A biological filter apparatus for biological degradation of contaminants in an air stream is operated by passing the air stream through a filter chamber containing a structure supporting a population of microbes and collecting water that drains from the structure and spraying the collected water onto the structure, whereby the sprayed water is recirculated. Upon start up or re-start of the filter apparatus, a nutrient composition is added to the recirculating water. The nutrient composition contains a buffering agent, sources of nitrogen and phosphorus, and sources of trace elements selected from manganese, calcium, iron and sodium.