Filtering harvested rainwater is a staged job, and the stages are decided by what you intend to use the water for. Garden irrigation needs debris removed and little else. Water going into the house for laundry, flushing or bathing needs sediment and carbon treatment. Water anyone will drink needs disinfection as well, because a roof catchment collects everything that lands on it between showers.
Keep the dirt out before you filter it
The cheapest treatment is exclusion. Gutter guards and a leaf screen at the downpipe keep leaves, needles and grit out of the tank; a first flush diverter dumps the initial volume off the roof, which carries the concentrated load of dust, bird droppings and whatever settled since the last rain, before allowing the rest into storage. A calmed inlet at the tank stops incoming water stirring up the settled sludge on the floor, and a floating extraction filter draws from just below the surface where the water is cleanest. Get these right and every downstream filter lasts several times longer.
Sediment and carbon for household use
Once water leaves the tank it needs mechanical filtration in stages: a coarse cartridge to take the bulk of the fines, then a finer one to polish. Trying to do it all in one fine cartridge simply blocks that cartridge. After sediment comes activated carbon, which deals with the organic compounds, colour and musty smell that harvested water picks up from a roof and a tank, and which also protects any downstream equipment. Fitting a pressure gauge either side of the sediment stage gives you an honest signal for changes, which matters more here than on mains water because the load varies with the weather.
Disinfection if anyone will drink it
Roof water is not potable by default, and no cartridge in the chain above changes that. For drinking, cooking or bathing, the water needs a disinfection stage, normally ultraviolet immediately before the point of use, with the water made clear enough beforehand for the light to penetrate. The same logic applies as on a private well: since private wells are not federally regulated under the Safe Drinking Water Act, and rainwater systems are likewise the owner's responsibility, nobody is monitoring the result but you. Local rules on indoor use of harvested rainwater vary, so check what your jurisdiction permits before plumbing it to fixtures.
Keeping the system honest over a year
Rainwater systems drift. Gutters silt up, the first flush diverter clogs and stops diverting, the tank accumulates a sludge layer, and cartridge intervals that suited a wet spring are wrong by late summer. Set a seasonal walk-round: clear the gutters and screens, check the diverter drains down between showers, inspect the tank inlet and overflow for insect ingress, and change the sediment cartridges on the gauge rather than the calendar. Sample the water at least annually for bacteria if it is used indoors. Our whole house water filter guide covers how the same sediment and carbon stages are sized for a mains-fed house.
Questions people ask about rainwater filter
Can I drink filtered rainwater?
Only with a disinfection stage such as ultraviolet after proper filtration, and only where local rules allow indoor potable use of harvested rainwater.
What does a first flush diverter actually do?
It sends the first volume off the roof to waste, carrying away the concentrated dust, droppings and debris that accumulated since the last rain, before the tank fills.
Do I need filtration for garden irrigation only?
Usually just debris screening and a coarse filter to protect pumps and drip emitters. Carbon and disinfection are only needed for water that reaches people.
Why does stored rainwater smell musty?
Organic matter breaking down in the tank, often on a sludge layer at the bottom. Better exclusion upstream, a calmed inlet and an activated carbon stage all help.