Influence of the granulation grade on the concentration of suspended solids in the effluent of a pilot scale sequencing batch reactor operated with aerobic granular sludge

• Anaerobic feeding phase with complete mixing yield quick granulation in a pilot scale SBR.
• The results show that even for low strength municipal waste water the aerobic granules work.
• A clear relation between the grade of granulation and suspended solids in the effluent is shown.
• Below 12 °C water temperature the existing nitrifying biomass in the granules is nearly inactive.

The operation of aerobic granular sludge reactors with a completely granulated sludge bed is often associated with effluent suspended solids concentrations above the required limits. In this work it can be shown that the amount of aerobic granules on the total biomass concentration is a key operation tool to control the suspended solids effluent concentration. Low effluent concentrations could be achieved only when the granule content was below 80%. A pilot scale reactor with a working volume of 4 m3 was used to treat municipal wastewater with aerobic granules and was operated for 247 days. The chosen sequential batch operation consisted of five phases: a short influent, anaerobic mixing, aeration, settling and effluent. The anaerobic mixing is innovative as it provides the same conditions for all granules. Additionally, by choosing an anaerobic mixing step the activated sludge granulation can be applied independently to the reactor geometry. Total suspended solids concentrations of 12 gTSS/L in the reactor and effluent suspended solids concentration below 50 mgTSS/L could be achieved. During the reactor start-up a high hydraulic selection pressure was not required to induce the granule formation. Hence, a complete nitrification was achieved already during start up. The added phosphorus was completely removed biologically by the granules.