Anaerobic digestion (AD) is a process where organic materials are degraded and converted into biogas; a source of renewable energy. The residual material from the process, i.e. the digestate, has a high content of plant available nutrients and can be used as biofertilizer. AD is widely used at industrial-scale for treatment of organic waste streams. Either so called wet digestion is applied (total solids (TS) <15%), or an alternative, less commonly applied technology, is high-solid digestion (HSD) (TS >15%). HSD has several advantages, such as reduced use of fresh water for substrate dilution, possibilities to use a relatively high organic loading rate, and a digestate with high nutrient concentrations. In the current study, a HSD process digesting a combination of nutritious and protein rich food waste, and more recalcitrant and fiber rich material, is studied. During degradation of proteins, ammonia is released, which at high concentrations is inhibiting to the methane producing community, which in turn can cause accumulation of volatile fatty acids and process failure. The aim in this study is to investigate gene expression at high ammonia pressure and which taxa that can survive and maintain a high activity at such conditions. RNA was extracted from HSD lab-scale reactors, digesting food waste as well as cellulose rich substrates such as garden residues and manure, supplemented with albumin to obtain NH4-N concentrations at inhibiting or close to inhibiting levels. In addition, RNA has been sampled from anaerobic digesters operated at wet conditions and in a series to be able to apply a high organic loading rate in a first step reactor while reducing short-circuiting in the system. The RNA, as well as DNA for metagenome analysis, was sampled during a period of process disturbances in the first-step reactors leading to elevated propionate levels. The aim is to investigate which taxa that are active under these conditions and that contribute to the metabolism of volatile fatty acids.