John -
As noted above, I have two, one in the darkroom and one for a sink in my workshop. In each case I have a reservoir under the sink to catch and hold the graywater before pumping it up to the household sanitary drain - which runs just below the basement ceiling. In both cases, the units are the fully submirsable design with a float switch that pivots at the end of a short length of cable.
The question of reservoir volume is important. You are concerned about both depth and the cross-section dimensions. In the darkroom in our former home, I used a 5 gallon plastic pail (from driveway sealer). The depth of that pail was just barely sufficient - the float switch on the pump caused it to start when the liquid level was only about a 1/2 inch from the top of the pail. When we moved to the new place, I shopped around for a container that was deeper than the driveway sealer pail. The Rubbermaid bin in the darkroom is about 2" deeper, so there is about 2" of "freeboard" when the pump starts. Incidentally, an advantage of the Rubbermaid bin option is that they come with lids. On the other hand, the Rubbermaid bin is rectangular, and for mechanical reasons, round containers are stronger when it comes to holding fluids. As a result, there is a pronounced bulge when the liquid level nears the point when the float switch kicks in.
For the shop, I chose a round laundry tub. It has 3-4" of freeboard, but it doesn't have a cover.
In my current setup, I have never had a problem with the float switch not starting the pump when the reservoir was full. That was not the case in the former arrangement using a plastic driveway sealer pail. What happened was that the float switch would touch the side of the pail, previent it from freely following the liquid level. I had a couple of instances when the pail overflowed - what a mess! When I chose the new reservoirs, I also made sure that they were large enough to not interfere with the float.
I will say, however, that in both instances I can visually check the liquid level, and subconsciously I do that frequently to make sure that the reservoir doesn't overflow.
I would be very hesitant about penetrating the concrete floor of the basement. Around here, there is a concern about the subsurface water level and wet basements, and poking a hole in the floor will aggravate that problem. In fact, when I built my darkroom, I used construction adhesive to attach the sole plates for the walls to the floor rather than cement nails to avoid any form of penetration of the floor.
There is one other option that you may have. If your home has a basement sump, and if it's convenient to the darkroom space, you could drain into the sump, and then rely on your sump pump to remove the graywater along with other seepage into the sump. Before taking that option, it would probably be wise to make sure that your normal seasonal flow into the sump can be readily handled by the sump pump - you don't want to be putting more volume into the sump than the pump can handle.
And to Georges' point, I think the key to both the temperature problem and the problem with darkroom chemicals attacking the pump is to make sure that there is enough volumn to dilute undesirable elements in the darkroom effluent.
One other point: for safety reasons I have both pumps on electrical circuits with GFI protection. Last year, I noticed that the darkroom pump (which is about 10 years old at this point) was tripping it's GFI. I suspect that a gasket is leaking and that I will need to replace the pump sometime soon. The pump on the shop sink is much newer, purchased whenI set up the shop when we moved three years ago, and I've already had to replace that pump once. As noted above, the newer pump has a much cheaper feel than the old darkroom pump, so either they have been cost-reduced or else the first unit I purchsed for the shop had an "infant mortality" failure. Having them on a GFI circuit provides a sense of security, but it also means that the ability to visually check the liquid level occasionally is important since you never know if the circuit has tripped.
Incidentally, sump pumps come in two designs - there are also models in which the motor is on a pedestile, and the float switch rides vertically on a rod. If you have enough clearance above the reservoir, these may be preferable to the fully submirsable design.
Have fun!
