When I became aware, a couple of years ago, that roof runoff may contain some heavy metals that could potentially find their way into your food garden, I took a bucket of biochar and dumped it into the bottom of my rain barrel. Problem solved. What I didn't realize is that I was solving another (bigger) problem of roof runoff - e. coli bacteria.
To be honest, my quick fix may not have solved the problems; (a) because I didn't really have a problem - my rain barrel feeds my ornamental rain garden, and (b) the biochar filter inside a rain barrel should probably be a little more meticulously constructed and maintained.
Now that I am about to set up a rain barrel that will be used to spray my mushroom logs and save money on my water bill, I definitely need to think about the contaminants in the water. The draft Certified Naturally Grown standards for mushroom growers does not address roof runoff specifically, but does cover surface water. Could a biochar filter be used to meet the CNG standard? Let's explore.
The water standards for mushrooms are the same as for irrigation water. A study done at Rutgers University found that the major concern for roof runoff is e. coli contamination, rather than heavy metals (though lead and zinc warrant measurement). This bacterial contamination could stem from dead animals and feces, particularly in gutters (but also in rain barrels and on the roof, itself). Occasionally, the study found e. coli in excess of the limit coming from a roof in their study.
It turns out that biochar captures and sequesters e. coli bacteria, as +Kathleen Draper points out, referring to research done at McGill University showing that biochar filters out e. coli. The excellent thesis by Shoieb Akaram Arief Ismail shows how large pore biochar with particle sizes up to 2 cm (such as the wood based product that I make with my pyramid kiln) is a significantly better e. coli filter (a couple orders of magnitude) than sandy soil alone.
Granted, the filtering action in this study took place over 24 hours, but some reduction of e. coli could be expected by less than a minute of residence time in a rain barrel partially filled with biochar. The practical limit is how badly the water flow would be reduced by the filter. This can be discovered empirically. The biochar should be rinsed thoroughly prior to adding to the rain barrel in order to prevent washout of fine particles which could clog the outlet. In addition, gutters should be cleaned regularly and the biochar filter changed annually when the rain barrel is scrubbed clean for the season.
The rain barrel volume you are sacrificing to the filter may require a larger barrel. I got a good deal on a 65 gallon barrel, which I think will do just fine. Even a 55 gallon barrel should be big enough. For my homeys, Baltimore is having a truckload sale of them and Calvert County has a half-price deal for residents that comes with a free workshop, both in April.
