Suspended biomass within the interstitial void spaces was considered to be a significant factor in substrate removal. On the other hand, excessive biomass growth results in a decrease in the available area for the organic matter to diffuse into the granules, and therefore, potentially decreases the removal efficiency. Wastewater containing high levels of suspended solids may cause a gradual accumulation of slow and non-biodegradable solids within the void spaces between the granules. The slow hydrolysis of entrapped solids at low temperatures also results in solids accumulation. Consequently, as the pores become occupied by entrapped solids and biomass, a decrease in the effective porosity will lead to a rapid buildup of head loss, channeling, and short-circuiting of flow through the reactor.