Today is about the stuff that's plugged our water pump intakes in the past. It may seem silly, but we lost $10,000 once due to a plugged pump intake.
We'd measured and remeasured everything in the system in question, and were baffled, for it all tested out OK. We didn't understand until we did a flow rate test on the troughs, and found almost no flow in two out of four large trough circuits.
Ten thousand dollars is not silly, and our "antennae" are now on alert for similar situations. We catch them now before they cost us money. In today's newsletter and the next, we'll tell you about several such so you don't have to figure them out the hard way.
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Aquaponics Nugget #77:
Pump Intake Filters, Part 1
The $10,000 plugged pump
Our $10,000 problem was an easy one to solve, once we'd figured out we had a flow rate problem. We had 7 gallons a minute coming out of a pump that should have been putting out 35.
How do you figure your flow rate? Easy: take a five-gallon bucket and stick it under the inflow fitting. We put ours under the trough inflow fitting, right inside the trough. Time it with a stopwatch, phone, or pocket timer, and see how long it takes to fill up the bucket. Then you do your math: if it fills the bucket in a minute, that is a flow rate of 5 gallons a minute. If it fills the bucket in two minutes, that is a flow rate of 2-1/2 gallons a minute. We've found for our 4-foot wide, 10 to 12 inch deep troughs, a flow rate of 5 gallons a minute in each trough is conservatively plenty.
Why was the flow rate so poor? It had been fine a year previously when I finished the system. When I'd installed the 2,000-gallon sump tank for that system, I knew I was going to put a couple hundred pounds of two to three pound fish into it (we use all our sump tanks as "real estate" to house fish; this means our main fish tank doesn't need to be as large). This sump tank had a 2-inch PVC intake that went to the pump, and these big fish couldn't get stuck in it, so I omitted the usual intake filter to "save a little time" (I saved ten minutes and $5 worth of parts).
Over the next several months, some 2-inch net pots came into the sump tank from the troughs, floated around, then fit perfectly into the 2-inch PVC pump intake line to the pump. Some plastic plant tags followed, and about month 8, the pump intake was so covered with solid plastic that it was pumping one-fifth the water it should have. We lost $10,000 due to poor plant growth that in turn was due to a poor water flow rate through the troughs over the next four months.
The fix was easy: turn the pump off, clean the junk out, then spend ten minutes making the filter I should have made in the first place. We haven't had a problem with that pump in the last year and a half.
The Right Filter Mesh Size
Your pump filter needs to have the correct filter mesh size for the "stuff" you're trying to filter out, for you can only let stuff through to the pump that the pump can "chew up" and spit out the other side without getting clogged. Generally, the smaller the pump, the smaller the mesh needs to be, and the larger the pump, the larger the mesh can be. For our 35 gpm 170 watt pumps with 1-1/2 inch fittings, we use 1/8 inch or 1/4 inch mesh for the filter screen. Here's some examples of the right and wrong size filter screens:
Both of these are "wrong" sized filter screens (2" net pot for scale). The 1/2" mesh on the right let grass stems through over several months that finally clogged the pump and cut flow to nothing; when it was replaced with the 1/16" mesh on the left, vermiculite, dead gammarus, and sludge clogged and collapsed it within about two weeks, also cutting flow to nothing. The correct filter size mesh for this application is shown in the next photo.The correctly sized 1/8" mesh for this application is shown installed on the filter at the bottom; the too-large 1/2 inch mesh filter is shown at the top, along with a flat piece of both sizes of mesh, and a piece of 1/16" too-small mesh next to the net pot in the middle. There's a piece of stiff 1/2" mesh inside the 1/8" mesh on the filter at the bottom to provide a "backbone" to keep the 1/8" mesh from just folding over. You should still check and clean this filter every three or four months or so to make certain you have good flow rates in your troughs.
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(Next week: Part 2, Instructions and photos showing how to make these filters, and more info on how to keep your aquaponic water flowing freely. Thanks for listening!).
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