Aerobic Granular Sludge Process

13-jun-2006, STOWA





The Netherlands

Stage of development

Full scale

Process  -





BOD removal  - Nitrogen removal



Raw wastewater  -  Effluent from primary treatment



Enhanced sludge settling by formation of aerobic granular sludge

Keywords: decrease area demand; water line; BOD removal; nitrogen removal




Granular sludge demonstrates high settling velocities facilitating efficient solid-liquid separation. With high biomass retention and biological activity a granular sludge reactor can be operated at high volumetric loading rates. In the past anaerobic treatment concepts have been developed and implemented which make use of granular sludge.

Recent research has shown that granular sludge can be obtained under aerobic process conditions. Further development of this technology may result in the design of compact secondary and tertiary treatment units with small footprints. Cost savings can be realised due to reduced space requirements.


Description and working principle

The aerobic granular sludge reactor is operated as a sequenced batch reactor (Granulated Sequenced Batch Reactor - GSBR). The SBR concept is necessary to achieve process conditions for the formation of aerobic granular sludge.

Batch feeding of the reactor induces a high substrate concentration at the beginning of a treatment cycle. Due to a high concentration gradient substrate can diffuse deeply into the granules preventing starvation of bacteria within the granules. With insufficient feeding (diffusion gradient) the bacteria at the centre of the granules will be starved and weakened which eventually leads to the disintegration of the granules.

In general the size of the granules will increase until the formation of stable granules is limited by substrate diffusion. Less stable granules are susceptible to shear forces and will be reduced in size or disintegrate. Weakened biomass in the granule centre will also decrease the granule density and inhibit settling processes,  causing washout. Thus, a dynamic equilibrium will eventually be reached between substrate concentration and the average diameter of granules.

Unlike bacteria found in anaerobic granular sludge, aerobic bacteria in general do not tend to naturally form granules. In order to achieve granulation under aerobic process conditions, short settling times are used to introduce a strong selective advantage for well-settling sludge (granules). Poor-settling biomass will be washed out under these conditions.

Accordingly, appropriate settling and decanting times in each treatment cycle are chosen. In pilot trials, the GSBR is operated at settling times that correspond to average settling velocities of about 10 to 15 m/h. These relatively high settling velocities allow high volumetric loadings of the reactor resulting in a compact reactor design.

It has been observed that high shear forces under turbulent flow conditions give selective advantage to the formation of stable granules. Both, an air-lift reactor and a bubble column reactor seem capable of achieving sufficiently turbulent flow conditions while maintaining the oxygen supply. Tests results indicate that the use of a bubble column reactor enables a better process control of the nitrogen removal (denitrification). In turn it will be possible to significantly lower the energy demand of the process.


Nitrogen removal

Research has shown that nitrogen removal rates of more than 80% seem feasible. Nitrification is taking place in the outer, aerobic layer of the granules. Denitrification will occur in the anoxic core of the granules with the necessary carbon source being supplied by substrate diffused into the granules.

Similar to conventional applications of the SBR concept one treatment cycle in the AGSR has four, more or less well-defined phases: Filling ‑ Mixing/Aerating - Settling - Decanting.

Phase 1 and 2 - Filling and mixing/aerating:

Continuous mixing and aeration are achieved through injection of oxygen-rich gas. Turbulent flow conditions are created giving the granule forming bacteria a selective advantage.

Phase 3 - Settling: The allowed settling times corresponds to settling velocities of 10 to 15 m/h, causing poor-settling biomass to be washed out and the granules to be retained in the reactor.

Phase 4 - Decanting: In the third phase the reactor is emptied through outlets at the middle of the tank. The selected volume exchange ratio (hydraulic retention time) determines the amount of effluent that is drawn off.

More technical information can be found in patent WO9837027 and related patents.


Design guidelines / Technical data

It has been demonstrated over prolonged periods of time that stable operation of the GSBR is possible. Based on findings of tests on laboratory scale and an economic and technical feasibility study a decision will be taken about realisation and funding of pilot scale trials. Further research topics will include:

Substrate: At this moment tests are carried out at lab scale with synthetic wastewater, using acetate as the main substrate because of its high biodegradability. By using acetate as a substrate, aerobic granules are easily obtained. Further research on pilot scale will focus on the formation of granules with municipal wastewater as substrate.

Hydraulic characteristics: Experiences on laboratory scale can provide insight into the basic hydraulic characteristics, such as the average settling velocity of the granules. However, the hydraulic parameters of semi‑ and full-scale installations are expected to be different.

Sludge treatment: Experience with anaerobic granular sludges in the past decades suggests that de-watering and sludge handling of aerobic granular sludge will be straightforward. However, research is required to verify this assumption. Also, the distribution of excess biomass that is washed out or being drawn-off as granules has to be considered.



Comparison with other treatment concepts

A feasibility study has been carried out to determine the potential of the process under Dutch circumstances.  It is expected that the process will perform well compared to other treatment concepts. In the table below a qualitative comparison of a number of major treatment concepts is shown.


Activated sludge

Membrane BioReactor

Compact Systems

Aerobic granular sludge






Operating costs





Effluent quality










Space requirements












Capital and operating cost

A feasibility study has been carried out to establish the economic viability of the technology. The aerobic granular sludge process was compared to a conventional activated sludge system. To achieve phosphorus removal, chemical precipitation was assumed for both systems. Likewise, sludge treatment (thickening, digestion, de-watering) is assumed to be the same for the various scenarios.

The results of the feasibility study are shown in the figure below. Calculated operating costs are based on estimations of the energy demand, the amount of chemicals used and final sludge disposal costs. Costs for maintenance and personnel have not been included.

It appears that the aerobic granular sludge process combined with preprecipitation would be less expensive than the conventional activated sludge system with preprecipitation at all design capacities between 70.000 and 170.000 p.e. The aerobic granular sludge process combined with post-precipitation proves to be less expensive at design capacities of more than 100.000 p.e.

Cost estimates for Activated Sludge Systems and the GSBR
whole lines denote the construction costs, dotted lines the operating costs-


Reference installations

Based on findings of tests on laboratory scale and an economic and technical feasibility study a decision will be taken about realisation and funding of pilot scale trials.


Suppliers / Patents

In the Netherlands research on aerobic granular sludge has been initiated by Technical University of Delft. A patent on the process configuration has been registered - patent number WO9837027.

Currently, a research and feasibility study is carried out by the Technical University of Delft (TUD) in co‑operation with the Dutch engineering company DHV. The TUD is focusing on research and development of the process, DHV is looking into the economic and technical feasibility of the technology.

The ongoing research and development efforts are supported and funded by Stowa - the Dutch Foundation for Applied Research in Wastewater Treatment.

Delft University of Technology - Faculty of Applied Sciences (Kluyver Laboratory for Biotechnology)

Julianalaan 67  -  2628 BC Delft  -  The Netherlands

Tel.: 0031 - 15 278 2342 - Fax : 15 278 2355 - E-mail:  -  Web:

DHV Water BV

Postbus 484  - 3800 AL Amersfoort - The Netherlands

Tel: 0031 - 33  468 2200  -  Fax: 33 468 2301  -  E-mail:  -  Web:


Literature references

[1]   Unpublished DHV report. 2001.

[2]   Etterer, T.; Wilderer, P.A.: Generation and properties of aerobic granular sludge. Wat Sci Tech, 3-43.2001

[3]   Morgenroth, E.; Sherden, T.; van Loosdrecht, M.C.M.; Heijnen, J.J.; Wilderer, P.A.: Aerobic granular sludge in a sequencing batch reactor. Wat Res, Vol. 31, No. 12, 1997.




Aerobic granules - Day 346 pilot trial at TU Delft

Aerobic granules - Day 406 pilot trial at TU Delft