More about the maintenance of a slow sand water filter used to purify roof water:
In this situation where rain water is captured and stored in a holding container to be later run through the slow sand filter and then stored in a separate “filtered water container”, a person must be willing to spend some time monitoring the filter they set up and making sure water is added regularly to the extent that the filter can adequately handle without overflowing and wasting stored water. If there is no electrical power available, this will mean physically manually adding water to run through the filter at least every other day.
If solar power is available, a very small pump will suffice to provide a trickle of water to the filter from the main filtered water storage container. The resulting trickle of output from the filter should flow back into the main filtered water storage container bypassing an overflow container (this way if it rains hard and the filter overflows with unfiltered rain water, or if the flow through the filter slows down resulting in the pump inadvertently supplying too much water flow, it won’t taint the main filtered water tank.
If you’re filtering rain water runoff from a roof or other impervious surface using a biological sand filter you absolutely must know that a biological sand filter is just that: biological. A biological sand water filter often called a “slow sand water filter”, or a “biosand water filter”, should really be called a “living water filter”. These filters are living mini-ecosystems, and they are dynamic. They must have water flowing through them always. If the flow stops, they will die. The microbes in these filters are aerobic, and they need oxygen and food. This can only be provided by flowing water, with some nutrients available in the water (most water has oxygen, and nutrients in it already – unless it has been treated with chlorine, or some other poison). The flow need only be several gallons per hour but it must be constant, and happen at least every other day on a regular basis – always. Recirculating filtered water will be enough to keep the filter alive in between rain events. The problem is that these filters almost always have a slower output rate than the possible input rate; consequently the input of water to the filter may be inadvertently topped off by too much flow from a recirculation pump, or an unexpected heavy rain shower. An overflow output on the filter will allow the pump to run without constant attention and still allow the trickle of water that makes it through the filter to flow into the separate filtered water storage tank. This requires that the overflow output be allowed to flow back into a storage container that is separate from the main filtered water storage container.
Alternatively, a low pressure system with a float valve on the input of the filter can be used. This is by far, the most effective way of providing recirculated water to the filter. Overflow does not occur and an extra container is not needed.