Household Cleaners and Chemicals
One of the things to consider with any sewage treatment system is what actually goes down the drain... Improving the water quality of black water (from the toilet) and grey water (from other water uses such as showers, sinks, washing machines etc.) would be a lot more straightforward if it was simply faeces, urine and the biological grey water bits from food preparation, washing dishes and cleaning clothes. But it's more than that. If you look at the ingredients list in any bottle of household cleaners in the house - even the most eco-friendly one - it will be a list of complicated looking chemical names. In the environment, bacteria and flora and fauna often find this list of ingredients equally complicated, and many can accumulate in the environment as toxins, leading to water pollution and environmental and health problems.
Constructed wetlands, reed beds and willow systems all help to get a host of pollutants out of the wastewater leaving the house, but actually turning off the supply of these is better still. With a little care while shopping you can take a significant step towards keeping your local groundwater, surface waters and wider environment clean and healthy.
The easiest policy is to buy your household cleaners in your local health food shop. That way you'll probably get the most eco-friendly products available without having to get a PhD in environmental chemistry first. However if you want to dig and deluge for more information then the following is a motley collection of links that may be useful in your searches: