Making a critical assessment
Black and Veatch's Frank Rogella looks at magnetised anionic exchange resinWater utilities face substantial challenges and tend to become trapped between two contradictory requirements: remove or inactivate waterborne pathogens and concurrently minimise disinfection by-products (DBPs). While increasingly stringent disinfection requirements and public concern about well-publicised pathogens like Cryptosporidium call for higher chemical disinfectant dosages, increasing concern about DBPs eliminates such seemingly simple solutions. Utility managers may feel trapped in a hopeless situation.
Extensive research identified these elevated levels of DOC and non-sulphide reduced sulphur compounds present in Wanneroo's finished water as the cause of the odour and taste. This presented a considerable challenge because further removal of these constituents would be very difficult and expensive.
Focused research identified ion exchange technology, using a magnetised anionic exchange resin (Miex) to be the most effective, economical solution for the problem.
The Miex process differs from conventional ion exchange in that it relies on continuous adsorption and intermittent regeneration. The particle size of the resin is considerably smaller than conventional resin beads, with a size range of 150 to 180µm. The resin is extracted from the process via settlers and regenerated in a separate part of the plant. High recovery rates of 99.9% are possible because of the magnetic properties of the resin. Once mixing energy is removed, the resin beads attract one another to form large agglomerates with a high settling velocity (25m/h). This enables the use of compact settling tanks with upflow rates between 10 to 15m/h.
The process consists of stirred resin contactors, where DOC is adsorbed onto the ion exchange resin, followed by settlers to separate the resin from the treated water and return it to the contactors. A proportion of the returned resin, between 5 and 15%, is diverted to a side stream to be regenerated with sodium chloride to remove the DOC. The resin is then returned to the contacters together with some make-up resin to replace losses.
A 1ml/d pilot was operated over two years to test various water quality characteristics and generate performance data for all likely operating scenarios. Removal figures showed that with 10-minute contact time at a resin dosage of 20ml/l, a removal of 60% of DOC could be achieved before coagulation, and with the downstream addition of alum, DOC concentrations could be reduced to less than 2mg/l. By using upstream Miex, the coagulant dose in the downstream settling could be reduced from 90mg/l to 30mg/l, and prechlorination can be avoided. The most interesting feature of Miex is that it preferentially adsorbs the organic matter with low molecular weights, which are hard to capture with conventional flocculation. Combining ion exchange and flocculation, the complete spectrum of organics is removed while reducing chemical dosages.
Following the successful trials, an engineering contractor was appointed to design, procure and construct the plant. For an overall treatment capacity of 112.5ml/d, 50% of the full-scale conventional flow, the total contract cost was in the region of US$8M.
With a new pumping station, the plant was integrated into the hydraulic profile of the works and retrofitted into the existing process train between the aerators and clarifiers. The design began in May 2000 and the plant passed its performance test in March 2002. The trials established that the main operating parameters had been achieved. Of particular importance was the resin loss from the system, which represents the largest part of the operating cost, and which was, like all other consumables, to be shown within the design guarantee.
In addition to controlling DOC and sulphide levels in the treated water, chlorine demand has been reduced, which enables chlorine residuals to be extended further into the water distribution system without supplementary dosing along the network.
The reduced DOC and lower chlorine levels enhance the water quality by contributing to lower levels of disinfection products, such as trihalomethanes. The chemical dosage of the conventional plant has also been reduced, in parallel with the amount of alum sludge produced.
This promising experience of the Miex process to reduce levels of difficult-to-remove
DOC, such as that occurring in the Highlands of Scotland and Scandinavia, inspired
an American Water Works Association Research Foundation (AWWARF) study to evaluate
the effectiveness of the process in treating a broad range of water qualities
at utilities across the US.