“…The toxicity of HFRs to living organisms has been described in previous reviews. , This review aims to analyze the impacts of HFRs and related transformation products on wastewater sludge microbiomes and the performance of biological wastewater/wastewater sludge treatment. The effects of some HFRs (e.g., TBBPA, PBDEs, and DBDPE) on microbiomes are well documented − and can be categorized based on their effects on (i) the entire microbial community, where exposure to HFRs can alter microbial composition (usually decreasing microbial diversity) and interactions of the microbiome via environmental filtering; ,,− (ii) individual microbial cells, where the integrity and function of cell membrane can be compromised by accumulation of these lipophilic pollutants, which directly affects cell viability; ,,, (iii) enzymatic activity and, therefore, activity of metabolic pathways can be altered by HFRs. , The ultimate effects of HFRs and related transformation products depend on a combination of these effects. Because there are few studies investigating the direct effects of HFRs on biological wastewater treatment systems and sludge anaerobic digesters, this review discusses demonstrated and putative effects of HFRs on biological nitrogen removal (i.e., nitrification, denitrification, and anaerobic ammonia oxidation) and methanogenesis by integrating the available information from both biological treatment systems ,,,− and other environmental matrices. ,− …”