sand filter - construction has begun!
 

I've started building a sand filter for our rain water collection system. It's to be a non-backflushable slow sand filter. The vessel is a 4000 litre (1000 gal) plastic cistern (I made a mistake on the tank size when I first posted - fixed now). I'll be using drain rock, pea gravel, and 20/50 filter sand as media and would like to hear what options I have for a barrier between the pea gravel and filter sand to stop migration of the sand into the gravel. Is landscape cloth food grade? Would a cotton sheet stand the test of time?

http://i194.photobucket.com/albums/z...ps9077e9a5.jpg

Depending what you're doing with your rainwater, seems like landscape fabric would be ok, its made for gardening after all. I personally wouldn't drink through it. Food grade plastic recycling is number 5, maybe you could find that on the package. I believe its nylon, aka plastic. Asphalt shingles leach carcinogens from the bitament . How will you clean your sand filter when its time? I used a few bio filters for pond pumps, many used a thick nylon filter in them. You can buy a generic cut to size washable a/c filter at the depot. Its blue plastic, 1"x 30x 36 or some thing like that.

Slow sand filters have been used for centuries. To clean the media when there's not much water flow is to backflush them. As long as the media isn't supersmall from the diameter it was originally and the organizms are still active, they will work for eions. BTW,the media simpky supports the microorganisms.

I made some progress on the filter today and thought I'd share some thoughts and pictures.

There's lots of information on the web about slow sand filters (bio-sand filters) but much of it concerns very large scale installations. The majority of pictures returned by Google are theoretical schematics. I could find very little nuts and bolts practical information about, or examples of, smaller filters within the realm of DIY.

As mentioned above the tank I'm using is a 1000 gallon cistern that will be just over half buried. The filter will be used for domestic water - yes drinking - filtered from rainwater collected off a steel roof. It's non_backflushable by design. Cleaning when necessary will involve scraping the bioscum off the sand.

I hope it works, it's a major project. The hole in the ground had to be dug by hand. The access lid on the tank is small. I fit through but barely. All the plumbing inside the tank (PVC - 1 1/4" and 6") was taken in in pieces and assembled. The aggregate is poured in a bucket at a time. I still have a ways to go.

Now to the interesting stuff,the pictures!

http://s194.photobucket.com/user/Gre.../sand%20filter

I was wondering what to use to separate the sand from the pea gravel. I ended up using nothing. The pea gravel is 10 mm and the sand I ended up with is 10-20. The photos show good stratification in a test jar- at least initially - even after gentle agitation. I'm encouraged. And happy I don't need some synthetic material in my filter.

sand_in_jar_zps157073c7.jpg Photo by Greg_M_ | Photobucket

sand_in_jar_closeup_zpsfda669e2.jpg Photo by Greg_M_ | Photobucket

I don't see anything wrong with the sand or the pea gravel. I would suggest getting some sand that's finer to go on top of what you already have.

400 Liters = how many gallons?

Steve

Sorry Steve, fat finger typing. That should be 4000 litres, (1000 gallons).

I was wondering as 400 liters is not a lot of water (but 4000 is!). Include diatomaceous earth as the most fine filter (on top of the sand). Put in about 3-4 inches of thickness.

DE (as it is known) is cheap, very effective and traps bacteria, pollen as well as all algae. You do not need the food grade. Get the kind that goes into swimming pool filters.

After looking at this, you might want to reverse the trapping layers. The last filter element might be the DE, then the sand, then the pea gravel.

Think about it.

Let us know how this works out!


Steve



Steve

[QUOTE=stevehull;40079]After looking at this, you might want to reverse the trapping layers. The last filter element might be the DE, then the sand, then the pea gravel.
/QUOTE]

Too late for that. There's 40 cm (51 bags at 0.44 cu ft ea.) of 10-20 sand in there already. I'm trying to find 40-70 sand for the remaining ~ 3/4 meter.

Cheers, Greg

Sunspot, am doing my homework prior to starting a slow sand filter.
You are right not to back flush, it destroys the filter.
The finest sand goes on top just like you are doing.
Did you pre-wash the sand?
What did you do to make sure you had no air pockets in the sand?
Have you got it working and how is it performing?.
Thanks.

Sorry for the very delayed response nibs, I haven't had much time at home lately.

I managed to get the filter completely full of sand last fall and topped it up with water. I've been letting the bioscum form since. I figured there wouldn't be much biological growth over the winter so plan to start using the filter this fall (fingers crossed). I didn't wash the sand nor did I concern myself with air pockets so I suppose it's possible there's air in there somewhere but it seems unlikely and I don't know that it would be detrimental in any case.

Still to do is a stand for the buffer tanks. As I plan to limit the flow rate through the filter to about 5 litres per minute -waaay less than pours out the downspout in a heavy rain - to ensure adequate time for filtration I bought three 450 litre tanks to use for buffering. We don't get much rain here in the summer so those tanks are seeing temporary duty in the garden for now.

http://i194.photobucket.com/albums/z...pspwurm9pf.jpg

If you've made any progress on your filter I'd be interested to know what you've done.

Apologies again for my tardiness in replying to your post.

Cheers, Greg

Am very happy with the results of my sand filter.
just a note to let you know we started pumping the hot tub through the sand filter.
Wound up using a 55Gal ex beer yeast barrel (good pedigree),
Bought some gravel and raked some up in the yard, then added crushed quartz, 300lbs & then 50 lbs playsand, then 1/2 pail of local very fine bank sand. Ran unchlorinated tap water through for 3 days, it was clear after about the first 4 hours.
We put a small RV pump to move the water from the tub up to the barrel. Right now the filter is doing about a gallon a minute, will work on slowing that down as the schmutzdeck develops and when we have the final pump working.

An interesting tid bit, We heat the tub with a wood fired stove which thermo syphons the water, the filter pump uses the same drain fitting and with the pump going the heater will not thermosyphon, pump just pulls hot water through heater, backwards.
Fixed that with a snap switch so that when the fire is hot (120 deg F) the pump wont cycle
We bought a float switch from Amazon, and will put a bigger pump on when we get time.
Tub water was a tiny bit cloudy, When we started the system, and was clear within 24 hours. We started the filter on Jun 8Th, normally we would have changed the water on June 9 or 10, so here we are 12 days later and the water is still crystal clear, and we have used no chemicals (H2O2) since around June 1 or 2.
There is a minor down side, the filter cools the tub during its run time, so it takes a little more firewood to heat it back up.

What a great use for a sand filter. I wouldn't have thought of that. Does the schmutzdecke tolerate the heat? I suppose it would be simple enough to halt the filtering while the tub was heated and resume once it cooled off again (like you've done to preserve the siphoning of the heated water).

Very interesting.

Sunspot.
If you are ever up this way, HWY 3 east of Grand Forks drop by.

Absolutely! And likewise, if you ever find yourself on the island give me a ring.

Cheers, Greg

I just want to add a couple of photos of progress made on the buffer tank portion of this filter installation. The four 400 l tanks are plumbed in parallel with 1" pvc discharging through a gate valve into the top of the sand filter. I welded the stand from 2" square tubing and flat bar with plastic wood boards for decking.

http://i194.photobucket.com/albums/z...psupzqmjrs.jpg

A view into the filter:

http://i194.photobucket.com/albums/z...psvkjfhti3.jpg

I'm waiting on a rubber grommet to make a nicer connection to the filter basket in the top of the first tank. Then a small amount of plumbing to complete on the discharge side of the filter and it'll be in service!