Simple sand 'biofilter'

So last week, we looked at a simple rain- barrel for gardening and DIY needs. Harvested rain can be used for anything that mains or well water is usually used for, as long as it is filtered.

To use it for washing dishes (or people) it would be preferable to filter the rainwater, even more-so if you want to drink it safely; rain carries particles of dust and airborne pollution, as well as picking up bacteria and chemicals from the roof and gutters. If you filter you are much less likely to need a chemical sterilizer and the filters are pretty simple to set up.

If you want to collect a reservoir of water you’ll need a little more space because there will be at one more barrel involved. If you have a barrel raised well up off the ground collecting rainwater, you can turn this into your filter and connect it to a barrel or two just beside using a drill, a piece of tube and a couple of inexpensive adapters.
This post is aimed at keeping the system small so people who live in the suburbs or even the city can still use it, so this filter will also be the reservoir.

Often called a bio-filter, the sand filter is a little slow and it’s mainly useful for drinking water. If you have other barrels for garden or cistern, they can be left filter-less. Basically, you pop the lid off your barrel and fill it almost to the top with sand, leaving a few inches between the sand and the mouth of the downpipe.

It’s called a bio-filter because there is a thin top layer of water quickly becomes biologically active; there are microbes eating the various chemicals and other microbes. This top ‘hypogeal layer’ is not potable but as the sand filters the water down to the tap it becomes cleaner and the bottom layer of water is remarkably pure and certainly good to drink. Now and then this layer builds up and will need to be scraped off to allow the process space to work; it looks pretty grim and the first time I saw it, I found it a little worrying but it’s fine and is simply a natural and passive way to begin the cleaning process without adding anything but sand. The one thing that you might need to alter is to put a finer mesh at the top of the downpipe because the active layer needs to be as clear as possible to keep it running well.

Next week we’ll take a look at a more developed filtering system that can offer water to a much larger reservoir.

I’d just like to credit Dennis “Mad Man” Pacheco for the image.

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