Dome flushed with success
Thames Water has supplied water to flush the Millennium Dome's toilets from a series of recycling systems - the project has been a useful testbed for grey and rainwater treatment
Thames provided a recycling system to supply the Dome with reclaimed water for WC and urinal flushing. The water came from three sources;
As well as testing treatment techniques, the programme assessed water-saving equipment and looked at visitors perceptions of water efficiency. The size and shape of the Dome's roof had the potential to collect huge volumes of rainwater. Run-off was channelled through hoppers into the surface water drainage system for treatment in reedbeds. The system functioned well but according to plant manager and Thames' communication and innovation manager Alan Smith, its effectiveness was limited by a lack of space for the reedbeds. Despite a large-capacity storage sump, the inability to guarantee rainfall made it an unreliable water source.
Another problem was a lack of research into rainwater quality, especially after periods of storage. Of all the water sources, rainwater gave the greatest increase in trans-membrane pressure because high levels of impurities were fouling the membrane.
The greywater was treated in a biological aerated filter (BAF) then passed through the same ultrafiltration (UF) system as the rainwater to remove soap and scum. Once the Dome opened it became clear that one of the problems the system faced was inconsistent inputs, as visitor numbers have been lower and more erratic than forecast. This affected the amount of water available for flushing the toilets. The volume of treated water proved a very accurate indicator of visitor levels.
A relative lack of research into the soap content of greywater meant the levels of contamination the system would have to treat were unclear. The lack of quality standards for greywater treatment also meant there were no obvious standards to aim for. In the end a target biochemical oxygen demand (BOD) range of 2mg/l to <20mg/l was set.
The tests were Thames' first large-scale trials of greywater treatment by BAF and UF, and although the treatment plant achieved BOD and pathogen kill targets, Alan Smith said its limits had become apparent and the system did not represent the preferred option to meet the greywater treatment needs of a site such as the Dome. He cited membrane bioreactors as more suitable, but since Thames had only trialled them on a pilot scale proven BAF technology was chosen for the Greenwich site.
Borehole water required a relatively sophisticated treatment process due to high levels of hydrogen sulphide, high salinity and trace elements from industrial processes carried out on the site. The custom-designed treatment process incorporated hydrogen peroxide dosing with granular activated carbon (GAC) treatment and reverse osmosis.
The investigation into visitor's attitudes to water conservation was based around two identically equipped 'behaviour' toilet blocks, one with water conservation information, one without. Thames is undertaking user surveys to validate quantitative data gathered from the blocks, and although it is too early to draw concrete conclusions, the conservation block showed lower water use, indicating a public receptiveness to water saving messages. On the other hand, it seems the infrared taps in the super efficient toilet block, which was also equipped with waterless urinals and dual-flush toilets, were something of a novelty. It seems children have been running past the banks of handbasins, passing their hands under the infrared sensors to switch on all the taps in one go.
Thames is very pleased with the Dome project which acted not just as a large-scale test bed for a variety of technologies, but also a high-profile showcase for the company.