A process and system are disclosed for the biofiltration of volatile organic compounds. The process involved recirculating contaminated effluent gas through a biofilter (1), the biofilter (1) having an inlet (20), outlet (15) and micro-organism laden filter media bed (3). The filter media bed additionally having moisture retaining properties. This process has been effective in removing high levels of VOCs from effluent gas streams and also in removing VOCs from an effluent gas stream where the VOC levels and/or volumes of effluent gas vary over time.