Design, operating and research experience at the Penneshaw Seawater Desalination Plant, South Australia

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Abstract
This paper provides a historical perspective of the plant, including performance problems and how they were overcome, as well as key learnings from selected process R&D studies. The South Australian Water Corporation (SA Water) is a wholly-owned public water utility, responsible for the management of water and wastewater supply, treatment and distribution infrastructure, for more than 90% of the state’s population (~ 1.1 million people). Amongst its infrastructure, SA Water operates a 300 m3 /day seawater reverse osmosis (SWRO) desalination plant for the island coastal community of Penneshaw. Desalination was established as the most cost-effective supply option, when the existing source water, an open dam on a farmer’s property, was deemed a very high microbiological risk to humans, particularly with respect to Cryptosporidium and trihalomethane disinfection by-product formation. SWRO was determined to provide lower cost water than constructing a new 60 kilometre pipeline to connect with treated water from an existing conventional water treatment plant in the mid-west of the island.

Built in 1998, the SWRO plant has provided an opportunity to develop a knowledge base for the design and operation of seawater desalination issues. Driven by the environmentally sensitive nature of the local marine environment, the requirement for the plant to be ‘chemical-free’ has resulted in numerous challenges for process design. The current operating recovery of the RO membrane system is low (by world standards), at 28%, in an attempt to mitigate calcium carbonate scaling. Mechanical integrity issues with the use of 15” diameter pressure vessels resulted in a shift to established conventional 8” RO membrane elements, with significant improvements in plant operation. An improved understanding of seawater corrosion issues and the critical importance of reliable and robust pre-treatment filtration and post-treatment conditioning systems have been positive outcomes from the various upgrades to the plant undertaken from 2001-2005.

In 2005 a research program was initiated to improve our understanding of the relevant fouling mechanisms of the open intake feed water on the RO membranes. The results confirmed significant biofouling activity, even with a pre-treatment system incorporating two stages of high intensity ultraviolet disinfection of the feed water. Pre-treatment efficacy was found to be reasonable, especially in light of the absence of coagulant addition. Most SDI measurements were below 4, with more than 50% below 3. Heterotrophic plate count analysis using marine agar yielded a removal efficiency near 90%. However, the removal of transparent exocellular particles was relatively poor for bacteria (< 5%). Removal of clumps was far more efficient (>85%). In relation to inorganic fouling, quantitative mass balances for key chemical species across the membrane system did not adequately predict the dominant inorganic foulants, when compared with the analysis of spent chemical cleaning solutions. An acid dosing trial to assess operational, water quality and environmental impacts from operating at a higher recovery (40%) revealed significant benefits.

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