G J Macdonald – Beca Ltd (Fellow - Water Environment Federation)
H Madon – Alimentary Systems Ltd (Fellow, Edmund Hillary Fellowship)
Wastewater treatment is a continuous process, with very high energy needs and a large carbon footprint across the wastewater network. Most often, at a New Zealand council level, there is mixing of domestic and industrial wastewaters. Similarly, most plants in NZ process wastewater to secondary treatment level, without nutrient reduction or tertiary treatment processes. However, more stringent national effluent standards will require additional higher levels of liquid stream treatment – incurring greater costs, larger energy needs and higher embedded and operating carbon footprints.
Higher treatment of the liquid streams will result in more sludge being produced which will need to be dealt with more innovatively and intelligently than our current practices, which are dominated by dewatering and landfilling, adding to landfill gas emissions. Some large NZ WWTPs operate sludge digesters, and while these plants recover significant energy, they do not export “green energy” as is common for many “wastewater recovery facilities” (WRRFs) elsewhere.
Many of our large primary industries operate their own wastewater treatment facilities, often to recover usable by-products, but they also produce residual sludge, which must be dealt with. Often, landfills will not accept DAF sludge from meat works or from dairy plants, and these problems will be compounded by expected higher environmental controls on liquid and solid waste disposal. On the other hand, most Councils operate separate facilities to manage green waste, food waste and other organic waste materials. This inevitably increases both capital and operational costs because of the lack of scale and the non-realisation of possible co-management benefits.