Aerobic granular sludge

In recent years, a significant part of research on wastewater treatment has focussed on development of compact systems. Wastewater treatment plants (WWTP) can be designed compact when biomass is maintained in the system without the use of conventional settling tanks as in flocculated activated sludge systems. Examples are developments in the field of Membrane BioReactors, Biological Aerated Filters and other biofilm systems. For conventional biofilm technology the surface area of the biofilms, needed for mass transfer and therefore the conversion processes, is limited by the reactor surface the biomass can attach to. When biofilms are grown in a granular shape, the maximum surface area per volume of reactor can be obtained. Most compact systems for the treatment of municipal sewage are based on continuously operated reactors. In this kind of continuously operated systems, it was possible to grow granular sludge with slowly biodegradable substrates (e.g. methanol or ammonia). In the case of easily biodegradable substrates, these systems can only be used when enough basalt was added to the system (as carrier material and for shear reasons, Kwok et al., 1998). Research concerning the formation of storage polymers (Van Loosdrecht et al., 1997; Krishna and van Loosdrecht, 1999) finally resulted in the idea of growing aerobic granules without carrier material on readily biodegradable substrates in a Sequencing Batch Reactor (Morgenroth et al., 1997; Beun et al., 1999; Dangcong et al., 1999). The conversion of readily biodegradable COD into a substrate yielding a lower maximal growth rate facilitated granule formation. In 1998, an international patent was submitted and granted (Heijnen and Van Loosdrecht, 1998). An extension of this first patent was submitted in 2004, including the description of anaerobic feeding (Van Loosdrecht and De Kreuk, 2004).