Using lower salinity SW in AWC to offset evaporation

Reef_Hobbyist · May 2, 2018

I currently do a 5 gal AWC per day and I have 2 gal of evaporation per day. Is it as simple as diluting my new SW by an extra 2 gal per day to offset evaporation and stop adding RODI? Two birds with one stone?

If so the new SW is going to be diluted by 40% !!! Seems crazy to me.... is there more to it than that?

So under this new approach my AWC would be set to pump out 5 gal normal SW and pump in 7 gal diluted SW per day.

For background, 2 gal evaporation per day may seem like a lot to you but this is a very large 900 gal system in a warm climate under A/C year round. Temp and evaporation are rock steady. After 5 years of experience, my salinity never varies... ah, the benefits of a large system but it does mean a lot of evaporation and RODI making. And I should also add that since I know my evaporation so well, I just dose 2 gal RODI per day (not based on a sump float).

I am thinking I would make 50 gal SW at 1.026. This would be a 10 day supply. Then add 10 days worth of evaporation (2 gal x 10) = 20 gal additional RODI for a total of 70 gal. Then test what that salinity comes to. Or, is there a formula to calculate what lower salinity to use when mixing the new SW?

PRO: I could mix up a much larger batch of SW less often. CON: this diluted SW is only good for this one specific purpose. However, I do have 55 gal of RODI always on hand in another storage container.

Thanks in advance for assistance on this. Feel like I am missing something....

DLHDesign · May 2, 2018

Diluting the SG will also dilute the trace elements that are being added to the tank - which partially defeats the purpose of doing a water change. It's not just salinity you're concerned about - it's trace elements, alkalinity, pH, etc.

I would stick with AWC for nutrient export/import and auto-top-off of RODI for evaporation control, myself.

EDIT: Oh; I see - you're just increasing your pump-in. Missed that the first read-through...
There's no guarantee that your evaporation is occurring in a linear fashion throughout the day (though it's likely the case). At the same time, I can't see that it would be any practical difference in the course of a day. I still think two different systems is a better idea, myself; or at least have redundant systems to ensure that your AWC pump doesn't over-fill the tank.

Reef_Hobbyist · May 2, 2018

DLHDesign said:
Diluting the SG will also dilute the trace elements that are being added to the tank - which partially defeats the purpose of doing a water change. It's not just salinity you're concerned about - it's trace elements, alkalinity, pH, etc.

I would stick with AWC for nutrient export/import and auto-top-off of RODI for evaporation control, myself.

EDIT: Oh; I see - you're just increasing your pump-in. Missed that the first read-through...
There's no guarantee that your evaporation is occurring in a linear fashion throughout the day (though it's likely the case). At the same time, I can't see that it would be any practical difference in the course of a day. I still think two different systems is a better idea, myself; or at least have redundant systems to ensure that your AWC pump doesn't over-fill the tank.

I'm sure evaporation is not linear during the day or even throughout a month or year. My addition of RODI is.... yet with 900 gallons of SW, salinity really doesn't seem to fluctuate no matter when I test it or how often (I guess its the drop in the ocean principle).

And yes, good thought on the redundant systems. I have float switches, etc. already built into my system.

What I would really love to do is to use wet skimming for most if not all the outgoing dirty SW. I have tinkered with it over the years but have never been able to achieve a reliable and consistent enough amount of skimmate to build a process around it. Managed to get it to a few days in a row but never even a full week. Its just to unpredictable, especially when feeding oily things like mysis shrimp. Having said that, I will continue to use wet skimming and reduce the amount I need to pump out for SW export.

Randy Holmes-Farley · May 2, 2018

Reef_Hobbyist said:
I currently do a 5 gal AWC per day and I have 2 gal of evaporation per day. Is it as simple as diluting my new SW by an extra 2 gal per day to offset evaporation and stop adding RODI? Two birds with one stone?

If so the new SW is going to be diluted by 40% !!! Seems crazy to me.... is there more to it than that?

So under this new approach my AWC would be set to pump out 5 gal normal SW and pump in 7 gal diluted SW per day.

For background, 2 gal evaporation per day may seem like a lot to you but this is a very large 900 gal system in a warm climate under A/C year round. Temp and evaporation are rock steady. After 5 years of experience, my salinity never varies... ah, the benefits of a large system but it does mean a lot of evaporation and RODI making. And I should also add that since I know my evaporation so well, I just dose 2 gal RODI per day (not based on a sump float).

I am thinking I would make 50 gal SW at 1.026. This would be a 10 day supply. Then add 10 days worth of evaporation (2 gal x 10) = 20 gal additional RODI for a total of 70 gal. Then test what that salinity comes to. Or, is there a formula to calculate what lower salinity to use when mixing the new SW?

PRO: I could mix up a much larger batch of SW less often. CON: this diluted SW is only good for this one specific purpose. However, I do have 55 gal of RODI always on hand in another storage container.

Thanks in advance for assistance on this. Feel like I am missing something....