Water & Wastewater Asia Jul/Aug 2018

WATER & WASTEWATER ASIA July / August 2018 42 / INSIGHT S o m e w a t e r s h a v e elevated concentrations of DOC (dissolved organic carbon), especially in sources such as surface waters that are under the influence of secondary effluent, recreation, heavy population, farming and industry. Climate change also results in higher DOC concentrations in surface water globally. Fo r wa t e r w i t h e l eva t ed concentrations of DOC, the usage of IX (Ion eXchange) as a pre-treatment will help in the removal of colour and increase efficiency in all downstream processes, including coagulation, membrane filtration, AOP and GAC. The affected water also contains suspended and colloidal matter, hence, making it nearly impossible to use the standard state-of-the-art fixed bed IX columns. This is due to the beds fouling quickly (e.g., head loss build-up) with the suspended matter and functioning as a filtration bed than as an adsorption media. To treat these heavy polluted waters, the technologies are based on fluidized bed reactors or totally-mixed reactors with very high concentrations of resin. In these processes, the bed volumes are treated until regeneration are designed to be as high as possible — the goal is to remove as much of the pollutants as possible to lower the salt consumption needed for regeneration [Slunsjki, 1999]. This approach has a few disadvantages as it makes the treated water less attractive to use (e.g., more expensive). But more importantly, for somewaters, the anion ion exchange process may not be feasible. This is so as the polluted waters often contain AOC (assymmable organic Figure 2: Schematic of the SIX ® process carbon) and phosphates, which will be adsorbed. Together with the large detention times (used to minimise the number of regenerations) and porous resin beads, the perfect environment for bacteria to grow is created. In addition, biofilm forms on the resin to blind the active groups of the resin, which is known as “resin blinding”. Resin blinding occurs slowly but can lead to serious problems. Besides losing adsorption capacity [Wachinski, 2006], it leads to the need to operate with a higher resin concentration or longer contact time [Verdickt, 2011; Cornelissen 2009], thus, increasing operational costs and/or lowering plant capacity. In fact, with time, the biofilm starts to release organic matter or adenosine triphosphate (ATP) that can be detrimental to downstream processes [Cornelissen, 2010], especially membranes. To overcome resin blinding, fixed bed or fluidised reactor systems are flushed with a high pH solution on a periodic basis to kill and dissolve the biofilm as much as possible. But these fixed beds cannot treat waters with suspended matter. For some commercial resins like the MIEX ® hydrophobic resin, it is not possible to use high pH (hydroxide ions) to control biofilm development as the resin is not resistant to hydroxide, and with exposure, would begin to fall apart over time; shortening the lifetime of this relatively expensive resin. These issues have led to the development of a new ion exchange process [Galjaard, 2009] by PWNT. Compared to the other ion exchange processes which treat waters containing suspended matter, the ‘single pass’ or ‘suspended’ ion exchange process (SIX ® ) achieves full control of the adsorption process without [serious] ‘blinding’ the resin or producing biomass, hence, producing optimumsorption kinetics and reduced contact times. This can be done with any resin that is commercially available, and the resins used to date can be treated with hydroxide for biofilm control, if necessary. raw water resin contacting separation regeneration resin storage salt re-use effluent fresh salt waste