Slow sand filters are totally sustainable, will work on gravity feed only, take out hydrocarbons, bacteria, viruses, and do not require addeed chemicals to operate. Check out the websites below. And check out the post just before this one if you want to see just how well a slow sand filter removes bacteria.
There are lots of websites, and YouTube videos showing detailed rain water harvesting systems, but very few include any mention of a slow sand filter. For the most part, websites for state governments fail to show any references to slow sand filters. They are so incredibly simple to set up. A container, some sand and drain pipes and mother nature does the rest. This has been happening since life began on earth in natural settings where water is filtered as it flows through sand beds, and more slowly through rock.
Set up a small slow sand filter on just one of your downspouts and let it run for 3 or 4 months and you will have very clean water. Check the websites for details. We have been doing this here for 7 years now – the websites have documentation. At the very least let the water that runs into your rain barrel come through a slow sand filter first, then the water will not go stagnant or be full of nasty bacteria. Check out the websites.
Here are three websites to check out with information about rain water harvesting with a slow sand filter:
And here are more:
Just Google slow sand water filter, or biosand water filter, or biological sand water filter. All you civil engineers out there designing rain water harvesting systems, why not take a look at small slow sand water filters? They work.