More notes on roof water quality required for a slow sand filter:
Experience here in the art of roof water harvesting and slow sand water filter construction is a continual learning experience. The newest roof water filter here (filter 5 – more info on filter 5 is upcoming as soon as I get time) has had some issues with adequate filtering. Three issues stand out:
1. First flush diverter operation
2. Gutters and downspout debris
3. The conditions surrounding the roof surface
We have discovered that a functioning first flush diverter, or some sort of pre-filter device is absolutely necessary for the proper operation of a slow sand filter used to purify roof water. The filter will plug up rapidly (within hours) if too much solid material flows into the filter. The filter 5 system here requires at the very least 50 gallons of diversion after a long dry spell. This is because of the location. This part of the roof is directly under a massive old growth fir tree. The tree is huge – more than 4 feet in diameter and about 125 feet tall some of the lower branches are as big as small trees. Huge amounts of organic material accumulate on this part of the roof yearly. This past month, the diverter failed to prevent muddy water from entering the filter. The result was catastrophic. The filter’s flow rate slowed to nearly non existent and the water was no longer clear, and was foaming up in the container – all bad signs. We cleaned the filter, drained the diverter, and re-cleaned the gutters and downspouts. It took over 50 gallons of runoff before the water was acceptable for input to the filter. Part of the problem was due to the fact that the automatic slow drain was not set so as to allow the diverter storage to gradually drain, and the diverter was totally full when the rainy season started. This was my error. Had I set the slow drain before the rain started, the worst water would have been diverted. This is an extreme example, but not for this area of the country. Trees are common around houses and rooftops, and we often have 2 month dry spells during the summer here – lots and lots of stuff builds up on the roof – dust, air pollution, organic material and pine needles.
If a slow sand filter is used to purify your roof water , it is critical that the downspouts and gutters be kept free of solid material. In most cases, normal maintainance will keep the system clean enough. Some debris is unavoidable, and the diverter should handle that ok.
The surroundings will have a huge effect on the water quality. If there are no trees near the roof surface and the roof is not near any highways, or subject to frequent poor air quality episodes, the runoff water may not contain as much solid material.
From this we have learned that slow sand filters are not perfect. The water input must be fairly clear to start with.
Another way to handle this situation is to have one or two settling barrels with an overflow on the last one before the water enters the filter. This is the way filter 1 is set up. (in the image at this preceeding link to filter 1, 2/3 of the way down the page, the settling barrels can be seen on the right, with the pitcher pump in the background on the 3rd barrel used as storage). Filter 1 has been in operation for 6 years, and there have not been any problems with too much debris entering the filter. It is necessary to clean out the settling barrels every 1 or 2 years.