Drying out of the media deactivates the microbes and also creates cracks or channeling resulting in a reduction of filter efficiency. Too much moisture can plug some of the pores in the media, causing channeling and limiting oxygen flow in saturated areas of the filter, thereby creating anaerobic zones in the biofilm. The open top biofilter design described in this report allows for adequate media drainage after a heavy rain or snow. The filter should be placed on a sloped well-drained area to prevent any water ponding in the air plenum. If adequate drainage is provided for this design, over saturation should not be a problem.
Moisture needs to be supplied by sprinkling water directly on the bed.
During the winter months and cooler temperatures, moisture transfer from the media to the exhaust air is slow enough to prevent drying requiring no additional water. In periods of rainy weather, no additional water is needed. Water addition can be automated with a soil moisture probe, a solenoid valve in the water supply line, and an electrical controller. Excess vegetative growth on the biofilter surface can reduce its efficiency. Heavy root growth can plug some media pores causing channeling and limiting oxygen flow.