Short answer is yes - it is safe to go in effluent pond and sewers. Simply because of the level of dilution

0.1mg of glyphosphate / 1 liter of water is known to be horrid to water based critters and plants.

Take your bog standard Glyphosate. And 1 litre of the stuff. Approx. 33% is the good /nasty stuff (depending on your view). The other 66% is filler.

You'll normally mix to a 1/10 ratio so there's your first dilution. 1 Liter of Glyphosate is actually 0.33l or 3.3Mg.

Say you have a 12/ knapsack's. Pour in your 1l of Glyphosate and 10 liters of water and you end up with about 0.33mg of glyphosate in about 11l of water. This is now in the non-hazardous zone

When you have finished you'll doing a triple rinse. So say you have a 10 litre (+/-) knapsack your will be diluting that 1/10 remnants with 30 litres of water. So now its super non dangerous.

However you are now sending your very diluted water way into a very large water system - the effluent pond. Which will dilute loads more. Leaving the remnants quite safe to water based critters and plants.

From a SDS here is the toxicity levesl

Acute Toxicity (fish) (glyphosate as IPA salt)
LC50 (rainbow trout) (96hr) 8.2-26mg/l
LC50 (bluegill sunfish) (96hr) 5.8-14mg/l (MCPB)
LC50 (carp) (96hr) 19.7mg/l
Acute Toxicity (daphnia) (glyphosate as IPA salt)
LC50 (48hr) 480mg/l
Toxic to fish and other aquatic organisms
Acute toxicity to birds (glyphosate)
LD50 (bobtail quail) 3850mg/kg
not toxic to birds
Acute toxicity (bees) (glyphosate)
LD50 100µg/bee
not toxic to bees

Thanks! The toxicity levels now (kind of) make sense.

are you sure those calcs are correct?

Round-Up (and the other glyphosates) all seem to be around 360g/L (36% so close enough to your rough 33%) - and the dilution ratio for use is typically 10ml per 1 L (or 1:100), this is simialr to your double dilution example of 1 L weed killer to 10L water, and then diluting that again with 1L in a 12L napsack sprayer, but this would give a diluted concentration of 3.6g per liter (not 0.33mg per 11 litres).
I would expect minimum residue to be left in the sprayers when washing out, lets say no more than 100 mL so 0.36g of residual glyphosate to go to the effluent pond (or 360mg) - to get under the 0.1mg/L concentration you mentioned the pond would need to have 3,600L for each sprayer being washed out.

So it is likely not an issue, however I'm not sure about how and how quickly it would break down in the effluent pond - so the concentration might increase over time if it is a regular occurrence.

to further mitigate the risk you could ensure that the sprayers are fully empty before washing and the first wash water (rinsate) is used to manage any weeds/grass in the proximity of the wash-down pad.

Here is an interesting read:
https://deepgreenpermaculture.com/2021/04/21/how-to-neutralise-glyphosate-roundup-herbicide-contamination-in-soil/

Correct @MattD2. You can see why I didn't pass math! So many decimal points.

So lets try again
10ml of herbicide coming out of the container = 100mgs. If 36% of this is glyphosate we then have 36mg's of poison

We've got 36mg going into our 1 litre sprayer. And we then spray 99% of our brew or 35.64mg's onto weeds.

This leaves us with 0.36mg to dispose of with our triple rinse

Rinse 1 we tip 99% into the effluent pond so there goes 0.3564mg. Leaving us with 0.0036mg

Rinse 2 we tip 99% into the effluent pond and there goes another 0.003564mg. Leaving us with 0.000036

And third rinse we get rid of the lot so the final bit gets tipped into the pond.

So in the end, for 1 liter of spray we have disposed of 0.36mg in 3 litres of water that now heads to the effluent pond. Or 0.12mg a liter which is a smidge above the danger level - before it hits further dilution.

That my homework done. I hope I pass. But I expect a fail! (And its why I'm not an accountant)

@robb - Back in the day Glyphosate was my weedkiller of choice. I went through hundreds of litres of the stuff. Killed the weeds but not the good micro critters in the soil. Its good stuff for humans and plant - providing you don't drink it straight out of the container. The one I'd be watching out for around the home for humans and plans is Dicamba - now that can be pretty nasty to non target plants.

And if you have broad leaf at home - a sprinkle of salt will do the job.

10ml of herbicide coming out of the container = 100mgs. — Andrew

This is where you are making an error. 1ml (of water) = 1g (=1000mg). So your calculations are out by a factor of 100 (10ml = 10,000mg not 100mg).

I think I'll score my self 1/10 for at least spelling my name right!

Some interesting information in there. Especially the research on the half-life of Glyphosate breaking down in the enviroment (mean half-life of 30 days). That's key in considering how much and how often you can wash out into an effluent pond without an issue.