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Neoerectus I think by-laws are different from one place to another when it comes to household sewage waste. Where my parent's farm is located, the local building code states that ALL household water-waste must end up in the septic tank, regardless of their source (sink, shower, laundry and of course toilet).
Before you divert your bath and wash water to a filtering system, make sure you aren't violating 'code', otherwise this could present itself as a real issue for you down the road.
The thing is, any water that comes in contact with the human body such as from showers and sinks, is considered sewage water, not waste water in some districts, depending on the soil composition of the land and the level of the water table beneath the surface.
The only real waste water than you need to divert away from the septic system if you can, is laundry water and kitchen sink/dishwasher waste water. Those two sources contain soap and often bleach will will kill the action of a septic system.
