Groundwater Treatment Plant
The Orica Botany Groundwater Cleanup Plan involves hydraulic containment from the three containment lines and treatment of the extracted groundwater in the Groundwater Treatment Plant (GTP). The GTP removes chlorinated hydrocarbon (CHC) (Fact sheet: 37kb) from the groundwater by an air stripping process, and the contaminated off-gas is treated in a thermal oxidiser. The ‘stripped water' is then processed through various conventional water treatment operations before being feed to reverse osmosis units that produce high quality Treated Water.
Design and construction of the GTP occurred over two years at a cost of $167 million. Commissioning commenced in late 2005 and groundwater was first introduced to the plant in January 2006. In March 2007, the GTP entered its fully operational phase and treats groundwater at an average rate of 6 ML/day. The actual daily treated volume is lower than the plant's design capacity as the Botany area's average groundwater flow rates were later found to be much lower than was originally anticipated. At times of high rainfall up to 7 ML/day of groundwater can be extracted in the short term. By extracting at around 6 ML/day from the containment areas, Orica achieves effective hydraulic containment of the contaminated groundwater. As of October 2012, the GTP has operated for nearly seven years of its 30 year design life.
Environmental monitoring results for the GTP
Night shot of the Groundwater Treatment Plant:
Air Stripping Unit (left) and Quench Tower & Stack (right)
The GTP was constructed using proven process technology from around the world.
The plant has successfully achieved containment of the contaminated groundwater, protecting Botany Bay and the local environment, and is consistently supplying five local factories with high quality treated water, replacing 4-5 ML/day of demand on Sydney's potable water supply.
GTP and Botany Groundwater Cleanup Project
The GTP plays an essential role in Orica's Botany Groundwater Treatment Project. As shown in the diagram below, the groundwater extracted from wells along the three containment lines is pumped and transferred through pipelines to the GTP for treatment. Treated water is then reused by local factories. Excess treated water is discharged to Botany Bay via a local stormwater canal.
GTP Layout and Key Units
The principal unit processes employed in the GTP are:
How the GTP Cleans the Groundwater
Extraction of Groundwater
The contaminated groundwater is extracted from the three containment areas (primary, secondary and BIP), which are equipped with multiple extraction wells to pump water from the shallow, intermediate and deep aquifers.
Simplified diagram of process flow
Extracted groundwater is pumped through the containment pipelines and enters the GTP Feed Tank where it is blended and dosed with hydrochloric acid (to minimise mineral scaling in the air strippers). It is then pumped through the Air Stripping Units, where the volatile CHCs are removed. The Air Stripping Units remove contaminants from the groundwater drawing air up through a falling column of groundwater in a closed structure. This process results in an air stream containing gaseous CHCs, and stripped water with negligible concentration of volatile CHCs. Chlorine dioxide is dosed into the middle of the Air Stripping Units to control biological fouling within the strippers.
The air stream containing gaseous CHCs is blown through a Thermal Oxidiser operating with an outlet temperature of 900ºC. The thermal oxidiser destroys the CHCs by converting them to carbon dioxide, water vapour and hydrogen chloride. The hot combustion gases pass through a waste heat boiler and energy recovery heat exchanger before being rapidly cooled in a quench tower. The rapid cooling is required to avoid dioxin formation. The hydrochloric acid absorber recovers the hydrogen chloride from the gas and the resultant solution is pumped back to the feed tank. The caustic scrubber neutralises any residual hydrogen chloride vapour, and the scrubbed gas is then mixed with hot air steam to improve dispersion and reduce plume visibility. For more information on the thermal oxidiser, view Fact Sheet No 12 Thermal Oxidiser (31kb).
Groundwater 'stripped' of volatile CHCs (referred to as stripped water) is treated in the Stripped Groundwater Treatment package section of the GTP to remove iron, aluminium, readily biodegradeable carbon, and trace levels of non-volatile CHCs. First, the stripped groundwater is pumped to an Actiflo® unit where sodium hydroxide and flocculating agents are dosed, and iron and aluminium are removed by precipitation and flocculation. Then, the water passes through multimedia filters (sand and anthracite media filtration) and then through Biological Aerated Filters (BAFs). The zeolite based aerated filters enable microbes to proliferate and remove readily assimilable carbon and some ammonia from the water. Granular Activated Carbon (GAC) filters then remove trace levelss of non-volatile CHCs - mostly chlorinated phenols, as well as some readily assimilable carbon. After the GACs, the water is dosed with chloramine to suppress biological fouling before passing through a second bank of dual-media filters. At the final stage, the water is pumped through two sets of Reverse Osmosis (RO) Units to remove dissolved solids and other impurities. The high quality permeate is partially dechlorinated and then supplied to neighbouring facilities for industrial reuse. The salty reject stream from the RO units is discharged to sewer.
The Treated Water is reused by neighbouring industries any any excess water is discharged to a stormwater canal, which then flows into Botany Bay. The discharged treated water is dosed with sodium bisulphite and sodium hydroxide to neutralise residual chloramine and maintain a stable pH before the water is discharged into the canal.
Although chloramine is a common disinfectant found in drinking water that has no health risks to humans, it may be toxic to aquatic life.
What Does Orica Do with the Treated Water?
Treated Water Recycling
The GTP currently treats an average in excess of 6 ML per day of contaminated groundwater and produce 4-5 ML/day of Treated Water for industrial reuse. This amounts to about 0.5% of total water consumption in Sydney. A condition of the Environment Protection Licence is that Orica is required to maximise reuse of the treated water. It does so by providing the water for use as:
water supply for cooling towers
water supply for demineralisation plants that use the demin water for steam production
industrial process water for use on equipment such as in pump seals
Orica is committed to maximising the reuse of treated water (Treated Water Recycling Program).
Excess Treated Water
From time to time, industrial facilities on the BIP are offline for maintenance or upgrades. When this occurs there is an excess supply of Treated Water, and it must be discharged to a stormwater canal so as to not affect groundwater extraction. The discharge point is into a stormwater canal near Perry Street, Matraville, and the water flows to Botany Bay via Brotherson Dock.
Why Doesn't Orica Return Treated Water to the Underground Aquifer?
The treated water is a valuable resource that can be used to displace consumption of Sydney's limited portable water supplies. Hence, Orica is endeavouring to maximise the reuse of treated water. In any case, it is not feasible to return treated groundwater into the underground aquifer. Since the groundwater is anaerobic (free of oxygen) returning the treated water (which is fully aerated) to the aquifer may cause significant disruption due to biological fouling in the vicinity of the injection points.