At various places in the biogas plant. exhaust air emerges in even larger quantities [202] .
The exhaust air pipeline from the hydrolysis reactor can be connected to the biogas pipe or, for example, to the central air discharge pipeline of the entire plant. When the exhaust air is added to the biogas, the gas quality deteriorates and the risk of explosions rises. Fluctuations in pH during the hydrolysis result in sudden CO 2 and H 2 production, which can sometimes cause a shutdown of the gas engine.
Each uncontrolled escape of exhaust gas into the environment represents a source of odor and also of danger. Therefore, the exhaust air must be cleaned. Bioscrubbers, chemoscrubbers, and biofi lters are approved for this purpose.
Bioscrubbers [203] work with circulating water and are preferred when the concentration of odorous substances is high and the concentration of H 2 S is less than 20 mg m -3 . For high H 2 S concentrations, a chemoscrubber is recommended, which operates with a caustic soda solution at a pH of about 10. However, for deodorization a biofi lter has to be installed downstream. Biofi lters [204] are suitable for H 2 S concentrations up to 20 mg m -3 in the exhaust air.
In order to avoid general odor problems, for example, from long pipelines for the return of the water from the bioreactor to the sewage treatment plant, it is recommended to connect an aerobic treatment of the waste water with nitrifi cation and denitrifi cation procedures downstream of the biogas plant (Figure 25.1 ).