A few sections of the sewers in Watertown were experiencing clogging due to an excess biomass growth. It was suspected that the wastewaters discharged from Fisher-Barton’s two facilities were causing excess biomass growth. Madison raw sewage had a slightly lower biomass growth than the mixture of industrial wastewater and domestic sewage samples. From microscopic examination of biomass, the main cause of excess biomass growth is thought to be toilet tissue. Biomass is attached to toilet tissue, decomposing it gradually and generating extracellular polymeric substances (EPS). Then, suspended particulates including graphite and precipitates are attached to the biomass and filamentous organisms, especially Sphaerotilus natans, leading to the increase in the biomass volume. We found that the filamentous bacteria were present in biofilm by FISH. A significant amount of graphite was embedded to biomass grown in sewer. Since the major cause of excess biomass appeared to be toilet tissue, it would not be economical to employ a filtration system for complete graphite separation. Instead, it is recommended that the manufacturing process where graphite is used be assessed and a best management practice is in place.