Improving Disinfectant Retention and Stabilizing Water Quality

April 3, 2018
Active mixing helps Australian utility prevent short-circuiting in long, shallow basin

Maintaining disinfectant residual levels in drinking water reticulation systems is a challenge, even under normal conditions. Water must travel through several kilometers of pipes and is often stored in water tanks and basins before reaching customers. However, when the distance between the treatment plant and customer is extensive, maintaining adequate disinfectant levels becomes even harder. Further challenges become apparent during times of low water usage as the age of water within the water reticulation system increases.

The Problem

Central Highlands Water—west of Melbourne, Victoria, Australia—faced this issue to an extreme. To service the southernmost part of its system, water produced at one of its main treatment plants in Ballarat is pumped and then gravity fed 68 km through a single water main to the town of Rokewood. At a distance of 42 km from the treatment plant, the water passes through the 2.3-megaliter Enfield basin.

“Maintaining adequate disinfectant residual levels has always been a priority,” said Jon Gooding, senior engineer for non-revenue water for Central Highlands Water. “With years of drought and substantial decreases in water use as a result of changed customer behavior, keeping our residual at the level we like at the end of the system became difficult.”

A common technique for improving disinfectant residual levels at the end of a reticulation system is to flush water. But while flushing or pouring out water is widely practiced—and in some utilities is seen as the “price” for maintaining water quality—operators at Central Highlands Water sought ways of improving water quality without the need to regularly flush water.

The Solution

Central Highlands Water started by conducting a dual process that involved high- velocity flushing and air scouring of the water main feeding this part of the reticulation system. This approach uses the high velocity of water and the action of air to dislodge sediment and biofilm that accumulate naturally in water pipes, as the presence of these elements can cause accelerated disinfectant loss. Central Highlands then conducted a thorough de-nitrification program for the reticulation system to further eliminate biofilm and nitrifying bacteria that can cause water quality degradation.

A part of the effort to improve water quality focused on the 2.3- megaliter Enfield basin. Operators normally kept this water reservoir as low as possible to minimize retention time. However, the basin has a separate inlet and outlet, and operators suspected that water could “short-circuit”—travel directly across the reservoir without mixing with the water on either side, causing some water to remain trapped in the storage basin.

In 2011, engineers designed and installed an automated dosing system into the Enfield basin in order to allow operators to boost disinfectant levels and enhance water quality. Shortly after this system was installed, the water operators saw a problem.

“Even though we were dosing adequate amounts near the inlet, we still saw low residual levels leaving the basin,” said Gooding. “It was as if the disinfectant was disappearing.”

Operators had a portable dosing system in a trailer that included a jet pump that could be used to mix the storage basin when applying a dose.

“When we used that system, we saw improvements in water quality right away,” said Gooding. “However, once we stopped using that pump, the improvements decayed away.”

Realizing that the hydraulics of the basin itself may be the source of the problem, operators began researching options and learned about the PAX Water Mixer, a powerful and lightweight mixer that can be installed into potable water storage tanks and reservoirs to provide constant mixing. Studies in the United States have shown that active mixing not only eliminated short-circuiting in water storage tanks, but also significantly improved disinfectant residual levels.

The Enfield basin is relatively long and shallow, and operators knew they needed to promote lateral mixing across the length and width of the basin in addition to top-to-bottom mixing. PAX Water Technologies provided a horizontally mounted version of the mixer that was well suited to achieve this goal.

The installation of the PAX Water Mixer took place in December 2012. Divers were used to install the system so that the basin would not need to be taken out of service. After the system was on and running, operators saw an immediate change. Whereas before, residual levels within the basin had a high degree of variability, after installation residual levels rose and stabilized.

“Residual levels rose to more than 1.0 mg/L after two months,” said Gooding.

More dramatically, the increase in the residual level from mixing allowed operators to turn off the automated dosing system in the basin, and the dosing system has remained off for the past 12 months. Operators were also pleased to see disinfectant residual levels rise at the end of the system in Rokewood. The regular flushing of the mains has not been necessary since the installation of the PAX Water Mixer.

The Results

“The PAX Water Mixer is a great tool that we can add to our water quality management program,” said Gooding. “We now have a lot more control of our system and reduced operating costs.”