Development of temperature criteria for marine discharge from a large industrial seawater supplies project in Western Australia

• Published in: Water S. A. 2004-01, Vol.30 (5), p.100-106
• Main Authors: Shackleton, Blair, Bath, Andrew, Botica, Chris
• Format: Article
• Language: English
• ISSN: 0378-4738

A multi-user industrial water supply system is under construction on the Burrup Peninsula in Western Australia, 1400 km north of Perth, to support a number of gas-processing plants that convert natural gas into ammonia, dimethyl-ether, methanol or liquid fuels. At full design capacity, the project will abstract 280 000 m3/day of seawater from King Bay. Seawater will be conveyed by pipeline to each processing plant and used for cooling (using evaporative systems) and process feed water (after desalination). The total return flow (comprising cooling tower blow-down and desalination concentrate) will amount to 210 000 m3/day. A pipeline collects the return flow from each industry and discharges into a submerged marine outfall in King Bay. As part of the project environmental approvals process, the Department of the Environment (DoE) prescribed discharge criteria for the temperature of the return flow entering King Bay. DoE requires the return flow, at the end of the outlet pipe, to be less than 2°C above the temperature of the intake (calculated over a 24-hour period and expressed as an 80 percentile). For the remainder (20% of the time), the return flow must not exceed 5°C above intake temperature. As more information from the project proponents became available, it was realised that industrial evaporative cooling systems could not comply with the temperature criteria proposed by DoE under certain climatic conditions. Key findings of this study show: the DoE temperature limits proposed in the Ministerial Condition are too stringent, practical design of evaporative cooling systems on the Burrup will yield a discharge of 6°C above intake temperature, discharge at 6°C will not influence the biota in proximity to the diffuser or the corals located 700 m from the diffuser, marine organisms in King Bay are exposed and tolerant to large natural variations in seawater temperature, relaxation of the return flow temperature from 2 to 6°C (above intake) will not influence the biota of King Bay, and environmental management systems are being developed to monitor and manage the temperature of the return flow.