The science behind sewage treatment plants

[Crunchynut]

Some people will want to install and forget, which is fine. Read no further.
 

Personally I want to understand how these things work, and that way I can fix it when it’s not working right or even prevent it going wrong in the first place. I’m not really talking mechanically - that seems obvious enough on most types - but at a biological level.

 

Finding information in the U.K. is difficult. It’s as if regulations for home sewage treatment  plants have been brought in but not really understood, and the real world running of plant to achieve these goals ignored. Some of the literature I have found - largely on manufacturer sites - is laughable. Clearly they don’t really understand, or they thing we are too unintelligent to understand, or both. That’s my opinion anyway.

 

I have gathered a lot of intel now from many websites - mostly in the US and mostly referring to large municipal plant, but the science is roughly the same. I understand much better now and feel able to predict, spot and correct issues. One site that has some good videos (I know, I need to get out more)  on You Tube is worth sharing for anybody interested.

 

Fill your boots!

 

 

https://youtube.com/@TeamAquafix

 

Edited March 15, 2023 by Crunchynut

[crooksey]

Its not too hard, aeration encourages break down, allowing water to settle will leave solids at the bottom and a clear water layer on the top to be pumped out.

 

Many old septic tanks and brick chamber tanks can be simply aerated that will produce the same quality of water as some sewage plants.

[Crunchynut]

Yep - that’s the simple view that the manufacturers seem to think is all we are able to understand.

 

The science, and the difference between a plant that works well versus one that doesn’t, is far far more complex.

 

So if your plant suddenly starts generating a lot of floating sludge that’s carrying over into the outlet, or a chocolate mouse looking foam, or suddenly isn’t settling, or the effluent has gone cloudy, what do you do? Increase aeration? Decrease aeration? Increase the rate of settled sludge return? Pump out? Chuck in some muck munchers?  Contact a ‘maintenance company’ who come and look busy?  All very tricky if understanding is limited to what the glossy brochures say.

 

 

[Kelvin]

Maybe you could write a summary for each scenario described. There’s almost 6 hours of video in that link alone. That’s a lot of shit to wade through. 

[dpmiller]

I'd caution against taking too much of what is in the Aquafix videos as gospel in the case of our domestic units, as they focus rather a lot on anaerobic systems and the whole point of air-blower plants is in adding oxygen...

[Crunchynut]

 

2 hours ago, dpmiller said:

I'd caution against taking too much of what is in the Aquafix videos as gospel in the case of our domestic units, as they focus rather a lot on anaerobic systems and the whole point of air-blower plants is in adding oxygen...

 

Sorry but I think you might have mis-read - they hardly cover anaerobic systems. The only time they do is when aerobic

systems turn bad and become anaerobic.
 

This can happen through lack of dissolved oxygen (blower not working sufficiently), or in old sludge where the floc eps (a skin that forms around the bacteria) has thickened and doesn’t allow as much dissolved oxygen to reached through the eps to the bacteria which then become anaerobic as a result, where the rate of settled sludge return is very low meaning it takes too long for the settled sludge to become re-oxygenated by returning it to the bubble chamber, or in big municipal plants where anaerobic regions in their huge tanks are used to force denitrification (where the nitrates are broken apart to release their oxygen because there is no elemental dissolved oxygen) which is desirable to reduce the amount of nitrates in the effluent.

 

Edited March 17, 2023 by Crunchynut

[Kelvin]

One of the most overlooked areas affecting STPs is if anyone in the household is on long term medication or undergoing therapies like chemotherapy. Considering that the majority of the population will be on some form of medication in their later years. It’s also a problem for commercial treatment plants. 
 

Do they cover this at all? 

Edited March 17, 2023 by Kelvin

[Crunchynut]

6 hours ago, Kelvin said:

One of the most overlooked areas affecting STPs is if anyone in the household is on long term medication or undergoing therapies like chemotherapy. Considering that the majority of the population will be on some form of medication in their later years. It’s also a problem for commercial treatment plants. 
 

Do they cover this at all? 

I haven’t found anything that specifically mentions that. They do talk about toxicity generally. There are certain types of bacteria (the problematic filamentous variety) and Metazoa (creepy crawlies) that are evidence of toxicity in the plant, but I guess that could be due to many things. The other thing they say is how sensitive bacteria are - how quickly they die but also how quickly they reproduce - and hence how the biology in your tank will rapidly ‘move with the times’, sometimes positively and sometimes negatively. So I guess (and this is very much a guess) if drug residue is killing off one of the native bacteria in the tank, another may start to prosper in its place. It may not be all bad, because, apparently, the bacteria in your tank don’t kill the bacteria in your poo. That dies just because the environment in your tank isn’t conducive to its survival. Who’da thought?!
 

One of their videos talks about additives and whether  they do any good. What they say is, that chucking in commercially available powder containing bacteria and enzymes might be beneficial if the contents have been carefully selected to thrive in the STP environment. If they are just any old ‘cheap to manufacture’ bugs then they just quickly die. 

[Roger440]

Im not sure i really understand what goes on in mine, but i do now know how to get the best outfall from it.

 

Mines a vortex and its very sensitive to the sludge return rate. Though thats partly becuase its also it weakest part of the design.

 

If the flow rate is too high, the outfall becomes more cloudy. Too slow, and the sludge return gets blocked. 

 

The real issue is how do i know whats "good" as ive nothing to compare to.

 

At the last emptying, i was there when the chap emptied it. He said it was exceptional. Hardly ever sees one that good.

 

I suspect there a lot of them out there not working well.

[Crunchynut]

I have a Vortex too and have experienced the same with the SSR (settled sludge return). How do you unblock it? I either poke around the inlet to the pipe with a long stick, or dismantle the cross pipe so I can jet some water from a hose down the pipe. It has solved the problem on the couple of occasions it has happened. What do you do? 

 

What you have observed re: cloudy effluent is explainable:

For good settlement in the settlement chamber you want the bacteria to stick together in a floc (lumps of sludge). The reason they stick together is because the bacteria form a sticky coating that is partly a defence mechanism to there being limited amounts of oxygen available. These flocs are more dense than water and hence sink in the settlement chamber, allowing clear effluent above to pass over the outlet weir. If there is too much dissolved oxygen (DO) then the bacteria don’t build up that sticky coating and hence don’t stick together, so you get fragments left over in the effluent making it cloudy. So, why does a high rate of SSR make the effluent cloudy? Well, because the longer the biomass is in the settlement tank it is obviously not being oxygenated by the bubble diffuser, so the DO balance is kept low. If you increase SSR then more of the biomass gets oxygenated per minute and that might lead too much DO and cloudy effluent.
 

As you know, the good thing about the Vortex is that the big lid allows you to easily see what’s going on (the reason I chose it over others) and that the SSR and aeration rates are adjustable. The frustrating thing is that they are fed from the same blower, so if you adjust one it affects the other. I understand why they have done that - you would otherwise need 2 blowers and twice the energy consumption - though it  makes adjustment slightly more intricate and if one goes up the other goes down. If you increase SSR you may need to reduce aeration a fraction more than it will naturally do. Personally I leave the fine bubble diffuser valve fully open and ‘throttle’ it by controlling how much air goes to the coarse bubble diffuser. 

 

[Cpd]

I only have a very large septic tank but I feed it with “muck munchers” every month, it’s been installed a few years now and I took the lid of to check it recently , virtually no smell at all, just some dark water and no scum layer. No maintenance beyond feeding it monthly, and no electricity usage 😁 I understand it’s not possible for everyone but for me it works just great. KISS - Keep it simple stupid was what I based my design on, but I have plenty of land  for a leach field which most plots don’t have. 

[SteamyTea]

I don't really understand bio-chemistry, it is probably the hardest of the real sciences to get to grips with, not like physics, which is pretty easy really.

One thing that I have often wondered is how well tanks and plants deal with small amounts of regular usage i.e. single person household.

Not much poo or wee, but quite a lot of bath/shower water.

[Crunchynut]

5 minutes ago, Cpd said:

I only have a very large septic tank but I feed it with “muck munchers” every month, it’s been installed a few years now and I took the lid of to check it recently , virtually no smell at all, just some dark water and no scum layer. No maintenance beyond feeding it monthly, and no electricity usage 😁 I understand it’s not possible for everyone but for me it works just great. KISS - Keep it simple stupid was what I based my design on, but I have plenty of land  for a leach field which most plots don’t have. 

I had a septic tank until a year ago, that was here when I moved in. The tank worked fine as you say, but unlike you, my drainage field was on level ground and in clay. It’s amazing it has ever worked at all - previous owners must have almost treated it like a cesspool and had it pumped regularly. The STP allows me to discharge to a ditch (I needed an EA permit since the ditch is seasonally dry). Only problem is that people out for a stroll on a Sunday walk right past the outlet, so I can’t get away with anything other than nice clean effluent - which is why I’m becoming quite a nerd on the subject!

[Crunchynut]

33 minutes ago, SteamyTea said:

I don't really understand bio-chemistry, it is probably the hardest of the real sciences to get to grips with, not like physics, which is pretty easy really.

One thing that I have often wondered is how well tanks and plants deal with small amounts of regular usage i.e. single person household.

Not much poo or wee, but quite a lot of bath/shower water.

I’m a bit confused by this too. Again, I’m no expert, but this is my take on it:

 

Firstly, the amount of undesirables in the inflowing effluent dictates how much dissolved oxygen in the treatment plant is needed to support the bacteria levels to cope.  So I could well imagine that if there is far too much DO for the incoming effluent then, as mentioned above, you would get a cloudy effluent. So, I guess you would want to have a low aeration rate in a tank in this situation.

 

Next, I don’t think it’s a case that the bacteria will all die and never come back - they will die if there is no food but I think they soon re-establish once the food returns. Their reproduction rate is super fast. But,  there is ‘sludge age’ profile and different bacteria do their stuff on different ages of sludge (eg  some sludge may have just arrived, and some sludge may be days or weeks old). As you say, if there is so much water washing the effluent through the plant, and the biomass isn’t settling because it is being over-aerated and hence partially being carried over the outlet weir, then the sludge will never reach old age and so I can imagine that the effluent is only partially tackled by bacteria that thrive on young sludge only. I guess the solution here is for the tank capacity not to be too small, so that the sludge has a chance of staying in there for at least a couple of days.

 

Finally, as I mentioned earlier, the bacteria in your poo isn’t eaten by the bacteria in your tank. It just dies because it’s not an environment conducive to its survival. But if the environment has never been able to get established because of so much water washing through, then maybe the poo bacteria don’t die.

 

Back to my opening comments on this thread - you wonder how well these plants are operating in the real world.

Edited March 17, 2023 by Crunchynut

[jamieled]

Glad to have found this thread. Any tips for a vortex where we're getting really high rates of floating sludge in the settlement chamber (and outflow chamber)? I manually push it all back through the floating sludge return every couple of days now. Started about 3 months after we last got it emptied.

 

There's no particular nasty smell, but I've tried a few combinations of settings with the blower valves and haven't had much luck. It looks like when water inflows into the tank this pushes some sediment dense water into the settlement chamber which the floating sludge return then can't cope with. I might be wrong on this of course.

[Kelvin]

32 minutes ago, SteamyTea said:

I don't really understand bio-chemistry, it is probably the hardest of the real sciences to get to grips with, not like physics, which is pretty easy really.

One thing that I have often wondered is how well tanks and plants deal with small amounts of regular usage i.e. single person household.

Not much poo or wee, but quite a lot of bath/shower water.

Invite the neighbours round for a poo and eat more fibre. 

[Crunchynut]

28 minutes ago, jamieled said:

Glad to have found this thread. Any tips for a vortex where we're getting really high rates of floating sludge in the settlement chamber (and outflow chamber)? I manually push it all back through the floating sludge return every couple of days now. Started about 3 months after we last got it emptied.

 

There's no particular nasty smell, but I've tried a few combinations of settings with the blower valves and haven't had much luck. It looks like when water inflows into the tank this pushes some sediment dense water into the settlement chamber which the floating sludge return then can't cope with. I might be wrong on this of course.

I had this problem a few weeks ago. Exactly the same. It’s what sparked me into doing some research. Here is my take on it :

 

If you observe what happens, and notice that lumps of floc (sludge) spring to the surface in a mass of fine bubbles, then it’s to do with consumption of dissolved oxygen (DO) in the settlement tank. The biomass in that tank is obviously no longer being oxygenated by the bubble diffuser, but the bacteria keep consuming it, so the amount of DO reduces. If it gets too low then they start to go after oxygen that’s in the nitrate molecules (Nitrogen combined with Oxygen). When they break these molecules apart to get at the oxygen, then nitrogen is also released. This accumulates in the floc until there is enough to make the floc buoyant and it floats to the surface.
 

So, you might get this effect if;  either overall dissolved oxygen is too low (eg the bubble rate it too low) meaning the biomass in the settlement tank will soon run out (because it was low to start with) and start going after the nitrates. Or, it may be that the bubble rate is fine, but the rate at which the settled sludge is being returned to the bubble chamber for re-oxygenation is so slow, that the DO diminishes just because of the length of time in the settlement tank.

 

What I did was increase the rate of SSR (settled sludge return) and the rate of aeration. Because these two rates work off the one blower and if you increase one the other will decrease, what I do is sacrifice the amount of air going to the coarse bubble diffuser. That’s because on newer Vortex tanks where SSR dumps into the inlet box, I find that the SSR flow agitates the inlet box pretty well anyway, and hence not too much coarse bubbling is needed. It took a couple of weeks to sort itself out, and to cope during this time I set the timer for the floating sludge return to run every 40 mins instead of every 60 mins.

 

That’s my experience anyway, and as I keep saying, I’m an enthusiastic amateur and no expert.

 

Edited March 17, 2023 by Crunchynut

[Onoff]

57 minutes ago, Kelvin said:

Invite the neighbours round for a poo

 

Bet that's a first ever sentence on any forum, anywhere! 😂

[SteamyTea]

7 hours ago, Onoff said:

8 hours ago, Kelvin said:

Invite the neighbours round for a poo

 

Bet that's a first ever sentence on any forum, anywhere!

This is Kernow, nothing is out of bounds in the off seasons.

[Alan Ambrose]

>>> The science behind sewage treatment plants

 

I think you've opened up a whole new can of ......... worms there.

[saveasteading]

22 hours ago, SteamyTea said:

how well tanks and plants deal with small amounts of regular usage i.e. single person household.

Not much poo or wee, but quite a lot of bath/shower water.

There is a formula. An assumption is made for a domestic digester, for the balance of poo, wee and water. Out of balance creates inefficiency. 

From experience, for a sports hall there is barely any toilet use, but lots of theoretical showers. They are barely used but could be.

Meanwhile the next door classroom block has toilets but no showers or other plain water.

Each on their own  need bigger units or they don't work properly. We combined them and it works fine.

Bleach kills the bacteria and I guess dishwashers are bad for that.

 

(Did you know that airports have exceptional poo content in the morning? Needs high stall proportion in the mens' toilets.)

 

I don't think anyone has mentioned the chicane of pipes and wiers between sections,  to allow only the grey, and clearing, liquid through.

[Crunchynut]

Still a lot I don’t understand.
 

A couple of days ago I peered into the settlement tank and beneath the top clear layer I could see the settled floc only about 15” below the surface. Perfectly settled but worryingly high. Next day I looked and the settlement level had dropped to the point where I couldn’t see it.  Nothing had changed. Same loading, same overall water level. It hasn’t done it since.

 

The only thing I can think is that that day the power had been off for an hour. Had this caused filamentous bacteria* to grow (the ones that thrive in low dissolved oxygen situations) in such a short time? And can they be destroyed quickly once other bacteria thrive again when the dissolved oxygen (DO) recovers? But if low DO was the problem I would have expected quite a bit of floating sludge because there had been excessive denitrification**, but there wasn’t - perhaps the floating sludge return had been doing it’s job well. I don’t know.
 

*filamentous bacteria form a very low density floc which consumes more volume and hence you would expect the volume of floc in the tank to rise and hence the settled level to be higher.

** denitrification is where, if DO runs out in the settlement chamber, the bacteria break apart molecules containing oxygen to get at it. So, Nitrates are broken apart and the Oxygen consumed and the Nitrogen floats lumps of floc to the surface causing floating sludge.

 

Still searching for more knowledge and answers that allow us to be able to manage our tanks better ……please comment if you have any.

Edited March 29, 2023 by Crunchynut

[SteamyTea]

31 minutes ago, Crunchynut said:

Still searching for more knowledge and answers that allow us to be able to manage our tanks better

I think there is two issues here:

How it work.

Making mine work.

 

I am sure your local agricultural college will have an expert on AD, it is becoming all the rage now, even Farming Today (which should really be called Middle of the night farming) was talking about pig poo and stuff this morning.

 

[dpmiller]

The power being off for an hour or two is an irrelevance- SBR tanks like our Solido do just this on a regular basis to settle the floc a bit before the clearwater discharge

[Crunchynut]

You could well be right. I don’t know much about SBR but quickly found some quite informative stuff on the internet. In the document I found they comment that the formation of filamentous bacteria during the ‘idle’ period is offset by a new ‘high food’ influx when the ‘fill’ period commences, which causes the beneficial bacteria (that produce floc that settles well) to prevail of filamentous.

 

So maybe we are both right!
 

Thanks for the comment - still learning…..

 

Edited March 29, 2023 by Crunchynut