Appropriate hydraulic characteristics of biofilter media, including flow rate and water contact time, along with pollutant removal, are important characteristics when selecting the media and associated subsurface drainage system. This presentation will describe a series of controlled laboratory column tests conducted using various media to predict changes in flow with changes in the mixture, focusing on media density associated with compaction, particle size distribution (and uniformity), and amount of organic material. The results of these tests will enable the biofilter designer to better estimate the flow rates and filter residence times for various mixtures of common biofilter media components. Future tests will examine the particulate trapping capabilities of these mixtures as a function of stormwater particle size.
As noted above, the laboratory columns used in the tests have various mixtures of sand and peat. The results of the predicted performance of these mixtures were also verified using column tests (for different compaction conditions) of surface and subsurface soil samples obtained from Tuscaloosa, AL, infiltration test areas, along with bioretention media obtained from actual Kansas City biofilters and standard samples of North Carolina biofilter media.
Three levels of compaction were used to modify the density of the media layer during the tests: hand compaction, standard proctor compaction, and modified proctor compaction. The infiltration rates through the media were measured in each column using municipal tap water. Particle trapping tests will be performed in selected intermediate compacted media columns using a mixture of fine ground silica particulates (Sil-Co-SiL250), medium sand, and coarse sand mixed with Black Warrior River water.
The preliminary laboratory compaction test results indicated that compaction and media particle uniformity have the most significant effects on the infiltration rates; however changes in bulk density resulting from amending the sand mixture with peat were observed.