Sanjay Kumarasingham (Watercare Services Ltd) discusses the use of chlorination and wetland filtration as alternatives to UV when polishing wastewater.
Simon Smit, Texcel
Patricia Johanson/Carollo Engineers
Wastewater can be treated in various ways in the tertiary polishing stage. This particular plant had an option of looking at chlorination, UV disinfection and wetland or sand filtration. This plant is unique in the sense that it is quite a huge 300,000 cubic metres a day volume that gets treated here. So it was a cost exercise. One of the issues of chlorine is there could be a residue of chlorine that is left in the treated effluent, which would find its way into the receiving environment. So you will end up having to use a lot of chlorine to get to the quality that you want for the receiving environment. Added to that, you would have the effects of carrying a certain amount of chlorine into the environment as well. The other one is wetlands, and that is more low technology. It is using wetlands to actually take up certain nutrients and as a polishing effect before it goes into the local stream. And there are plants in New Zealand which have got wetlands as well. It is a more natural way of doing things. To manage a wetland would be huge with the volumes here. You would need acres and acres of land to do that. And living in a big city like Auckland, the footprint is always a criteria – what I mean by footprint is the space – so if you have lots of space and you don't want to use too much power and if your volume to be treated is relatively less, then certainly you could look at wetlands as an option.