A foul-flush device or detachable down-pipe can be fitted that allows the first 20 litres of runoff from a storm to be diverted from the storage tanks. This is because runoff is contaminated with dust, leaves, insects and bird droppings. To prevent the use of dirty water, the runoff is then led through a small filter of gravel, sand and charcoal before entering the storage tank, or a filter is placed between the catchment structure and the storage tank. Where there is no foul-flush device, the user or caretaker has to divert away the first 20 litres at the start of every rainstorm.
====The EMAS filtration system====
[[Image:Filter.jpg|thumb|right|150px|Filtering the water coming from the gutter]]
The EMAS system for rainfall collection uses various EMAS technologies as well as simple tools to convert rainwater into usable drinking water. If roof rainwater is being used, it is collected through a regular gutter. To filter the water, at the bottom of the gutter, a pitcher or ferrocement tank is placed, with an outlet pipe. A synthetic cloth bag is attached to the rim of the pitcher using an iron ring or wire , which fits around the edge. The bag should be cleaned every 3 months.
As water begins to collect, to avoid too much garbage collecting here, first some amount of water is deflected, along with most of the garbage. Hereafter, water can be directly sent to an [[EMAS Cistern ]]. It is advisable for multiple cisterns to be available for storage, depending on the size of the roof. Connect one cistern at a time to the outlet pipe. From here water can be pumped and distributed using a regular EMAS pump. The pump can also be connected to faucets and tanks around the house.
====Maintenance====