Guideline VDI 3478 deals with the cleaning of waste gas flows polluted with gaseous substances foreign to air. They apply primarily to the reduction of organic emission components that biodegrade at sufficient speed as well as hydrogen sulphide and ammonia. Their typical field of application comprises mass concentrations of organic compounds up to approx. 1 g/m. The process is designed to reduce emissions that, by virtue of their action principles, are potentially hazardous to health and/or a source of nuisance. The guideline presents an overview of possible solutions and of the associated principles and assessment criteria necessary for correct design. The descriptions of processes given in the guideline are examples of applications of systems, some of which can be operated as pilot plants. All volumes for gases in this guideline refer to standard temperature and pressure (273 K, 1013 mbar) after deduction of the share of water vapour. Special reference is made to exceptions. Reference is made in due course to the acts, ordinances, administrative and other regulations specifically applicable to plant construction and operation and to technical rules (see also Bibliography). Guideline VDI 3478 Part 2, defines the state of the art for the biological trickle bed reactor described in the following. The suitability of the process for the waste gas of the upstream plant must be checked in each case. The following technical and economic factors must be considered: efficiency in terms of emission control,plant availability,plant service life, plant safety, avoidance of new emissions and emission displacement (e.g. air pollution becomes water pollution),investment and operating costs (e.g. water/waste water, excess sludge, energy and maintenance), required space, consideration of start-up and shutdown processes The guideline describes biological trickle bed reactors that adsorb and biodegrade waste gas components The process is based on the substance conversion of the solvents or odorants with the aid of biologically active aerobic microorganisms that form a biofilm on the packings of the biological trickle bed reactors. All these processes for substance conversion take place in a matter of seconds. For the application of biological trickle bed reactors, the scrubbed waste gas components must be soluble and microbiologically degradable in the presence of air.