It is always amazing how much is not known about filter-grade biochar, as well as the amount of misinformation about the product. So here is a quick rundown…Biochar is typically sourced from high-carbon fly ash from wood-fired power plants located at lumber mills and other wood product operations. For filter grade quality for stormwater processes, one needs to be selective on what type of wood is being burnt at that facility. Different types of wood will produce different performances.
Plus it is generally, but not always, best to seek out older power plant processes as opposed to ones recently installed, for their fly ash consists of larger particulates. The newer power plants typically produce a powder style of fly ash of which will choke off filter areas. The age and/or the type of the power plant will produce different performances.
To maintain adequate flow rates, filter-grade biochar must first be thoroughly rinsed before it can be used in stormwater applications. Some biochar companies sell biochar from older facilities that utilize a wet fly-ash separation method. The biochar in these facilities enters a water bath and then falls over a weir.
Sometimes this type of biochar is sold as a “washed” product, which is completely false and far from the truth. This “washed” biochar will be loaded by fines and clays, either from the mill’s staging / loading areas or from the power plant burning processes. The initial water bath does not separate these filter contaminants out of the product.
While thorough washing and rinsing is time-consuming and expensive; it is absolutely critical for achieving a filter-grade quality of biochar.The next step in processing biochar is to grade the biochar into a variety of sizes by sequential screening and sieving. Here is some of the alchemy needed to measure how much media permeability is needed versus how much exposed contact areas are available within the media. When selecting the best particle size mixture, there is often a tradeoff between permeability and contact time:
- The more exposed contact areas provide more intimacy with the stormwater contaminants; hence more contaminants are removed.
- The smaller the size, the more exposed contact areas within the biochar, the increased resistance to flow permeability.
- Whereas, the larger biochar particles will allow for greater flow permeability. The larger the size, the less resistance to flow permeability, the less intimacy (therefore less removal).
As you can see, there needs to be a balance with both intimacy and permeability depending on the structure of the filter design.
- Typically with our biochar filters, we can utilize a granular-sized media if there is a small amount of head available to carry the stormwater through the media, as typical with our upflow and downflow biofilters.
- However with the lateral flow patterns in our Blue Filter Gabions, the biochar needs to be sized larger in a flake-style which will allow for greater flow permeability.
Filter grade biochar requires certain organic additives in order to achieve a long activation life prior to the media beds being exhausted. A small portion of certain organic acids will activate within the biochar’s alkaline nature, thus increasing bed life and robust removal of metals. In addition, by having aerobic microbes naturally feed on the biochar during the non-rainy seasons; the microbes will expose more exchange sites for metals removal.
We here at Gullywasher strive to offer the best of what a filter-grade biochar can offer within our organic stormwater filter processes require to maximize metals removal, flow permeability, and bed life sustainability.