Background & objectives: Formaldehyde is one of the volatile organic compounds and classified as probably carcinogenic. The aim of this study was to determine the dominant microorganism in biodegradation of formaldehyde from the air.

Methods: This study was an experimental study that an aerobic biofilter reactor made ​​of steel with mixture of compost and wood chips (50/50 percent in terms of volume volume) in the first step of operation and pumice ston and compost mixture (80% pumice and 20% compost in terms of volume) in the second step were used as beds for microbial growth. First and second step experiments started with concentrations of 20±2 and 24±3 mg/m³, respectively, and completed with concentrations of 276±4 and 224±5 mg/m³, respectively. Sampling and measurement was done using the 3500 method National Institute for Occupational Safety and Health.

Results: The maximum of formaldehyde removal efficiency in both the first and second in empty bed residence time of 180 seconds was achieved at about 91% in the operation first stage and 88% the operation second stage. Also, after some tests done to determine the dominant microbial population it was found that the Citrobacter freundii was the dominant microorganism to formaldehyde biodegradation in the study.

Conclusion: The results showed that by increasing the inlet concentration of formaldehyde and reducing the EBRT, the formaldehyde removal capacity of the system decreases. Conclusion: The present study introduced this process as an effective method for removing formaldehyde from the air stream. As well as the results, significant differences in terms of the removal efficiency with different substrate materials in two operation steps do not showed. However that the sustainability of the system capacity to remove formaldehyde in the operation first stage was observed more than the second stage.