Backyard Aquaponics
A place to discuss aquaponics

Hello fishy people,
We are starting a commercial system and are concerned about the 77 ppm Sodium content of our source water (50 ppm is supposed to be the upper limit for hydroponics). To complicate things the 360 ppm hardness would quickly foul an RO membrane without a water softener, sooo... I've read that KCl in a softener will remove most Na along with Ca and Mg cations, which would be much simpler and cheaper than adding the second filter. I'm hoping to get feedback from someone with a little more chemistry expertise/filtration experience than me, or maybe just some ideas.

In case you all are curious, we are setting up a Nelson and Pade 4-500 system; I estimate around 8,000 gallons capacity. And if you're still curious, I will share more details later.

Thanks, and Grow Global!

Ideally water should pass through a potassium chloride water softener then a carbon filter then your RO system. The potassium water filter will remove sodium and calcium. The carbon filter will remove chlorine. The ro will remove total dissolved solids. A word of caution, potassium chloride water softeners will collect sodium to a point, once that point is reached they tend to start releasing sodium and need to be regenerated or replaced. I have 6 years experience in operating and maintaining industrial size RO systems at a pharmaceutical laboratory.

From a commercial standpoint, make sure to take in consideration the extra costs of these filters, the replacement filter costs and the cost of the extra water waste from each system. RO and water softeners uses LOTS of water to perform their duties. Also by using potassium chloride, you will increase the amount of potassium in the water - which is useful for growing plants(especially fruiting ones) but you will need to measure the amount of potassium to ensure your additions of calcium/magnesium will match the potassium additions from the softener.

Hope that helps!

Wow, thanks to both of you, that is really helpful. Yes, Max 47, I found an academic article on the uptake and release of sodium from the cation resin which showed basically all Na bound on the exchange sites when the resins are first recharged, diminishing to zero Na removal when K is half spent, and then the resins begin rereleasing that Na into solution, but I am glad to have your confirmation that it is possible.

And as you've suggested, Eddie, I have been trying to critically evaluate the balance between initial cost, maintenance costs, ease of maintenance, and environmental impact. That sure leaves a lot of room for interpretation! I would love to skip the extra RO step, but even if I could cut sodium in our source water (from a well) in half through doubling softener recharge frequencies, I am concerned about eventual Na/K buildup.
It sounds as though you're saying I can balance extra K with added Ca and Mg, which I did not know. Do you think the carbon filter is necessary for residual chlorine left from KCl?

As for waste water, rather than the septic tank, I was considering using it for some type of evaporative cooling, and/or creating a mini sodic/saline desert around the facility's perimeter to eliminate weeds and pest habitat. I figured that would create a compelling and tangible example for teaching about ecological impact of salts in irrigation waters!

In the end, I think we will need the RO to reliably manage ion inputs, but that we might reduce maintenance and costs, and hold on to some of the valuable nutes, by tweaking softener recharge frequencies and RO bypass flow?!?! Am I making this more complicated than it needs to be? Ha, ha, ha...

The key with potassium/calcium/mag supplementation is you want to try and maintain the same amount of each nutrient in solution (usually between 40-70 ppm). As long as they are still around equal parts then you won't have nutrient lockout.

Have you tested the pH of your water source? (remember to let it sit with an air stone over night before measuring since the water is coming from a well and might have CO2 dissolved in which will mess with the numbers initially). I think your pH will also dictate what you need to do.

Remember that the water output from softener will be full of Na, Ca and Mg ions so evaporative cooling might be tricky due to scaling buildup - just make sure to use larger holes in your water manifold. I think you are on the right track with using the RO + softener to adjust your nutrient loads - it just might take some tuning.

I would suggest putting the softener in place and then sending the water to get tested so you know what you are dealing with - look for irrigation testing service (usually around $60 bucks)

I use a NaCl water softener myself and it adds a decent amount of sodium but the chloride levels are still pretty low and will be taken up by the fish and plants over time as long as I am not adding too much.

Please note that there is no chlorine in the water from adding KCl - only chlorides so you don't need the carbon filter unless you are adding chlorine to your well water for disinfection or you are trying to remove other chemicals from the water.

Well, after a few weeks agonizing over our filtration questions, I found a thread where people discussed salting their systems for fish health at concentrations of 1-2 ppt without any adverse effects on plant health, and going as high as 3 ppt before seeing problems with their strawberries and cucumbers. Since this probably translates to somewhere in the ballpark of 500-1,000 ppm Na, I am now assuming I won't see any potential problems for a long time, especially if I am utilizing rainwater as well. I suspect that just as a mineral soil behaves much differently than a humus rich soil in regards to nutrient deficiencies and imbalances, so does a hydroponic as opposed to an aquaponic system. After further research, I am now considering slowly increasing salt concentrations with careful monitoring, because of the immense benefits to fish health and growth, and the potential for increased stocking rates because of reduced oxygen demands. As Eddie points out, and as I know from soil science, cation balance, especially Ca/Mg/Na ratios, is critical for mitigating impacts of high sodium.

If you're trying to grow strawberries in your AP system a lot of people have had problems with them at lower salt concentrations (1 ppt NaCl), they are usually the first plant to go belly up. That's about the level you'd salt for fish health. I don't know that anyone has looked at salt tolerance in different strawberry varieties and there may be other things going on that help it work in some systems (ion balance as mentioned for instance).