Stable granulation of seawater-adapted aerobic granular sludge with filamentous Thiothrix bacteria

Many sources of wastewater contain sulfides, which can cause excessive growth of filamentous bacteria such as Thiothrix sp. resulting in bulking sludge in conventional activated sludge systems. Granular sludge systems could potentially also suffer from the growth of filamentous bacteria. Uptake of easily degradable COD by the relatively slow growing Ca. Accumulibacter phosphatis bacteria and the absence of strong diffusion gradients due to plug flow feeding through the settled granular sludge bed are assumed to be the dominant factors for successful granulation. Sulfides will remain after this anaerobic phase and cause growth of sulfide-consuming bacteria such as Thiothrix sp. Here we observed the impact of growth of Thiothrix sp bacteria in a laboratory aerobic granular sludge reactor by feeding a mixture of acetate and thiosulfate in the influent. Thiothrix sp, proliferated when 18% of the influent COD was due to thiosulfate, forming 51.4 ± 8.3% of the total granular biomass. Despite the strong presence of these filamentous bacteria a well settling sludge was maintained (SVI₁₀ equal to 13.3 mL/g). These results confirm that sludge morphology is not necessarily a reflection of the cell morphology of the bacteria, but is highly influence by reactor operation. It also reiterates the fact that compact biofilms are formed when the substrate consumption rate is lower than the substrate transport rate.