Biotrickling filters (BTF) are irrigated reactors with a packed bed colonized by microbial growths capable of degrading contaminants from a passing gas. Essentially BTFs are a tank with raw biogas plumbed in to permeate filter media and upgraded biogas exiting. Air and recycled nutrient water support the microbial activity on this media. The liquid phase also aids pollutant capture and removal of the microbial breakdown products.