Just curious what the stormwater design/maintenance community has to say about the Austin sand filter design spec for media- they call for C33 sand, but limit the grain size range to .02"-.04". I've been in the construction materials business for over 20 years, and in stormwater design and maintenance for an additional 8-10 years or so. I have yet to find an aggregate supplier in my area that will produce a sand filter media that meets the .02"-.04" limitation. From what I can tell, EPA used the Austin sand filter design and promoted it, so many jurisdictions have this .02"-.04" spec for sand filter media. Basically, the media must pass a #18 sieve and be retained on a #35 to get a product that is close to .02"-.04".
Just the logistics of producing a product that meets this range seems daunting- we're looking for natural sand, not crushed material. How much sand and gravel do you have to mine to generate any quantity of this narrow band? Is a typical screen setup able to be fine tuned to produce this range? Screens that fine must use a wet wash process, seems like a dry screen would clog constantly. In fact, at the level of production needed in a sand/gravel mine, I would be surprised if any sieves smaller than #10 are viable.
My jurisdiction(I'm a public employee now, it's a bit more stable as I count down toward retiring) is considering dropping the .02"-.04" spec in favor of the generic C33 spec. My opinion, after inspecting and maintaining several hundred above and below ground sand filters over the years, is that a wider range of particle sizes, a "well graded" blend that meets C33, will be less likely to clog. It may not have the pollutant removal efficiency of the narrow spec, but, taking the long view- I'd rather lose a bit of filtering efficiency over the long term than lose it all due to premature clogging. And we all know how thrilled a property owner is to pay for the reconstruction of a failed underground sand filter(especially when the designers only provide access via manhole- ever move a few hundred tons of dirty sand via single manhole on a filter the size of a basketball court? It gets expensive from a labor perspective.)
I hope I'm not operating in a bubble, hence this post. Bosses ain't gonna pay to send any of us to StormCon to discuss this over drinks with our fellow regulators- so here I am.
Just the logistics of producing a product that meets this range seems daunting- we're looking for natural sand, not crushed material. How much sand and gravel do you have to mine to generate any quantity of this narrow band? Is a typical screen setup able to be fine tuned to produce this range? Screens that fine must use a wet wash process, seems like a dry screen would clog constantly. In fact, at the level of production needed in a sand/gravel mine, I would be surprised if any sieves smaller than #10 are viable.
My jurisdiction(I'm a public employee now, it's a bit more stable as I count down toward retiring) is considering dropping the .02"-.04" spec in favor of the generic C33 spec. My opinion, after inspecting and maintaining several hundred above and below ground sand filters over the years, is that a wider range of particle sizes, a "well graded" blend that meets C33, will be less likely to clog. It may not have the pollutant removal efficiency of the narrow spec, but, taking the long view- I'd rather lose a bit of filtering efficiency over the long term than lose it all due to premature clogging. And we all know how thrilled a property owner is to pay for the reconstruction of a failed underground sand filter(especially when the designers only provide access via manhole- ever move a few hundred tons of dirty sand via single manhole on a filter the size of a basketball court? It gets expensive from a labor perspective.)
I hope I'm not operating in a bubble, hence this post. Bosses ain't gonna pay to send any of us to StormCon to discuss this over drinks with our fellow regulators- so here I am.
