity1. The method of producing fresh water from the sea or brackish groundwater has been evolving from a distillation process to Reverse Osmosis (RO) process, where water is forced at high pressure through a membrane that separates salts from the water1. New techniques being researched to reduce the energy required by the desalination process include forward osmosis, carbon nanotubes and biometics1. With each change in technology there are associated durability challenges for construction materials. This paper is an abridged version of a publication presented at the 2010 Corrosion & Prevention conference in Adelaide2. It considers the guidelines provided in ISO 13823 -- general principles on the design of structures for durability3. Environmental Exposure: Brine As the reject stream from the Seawater Reverse Osmosis (SWRO) process, brine contains concentrated levels of the ionic species present in the seawater feed stream. The concentrated seawater or brine has a resultant increase in aggressivity to materials compared with seawater. Equipment and structures typically found in the brine circuit includes: pumps, valves, storage tanks, clarifiers, outfall tunnels and diffuser systems. The materials for all these components need to be resistant to the effects of brine or be maintainable for the nominated design life. SWRO desalination plants operating in Australia prior to 2008 typically produce brine with seawater ion concentrations of 1.5 to 1.8 times that of seawater4. This concentration factor is gradually increasing as membrane and processing technologies improve, and modern plants are approaching a brine concentration of 1.9 times that of seawater. For the purpose of this paper, a 1.9 concentration factor is considered; that is, a brine solution with a chloride concentration of up to 39,500 ppm and a sulphate concentration of up to 5,400 ppm, as shown in Table 1. According to ISO 13823, "in designing for durability, the structure environment (the macro-environment) contains influences outside the structure (atmospheric and ground conditions, including pollution) and inside the structure (indoor atmosphere and materials), that are transformed into one or more agents on the surface of or within a component (the micro- environment) causing environmental action". In the case of SWRO brine, the influences (structure environment) would be defined as outside and inside water (that is, the fluid) and the agents causing environmental action are chlorides, sulphates and magnesium, as listed in Table 1. However, this environmental exposure is not easily classified using the key Australian Standards for concrete and steel structures, as illustrated in Table 2. While the exposure classifications for concrete elements exposed to brine can be defined for structures with a 40- to 60-year design life in accordance with AS 3735, no specific guidance is given when a longer life is required. In addition, none of the standards listed above propose an exposure classification for steel elements in contact with brine. Reinforced Concrete in Contact With Brine Typically, concrete elements in contact with brine produced in desalination plants are reinforced using either steel bars or steel fibres. While unreinforced concrete may be used, the cement matrix is still susceptible to attack by sulphate and magnesium ions as described below. Within brine solutions, the key agents causing deterioration of the concrete matrix or steel reinforcement are sulphate, magnesium and chloride ions10, 11. More information on the deterioration mechanisms can be found in the literature2. The following discussion on durability measures to mitigate the risk of deterioration of reinforced concrete elements (using steel bars or fibres) exposed to SWRO brine is based on information obtained from literature as F Blin, S Furman A review of the corrosion processes involved in SWRO plants MATERIALS SELECTION FOR SWRO BRINE ENVIRONMENTS Table 1. Typical composition of seawater and brine (approximate values). Environment pH Chlorides (ppm) Sulphate (ppm) Magnesium (ppm) Seawater 7.5--8.5 19,300--20,900 2,950--3,050 1,300--1,450 Brine 6.5--7.5 39,500 5,400 2,500 Table 2. Environmental classifications to Australian Standard. AS 5100.5  AS 3600  AS 4997  AS 2159  AS 3735 2 Design life (yrs)1 100 50±20% 50 50 & 100 40--60 Concrete exposure classification U U Not defined Not defined B2--C3 Steel N/A N/A Not defined Not defined N/A Notes: 1. The design lives specified in this table are as defined in the respective Australian Standards. 2. Guidance is provided in Supplement 1. 3. The classification depends on whether elements are predominantly submerged or in alternate wet and dry conditions.
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