The Dark Side of Drinking Water - Managing Residual Streams

S Pearson and P La Roche (Beca)

The landscape of freshwater in New Zealand is changing with the introduction of Te Mana o Te Wai into to our regulatory frameworks, and the National Environmental Standards for Freshwater driving improvement to the freshwater quality in New Zealand. 

This is bringing an increased focus on the availability and efficient use of water, and requirements for improved water quality for any discharges to freshwater bodies or the surrounding environment. This is driving water suppliers to look for:
• greater efficiency through recycling of waste streams to reduce discharges from water treatment plants, and
• improved water quality of discharge streams for the betterment of the environment

Upgrades from cartridge filtration and ultra-violet disinfection to processes using membrane filtration are becoming more common in New Zealand due to changing source water quality, particularly during heavy rainfall events. This process change introduces liquid residuals streams from backwashing and chemical cleaning of the membranes, which require management as part of the plant upgrades.

This paper summarises the typical residuals process streams used across New Zealand ranging from basic tank or pond settling systems to zero liquid discharge facilities with dewatering of solids.

As the amount of water recycled back into the process increases, the complexity of the process and operation increases. For each waste treatment option, this paper discusses:
• challenges in meeting drinking water regulatory compliance with rules around the recycle of water through the process,
• the increased operational requirements, and associated risk profiles,
• the overall plant operation including how plant start-up and shutdown are managed and challenges with recovering from an abnormal event or process failure,
• specific process constraints.

This paper also discusses challenges experienced through consenting of discharges with the lack of a national standard resulting in inconsistent approaches and standards throughout the country.