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Increases in population and water use are increasing pressures on conventional water sources. The use of alternative sources for non-potable applications can relieve some of this demand. Poor quality ground water, combined with quantities of grey water and rain water, was identified as potential alternative to potable water for toilet flushing at the Millennium Dome, a large entertainment complex in London, UK. An 87m deep borehole was sunk on the Millennium Dome site. On abstraction, the ground water was found to be brackish, with further contamination in the form of hydrogen sulphide, ferrous iron, and aromatic hydrocarbons. The chosen treatment process utilises oxidation of the sulphide and ferrous iron with hydrogen peroxide, adsorption of the organics and filtration of the precipitated ferric hydroxide on granular activated carbon, further pre-treatment by hollow fibre ultrafiltration (UF) membranes prior to desalination on Reverse Osmosis (RO) membranes. The final RO product water undergoes re-hardening and residual disinfection prior for use in flushing toilets inside the Millennium Dome. Results for the pre-treatment and membrane processes over a 6 month operational period are discussed. A saving of approximately 50% of the drinking water supply has been demonstrated

Oxidation with hydrogen peroxide is an efficient method for sulphide removal prior to membrane treatment. A stoichiometric excess is required to ensure the oxidation product is sulphate and not elemental sulphur. Removal of sulphide is essential to prevent fouling of the membranes by elemental sulphur. Where other compounds are present which exert an oxygen demand, as in the case of the ferrous iron also found in the groundwater, additional peroxide is required. The GAC filters provided a dual duty for the filtration of precipitated ferric hydroxide and adsorption of the aromatic hydrocarbons found in the groundwater. The chalk aquifer provides efficient bank filtration in removing silt and particulate matter from the brackish water entering the aquifer from the River Thames estuary. The “Ultrabar” UF process was highly effective in removing particulate matter and bacteria from the feed stream. Increases in transmembrane pressure were seen when the groundwater was mixed with water from the other sources. High values of TMP, typically over 1 bar, required the membranes to be soaked with a solution of sodium hydroxide and sodium hypochlorite to reduce the TMP to the 0.3 bar baseline. The RO plant worked efficiently and reliably for the 18 month duration of the project. No increase in inlet pressure was observed and no cleaning of the membranes was carried out. This demonstrates the effectiveness of the UF process as a pretreatment to RO membranes. Reclamation of water from the alternative sources available and recycling for flushing the toilets on the Millennium Dome site has saved approximately 50% of the potable water supply.