I have taken video instructions for the new drain valve but until this is edited and published, here is the low down:
The Drain Control Valve should be installed on the main (centermost) drain, as this is the one that will handle the majority of the flow.
The purpose of the valve is to restrict flow just enough to allow the drain to convert to a full siphon and purge out the air. This concept is referred to as a “tuned siphon”. If you leave the valve all the way open, you will 1) not get as much flow through the drain as you would if the valve was closed, 2) you will get excess flow through the secondary drain, and 3) it will be noisy (sucking air) and you will get a lot of bubbles. Start with the water running and valve open (screw out all the way) and then turn it until the unit goes silent. That’s it!
The purpose of the drawstring bag is not to filter, it is to prevent bubbles from going everywhere, especially for the first few weeks. The top of the bag (and coupler) can be underwater. This will force the bubbles to rise to the surface while not trapping life such as copepods.
You may be able to remove the filter bag after the screen matures. I do not run filter bags on any of my mature scrubbers – with a mature screen and a tuned drain, you will have little to zero bubble issues.
The flapper is opened and closed by turning the thumbscrew. This replaces the bulky ball valve or a gate valve. It’s very simple. The valve should be at the low end of the run of drain plumbing, it is installed in one of the 1.5″ pipes such that you can cut the top part of the pipe to length so that the bottom of the valve-end of the pipe is about 0.5 to 1″ below the water surface (sump operating water level under the scrubber) but you can place it lower under the water: I would not recommend more than 2″ below, otherwise air may not easily/quickly purge from the drain.
Once that is cut to the correct length, slide the 2″ coupler with thumbscrews over the submerged end of the pipe and tighten it down. The top of the coupler can be (and actually is recommended to be) fully submerged.
Next, slide the drawstring filter bag over the coupler (over the thumbscrews) and tighten down so that it doesn’t slip off – it doesn’t have to be cinched way down tight, just enough to keep it in place.
Put all the pieces together (if you didn’t install them in place) and you are ready to fire up the scrubber and tune the drain!