Making a critical assessment
Black and Veatch's Frank Rogella looks at magnetised anionic exchange resin
Water 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
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.