Real-time desalination plant outfall monitoring using acoustic telemetry modems

Desalination is a growing technology to produce clean, safe drinking water. In locations where these plants are beginning to come online to produce water for the local population local government agencies are requiring these plants to monitor salinity levels at specific offshore locations and cease saline water discharge immediately if preset levels are exceeded. One way to monitor this saline water dissipation and corresponding salinity levels in the waters surrounding the desalination plant are by using underwater observatories with a variety of sensors. Sensors that measure conductivity will give the plant operators the information needed to determine if salinity levels are exceeded or not. The sensor data needs to be telemetered to shore for analysis and this analysis must be done in near real-time since the plant is constantly producing saline water discharge. Using acoustic modems the data can be transmitted to surface buoys from these underwater observatories and then from the buoy to shore using radio frequency (RF) telemetry. In the case of AdelaideAqua D&C Consortium (AA), the consortium licensed to operate the Port Stanvac Desalination Plant in Adelaide, Australia, the Australian Environmental Protection Agency (EPA) has required that such a system be installed in the coastal waters of Gulf St Vincent in Lonsdale, Australia. This system will be used to monitor, in near real-time, salinity levels at nine locations scattered throughout the discharge area. If salinity levels rise above preset limits 100 meters from the discharge diffuser the plant must suspend operations. Using sensors attached to acoustic modems these data are transmitted from the modem data logger upon request on an hourly basis. There are a total nine underwater observatories and thus nine surface buoys. Each buoy has a surface modem which is paired with a single modem on the seafloor which stores data from a sensor measuring salinity and in one case a current monitoring profiler as- well. The controller on the buoy, on an hourly basis, requests these stored data and sends them to shore via RF modem. Once the data arrives on shore analysis is done and if required actions are taken. Acoustic modems are in use for this project because cabling these sensor packages to shore is expensive and would be difficult to maintain. Given the short distance from the sensor/modem package acoustic data telemetry could be less of a challenge. The systems will be recovered on a quarterly basis and serviced before being redeployed. A polyvinyl chloride (PVC) housing was selected for use on the acoustic modems due to the shallow water environment where corrosion could be a problem for many other materials. Each modem was optioned with the available second serial port to allow for future expansion. The system is being commissioned from April to May 2011.