Xylem Water Solutions - How Sydney water authorities mix a good drink

How Sydney water authorities mix a good drink

Inadequate mixing was identified as the problem behind low levels of chlorine in reserve drinking water reservoirs in Australia. Tests indicated that, of all available alternatives, the installation of submersible mixers was the most effective solution.

"We just weren't getting adequate mixing"
Sydney Water Board has a network of around 180 service reservoirs for storing water, ready to be delivered to their customers. These reservoirs generally consist of above ground, steel tanks with capacities of up to 70 million liters. In order to maintain the quality of the water, these tanks have to be regularly dosed with chlorine, especially because of the area's warm climate. Investigations of water samples from the tanks revealed that chlorine levels were often lower than required, which meant that additional chlorination had to be carried out.

The main problem was a lack of adequate mixing: In nearly every reservoir sampled the chlorine content varied considerably and there were often dead spots that received virtually no chlorine. Standard procedure at that time relied on natural mixing to distribute the chlorine, but this was obviously inadequate.

"Natural mixing was just too slow for us"
Tests showed that natural mixing does work - in time. Unfortunately, service reservoirs are generally not isolated from the distribution system so time is a critical factor: In 48 hours not only would much of the chlorine have undergone natural decay but more importantly bacteria would have a much greater chance of being drawn into the distribution system. Ideally, the mixing process should take the same time as it takes to dose the reservoir with chlorine.

"Mechanical mixing proved the answer"
Of the three alternatives - air diffusers, inlet pipe nozzles and mechanical mixers - the mechanical option proved the most attractive. This consisted of installing a submersible mixer, from ITT Flygt, inside the reservoir, fixed to the wall at a predetermined position. The arrangement mixes the water in the reservoir and is also highly flexible: it can be controlled automatically, through the reservoir pump's controls, run continuously, or operated manually and by telemetry.

"We hedged our bets and tested the system off-line"
Extensive field tests were first carried out using the submersible mixer. A suitable, "off-line" reservoir was selected for the tests. The initial measurement of chlorine in the reservoir indicated that, while the chlorine was evenly distributed, the level was inadequate to guarantee a safety margin against bacteriological contamination. The results of adding chlorine by the usual method indicated that the recommended dose of 1 mg per 1 is not sufficient to ensure a residual approaching 1 mg per 1 of chlorine. Use of the submersible mixer greatly improved the degree of chlorine mixing throughout the reservoir. The tests established that the previous manual chlorination procedure, which used a dose of 1 mg/1 and aimed at achieving chlorine residual of approximately 0.7 to 0.8 mg per liter, evenly distributed throughout the reservoir, was not proving effective. The results showed that, not only is the current dosing rate insufficient in providing an acceptable chlorine residual, but also the mixing of chlorine is unsatisfactory. The installation of submersible mixers in service reservoirs, however, solves this problem by ensuring even dosing of chlorine throughout the reservoir.

Submersible mixers not only ensure more thorough mixing, but also facilitate the work of the chlorinators. Instead of having to dose at three levels at each roof hatch the operator merely has to lower the dosing hose through the hatch closest to the mixer - it will do the rest. In addition, overall chlorine doses can be lowered and more reliable results can be achieved. (Flygt Australia have now developed a method of installing their mixers which does not require the reservoir to be taken out of service.)

"We use less chlorine and we get a better result"
So far, Flygt Australia have installed 39 mixers in Sydney Water Board's reservoirs. Ultimately, most of their 180 reservoirs will be fitted with mixers. The Board is extremely satisfied with the results, having saved chemical costs and reduced lead time necessary to bring reservoirs on stream.