bThe coexistence of sulfate-reducing bacteria (SRB) and methanogenic archaea (MA) in anaerobic biofilms developed in sewer inner pipe surfaces favors the accumulation of sulfide (H 2 S) and methane (CH 4 ) as metabolic end products, causing severe impacts on sewerage systems. In this study, we investigated the time course of H 2 S and CH 4 production and emission rates during different stages of biofilm development in relation to changes in the composition of microbial biofilm communities. The study was carried out in a laboratory sewer pilot plant that mimics a full-scale anaerobic rising sewer using a combination of process data and ). This contrasting trend coincided with a stable SRB community and an archaeal community composed solely of methanogens derived from the human gut (i.e., Methanobrevibacter and Methanosphaera). In turn, CH 4 emissions increased after 1 year of biofilm growth (327.6 ؎ 16.6 mg COD-CH 4 liter ؊1 day ؊1 ), coinciding with the replacement of methanogenic colonizers by species more adapted to sewer conditions (i.e., Methanosaeta spp.). Our study provides data that confirm the capacity of our laboratory experimental system to mimic the functioning of full-scale sewers both microbiologically and operationally in terms of sulfide and methane production, gaining insight into the complex dynamics of key microbial groups during biofilm development. W astewater collection systems, or sewers, consist of an underground network of physical structures-installations composed of pipelines, pumping stations, manholes, and channels that convey wastewaters from their source to the discharge point, usually a wastewater treatment plant (WWTP). Sewer systems thus prevent the direct contact of urban populations to fecal material and potential microbial pathogens, greatly reducing the spread of infectious diseases. Sewers have traditionally been considered only hydraulic transport systems for sewage, although they are in fact "reactors" where complex physicochemical and microbial processes take place. Wastewater microorganisms are diverse and abundant, and they are exposed to a wide range of both inorganic and organic substrates as well as changing conditions along their transport through sewers (1). In this regard, wastewater transport through the pipes facilitates the formation of microbial biofilms that grow attached to the inner surface of sewer pipes (2). Different factors, such as large surface area, low flow velocity near pipe walls, and nutrient availability, may favor microbial colonization of pipe surfaces and biofilm growth. Formation of fully functional biofilms occurs in different steps, from surface conditioning, adhesion of microbial "colonizers," initial growth, and glycocalyx formation, followed by secondary colonization and growth (3).Anaerobic conditions in sewer pipes favor the accumulation of both sulfide (H 2 S) and methane (CH 4 ) as end products of different microbial metabolisms, i.e., anaerobic respiration of organic matter by sulfate-reducing bacteria (SRB) and methanogenic archae...