Aerobic granular sludge: State of the art, applications, and new perspectives.

The aerobic granular sludge (AGS) process has attracted significant interest over the last decade and is one of the most promising wastewater treatment technologies. AGS offers several advantages over conventional activated sludge (CAS) including excellent settling, and higher volumetric loading capacity. Because of porosity, concentration gradients develop and stratified aerobic, anoxic, and anaerobic layers develop throughout granule depth. This is the reason for simultaneous nutrient removal in a single tank. Aerobic granulation is influenced by many parameters including wastewater characteristics and operating conditions. Among these, the anaerobic upflow feeding strategy and a properly balanced feast/famine regime have most significantly influenced granule stability and nutrient removal efficiency. Hydraulic shear forces can improve the physical characteristics of the granules, whereas selective wasting also allows undesirable biomass with low density to be washed out. Implementation of AGS in full-scale requires some important retrofitting, including adequate grit and particulate organics removal from the wastewater. Furthermore, aerobic granular sludge cycles should be designed with flexibility to address variable influent loads. Considerations include equalization and flow balancing, dissolved oxygen control strategies, multiple draw points for selective wasting and swing-capabilities. Several studies and full-scale implementations have demonstrated that AGS is suited for the treatment of a wide variety of industrial wastewaters, as well as municipal wastewater. Full-scale implementations of AGS treating municipal wastewater have improved process stability, attained more stringent effluent quality (i.e., total nitrogen (TN) <5 mg/L; total phosphorus (TP) <0.3 mg/L), resulting in 30% to 60% energy savings compared with CAS.