A grey water settling tank is a reservoir in which effluent is held and allowed to separate into three basic components. Solids denser than water will settle to the bottom of the tank. Fats and solids less dense than water will float to the top. Water with similar density particles suspended in it will form a middle layer – the grey water.
You need to consider the amount of effluent you will divert to the tank to work out how big it needs to be.
Ease of Operation
A major consideration will be the ease with which you will be able to scoop the less dense layer off the top of the tank and the denser layer from the bottom of the tank.
Test the Idea
Use any large container to separate your effluent. This will give you an idea of the difficulty of extracting the top and bottom layers.
Discover the types and sizes of tanks available to you by visiting suppliers. If the benefits are great enough you might consider a grey water septic tank. This will make extracting the grey water easier. The solids layers will be removed periodically.
There have been some arguments about the wisdom of using grey water so read some of the reports as part of your decision making.