PandaAuthority comments on /r/Austin Water Megathread - Discussion and Rule Updates

[–]PandaAuthority 396 points 397 points 398 points 19 hours ago  (20 children)

An important measure in water treatment is turbidity. This measures water clarity. The higher the number, the “dirtier” the water. The typical NTU (turbidity units) of the water we pull from the lakes is <1. The plants are designed to treat water up to 100 NTU. We have seen sustained NTU over 200, as high as 500, since Thursday. This is completely unprecedented. While the city has, of course, dealt with flood waters in the past, historically NTU has been this high for a maximum of a few hours, never a full day, much less several. Understanding this is step one. But why does it matter?

Well the goal for the end product is <0.1 NTU. Austin is typically around .02-.06, which wins awards from the Partnership for Safe Water. In order to filter out particles, first ferric sulfate/lime are added to the water, then it’s sent through clarifiers where larger particles settle out, then the water is sent through filters. This all occurs before disinfection can happen, because high turbidity affects chlorination. The problem right now is the massive amount of silt in the water, combined with the particle size (extremely small), means the typical combination of ferric/lime is not working. Consultants with PhDs have been trying to sort out the right combo & haven’t been successful.

If the clarifiers can’t remove the silt, more of it moves to the filters... Think of these filters like a Brita filter. They can only remove so much before they fill up and aren’t as effective. They can backwash these filters, but that means shutting that filter down, and using water to clean it. Now that water has to be moved into a waste stream. These filters are having to be washed over and over again. That water has to go into a holding tank, where it’s eventually hauled off to a wastewater plant. But while these holding tanks are designed to hold 3x the water used for filter washing in normal ops, it’s not enough for this. So not only do they have to slow down ops to clean the filters, they have to slow it down when there’s no more room in the holding tank. There are only so many trucks that can haul the water away.

Because it’s taking so long to filter the water because of these issues, there are two options. Continue to filter the water to the typical highest standards and risk a water shortage (where the system would lose pressure necessary for emergency services) or release water at a lower standard to maintain minimum supply, while issuing the boil notice IN CASE the higher turbidity prevented the chlorination from being fully effective against contaminants.

If anyone has questions, feel free to ask. I’ll answer to the best of my ability. I can assure you, however, that employees are working around the clock to meet demand. There’s just nothing that they can do to control the conditions in the lake.

https://np.reddit.com/r/Austin/comments/9ql958/raustin_water_megathread_discussion_and_rule/e8gj5vv/?context=3