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Compaction-Induced Biochar Loss from Stormwater Biofilters


Roadside biofilters can transform the nation’s millions of miles long road infrastructure into an integrated stormwater treatment system. However, unlike traditional biofilters, roadside biofilter media may need to be compacted to maintain required soil stability. Thus, it is critical to examine how amendments used in biofilters behave under compaction. The objective of this work is to examine the effect of the size of biochar, a carbonaceous soil amendment, on the performance of biochar-augmented biofilters subjected to compaction. To examine the effect of biochar particle size on hydraulic properties and contaminant removal capacity of biofilters, a mixture of sand (0.6- 0.8 mm) and biochar (5% by weight) with three size ranges (150 μm < d < 833 μm, 833 μm < d < 1180 μm and 1180 μm < d < 2000 μm) was packed in plastic columns (5.1 cm I.D. and 30.5 cm media length) using compaction energy similar to Standard Proctor method, and subjected to intermittent infiltration of stormwater. Results showed that the particle size of biochar did not significantly affect the quantity of particles released during intermittent infiltration of stormwater, and the quantity of biochar released was insignificant compared to the biochar remained in the biofilter. To examine the dominant mechanism of particle release after compaction, biochar particles of different sizes were coated with a dye (acridine orange), subjected to compaction, and the dye concentration in the released biochar particles was compared against the concentration in packed biochar from where the fine particles were released from. The results showed that disintegration, not abrasion, is the dominant mechanism for biochar release under compaction, and disintegration is particularly prominent when biochar size was small. Biofilters with larger biochar size exhibited the highest initial hydraulic conductivity and the least potential to clogging. However, biofilters with high particle sizes had the lowest E. coli removal capacity. Overall, these results indicate that the negative impact on compaction can be mitigated by using the size of biochar similar to or greater than the size of other biofilter’s media such as sand.

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