Sand size and its effect on flow rates & biofilm development

[Bill Duke]

#640

Thank you for the article from Cornell about the relationship between media
ES and UC and the biofilm layer (cf. Nam, T.K.; Timmons, M.B.; Montemagno, C.D.; Tsukuda, S.M. (2000). Biofilm characteristics as affected by sand size and location in fluidized bed vessels. Aquacultural Engineering 22, pp.213–224).

I have resisted attributing too much importance to this because it can be a significant impediment. However, I have come to accept that the nature of the filter media is indeed very important for the reasons you point out in the biosandfilter.org summary (cf. Filter Media http://biosandfilter.org/biosandfilter/index.php/item/314). I agree, really, with all you have written there. Unfortunately, there is a paucity of information in the peer-reviewed literature about this entire area, and there are many features of the biofilm layer that still require
definition. I do think that particle size and shape are important, not only because of the resulting influence on flow rates and mechanical trapping, but also because the size and shape of the granules influence the volume and vigor of the biofilm. The uniformity coefficient is also important for the same reasons. The Cornell study I think su2000)?pports these theories, but how
much is applicable to the intermittent slow sand filter where flow rates, and thus shear forces, are much less, and the micro-environment that supports the biofilm layer is much different?

Although I still see this as an impediment, mostly because of the inconvenience of finding and preparing the media, I now think that the better the media, the better the function. How much better, and what the ideal mix is are questions for further studies. The water lab at the Water
and Watershed Research Program at the University of Victoria is gearing-up to be able to do some of these analyses. We will be working with CAWST to obtain some filters, but that will not be until the fall term.

Please stay in touch and keep up the good work. Biosandfilter.org is a great information resource.

Bill Duke, University of Victoria