While it might be obvious to you if you have a plumbing background, it's possible that there are others in the same situation as me, not really understanding the difference. I finally asked the question, and ended up under the house being shown exactly why in some situations the 100mm unit is necessary.
In this picture you can see 50mm drainage pipe from the bath, which joins into a 100mm pipe. You can see this because our house is built off the ground. So if your house is elevated or not built on a slab, it's possible that plumber can easily access the 50mm pipes, and the Greywater Gator Pro suits this situation.
However if your house is on a slab these pipes are generally buried, so you will need to install a Greywater Gator Maxi100. In Western Australian most of the homes are built on a concrete slabs so it's only possible to access the 100mm pipes.
You may also need to install the bigger unit if all your 50mm pipes join up to the 100mm pipe.
The inlet, overflow and outlet points in our Greywater units are easily accessible for the plumber. Its also possible to change the inlet and overflow around to suit your situation.
Another thing I discovered about the Maxi 100 is that it is necessary for the 100mm unit to be as deep as it is because of the standard depth of 100mm Greywater pipes coming from your house, usually 300mm minimum depth. So to physically fit all the components in and to have them at the correct level we use the 800mm deep pit.
I also found out that the diverter valve in the bigger unit (which is visible in the picture below & looks like a guillotine) can be installed either in the unit as its supplied or externally. This flexibility in the installation makes it easier for the plumber. The diverter valve in the Greywater Gator Pro is installed externally and can be on either side, depending on the positioning of your existing plumbing. These valves are a necessary addition as regulations state that Greywater Diversion units must be able to be diverted to waste water in the event of high rainfall.
However if you forget to do this, just switch the power off and the water will just go to waste via the overflow anyway 🙂
The surface area of the filters in the larger unit is also greater, to cope with larger flows, say from a 5 bedroom house or commercial situation. This means that a greater volume of water can be filtered before the filters need to be cleaned. Incidentally, cleaning the filters really doesn't need to be done very often - at the very most every six months, but more likely every 12 months. And they just need to be hosed off with a standard hose and water nozzle..
The new lid is really heavy duty and can withstand weight of up to 450kg. I guess that means you can walk on it, push the mower over it, the kids can ride their bikes over it, but you really shouldn't drive the car over it.
*This article was originally posted on 28/6/2012, but I have moved it here for continuity.