Sand filters are often used to decontaminate the water from rain water collection. Basically its a layer of gravel and a lot of sand. Bacteria and micro-organisms live in the sand that eat all the junk in the rain water. They do need to be cleaned every now and then though, but if I remember right it's basically purge, stir and rinse.

like a swimming pool filter. I have a sand fiter on my pool, and it's recommended to purge and rinse once a week. I'm sure you could do it less often with a water barrell set up since you wont be swimming in it or drinking it.

I made one last spring. 55-gal plastic drum - got it for free from an auto shop - used to contain windshield washer fluid, and as a bonus, it had about 3 liters in the bottom still when I got it home!
I cut a hole in the bottom for a 3/4" threaded copper pipe, put a 90 degree piece, an extension, and a spigot on the outside. I taped the threads with teflon tape, and used rubber gaskets and large metal washers on both sides to make sure the seal was good. (I read online that simply threading the pipe into the barrel would sometimes leak)
I cut half of the lid of the barrel off, then reached inside and put a nut on the backside of the rubber gasket/washer.
The massive hole in the top is covered by window screen and some rubbery gasket stuff I had lying around, secured with screws. My gutter drains in almost parallel to the ground across the screen. Sediment mostly stays atop, and water goes down. After a rainstorm or two I usually brush the top off with my hand, or it blows off when it dries.
I have a pvc drain tube inserted close to the top of the drum, which runs down to the bottom of my gutter after the barrel is full.
I painted the barrel with Krylon Fusion plastic paint to match the red brick color of my house, and have it set up on bricks stacked knee high.

The gutter services 1/2 of my roof, so it fills with the slightest rainfall. If I had the space, I would put more up, but my wife/neighbors would complain. Next step is to put a barrel on the garage - it's more out of the way.

One more thing - although several manufacturers of rain barrels leave water in the bottom of their barrels to weigh them down and keep them from moving, I'm paranoid of mosquitoes and we have far too many already. To supplement my screen - covered openings and draining within 5 days of filling, I filled the bottom 5cm with concrete - right up to the top of the pipe on the inside of the barrel. It won't evaporate, leaving a flight-risk barrel, and it'd be rough for larvae to grow in it.

Art Ludwig of Oasis Designs is the authority on gray water; even he has nearly given up on the notion. The thing about gray water is you have to use it right away. If you don't use a holding tank, suspended particles (skin, hair, macaroni) stay afloat and tend to clog stuff downstream. If you do use a holding tank, gray water turns to black water in about two hours.

I got no beef with septic systems -- they are as organic as organic gets.:D almost all our waste water goes straight into the septic, down thru 130 ft of limestone sand, and presumably back into our well.

As for rain catchment: it works brilliantly. Hard to describe how much water comes off a typical roof in even a short downpour. Many posters here got the challenges pegged: initial runoff nastyness, fine sediment & sludging, algae and microorganism growth, and diversion when full.

Dark barrels warm the water; that's great in winter, but it actually speeds algae growth in summer. We dose ours with unflavored Chlorox once in awhile. Doesn't take much -- half a cap will treat 55 gallons, given time. It may or may not kill certain other beasties, tho -- especially protozoa. So we pump our drinking rainwater thru a 0.5 micron carbon block filter.

We use 55 gallon drums for our drinking water so we can wheel one indoors for winter. For gardening, we have a buried 500 gallon steel tank (former diesel storage) and a 400 gallon poly farm tank. If we run low on irrigation water, we can toss the latter on a flatbed trailer and fill it in town for $1.

Metal roofs are fabulous for rain catchment. We have a fine screen on the barrels which we can knock the bugs out of. There's a bit of sludging, but people have lived off cisterns for millennia with limited trouble. It's open watercourses that breed dysentery and cholera. When a barrel is nearly empty, we take it to the patio, slosh it good, and pour out most of the sediment.

One interesting side effect of drinking rain water is its lack of salts or trace ions. Our bodies are used to gleaning certain micronutrients from drinking water. Shouldn't be an issue if a person is eating a balanced diet. Just thought I'd mention it. Oh -- and watch out where you get used barrels. Some of them may have held concentrated pesticides, and those residues may be impossible to rinse out. Oh -- and rain catchment is illegal in some places, like Colorado.:D Pipples take their water rights seriously in the West, and the rain that hits your roof is owned by somebody. Fort Collins was threatening to cite people with rain barrels, although they have since backed down in the face of jeering.

Wow that's weird. I can't believe they do that in certain places. That's like saying you have to put a blanket over your grass when it rains because someone owns the rain and YOU can't use it.

So is there an issue with gray or black water if you don't use it for drinking? What if you just use it to flush your toilets and water your grass and gardens?

My sump pump runs all the time. I want to bury a 50 gal plastic barrel and have the sump pump into it then an overflow to my drainage system. I want to use the water from the barrel to water the yard. I am trying to find a inexpensive dc or solar powered pump to use with a garden hose. If the pump is dc I can build my own panel for it. I am having trouble finding the pump I am looking for. I saw one in a google search but now I can't find it. A very simple pump is all I am looking for. Any help would be appreciated.

I'd probably check out pumps used for RVs, they'll run on 12 volts. SHURflo comes to mind.

I found a place here that sells the blue plastic barrels for $11 each. I posted at work that I needed deep cycle batteries and got 2 for $40. I'm going to build my own solar panel to keep the battery charged. I will use a bilge pump in the barrel because I am going to bury it. my sump pump runs about every 3 minutes and that will be my water source. I'm going to set up a water protected switch since this will only run when I am there to watch it. the pump is $19 which fits in my budget but it pumps500 gal an hour so it will probably empty pretty fast.

b4u2 - What kind of business (or is it a chain?) sold you the barrels? I need to expand my rain barrel capacity and that is a great price. I'm not exactly in your neighborhood, but maybe a similar store could sell me a barrel or two for a similar price.

Thanks,
Tim