FarmHack Tools: Back-flushing irrigation water filter

*Haga clic aquí para leerlo en español*

Project is for: Vegetable and fruit growers who need to filter irrigation water

Range of cost: $325 – $450

Skills / tools needed: Pipe wrenches, ability to glue PVC fittings together

Summary: When using drip irrigation, it’s important to have a good water filter system, especially if you are using surface water (from a pond or stream).  At Hearty Roots Farm, we pump from irrigation ponds that have silt, algae, pond weeds and other particles that would clog up our drip lines if they didn’t get filtered out.

When our farm was smaller, we just used a simple disc filter and once in a while we would take it apart and clean the filter element with a hose, since it would get clogged up with algae and dirt which reduces the flow rate and water pressure.  However, as our irrigation systems grew, we needed a more efficient way to filter our water without breaking the bank.  Sand filters work well but they are not easily portable and they cost upwards of $1,000.   We built our own easy-to-back-flush filter using two disc filters and some pvc pipe fittings, for just over $400.  Now we can clean out our filter in a matter of seconds, while the system is still running, and therefore we have very little water pressure and volume loss due to clogged filters.

* Note that Amaid also makes a “Scan Away” disc filter, for about $410, which achieves similar results to this.  I haven’t used it, so I can’t vouch for it.  I built this version because I already had a regular disc filter–  I bought mine before the Scan Away model existed–  so this approach was more economical for me, and now I have two usable filters.  Plus, there are fewer moving parts to break.

Materials needed:

  • two disc filters, such as Amaid filters, available from Rain-Flo irrigation (in this case, 2″ diameter) for about $190 each
  • 2 pressure gauges
  • about 8′ of 2″ Sch. 40 PVC pipe
  • PVC pipe fittings:  6 2″ ball valves, 4 2″ tee fittings, other fittings that will depend on your application / threadings.
  • A pallet and 2″ pipe clamps
  • Pipe wrenches / teflon tape / PVC glue

How to build it:

The purpose of this filter system is to be able to use one filter to “back flush” the other one, and vice-versa.  That is, you want to be able to run water through one filter and then send that filtered water backwards through the other filter, flushing out all of the accumulated dirt and algae from that filter and sending it out a drain.  You can reverse the process to clean out the other filter.

To do this, you need a series of ball valves so you can change the flow of the water.

The following diagram illustrates the set up of the filters and fittings:

Here is a photo of the filter we use at Hearty Roots.  Note that we left out one ball valve, which means that we can’t clean out both filters, just one of the two.  This is ok, but the method in the diagram would work better, and one of these days we’ll get around to fixing ours.

Note that we used some threaded fittings (toward the top of the photo) and some glued PVC slip fittings (bottom).  We clamped it all to a pallet, into which we had to cut out some space for the filters to fit flush with the pipes.

How to use it:

Note the numbers on the ball valves and A and B on the filters in the illustration above.  When using this system, you will want to run water mainly through one of the filters (in this case, filter A), which we will call your main filter.  Filter B in this case will be your “back flush” filter.

There are three configurations of open/closed valves that you will use:

  • For typical operation, i.e. sending water from the pump, through filter A, and out to the irrigation system, configure the valves as follows:  1 closed; 2 open; 3 closed; 4 closed; 5 closed; 6 open.
  • To flush out filter A, by running water through filter B and then back flushing filter A, configure the valves as follows:  1 open; 2 closed; 3 open; 4 closed; 5 open; 6 closed.
  • To flush out filter B (only needs to happen once in a while, if you use filter A as your main filter), configure the valves as follows:  1 closed; 2 open; 3 closed; 4 open; 5 open; 6 closed.

How do you know when to back flush your filter?  We put a pressure gauge on each of the two fittings on our main filter, which gives us a reading of the pressure on the pump side of the filter, and the irrigation side of the filter.  In our case, a totally clean filter knocks the pressure down about 10psi;  so when we run our drip irrigation, we run our pump at a throttle level to give us about 20psi on the first gauge, and 10psi on the second.  As the filter gets clogged with debris, the pressure on the second gauge decreases.  When if falls below 5psi (usually after an hour or so) we flip our valves to back flush the main filter for 15 seconds or so, then flip them back, restoring the pressure to the original levels.

Suggestions for improving or modifying this tool:

If you wanted to save a few bucks, you could probably use a smaller filter for filter B;  a 1 1/2″ disc filter from Amiad costs about $74 and will supply 40gpm to flush out your main filter, which seems like enough to me;  it would just mean substituting some adapters in your assembly to go from 2″ to 1 1/2″ at certain points.  But I like having both 2″ filters so if something goes wrong with one, I can swap it for the other.  Of course, you could also use bigger filters or smaller ones for the whole shebang, depending on your scale.
Credits for this post:

Posted and written by Benjamin Shute of Hearty Roots Community Farm, who grows vegetables in New York’s Hudson River Valley.

9 Responses to “FarmHack Tools: Back-flushing irrigation water filter”
  1. Bob says:

    Very interesting, I plan on using your example but do not see the need for valve #5.

    your example configurations:
    1) typical use
    2) flush out filter A
    3) flush out filter B

    Per you three configurations, the #5 valve is open (hence not in use) in configurations two and three. Valve #5 is only closed (hence in use) in configuration one but valves #3 and #4 are closed in this configuration so negate the reason for valve #5.

    Maybe I am missing something, if so please advise. Thanks for you excellent post. Regards.

  2. Benjamin says:

    Yes you are totally right about not needing valve #5 in the drawing. I had it in there because I wanted a way to bypass both filters and send water directly to some sprinklers (which don’t need filtered water), so I was closing off valves #2 and #5 and leaving open valve #4 to do that.

  3. Jen says:

    Hi Benjamin,
    Would there be any problem if I were to use a 3″; filter as my main filter, and a smaller one (say 3/4″;, 1″;, or 1.5″) for the back flush filter? You mentioned using a smaller one for the secondary filter, I’m just wondering if there’s a point where the size difference would become a problem.
    Thanks for the post.

  4. Benjamin says:

    Hi Jen– Actually I am planning on trying that exact thing this year, because I got a bigger pump and so I wanted a 3″ disc filter as my main filter. So I am using reducers to back flush through a 2″ filter. I haven’t put it all together yet so I can’t say how well it will work. I am guessing that the smaller the flush filter is, the less vigorously it will flush out the 3″ filter. Also I would say that maybe if your water source isn’t too crazy dirty, you might be able to get away with just back flushing with unfiltered water. That could introduce some dirt onto the “clean” side of the filter, but if a little bit of that is tolerable to you, you might be fine and save some money.

  5. Daniel says:

    Interesting post. Im getting ready to move into a 5 acre field and irrigate with creek water to drip tape. Have you found the disc filter to be pretty sufficient for filtering surface water? Also, with the 2″ filter what are the gpm you are moving through it? I trying to decide between sand and disc filter and I’d like to be able to pump 150 gpm.

  6. Benjamin says:

    I have irrigated using a disc filter from several ponds. I have used both a 2″ (rated for 40-110gpm) and a 3″ filter (rated for 220gpm). At one pond that was pretty shallow and mucky I had problems with the filter clogging up very quickly; at other cleaner ponds I would see a pressure drop after a few hours, and would back flush quickly to restore the psi.
    A sand filter can probably go for much longer without back flushing, and can handle more GPM (depending on size of the filter), but is probably more expensive.

  7. Lorie says:

    Using Line Blaster may completely eliminate the need to do all of this. It breaks up debris and buildup into tiny microscopic dust that can easily pass through emitters. See our website for more details. You can contact us at or 800-CHEM-911

  8. Scott Raney says:

    Big disk filters = big bucks and it’s a shame to need 2 of them. And both do need to be big: According to Netafim/Arkal you need about 80% of max flow to backflush, which means you can’t downsize the flush filter. Indeed it means you can’t even oversize the filter for the pump to get longer runtimes between flushes. A couple of other ideas:
    1) If you’ve got a pressure tank, how about just turning off the pump, opening a valve ahead of the filter, and using the pressure tank water to clean it? According to Netafim documentation you can flush their 2″ filter with less than 40 gallons of water (albeit passed in 25 seconds), and you should have at least that in a pressure tank for that size line. For bonus water, shut a valve ahead of pressure switch, run pump to maximum pressure in tanks, then shut off pump and open dump valve between pump and filter.
    2) Unless the water is extremely dirty, why not just run unfiltered water to backflush, then dump the first 40 gallons of water when you start running forward again? Or, if you’ve got a tank or cistern, fill it with output from the pump/filter and switch the input line to use that as the source when backflushing.

  9. MIchael says:

    I’m pumping water from a trophic lake, about 120′ from the inlet to the pump. There’s a simple screen at the end of the line which clogs pretty rapidly and is hard to get to, so I’d like to remove it and use something like your setup onshore. I’m wondering if your setup will work on the upstream (suction) side of the pump, or if I need to put it on the downstream side. If the latter, will the crud clog an impeller-style pump? The pump is working near its limits now, I think, due to friction loss in the suction line.

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