Cycling rainwater through a sand filter system - thoughts on pumps, solar panels etc?

Just realised I did start this topic off a year or two back and got some good suggestions which I implemented, so this post is kind of a follow-on from this one: https://permies.com/t/190015/Organic-gunk-rainwater-tank-making

So we have an underground concrete-lined water tank that takes rain from the roof, we use it for the household and for a bit of garden watering (but not drinking).

Although I do a bit of basic filtering - a "leaf pooper" type of grid to get rid of the really big stuff, followed by a first flush, followed by a finish mesh filter, otherwise known as women's tights/pantyhose. There's not as much filtering as I would like because too much filtering on the intake means you have problems when there are big rain events (which we rely on) because the system can't cope and you lose a LOT of water. So inevitably a fair bit of crap (organic material) gets in the main tank and gets stinky at some point (I am not sure it should, but it does). I resolved this by adding ceramic and carbon filters after the pump, before it gets in the house, and this has resolved the smell problem (though there is still a bit of sediment). However, the filters get clogged very quickly this way.

I was thinking to set up a slow sand/biosand filter of some sort outside the tank whereby a pump would constantly draw water up out of the rainwater tank, filter it through the slow sand filter and then send it back to the tank, thereby over time gradually filtering the tank water, especially in the winter and spring months when we are not actually resident at the property mostly but the rain events are (hopefully) happening, and thereby cleaning out the tank water and lessening the load on the house filters. Something like this:



As I see it, such a set-up would require a pump that could be:

- (direct?) solar-powered (wouldn't need to run all the time) - how would you set that up?
- able to cope with some debris
- able to lift say 4 m
- have a low throughput, you don't want or need 1000 lit/minute for a slow sand filter!

I've been looking at various inexpensive 12 v bilge pumps but most seem to fail on at least one of the counts above. Any thoughts setting something like this up, and what kind of pump/solar setup would work?

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Could a gravel and sand filter not go in-line above ground before the tank and just be gravity fed? If you make the surface area large enough it shouldn't be a problem even during downpours. You could easily repurpose an IBC tank for this task.

Ezra Beaton wrote:Could a gravel and sand filter not go in-line above ground before the tank and just be gravity fed? If you make the surface area large enough it shouldn't be a problem even during downpours. You could easily repurpose an IBC tank for this task.

Well, you're right, that's definitely a possibility though it would mean changing the location of my downspouts and everything quite a bit, though not disastrously so and I do have a space above the tank (outside) where such a tote could sit.

That's a hell of a lot of sand and gravel, though I guess not that expensive. However the thing that concerns me is still the throughput - a slow sand filter would still be WAY too slow. A square metre surface of slow sand filter could barely clear 200 litres in an hour (according to what I have read), whereas we can get that in a few MINUTES in a heavy rain. I think of necessity it would have to be a faster system, thus less effective, certainly without the purifying effect a slow system provides. Also, I would like to purify what's already in the tank without emptying it (yet again) and cleaning it all out, as that is a REALLY thankless job...

Maybe I need to look into the different types of sand filter and find a compromise. I know the fast, coarser type of filter needs frequent cleaning/backwashing, so that's not great either.

Try two tanks: one collects directly from the roof, and the other is fed from the first through a small sand filter at a reasonable rate.

Mark Danilovic wrote:
As I see it, such a set-up would require a pump that could be:

- (direct?) solar-powered (wouldn't need to run all the time) - how would you set that up?
- able to cope with some debris
- able to lift say 4 m
- have a low throughput, you don't want or need 1000 lit/minute for a slow sand filter!

I've been looking at various inexpensive 12 v bilge pumps but most seem to fail on at least one of the counts above. Any thoughts setting something like this up, and what kind of pump/solar setup would work?

-

I have a few ideas:

- Put a float valve  with a timer in the sand filter container that will shut the pump off for a least ten minutes once the sand filter is full of water.
- Install an overflow from the sand filter back into the cistern
- Install a variable speed controller on the pump (Not sure if this would work)
- Use a pond or aquarium pump. They aren't dirty water pumps but they are designed to deal with some particulate matter in the water and have a much lower flow rate than most sump pumps.
- Instead of using a slow sand filter install an solar air pump attached to several airstones and aerate the cistern water.

Solar for pumps is tricky, they use quite a bit of electricity and it's variable based on the head distance you are trying to pump.  I have set up a few solar installations including our entire 1400 sqft lake house and pumping water uses the most power out of anything we do.  

A 12v pump would be where I'd start, then size a solar panel accordingly.  Then add the appropriate solar charge controller and a marine battery and you're good to go.  A basic timer could control the pump but some solar charge controllers have an output that will be live when the sun is out or live when the sun is not out so that may be able to be leveraged.  This is completely do-able, just may need some experimentation to get there, good luck!

Aaron Yarbrough wrote:

Mark Danilovic wrote:
As I see it, such a set-up would require a pump that could be:
...

I have a few ideas:

- Put a float valve  with a timer in the sand filter container that will shut the pump off for a least ten minutes once the sand filter is full of water.
- Install an overflow from the sand filter back into the cistern
- Install a variable speed controller on the pump (Not sure if this would work)
- Use a pond or aquarium pump. They aren't dirty water pumps but they are designed to deal with some particulate matter in the water and have a much lower flow rate than most sump pumps.
- Instead of using a slow sand filter install an solar air pump attached to several airstones and aerate the cistern water.

Thanks for the suggestions! So you mean possibly not filter at all but rely on aeration to break down the organic matter aerobically? I actually thought of doing that just by using any old pump running round in a circle pumping the water out and back in again from a height, which would provide a fair bit of aeration by itself.

Pond or aquarium pumps unfortunately seem to have a very small lift on the whole (like max. 2m), and the more powerful ones you then have the problem of them running way too fast, though your float valve is a great idea and would maybe work better with a solar panel and battery setup as it would give the battery time to recharge, not sure.

Jason Nault wrote:Solar for pumps is tricky, they use quite a bit of electricity and it's variable based on the head distance you are trying to pump.  I have set up a few solar installations including our entire 1400 sqft lake house and pumping water uses the most power out of anything we do.  

A 12v pump would be where I'd start, then size a solar panel accordingly.  Then add the appropriate solar charge controller and a marine battery and you're good to go.  A basic timer could control the pump but some solar charge controllers have an output that will be live when the sun is out or live when the sun is not out so that may be able to be leveraged.  This is completely do-able, just may need some experimentation to get there, good luck!

Thanks for these tips. Maybe also using a floatation valve on the sand filter like someone suggested would ensure that the whole thing was only cycled periodically.

If it's a 12 v pump do I need to look at its maximum current draw and size off that? Because solar panels generally have a power rating, don't they? But if there is a battery and charge controller then the actual power output should be taken care of I presume? Or maybe you can just trickle-charge the battery with a small panel and use a timer or whatever to give it plenty of time to recharge, and avoid needing a proper charge controller? As I say, I am really not very clued up on solar setups.

Sean at Edible Acres just posted an aerated compost tea setup.
Instead of an aerator he uses a pump , and not to drive a fountain or waterfall.
Instead he inserted the hollow tube from a ball point pen into the hose that carries the water, and via the venturi principle air is drawn into water.
It can operate at no head whatsoever.

William Bronson wrote: Sean at Edible Acres just posted an aerated compost tea setup.
Instead of an aerator he uses a pump , and not to drive a fountain or waterfall.
Instead he inserted the hollow tube from a ball point pen into the hose that carries the water, and via the venturi principle air is drawn into water.
It can operate at no head whatsoever.

Never miss an Edible Acres video :). Watched that one but didn't look up exactly how this works - I understood it's basically sucking air through the ballpoint and aerating the water as it goes through?  I know aeration is one way to mitigate organic material (and anaerobic bacteria) in water as well as volatiles (basically what they do in water treatment plants), though I believe there will still be a build-up of sludge (floc) in the tank which I guess I was wanting to get rid of. Neat system, yes, it wouldn't require lifting the water anywhere much.

Mark Danilovic wrote:
Thanks for the suggestions! So you mean possibly not filter at all but rely on aeration to break down the organic matter aerobically? I actually thought of doing that just by using any old pump running round in a circle pumping the water out and back in again from a height, which would provide a fair bit of aeration by itself.

Yes, that's what I was thinking. It's easier than a sand filter and if it doesn't work it's a step toward a system with a sand filter.