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. A few previous studies have also indicated an effect of high ammonia on cellulose degradation and a decrease in abundance of cellulose degrading bacteria (Ahlberg-Eliasson et al. 2023, Sun et al. 2016). However, it is not known how and why the cellulose degrading bacteria are inhibited and if it is firstly the cell viability, hydrolytic enzyme expression or excretion, the function of extracellular hydrolytic enzymes, cellular uptake of substrates, or something else that leads to a reduced cellulose degradation. The aim in this study is to investigate one of the above mentioned aspects, i.e. the cellulase gene expression at high ammonia pressure. Currently RNA is 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. The process parameters are monitored to do extractions at two different ammonia levels; 3 and 4 g NH4-N/L. The plan then involves performing an mRNA sequence analysis to compare the gene expression at the two different ammonium levels and in more depth look into the expression of cellulase encoding genes using the CAZy database. References Ahlberg-Eliasson K, Singh A, Isaksson S, Schnürer A. 2023. Co-substrate composition is critical for enrichment of functional key species and for process efficiency during biogas production from cattle manure. Microbial Biotechnology, 16:350–371. Kalamaras SD, Vasileiadis S, Karas P, Angelidaki I, Kotsopoulos TA. 2020. Microbial adaptation to high ammonia concentrations during anaerobic digestion of manure‐based feedstock: biomethanation and 16S rRNA gene sequencing. Journal of Chemical Technology & Biotechnology. Sun L, Liu T, Müller B, Schnürer A. 2016. The microbial community structure in industrial biogas plants influences the degradation rate of straw and cellulose in batch tests. Biotechnology for Biofuels 9:128.