Xylem Water Solutions Canada - No more pong from Myponga. Improving the reservoir's water quality.

No more pong from Myponga. Improving the reservoir's water quality.

The myponga reservoir serves the most southern area of metropolitan Adelaide with unfiltered water. It's 35 m deep and contains 26,800 mega liters. Unfortunately the reservoir has had a history of taste and odor problems due to blue-green algae blooms (particularly of Microcystis and Anabaena). In addition, Myponga has suffered from the rarer problem of phantom midge larvae (Chaoborus Sp.).

Destratification, that is the exchange of water between the warm upper layer and the cooler bottom layer, provides an answer to such problems. And submersible mixers from ITT Flygt provide the tools for the job.

Stratification in water bodies reduces eater quality in a number of ways, the two main ones being increased algae growth and/or an increase in the release of substances from the sediment. Algae populations may increase with stratification and particular types of algae may adversely affect water quality. This is of special concern in water supply reservoirs due to the release of toxic as well as odorous substances.

"Destratification gives us better, odorless and fresher tasting water."
Stratification also facilitates the release of undesirable substances from the sediment and this usually associated with a decrease in the concentration of dissolved water layer (hypolimnion). Mixing brings oxygen to the sediment via water which has been aerated at the surface. Stratification prevents this from occurring, and confines aerated water to the upper layer (epilimnion). The hypolimnion has a limited amount of dissolved oxygen trapped in it at the time of stratification. Without light this oxygen is steadily depleted by the natural biological processes in the water and upper sediment. Bacterial respiration at the sediment-water interface is a major consumer of this limited supply of oxygen. Even in the absence of dissolved oxygen, bacterial respiration continues in an anaerobic state causing compounds other than oxygen to be reduced. This process releases elements such as iron, manganese and phosphorus into the overlying water.

Consltant´s study
The Engineering and Water Supply (EWS) department of the water authority for the State of South Australia, therefore, commissioned a report from a firm of consultants (Montgomery Hoskins and Stone) to evaluate destratification methods at Myponga.

Destratification problems
The conventional, air-diffuser method of destratification had been tried at Myponga and was reasonably successful in reducing the phantom midge larvae population, but had no destratifying effect in the reservoir. However, at Myponga, the dam is itself a quite heavily trafficked main road with very little space for winches, compressors and airlines. In addition, the area is a well developed tourist location, with many scenic views and picnic areas, and an air compressor would require sound insulation in a building.

Going submesible
The Investigations and Technical Policy branch of the EWS, however, came up with the solution of using a number of submersible mixers mounted on the sides of the dam wall. Flygt Australia were approached to put forward a proposal using three of their 4430, 3.1 kW mixers attached to mounting frames, fixed to the vertical wall of the dam.

Varying depth & angle
Three Flygt 4430 mixers, equipped with a blade configuration 2.5 m in diameter, were used. Each of the mixer units consists of a Flygt type 4430 mixer, a guide rail, a motor cable rail (with associated mounting bracket) and a davit arm (with which to raise and lower the mixer for maintenance and other purposes). The mixer's can be positioned at any depth and directed at selected angles to the horizontal. The guide rails, which reach 32 m down the dam wall, were assembled on site before installation and were lifted into position by a 30-ton crane.

"Capital costs are 25% lower and running costs are down too"

The Myponga mixer units began operating in March 1988. Raising the mixers after two weeks revealed their condition as good except for minor rust spots where mechanical damage on the surface had removed the paint. The submersible mixer solution has proved to have several advantages over the other alternatives considered: the capital cost of the Flygt proposal was approximately 25% lower than that of the two alternative proposals. Its total power consumption is approximately 15 - 30% of that required for the aeration option. No buildings or other sound attenuation arrangement are necessary. It is a flexible solution too; once the reservoir has been destratified some of the mixers can be turned off.

Building on success
So far, Flygt Australia has installed a total of 8 "banana" blade mixers for lake destratification. Other sites include the Lake Medlow storage dam, in Sydney's blue mountain area where a single Flygt 4430 mixer achieved a uniform water temperature in just one hour. Other successes include the Manly recreational dam in Sydney. 1991 was its first full year of operation and it was also the first in the last nine years that there was no significant outbreak of bluegreen algae at this dam. Bluegreen algae blooms are not simply unpleasant, they kill livestock and, if swallowed, can cause liver disease in humans.

International research
Flygt Australia is currently doing a feasibility study of further cooperation with the Centre for Water Research (CWR), in Western Australia, a world authority on destratification. They are currently carrying out 17 projects on stratification and water quality throughout the world. They have developed a technique for assessing the destratification need in lakes by measuring temperature at various depths and the wind velocity. This data can then be used to control the destratification equipment. The CWR is currently involved in many international water quality research programs, including those in progress at: the Venice lagoon, Lake Constance in Germany, Sapella Island in Georgia, USA, Lake 'Kinaret in Israel, the Adriatic sea and at several lakes in Japan.

Stratification of a body of water is often evidenced by increased algae growth and reduced water quality. Mechanical destratification of water serves to cycle surface water (the epilimnion) to the bottom layers (the hypolimnion), aiding the natural digestion of organic matter. With submersible mixers large volumes of water can be transferred at low cost and with minimal unwanted disturbance of the ecobalance.