Tuning a Skimmer / CO2 Scrubber for pH Control

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On my reefing journey, I found Kalkwasse + Skimmer + CO2 Scrubber to be very effective for bringing my pH from 7.8 - 7.9 to 8.26 - 8.36.

I guess most people would just leave it at that. However, I wanted to be more efficient with the CO2 scrubbing material so I wanted to implement a recirculating CO2 scrubber and control the airflow through the media to the skimmer. So I upgraded to an Aeraqua Duo.

Please note, I am not really looking to use my skimmer for filtration. So the question is how to tune this for efficient pH control.

My basic understanding of skimmers (which may or may not be true):
1. Foam head needs to break at or above the collar (base of the cup)
2. For wetter skimmate raise the foam head
3. Needle wheel venturi generally perform best when air is appropriately restricted to the pump as this actually increases pump speed and air draw.

Controls on the skimmer:
1. Exit gate valve - closing the gate raises the water level in the skimmer
2. Pump speed - increases water and air draw
3. Left-right external/recirculation control - pulls mix of internal and external water

I added the following controls:
1. An air solenoid valve that can switch between CO2 scrubbed air and house rich CO2 air
2. An inline shut-off valve that I can manually adjust to restrict airflow (not sure this is needed because of #2 above)

So how do we optimize this for pH control? There are so many dials to adjust.

A couple of thoughts:
1. Setting it to DC motor speed to 100% maximizes air-water mixture and should maximize pH impact
2. Maximizing recirculation would then produce higher pH water exiting the skimmer
3. Minimizing recirculation would move more tank water through the skimmer
4. Would running the DC motor at 50% with some recirculation be better and help not over skim the tank
5. The air solenoid is set to maintain the pH at 8.35 with a .05 hysteresis
6. I'm not sure I need an airflow restricted with the DC pumps
7. I don't think 2 & 3 make much difference
8. I think I may be better off shutting off the skimmer instead of switching to unscrubbed air which will drive pH down

Thoughts?
 
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Randy Holmes-Farley

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A couple of thoughts:
1. Setting it to DC motor speed to 100% maximizes air-water mixture and should maximize pH impact
2. Maximizing recirculation would then produce higher pH water exiting the skimmer
3. Minimizing recirculation would move more tank water through the skimmer
4. Would running the DC motor at 50% with some recirculation be better and help not over skim the tank
5. The air solenoid is set to maintain the pH at 8.35 with a .05 hysteresis
6. I'm not sure I need an airflow restricted with the DC pumps
7. I don't think 2 & 3 make much difference
8. I think I may be better off shutting off the skimmer instead of switching to unscrubbed air which will drive pH down

Thoughts?

#2/3. I do not think there is a rationale for wanting ANY recirculation of the water. Yes, recirculation of water raises the pH most/ removes CO2 most for a single pass through the skimmer, but a skimmer on a reef tank is not single pass device.

If the goal is to drive out CO2, it is always best to have lower pH/higher CO2 water in the skimmer to maximize the removal of CO2 (by maximizing the difference between the water and the air CO2 level). Hence, no water recirculation will export the most CO2 over time.
 
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#2/3. I do not think there is a rationale for wanting ANY recirculation of the water. Yes, recirculation of water raises the pH most/ removes CO2 most for a single pass through the skimmer, but a skimmer on a reef tank is not single pass device.

If the goal is to drive out CO2, it is always best to have lower pH/higher CO2 water in the skimmer to maximize the removal of CO2 (by maximizing the difference between the water and the air CO2 level). Hence, no water recirculation will export the most CO2 over time.

Thanks. So minimize recirculation it is.
 
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Other than my solenoid valve being down, I am able to maintain pH within 8.325 and 8.375 (8.35 setpoint w/ .05 hysteresis) by manually switching between house air and CO2 scrubbed air.

I've actually left the skimmer set for Maxpects recommendations for skimming my size tank/bioload which is 80%/0% right/left external circulation, 80% power, and gate adjusted to skimmate preference (I went wet since it on intermittently). Per their recommendations, an airflow restrictor isn't needed.
 
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@Randy Holmes-Farley

Other than having fun dialing in my system parameters. Is there any known benefit to locking in pH in a tight range as I have? Or is this just total overkill?

With this beast of a skimmer, I am able to quickly drive up or down pH. I'll try to get some numbers, but with a 3ml/min Kalk drip and the dynamic nature of the tank, I'm not sure what they will really mean.

After hooking up the skimmer, before I knew it, the pH went from 8.31 to 8.53 in 9hrs (~0.024 pH/hr). Then I removed the CO2 scrubber and was surprised again when the pH went from 8.53 to 8.30 in 4hrs (~0.058 pH/hr)

I'm sure there is a chemistry explanation why it was about twice as easy to drive down pH than drive it up, but I'll leave that to @Randy Holmes-Farley

2021-03-15 (3).png
 
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BLUE arrow indicates when I switched up skimmers. RED arrow indicates when I started to use the skimmer to help regulate pH in the tank.

2021-03-19 (2)_LI.jpg

There are a couple of things to note:
  • Very effective at maintaining my pH within a 0.05 range (8.32 low and 8.37 high)
  • For me, I believe kalkwasser is providing the majority of the daily pH boost. My Kalk reactor turns off 4 times a day to mix and settle the Kalk (1hr). Almost always during this time, the skimmer will come on with the CO2 scrubber to boost the pH.
  • During peak lighting hours, the pH will go high enough (once or twice) for the skimmer to come on without the CO2 scrubber to lower the pH
  • On average, so far, the skimmer comes on roughly 3-6 times a day for about 1-2hrs each time. I'd estimate roughly 8hrs of skimming. Surprisingly, the skimmer output has remained roughly the same ~1/4 cup of dark tea skimmate
  • I haven't been able to find an affordable 3-way solenoid and will probably go with installing two regular solenoid valves in parallel. However, I'm really not sure the downregulation is needed as I'm not sure my pH will go above 8.37 without the skimmer and CO2 scrubber assisting. I'll find out this week as my skimmer will be off for the next 5 days.
 

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Interesting data you have found here.
It is a good call to switch off your skimmer instead of changing the air inlet to a more CO2 rich environment.
I also believe that this would make the pH fluctuation change faster and you will need more switching. It might however also make it possible to keep the pH in a smaller range. Maybe something to try?

I found this website a long time ago when I was working om my ATO/AWC system.
it might be of use for you.
https://www.electricsolenoidvalves.com/3-way-valves/ (They used to have 220V valves which are now discontinued)
 
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Interesting data you have found here.
It is a good call to switch off your skimmer instead of changing the air inlet to a more CO2 rich environment.
I also believe that this would make the pH fluctuation change faster and you will need more switching. It might however also make it possible to keep the pH in a smaller range. Maybe something to try?

I found this website a long time ago when I was working om my ATO/AWC system.
it might be of use for you.
https://www.electricsolenoidvalves.com/3-way-valves/ (They used to have 220V valves which are now discontinued)

Thanks! I'm not sure I need a 3-way anymore. I'll post some more recent findings soon.
 
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So just my final observations after weeks of excellent pH control.

For my setup, a solenoid wasn't needed. It turns on when pH drops and turns off when I hit my setpoint.

I highly recommend using a recirculating CO2 scrubber. It provides 3 benefits.
1. More effective at raising pH
2. More efficient use of media
3. You don't need to add water as the incoming Ir is damp for the media

One drawback with the higher pH has been abiotic precipitation of Calcium Carbonate. My tank is only 5 months old with a few frags. So actual coral Ca/Alk demand is low. The abiotic precipitation has manifested in clumping my sand. Pumps and other items do not seem to have buildup.
 
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So just my final observations after weeks of excellent pH control.

For my setup, a solenoid wasn't needed. It turns on when pH drops and turns off when I hit my setpoint.

I highly recommend using a recirculating CO2 scrubber. It provides 3 benefits.
1. More effective at raising pH
2. More efficient use of media
3. You don't need to add water as the incoming Ir is damp for the media

One drawback with the higher pH has been abiotic precipitation of Calcium Carbonate. My tank is only 5 months old with a few frags. So actual coral Ca/Alk demand is low. The abiotic precipitation has manifested in clumping my sand. Pumps and other items do not seem to have buildup.
I also put in a CO2 scrubber and I noticed the build up of calcium carbonate film on my UV quartz sleeve. Unfortunately a pain in the neck to clean.

Have a high demand Alk/Calc aquarium and was dosing 110ml of 2 part daily. Just switched to a calcium reactor and noticed after 3 weeks little build up on the UV quartz sleeve.

The high pH from CO2 Scrubber and high Alk/Calc from calcium reactor has really helped increase the grow of my corals!
 
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As I continue to monitor the tank pH and performance, what I have found is that the Kalkwasser does a very good job of maintaining the pH. Meaning, once the pH is at the target level Kalkwasser can maintain it for hours. When my tank pauses Kalkwasser for mixing (4x day), that's when the Skimmer/CO2 will deffinately kick on. Additionally, I don't see a pH drop off at night. In fact the refugium has been maintaining a higher pH then that of the day-time display tank. Again, this means that the CO2 scrubber kicks on less at night. I should also note, that I have no algae in the DT...but a ton of GHA in the sump along with some Chaeto and Calerpa.
 
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Just a quick update:

1. I'm maintaining target pH now at 8.3
2. I'm now seeing day/night pH swings as high as 8.27 to 8.34. Although it normally swings between 8.28 and 8.32. I believe this is due to a maturing tank (6 months old) with more CO2 consumers during the daylight hours.
3. Weekly Kalkwasser top-off
4. 6 week CO2 scrubbing media replacement
5. I've had a couple of issues with water blocking the recirculation tube.
6. I've had a couple issues with the Kalkwasser RODI water running dry...
 

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