Disinfecting wastewater

At the Mangere Wastewater Treatment Plant in Auckland they use UVC light to disinfect wastewater. Before disinfection, the treatment plant removes solids and dissolved chemicals from the wastewater effluent. Sanjay Kumarasingham, a Senior Process Engineer at the treatment plant, monitors this process.

Wastewater exiting a channel

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Water flowing out of one channel after being disinfected. The invisible UVC light makes the water glow green and can be seen in the distance deep inside the channel.

Rights: The University of Waikato Te Whare Wānanga o Waikato

The building that houses the UV treatment operation is called the UV disinfection gallery, where the clear wastewater effluent is directed into 12 channels containing banks of waterproof UV lamps that shine with intense UVC light. In the time that it takes for the effluent to pass along these channels, the energy in the UV light is able to inactivate the bacteria and viruses that are present in the water. Most of the bacteria and viruses are not actually killed by the UV but their DNA is changed so that they are unable to reproduce and infect people.

Disinfection by UV is called a tertiary polishing treatment since it is the third in a line of processes and it has the effect of finishing off or ‘polishing’ the final wastewater effluent.

UV disinfection channel

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One channel in a UV disinfection process.

Rights: Watercare Services Ltd

Although there are 12 channels through which the effluent can flow, only about five are normally used at any time. Inlet penstocks are used to control the flow rate of effluent through each channel. In dry weather, about 3.3 cubic metres of effluent flows in total per second through the UV disinfection gallery. Before it enters the disinfection gallery, the effluent is filtered so that only particles smaller than 15 microns (15 thousandths of a millimetre) are left in the water. Filtering also helps the disinfection process because the UVC light can penetrate further through the clear effluent and so more microorganisms are inactivated.

A large number of lamps are used in the UV gallery. Each channel has three banks of 216 lamps, a total of 7,776 lamps in all. Each lamp costs approximately $200 and, on average, can run for about 12,000 hours before its brightness dims significantly. It is impractical to replace individual lamps, so whole banks of lamps are replaced when the overall UV light from the bank falls below a set level.

Even though the particles in the effluent are tiny, a thin film of organic matter slowly builds up on the smooth glass surface of each lamp, and this reduces the lamp’s brightness and efficiency. To eliminate this effect, each lamp has a rubber ring fitted to it that is made to slowly slide back and forth along the lamp and wipe its surface clean.

Each lamp is rated at 300 watts and is designed to emit UVC light around the 254 nm wavelength found to be most effective in penetrating microorganisms’ cell walls and damaging the cellular material inside.

Overall, the disinfection process is able to reduce the number of active microorganisms in the wastewater by 10,000 times. This is enough to ensure that the purified wastewater that is released into the Manukau Harbour has little effect on marine life in the harbour and humans who swim at nearby beaches. Because UV disinfection leaves no residue, it has an advantage over disinfection by chlorination (where chlorine is added to the effluent to kill microorganisms), where a proportion of the chlorine will remain in the discharged wastewater and will damage various kinds of marine life.