Organic gunk in my rainwater tank making it smell iffy, ideas for improving filtration?

So we collect rainwater in a 7,500 litre concrete tank that's underneath our garage. We really rely on it in the summer months as we are semi-off grid water-wise. We don't need it for drinking or cooking, but we do wash dishes, shower, run the washing-machine etc. off it.

It's pretty rudimentary, I did a video about it a year or two ago ( https://www.youtube.com/watch?v=fvD6kwwOofE),  but the gist is it comes off the roof and down a single downspout, straight into the cistern. I have tried various simple forms of filtration - putting it through muslin cloth, and even panty-hose, and it all works up to a point, but eventually the finer particles start to build up in the cistern and the water starts smelling a bit swampy, especially because we can have rainfall in, say, November, and then virtually nothing through spring until summer, when we are actually at the property and need the water. Yeah, we have had incredibly dry winters, springs and summers the last few years. Anyway, so that water is sitting in the tank for months at a time, and I guess it just starts to smell, even though it shouldn't (it's in complete darkness). I had to clean the tank this summer and it was NOT fun, there was sludge built up in our hot water heater as well, so that was a fun job too.

I need to step up the filtration, I have added a basic first-flush, finally, as a lot of crud does come off the roof during sudden rain events. I am also going to add mechanical filters coming OUT of the tank, into the house system.

But in addition I've been looking at sand filters for the roof water. Thing, is they are called SLOW sand filters, and there's a reason - they are very effective, but very slow. We rely on big rain events where we get at least 1000-2000 litres of water coming in in a fairly short space of time. You have to have a surge tank feeding the sand filter as a "buffer", but then that surge tank has to be like 2-3 IBC totes to cope with the volume, which makes no sense really. Or just make the sand filter really HUGE, like a really big surface area.

Any thoughts on how I should approach this. Big sand filter? Big surge tank? Is there any way to get a rough estimate of flow rate through a sand filter without actually building one, so I can figure out how to dimension it!

Where you have first flush diverter system currently I think a 55 gallon (208 Liter) HDPE drum surge tank plumbed like a septic tank would work well (See drawing attached). Also, if you can you get your hands on a Leaf Eater and install it in front of the surge tank it would work wonders keeping stuff out of your cistern.

Regarding, chlorine I found I initially had to give my tanks a shock dose  to take care of the swampy smell. Subsequently, now after a rain I'll add 1 - 1.5 cups (~.25 L) of bleach to our 1500 gallon (5700 L) tanks. I also have the luxury of having two tanks so I can dose one and use the other for a week and then dose that one.

Thanks for the ideas, yes, I looked around for a Leaf Eater, but they aren't sold in my country, so I bodged something myself and it kind of worked but wasn't very permanent, and only got the big stuff.

Yeah, I put chlorine in the tank, but I always put the minimum recommended, I think probably I needed to give it an overdose to start with as it never seemed to make much difference. I was using a commercial well-sanitising agent, it said like 12-20g per m3, but that didn't seem to be near enough and I was concerned about health and didn't really want to go crazy, plus the sludge was still a problem at the end of the day..

So your idea with the septic-style tank is that any debris should settle out and the cleaner water from the upper portion of the tank would enter the main tank? Septic tanks usually have two or three chambers, don't they, so only the overflow goes into the next one and the debris settles out as it goes through the chambers. Not sure how it would perform in a surge, though, doesn't it all just get murked up?

Mark Danilovic wrote:Thanks for the ideas, yes, I looked around for a Leaf Eater, but they aren't sold in my country, so I bodged something myself and it kind of worked but wasn't very permanent, and only got the big stuff.

I only have leaf eaters on the tanks I use for drinking water. On the other tanks I use a PVC Adapter and window screen. If only really small particulate makes it into your cistern I think it will significantly alleviate your sludge problem.

Mark Danilovic wrote: Yeah, I put chlorine in the tank, but I always put the minimum recommended, I think probably I needed to give it an overdose to start with as it never seemed to make much difference. I was using a commercial well-sanitising agent, it said like 12-20g per m3, but that didn't seem to be near enough and I was concerned about health and didn't really want to go crazy, plus the sludge was still a problem at the end of the day..

You might trying adding a cup of bleach a day until you can just faintly smell chlorine when you run the water and then just go do a small dose after a rain. If you're not drinking the water it shouldn't be an issue.

Mark Danilovic wrote:So your idea with the septic-style tank is that any debris should settle out and the cleaner water from the upper portion of the tank would enter the main tank? Septic tanks usually have two or three chambers, don't they, so only the overflow goes into the next one and the debris settles out as it goes through the chambers. Not sure how it would perform in a surge, though, doesn't it all just get murked up?

With the inflow outlet close to the bottom of the HDPE drum you get a less churn than if the incoming water was just pouring in from the top.  You could also screen or use an effluent filter with the outflow pipe. Or you add another HDPE drum after the first one to act like a second chamber in a septic tank. However, I think if you had a more effective leaf eater and a single chamber surge tank it would fix your sludge problems.

We also have two whole house filters directly after our pump and pressure tank. I'm sure that also helps filter out any sludge.

Slow sand filters are very much not suited to surge type filtration. They are incredibly effective if you can use them in a recirculating setup, though, and if the water is pretty clean to start with (rainfall runoff fits this category) then you should be able to get away with a single 200 l barrel. I'd also consider a second barrel with activated charcoal or biochar to do final polishing before consumption.

A small pump, like a 12 or 24V DC one suited to running a fountain, is a good match for a slow sand filter.

Mark, where are you living?
I may be able to help with a first flush unit.
Secondly, its unusual for tanks to smell.
I am guessing you have leaf matter in the tank and I will think of a way around that.
In the meantime install a  Tank strainer.
Here is a leaf pooper you can copy

Is it possible to give it a hit of ozone, knocking back the bio-bits without making the water unusable (chlorine).

I'm not promoting this per se. Rather, I'm curious is anyone has owned/used an O3 generator. I have a number of applications here.

Phil Stevens wrote:Slow sand filters are very much not suited to surge type filtration. They are incredibly effective if you can use them in a recirculating setup, though, and if the water is pretty clean to start with (rainfall runoff fits this category) then you should be able to get away with a single 200 l barrel. I'd also consider a second barrel with activated charcoal or biochar to do final polishing before consumption.

A small pump, like a 12 or 24V DC one suited to running a fountain, is a good match for a slow sand filter.

OK, so you pretty much confirm what I was thinking, that a slow sand filter isn't going to work on the intake. It also occurred to me to recirculate the water (I'd already tried it for oxygenation purposes, but it didn't seem to have much effect), I have a small 12V camper pump that I could hook up to a solar panel maybe. Though I'd rather avoid the material getting in the tank in the first place, rather than trying to refilter it - the pump is in danger of getting clogged, plus the tank gets gunk in it again... It might have to be a proper bilge pump for that purpose, or is there another type of pump that can handle debris OK?

Actually, you've got me thinking, with a setup like that I could have the whole sand filter indoors in the garage (where the service hatch is for the tank), where it's nice and dry, and just pump the water up into the sand filter and drain it back down into the tank, that would be quite nifty, though still not sure of a pump that could cope with possible debris, probably best to have a mesh on it. Plus after a big rainfall it would take a fair while to get the whole lot filtered, but since rain seems to be an increasingly rare occurrence, maybe I shouldn't worry too much...

Douglas Alpenstock wrote:Is it possible to give it a hit of ozone, knocking back the bio-bits without making the water unusable (chlorine).

I'm not promoting this per se. Rather, I'm curious is anyone has owned/used an O3 generator. I have a number of applications here.

I am always keen to avoid solutions that require "special" equipment, but I'd be interested in hearing about this. I tried recirculating the water to oxygenate it but that didn't seem to help much, or maybe I didn't do it for long enough...

John C Daley wrote:Mark, where are you living?
I may be able to help with a first flush unit.
Secondly, its unusual for tanks to smell.
I am guessing you have leaf matter in the tank and I will think of a way around that.
In the meantime install a  Tank strainer.
Here is a leaf pooper you can copy

Thanks for the suggestions. I live in Serbia, i.e. south-eastern Europe, a lot of stuff isn't available here but I CAN order stuff from Amazon UK at a pinch. Those drain wedges CAN be found on Amazon, but not the leaf pooper, sadly. I love it though! I have seen similar designs, but this is the most compelling somehow! Yes, I think something like that could be made fairly easily by cutting a section out of a pipe and lining with steel mesh. The first flush I made is a metre-high section of 110 mm pipe with a closed bottom (with a pinhole) and a constriction near the top so a ball can float up and redirect the flow once the first few minutes of (heavy) rain have fallen. It's no good for light rain as you lose the first 30 litres of water, or whatever, or it will even drain out too fast. I have yet to test it properly.

I think you may be right about leaf matter - here's the problem, we have looong periods with no rain, so debris does build up on the roof and in the gutters. We have a big linden/lime tree shedding a lot of leaves and seeds to one side of the house. The gutter to that side is somewhat accessible, and I should put a wedge in that one, but the gutter on the other side is only accessible from the (fairly steep) roof, and that's a job for a professional, wife would kill me if she saw me climbing around up there. I guess I could get someone to at least install a wedge there, but regular cleaning is very difficult, I have to use a hook-shaped brush from the upstairs floor.

I agree that probably too much leaf matter is getting in, and not only leaf matter but ALL sorts of crud: dead bugs, even unfortunate lizards, moss that built up from last time, etc. That's why I feel like I need some fine filtration, but I bet even eliminating the big stuff would be a big step forward. It's telling that none of my neighbours, who all have similar systems, have this problem, and they are doing nothing except some basic mesh filtering. So you are right about it being unusual. Neighbours tell me the only time it smells a little is after rain, and then they add more chlorine. But my problem seems to be too much for chlorine to deal with - or maybe I just need more chlorine, but I don't think I should need to...

I am happy to try and help you design aND BUILD SOMETHING.
Perhaps if I can get an image of the problem area, we can devise something. Even if I send it to you or we build it out materials you can get.

This first flush unit can be made locally;
first flush units explained

Hi Mark,
I think based on what you are saying, and the suggestions people have given already, that you best bet, may be to try to get another tank. With two tanks, you can dump the water into the first tank with basic filtering just to handle the surge in water. Then in between rains, pump it through a sand filter into your second tank that you actually use the water out of.

I think the first flush is a great idea.

If money were not too big of a problem (and it usually is for most of us), you might look into gutter guards of some sort. It seems to go beyond those wedges. It would cover your entire gutter with a fine mesh at a slight slant. The idea is that any large debris (leaves, chunks of shingle, dried bird poop, etc) will get washed overboard and fall to the ground, while the water falls into the gutter and eventually into your tank.

John C Daley wrote:This first flush unit can be made locally;
first flush units explained

Right, that's pretty much exactly what I have made, still looking for a rubber ball just the right size and conditions to test it in!

Matt McSpadden wrote:Hi Mark,
I think based on what you are saying, and the suggestions people have given already, that you best bet, may be to try to get another tank. With two tanks, you can dump the water into the first tank with basic filtering just to handle the surge in water. Then in between rains, pump it through a sand filter into your second tank that you actually use the water out of.

I think the first flush is a great idea.

If money were not too big of a problem (and it usually is for most of us), you might look into gutter guards of some sort. It seems to go beyond those wedges. It would cover your entire gutter with a fine mesh at a slight slant. The idea is that any large debris (leaves, chunks of shingle, dried bird poop, etc) will get washed overboard and fall to the ground, while the water falls into the gutter and eventually into your tank.

Yes, always looking to cheap out, though the bigger problem is actually getting on the roof to install them, a scissor lift is going to cost way more than the guards themselves. I can always make something myself out of steel mosquito mesh or something.

I guess the idea of a second tank means I can't collect more than the size of that tank in any one rain event, that's kind of the problem, whereas I would hope for quite a bit more. I.e. once I have collected a cubic metre, say (assuming I use an IBC), I have to overflow it somewhere. The other possibility might be to do basic mesh filtering into the tank and then do slow sand filtering at my leisure by recirculating the water at a much lower rate through the sand filter and back into the tank. But I would have to specifically use a dirty water pump, maybe solar powered, that could handle a few bits of leaves and things going through it (assuming some will always get in there). It would be a bit like a swimming pool, just keep pulling the water out, through the sand filter and back in the tank again and hope to continually clean the water that way. But for sure the trick is to let as little crap in there in the first place, I think I have been sloppy about that until now.

Another thing about slow sand filters is that they are a living thing, so dry spells and freezing both could be problems do deal with, one would just mean recirculating the water, the other, locating the filter in a warm place.

A radial-flow filter (used in aquaculture/aquaponics to separate fish waste) might also work well. Similar to Aaron's septic-style surge tank, it separates floating and sinking debris from the main flow. It is basically a barrel without a bottom inside a barrel with flow entering into the center, flowing down under the rim of the inner barrel, then up the outer space. Debris flows down, but doesn't turn the sharp corner around the inner barrel and continues down to settle on the bottom.

Keeping the debris out in the first place, is probably the best. Gutter guards, downspout screens, first-flush diverters, a filter sock...

Another option, is to have a smaller tank that holds the cleaned water ready for use. As long as your sand filter could keep pace with you usage, you wouldn't need more than maybe 3 days of storage of filtered water?

Mark, what is the height of the gutter your wife is freaking out about?

Kenneth Elwell wrote:Another thing about slow sand filters is that they are a living thing, so dry spells and freezing both could be problems do deal with, one would just mean recirculating the water, the other, locating the filter in a warm place.

A radial-flow filter (used in aquaculture/aquaponics to separate fish waste) might also work well. Similar to Aaron's septic-style surge tank, it separates floating and sinking debris from the main flow. It is basically a barrel without a bottom inside a barrel with flow entering into the center, flowing down under the rim of the inner barrel, then up the outer space. Debris flows down, but doesn't turn the sharp corner around the inner barrel and continues down to settle on the bottom.

I wasn't familiar with this design, I will have a look! I wonder what sort of flow rate that can handle.

Yes, I realise sand filters have this biofilm and it's a bit of a problem if you aren't using the system at certain times of the year like us. I guess I don't REALLY need drinking water, I just need fine particle filtration, so maintaining the biofilm isn't essential. Not sure if that is a valid approach. I mean, you can make a fine screen with other materials as well, so maybe the sand filter isn't even essential, a large, fine mesh surface might do just as well.

Kenneth Elwell wrote:Keeping the debris out in the first place, is probably the best. Gutter guards, downspout screens, first-flush diverters, a filter sock...

Another option, is to have a smaller tank that holds the cleaned water ready for use. As long as your sand filter could keep pace with you usage, you wouldn't need more than maybe 3 days of storage of filtered water?

Oh, that's an idea. Like a holding tank. Absolutely, an IBC's-worth of water would be more than enough, in fact even less than that probably, I think a slow sand filter could probably replenish an IBC in 24 hours and we definitely don't use a cubic metre of water a day. Only problem would be having to replumb the house water pump into that tank instead of the concrete tank that it's currently plumbed into, but that could just be spliced in and we could still choose between the two with a valve. An IBC could be located in our garage where it shouldn't really freeze enough to be a problem, but not sure about a smaller sand filter, whether that would stay thawed. We aren't at the property in winter anyway, so just shutting down the whole system for the winter might be the best option anyway. Either that or putting the pump on a timer and having it run intermittently to keep things moving. The IBC could just overflow back into the main tank when it gets full.

Also, a classic aquarium filter could be used to replenish the sand filter and IBC (provided enough head - I need about 3-4m) - they already have basic biofilm-type filtration, I mean, you have to buy cartridges for them which isn't sustainable to me, but maybe you can make your own.

Hmm, lots of ideas there, thanks!

Typically, the stinky water is on the bottom of the tank. Draw your water at about 12" off of the bottom and you might find that your problem is solved.

Actually, our intake is something like that height, maybe a little lower. But I think unfortunately the problem is such that it is tainting all the water. I am working on and off on making a much better first flush, possibly the "leaf pooper"-type arrangement someone suggested, plus some more rigorous filtration, though there's only so much you can do when it's coming off the roof because of the volume...

I have seen 'cyclone' systems used to cause sawdust to fall out of the exhaust systems in factories and also the same to get leaves etc out of rainwater. I will look around for you.
For finer material

This unit may be a great answer to your situation, its not budget priced, 306 English Pounds in cash, but it may solve all the problems in one hit.
Its a self cleaning high flow filter available from the UK. I have attached an image.
graf-optimax-pro-filter/

I'm rather late chiming in but I have had great experience with a "fast" sand filter.
It consists of a bucket, with slots cut in the bottom, filled with sand.
It served well, without issues, and seemed to prevent any detritus from entering the barrel.
I occasionally removed leaves from the top of the sand, but it never seemed to clog.
I've dismantled that filter but I've built another such filter, this time from a water heater tank.
The new one includes a layer of charcoal.
That lone could be perfect for removing the smell from the water.

For any filter, maybe consider employing an air lift pump?
They are powered by compressed air,  they can be used to move debris, and they are aerate the water they move.

https://en.m.wikipedia.org/wiki/Airlift_pump


You mentioned that you had circulated the water some already.
I wonder if there is any healthy population of  aerobic  bacteria present, or is this tank of water dominated by anaerobic life.
Maybe you could spike the water with lactic acid producing bacteria or acetic acid producing bacteria?
Both are known for creating environments that exclude pathogens.
Alcohol is disinfecting while also being food for vinegar making, oxygen loving critters.