I have been attempting to work through this question myself.
I believe that I have an answer, but I still have not been able
to test it due to barriers (deth in the family, and all the chaos
that it brings).
Here is what I have come up with, although it might be tough
to visualize without a drawing (I'll try to get to that soon).
Start with a plexy/acrylic box. Place a divider in the box that
creates a top and bottom half (looking down into the box).
The divider should come up about 6-8" in a 12" deep box.
Now, create an outlet from the bottom half that feeds the main
circulation pump of your geothermal cooling unit. The other end
of the geothermal cooling pipe enters the top half of the box.
When the pump is on, water will be sucked from the bottom half,
pumped into the top half, and cascade over the divider.
Now place the box next to you sump so that the top of the
divider in the box is at the same height as your sump water
level. Run a line between the bottom half of the box and the
sump. Run a line with a small pump (pump2) between the top
half of the box and the sump.
Here's how it works. When pump2 is off, the mail geothermal
pump will suck water from the bottom half of the box and dump
it into the top half. The resistance of pump2 being off will cause
the water in the top half to spill over the divider and replenish
the bottom half where the pump is sucking. There will be a small
amount of water exhange between the bottom half of the box
and the sump through the line that runs between them , but it
should be only slight. With pump2 off, there is no colling of the
sump or the tank.
OK, lets turn pump2 on. Pump 2 is now sucking water from the
top half of the box. As the water level is reduced, water in the
top half of the box stops flowing over the divider. The main
gothermal pump is reducing the water level in the bottom half,
so water now starts flowing from the sump into the bottom
half of the box.
Sorry if it's not clear. However, this will allow you to continue to
circulate water through the goethermal pipes without cooling
the sump/tank if they are at temperature. Choosing pump2 carefully allows you to decrease the tank temperature at a rate
that will not shock the system. If you are running a controller, it
can cycle the pump with very tight temperature tolerances.
To the nitrification, if you want a slow flow when not cooling, you
can run a small pump in parallel with the main geothermal pump
to keep a very slow flow moving though the pipe and the box.
Hope this halps. If not, I hope to see you at one of the BARE
events sometime soon. I still want to purchase some sand/rock
if you have any left, but it will be a week or do before I'm free
from the family chaos going on.
The Other Jim