Water Pumps And Flow Rate In The Vegetable Troughs

The Aquatic Eco Systems/Pentair Aquatics catalog gives a great example of a “cheap” $259 water pump that only uses $813 more of electricity per year than the “expensive” $479 pump. So, be on the lookout for ways to do things more economically and more easily, but also try to see everything that is affected by your choice, to make sure that your choice isn’t costing you more in the long run.

Since building our first systems, we did several experiments with water flow rates in our system troughs in an effort to make them more energy-efficient. Why would we want to do this? The water pumps run 24/7, and even a small improvement in energy efficiency could mean a large savings over a period of years. We did our most useful flow rate experiment accidentally!

(Below) What’s your flow rate?

We have a large aquaponics system with four separate sets of troughs. We noticed for a long time that growth in one of the trough sets was excellent, growth in another was good, in another barely OK, and growth in the fourth was poor: in that trough the vegetables were sickly with lots of bugs.

Susanne knew the problem must be connected to flow rate, so I got out there with a 5-gallon bucket and a stopwatch. I put the bucket under each of the trough inflows and timed it to see what the flow rate into the troughs was in gpm. It came as no surprise to see that the “excellent” trough had a flow rate of 5 gpm, the “good” trough had a flow rate of 1-1/3 gpm, the “OK” trough had a flow rate of 2/3 gpm, and the “poor” trough had a flow rate of 1/3 gpm! We have a WaterPik that puts out more water than that!

What didn’t make sense was that this systems pump was rated at 35 gpm at that system’s head, and all these flows only added up to 7-1/3 gpm, not 35! So next, we had to figure out why the pump wasn’t working at its rated capacity.

It turns out that when you neglect to put any pump intake filter on the sump tank because you think the fish you put in the sump are way too big to ever get into the intake pipe, then somehow some 2” net pots along with some of those flat plastic plant tags do get in there and clog the pump intake! After taking the pump apart, cleaning this mess out, putting a filter on the intake this time like I should have originally, and starting it back up, the pump pumped 35 gpm just like it was supposed to, and all the troughs got 5 gpm or more.

The most important thing about this experience was that we saw good growth and poor growth in the same system that could only be attributed to different water flow rates in the troughs. ALL the troughs in question had the same DO going in and the same DO (to within a couple of tenths ppm) going out, the same water from the same fish tank, the same vegetable varieties, the same sun exposure; it was the flow rate that caused the fluctuations.

SUPER IMPORTANT!! So, we came to the following conclusions: for leafy greens, in 4-foot wide by 10-inch deep troughs, in 70-76°F water temperatures, in an ORGANIC aquaponic system (more about this later), with trough DO’S over 4 ppm, 5 gpm of flow into a trough was EXCELLENT, 1-1/3 gpm was DECENT, 2/3 gpm was not quite enough, and 1/3 gpm was definitely too little. Even though, in other experiments, we’ve gotten very good growth rates with flow rates as little as 2 gallons per minute, we’re now recommending 5 gpm as a minimum flow rate for a 4-foot wide trough. If you use double- or triple-wide troughs (there are other problems with those, also), you’ll have to double or triple this flow rate to get the same velocity of water past your vegetable roots.

To take advantage of this new information, the first thing we did was hook all the troughs in a new system we were building together in a SERIES. The water flows from the first trough in the system into the next one, then from that one into the next one, and so on until it flows out of the last trough into the sump tank. In contrast to the 2 or 3 parallel pairs of troughs other systems use (that take the flow from their pump and split it into 3 separate streams, one for each trough pair), we essentially have a single trough in our current system designs. That allowed us to use a pump one-third the size of the one required if our pump’s output was split into three pairs of troughs. And that pump costs less than half as much to purchase, and costs 1/3 as much to run, forever, as the bigger pump that is required when you split your flow into 3 trough pairs.

IMPORTANT- SERIES TROUGHS! In the first system we built this way, we found that there was no measurable nutrient drop from the beginning to end of a single trough series that is 316 feet long. In other words there was no reason based on nutrient availability that this plumbing scheme wouldn’t work on a really loooong trough. There is a 900-foot long trough series at a student’s farm now, based on this information, and it works fine. He found that problems began when he increased this length to 1,200 lineal feet, so cut it back to 1,000 lineal feet. We still recommend you make each trough “circuit” no longer than 600 lineal feet; but this is still 2,400 square feet of aquaponics system! The 2,400 square foot UVI system we learned on had an 85 gallon per minute pump that sucked incredible amounts of electricity! Compare that to 5 gpm!

THIS IS REALLY IMPORTANT! It doesn’t matter how long the trough is!!! You can have a trough 800 feet long or 8 feet long (as in our MicroSystems), and if they both have 5 gallons per minute of water coming in, then the speed the water flows past the plant roots in both troughs is exactly the same. The water flow into the trough in gallons per minute is what establishes how fast the water flows past the plant roots!

ALSO SUPER IMPORTANT!! If you build one of the standard systems (256; 512; 1024; or 4,096), even if you rearrange the troughs to have some longer and some shorter (to fit inside a greenhouse, for instance), as long as you use the same “series” plumbing arrangement, pipe sizes, and water pump specified in the equipment list (remember to check your “head”, and get a bigger pump if you have a bigger “head), things WILL WORK FINE!

HOWEVER, If you decide to do something like: quadruple the size of the water pump, decrease the size of the PVC piping anywhere in the system to “save money”, purchase a blower five times the capacity of the one we specify for that size system, OR custom design your own system, but ignore the comprehensive information on how to design and specify components for your custom system in “How To Scale A System Larger Or Smaller” (next), you’ve embarked on an interesting and exciting experiment in aquaponics, and we can’t predict the results. Just know that several of our students have tried “experiments” such as this; have spent MUCH more money on their systems, and have not only ended up with systems that didn’t work as well, but also cost MORE to run as a result.

OFFER! If you read the post How To Scale A System Larger Or Smaller” and it’s not completely clear to you, we’ll happily design a custom system for you that meets your needs exactly, and we’ll guarantee it will work if built to specifications. Please understand we have to charge a fee for this.