9 September - The ponds are turning orange (and that's a good thing).
Why have ponds 5 an 6 turned 'orange' first?
Now that the new activated sludge system is allowing better-quality wastewater to enter the ponds, we're starting to see the health of the ponds recover.
Ponds 5 and 6 are the first to reach our thresholds to turn orange this week, and over the coming weeks we expect to see more ponds also turn orange.
However, the oxidation ponds earlier in the wastewater treatment process (ponds 1 and 2 for example) will take a bit longer to recover; they have received higher organic loads since the November fire, so they're therefore in the poorest health. Pond 1 is also a large pond, so will take longer to improve than some of the smaller ponds (such as 2a, 2b and 3).
We change a pond's colour when its water quality meets our thresholds for biological oxygen demand (BOD5) and dissolved oxygen. We’re monitoring a range of other parameters including suspended solids, pH levels, sludge levels, nitrogen and phosphorous levels, enterococci and faecal coliforms.
As wastewater moves through each pond, any remaining sludge sinks and the clearer water flows to the next pond, meaning the quality of the wastewater continually improves through the process until it leaves the treatment plant.
However, it will take another couple of months for the biological health of the ponds to return to a level of resilience where they’re capable of handling increased organic loads if there’s a temporary failure somewhere in the treatment plant process.
We intend to only turn a pond 'green' when there are higher levels of dissolved oxygen present and its biological health reaches this level of resilience.
Ocean outfall water quality showing improvement
Our water testing at the ocean outfall is showing that a lot of the indicators - such as fecal coliforms, enterococci and total suspended solids - are now decreasing.
This tells us the quality of the wastewater across the ponds is improving as a result of the new activated sludge system we have put in place.
We've also observed that midges and bird life is returning to the ponds, which is a welcome indication that the water quality
is improving.
Home interior hydrogen sulphide findings
Over the past couple of weeks we've been testing hydrogen sulphide levels inside a home near the wastewater treatment plant to understand how - or if - the levels differ from our external monitoring.
Our findings are that indoor hydrogen sulphide levels replicate the patterns outside, but at a lower level.
We can also confirm that we have measured no build-ups or residual hydrogen sulphide levels in homes after the wind changes and the levels outside of the homes drop.
Updated air quality testing results
We’ve uploaded last week's data for the six continuous air monitors that have been set up near the Christchurch Wastewater Treatment plant.
The monitors are to detect the levels of hydrogen sulphide in the Shortland Street, Maces Road, Estuary Road South, Rudds Road areas, as well as at Bromley and South Brighton Schools.
Note the readings online are only up until Friday 2 September – we'll publish graphs showing this week's increased levels of hydrogen sulphide early next week once the data from all of the sites has been collated.
You can read the data for all six locations, and an explanation about how to read the information by clicking the link below. You can also find past readings for each site.
Stay updated
We want everyone to be able to stay updated on our response and recovery from the treatment plant fire, and we regularly post new information and reports on ccc.govt.nz/wastewaterfire
If you know of anyone who would like to receive this e-newsletter they can subscribe online.
If you have any questions, or would like further information on the project, please call us on 03 941 8999 or 0800 800 169 or email wastewater@ccc.govt.nz
Thanks in advance for your patience and understanding as we continue to work on minimising the unpleasant smells after the fire. We’ll be in touch again soon.
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