Wall Pressures Caused by Wet Woodchips in a Model Biofilter Bin

C. S. Ima, D. D. Mann


Bulk materials, such as the woodchips used as media in a biofilter, exert pressure on the walls of
their containing structure. The magnitude of lateral pressure caused by wet woodchips on the
walls of a biofilter structure are unknown. Tests were conducted to measure the lateral pressure
caused by wet woodchips in a model bin to determine whether existing pressure prediction
equations are applicable to biofilters. Three model biofilter bins (0.5 m by 0.5 m, and 1.2 m tall)
were employed. Lateral pressures were measured with pressure sensors mounted on the bin wall
at 0.2, 0.5, 0.7, and 0.9 m above the bin floor. Woodchips of four different moisture contents
were tested (37, 45, 58, and 60% wet basis). Three replications of the test were performed for
each moisture level. The results showed that wall pressures increased as the moisture content of
the woodchips increased. At any sensor location, the lowest and highest observed pressures were
measured during the 37 and 60% moisture content tests, respectively. Analysis of variance
(Duncan’s means comparison test) performed at 5% confidence level revealed significant
differences (p < 0.0001) between pressures obtained at different moisture contents. The
percentage increase in pressure from the lowest to the highest moisture content was 80, 33, 100,
and 67% at 0.2, 05, 0.7, and 0.9 m locations, respectively. There were large differences between
observed pressures and the pressures predicted by the existing prediction equations, indicating
that none of the existing pressure equations are appropriate for predicting lateral pressures in a
biofilter bin. Multiple regression analysis was used to develop an empirical prediction equation
relating lateral pressure to moisture content and height.

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