Addition of ferrous and ferric iron salts to wastewater is a commonly used practice for sulfide abatement in sewer force mains. When iron is added to wastewater where sulfate respiration takes place, it produces ferrous sulfide precipitates with the formed sulfide. The effect of iron addition has traditionally been focused on solely from the perspective of reaction stoichiometry. Possible influences on the microbial communities in biofilms growing in force mains have largely been neglected. In this study the activity and microbiome was examined in three pilot scale force mains conveying real wastewater, two subjected to iron treatment and one operated as an untreated control. Activity was measured on suspended biofilm samples extracted from the experimental setup. The microbiome of the biofilm was analyzed by V3 + V4 16S rDNA sequencing. Correlation analysis of chemical composition of the biofilms and activity measurements for operational taxonomic units of relevance to sulfide and methane production were performed. In conclusion, it was found that both ferrous and ferric treatment reduced sulfate reduction and methane production, and that both iron salts induced significant changes to force main biofilm microbiomes.