Oxygenating the Swan and Canning rivers

Linde Gases has developed SOLVOX systems for the efficient introduction of oxygen into the treatment of drinking water. The technology has been proven as a primary remediation technique for the Swan Canning Estuary System, Perth.

Oxygen serves as a necessary component in removing contaminants in water. Not only does adding oxygen to water help ensure its purification, it is also an efficient and cost-effective way to control contaminants. Pure oxygen has a saturation rate in water that is nearly five times higher than air; thus, the increase of the dissolved oxygen content is readily achievable without high energy input and the associated foaming that can be caused by excessive aeration intensity.

With SOLVOX, oxygen is fed directly into a raw water feed upstream of a filtration system or into a sidestream, which is then introduced back into the main flow. SOLVOX D is specially designed for the injection of oxygen into pressurised pipelines, where oxygen is fed directly into a raw water stream through a stainless steel distribution nozzle — often in combination with the installation of a static mixer downstream to ensure complete mixing. In the SOLVOX R process, oxygen is dissolved in a pressure vessel for oxygenation to be carried out in an inline or bypass operation.

The Swan and Canning river estuary is a catchment that collects fresh water flowing downstream in winter and salt water moving upstream in summer as freshwater flow subsides. Rainfall run-off and stormwater are collected in the area, which has built-up sediment and other particles. Perth’s urban environment has contributed to the increase in nutrient loads through the channelling of drainage water directly into the river, while potentially toxic blue-green phytoplankton blooms in concentrations exceeding guidelines set down by the WHO for recreational exposure.

The Swan and Canning Cleanup Program has been operational for the past 15 years, installing oxygenation methods on the river system. Objectives of the program are to increase a viable water column and create a habitat for aquatic organisms, enhance nitrogen removal through oxygenation and reducing stored organic matter and reduce odours resulting from the production of hydrogen sulfide gas associated with anaerobic environments.

There have been multiple oxygenation methods tested on the river over the past 15 years, including chemical injections. To make a final push for the clean-up of the system, the program determined that the only answer was adding oxygen. Linde had worked with the Swan River Trust on chemical solutions in the past and was the natural choice to provide in-river oxygenation as it could deliver hydraulics to distribute oxygenation over a vast distance.

Currently, there are three oxygenation plants on the Canning River and two on the Swan River. The most recent system to be installed was the Nicholson Road Bridge system on the Canning by Linde. Process design objectives sought to design a capacity for oxygen dissolution at 5–10 kg O₂/h, with maintenance of river dissolved oxygen concentrations above 4.0 mg/L through the depth profile of the entire stretch of the river. The plant is to operate both automatically and remotely in both fresh and partially saline water conditions with a minimum operating life of five years.

Finalised earlier this year, Linde provided technology for the plant that included detailed hydraulic design for equally distributing oxygenated water across 2.3 km of the nominated river area. This included supplying:

• a liquid oxygen storage vessel and ambient vaporisers,
• a SOLVOX C110 technology and pump,
• an oxygen control panel,
• a motor control panel including PLC, HDMI and remote operations,
• an interconnecting water pipework,
• oxygen pipework.

The oxygen dissolution design of SOLVOX C would achieve complete dissolution of gaseous oxygen using a cone dissolver. The design methodology requires that the pressure throughout the distribution pipeline is held at a required pressure in excess of the saturation pressure at the maximum rate of dissolution. In this case, the head-loss objective is to ensure minimal loss from the distribution ‘T’ junction after exiting the dissolver.

In its first few months of operation, the project has been a success, which the clean-up team is looking to duplicate in the other two systems along the Canning. In addition, there is scope to add in an emergency response system that can be deployed in the event of a localised problem.