Water Journal : Water Journal May 2011
refereed paper membranes & desalination water MAY 2011 105 Abstract The Botany Groundwater Treatment Plant was plagued with severe biological fouling from start-up. Chloramine dosing was introduced, and high dosages achieved moderate success. Water re-use was limited by organic carbon and chloramine passage through into reverse osmosis permeate. Retrofitting to include biological treatment partially reduced organic carbon in the reverse osmosis feed, and allowed for the reduction of chloramine dose. The lower chloramine and organic carbon concentrations in permeate facilitated its re-use as feed water to demineralised water plants. Periodic flushing and reduced flux and recovery were implemented to extend the reverse osmosis run times. Introduction As a result of historical manufacturing activities at Orica's Botany site in Sydney, NSW, dating back to the 1940s, there is a legacy of groundwater contamination within the Botany Sands Aquifer. After trials of passive remediation proved unsuccessful, the Botany Groundwater Cleanup project was initiated to achieve hydraulic containment of the contaminant plumes. A key part of the project was construction of the Groundwater Treatment Plant (GTP) in 2005 to treat the groundwater contaminated with chlorinated hydrocarbons (CHCs). The spread of the contaminants and relatively fast-moving aquifer required a fast-track implementation of the project to prevent the contaminants reaching Botany Bay. This limited the time available for pilot trials, and hence only proven technologies were considered for the design of the plant. A number of options were considered for the management and disposal of the treated groundwater. Reinjection into the aquifer was not preferred, as there are significant challenges in performing this reliably without disturbing the aquifer and causing flooding. Discharge into the sewer system was not feasible, because the sewer does not have adequate spare capacity. At the same time, Sydney in New South Wales, along with most of south- eastern Australia, was experiencing a prolonged and severe drought. Re-use of the treated water by users on the Botany Industrial Park was identified as the preferred option, as it provided a sustainable outcome by reducing the demand from Sydney Water supply. Any treated water not re-used is disposed via a stormwater canal into Botany Bay. Feedwater and Treated Water Quality Groundwater is extracted through 114 wells along three containment lines. Figure 1 shows the containment lines intercepting the groundwater flow before contaminants reach Penryhn estuary in Botany Bay. The feed water specification was derived from samples drawn from monitoring wells installed previously to monitor groundwater contamination, along with hydrogeological modelling to predict extraction rates required to achieve containment. The Treated Water specification was based on disposal considerations, but did not at the time consider the requirements of potential users. Table 1 (see overleaf) summarises key parameters used for the design of the treatment plant. The capacity of the plant was set at 15MLD, with 91% recovery of feed as product water. The high recovery and consequential reduced reject flow allowed the reject stream to be disposed via the sewer without hydraulic overload. Treatment Process The GTP commissioned in 2006 included the following major components: • Acidification of extracted groundwater with hydrochloric acid to maintain iron solubility; • Air stripping to remove volatile CHCs and sulphide; • Thermal oxidation of off-gas; • Sodium hydroxide addition to stripped water to precipitate iron; • Polymer dosing to sand-ballasted clarification and multimedia filtration (10 filters) to remove iron; • Thickening of clarifier and filter backwash effluents, with recovery of thickener overflow; • Two stages (lead/lag) of granular activated carbon (GAC) contactors (five per stage) to remove trace non-volatile CHCs; • pH correction of RO feed with hydrochloric acid; • Dosing of antiscalant; • RO cartridge filters fitted with 20 micron Ultipleat® elements; • Four-stage Reverse Osmosis (RO) F Barendregt, M Selleck Improved pre-treatment reduces severe bio-fouling problems TREATING CONTAMINATED GROUNDWATER WITH REVERSE OSMOSIS Figure 1: Hydraulic containment network.
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