This is relatively big, encouraging news for us.
pH Raising Test Results
After finding out how much of a roughly 10% Potassium Hydroxide (KOH) solution is necessary to add to our water to raise the pH to 11.0 – a 250:1 proportion was about right – we ran it through the RO system. It took a while to tune the little EZ-Flo injection tank to work with such a low flow (less than 3 gallons per minute)… but eventually we got it such that the waste water pH was around 11.7, which hopefully would mean the inlet water was somewhere around 11.0.
After running it a bit longer, we took samples and sent them to the lab. The lab’s equipment had trouble so it took a bit of time before we got the results… but the results were encouraging! Finally
Inlet water boron level: 47.1
Outlet water boron level: 6.7
We are still waiting for test results to see how much potassium we ended up adding to the outlet water, and also waiting for the membrane manufacturer to reply with whether or not a pH of 11.0 will severely deteriorate the membrane life.
An additional thing we can do is make the waste water to product water ratio higher. The RO system uses a 2:1 ratio (2 parts waste water for every 1 part product water; i.e., it takes 3 gallons of water total to make 1 gallon of RO water – 2 gallons contains waste, 1 gallon is filtered). Increasing this to 3:1 (which is easy) will increase the flow of waste which means the contaminants have less time to sneak through the membrane. When we tested our under-sink RO (which is 3:1 by default and we chose not to put in a 2:1 part) system, we got somewhere between 45% and 50% boron removal at our normal inlet water pH; when we tested the irrigation/1000-gallon-per-day system, which is 2:1, we got about 30%.
Basically, the removal rate increased by about 15% by increasing the wast ratio. If we assume that would hold true, we could get as low as 90% to 95% removal rate… which would mean that even at the end of summer, when the boron concentrations are highest in our well water, we’d end up with somewhere around 2ppm or 4ppm, and possibly as low as somewhere around 1.5ppm to 3ppm at the beginning of summer (when the boron concentration was around 30 to 32ppm).
Costs
In case anyone is interested, here’s an estimate of our initial costs and running costs:
RO system and pressure pump: ~$1300
EZ-Flo injection equipment: $80 (may switch to using a better injection system, such as an actual holding tank + injection pump, but not sure yet).
RO membrane replacements: ~$300… possibly per year and possibly per 6 months, not entirely sure yet.
Misc other filter replacements for irrigation RO: ~$50/year
Potassium Hydroxide costs: ~$2/lb for 75% purity KOH flakes. At our current mixing level (2:1 waste:product ratio, roughly a 10% injection solution), we used approximately 2.5lbs to make 3 gallons of injection solution. It would take approximately 750 gallons of flow to use all 3 gallons of solution. Since we end up getting approximately 1/3 of the actual flow as product water (the 2:1 ratio), we would end up getting approximately 250 gallons of water. Assuming we irrigate with an average of 500 gallons per week (that would be close to peak usage), we need 2000 gallons per month of irrigation water, which means we would be using approximately 20 pounds of KOH, so about $40. That would significantly fluctuate, of course, because we aren’t going to be watering much at all during the rainy season; assuming an 8 month “irrigation” season and a 4 month non-irrigation system, that means approximately $320/year for KOH.
Holding tank, pressure tank, pressure pump: ~$700 combined.
Total initial costs: ~$2000
Total annual costs: Probably around $700/year for approximately 16,000 gallons of water.
Cost per gallon (not including initial equipment costs): $.04
For comparison, it costs approximately $200 to $240 to have 3800 gallons of water brought in, which equates to approximately $.05 to $.06 per gallon. It’s about $92/hr for the truck, $30 for the water, and $25 for fuel… and takes approximately 1 to 2 hours, depending on distance/access/etc. I have no clue about the irrigation suitability of that water, nor what contaminants would be in it.
Other Options
There are commercial systems available for removal of boron (typically coupled with desalination). The problem is the membranes usually require 150 to 200+ psi… which would mean a relatively large pressure pump (electricity costs + upkeep costs + initial equipment costs). The membranes themselves can be quite expensive ($500 to $1000+). They may not require a secondary storage tank, as their output flows are much higher, and they also may not require pH modification. It would likely require a two stage membrane process, system design and whatnot, etc. I’m really not sure what the estimated cost would be to implement, but I would guess it’d be more than $2000, and have no clue how much the annual costs would be.
Rainwater/natural spring water catching is also an option, and one that we could even do as a supplement. We have a seasonal underground spring that comes out on our property that we could collect (pump into a storage tank). However, estimating that we need approximately 15,000 gallons of water for the dry season, the water tank costs alone, including freight but not including any grading/concrete pads necessary, would be approximately $6500.
On the other hand, the tanks would likely last for 25-30 years and the only upkeep would be a few sediment filters and a sump pump of some sort… but it would be a pretty steep initial cost – closer to $10,000. Ouch.
Next Steps
Since it appears that our current setup will potentially work, the next steps are:
- Figure out a reliable way to inject the KOH solution, and make it such that we would not have to mix the solution once or twice a week.
- Get the 500 gallon storage tank as a buffer between the RO (~.7gpm/~1000gpd flow).
- Replace our delapidated water “shed” with a better one, re-locate the equipment there, re-do the necessary plumbing for the new locations of the pressure pump, pressure tank, etc.