Nereda aerobic granular sludge plant, Epe, Netherlands
The Nereda aerobic granular sludge plant in Epe, Netherlands.

The AquaNereda® aerobic granular sludge system (AGS) from Aqua-Aerobic Systems, Inc. is an innovative biological wastewater treatment technology that provides advanced nutrient removal, significantly smaller footprint, and up to 50% less energy, using the unique features of aerobic granular biomass.

One of the defining characteristics of the AquaNereda aerobic granule is rapid settling which in turn allows for a significant increase in biomass concentration. Granules are comprised entirely of true biomass and therefore do not require carriers. The layered microbial community that forms the granule is able to achieve enhanced biological nutrient removal, including simultaneous nitrification/denitrification and phosphorus reduction.

Enhanced settling properties

In an AGS system, bacteria are present as large dense granules, as opposed to fluffy suspended flocs. The density of these aerobic granules increases settling up to 15 times greater compared to conventional activated sludge systems. By overcoming settling limitations seen in conventional activated sludge systems, the mixed liquor can be increased significantly, which allows for a greater treatment capacity. Sludge volume index decreases to values of 30 ml/g – 50 ml/g, which allows the bioreactor to operate at mixed liquor suspended solids concentrations of 8,000 mg/l or higher.

Reduced footprint

Footprint is reduced up to 75% due to the full nutrient removal process taking place in a single reactor. Supplemental tanks such as primary clarifiers, selector basins, separate anoxic and aerobic compartments, and secondary clarifiers are not required. Also, the biomass operates at a higher concentration, allowing for increased treatment capacity within a single tank.

Energy savings

Reduction of air requirements in the bioreactor tank allows for substantial energy savings. The substrate and oxygen utilization rates in the aerobic granular system lower the specific energy and airflow. Based on the operation in a single reactor, the equipment required for the aerobic granular sludge process is significantly reduced, when compared to conventional activated sludge systems. Also, sludge recycling pumps are not needed. The combination of reduced aeration and mechanical components results in an energy savings of up to 50%.

Low life cycle cost

The reduction in mechanical equipment necessary to achieve enhanced biological nutrient removal requirements lowers construction, operating and maintenance costs. In addition, chemical consumption is reduced due to the high bio-P uptake in the aerobic granular biomass. These savings in operation and maintenance, as well as the lower construction costs, result in attractive whole life cycle costs.

How it works

Aerobic granular sludge combines the technical advantages of biofilm systems, including compactness and high metabolic rates, with those of activated sludge. This includes the ability to achieve full biological nutrient removal in one mixed biomass population, without the need for biofilm carriers. Aerobic, anoxic, and anaerobic biological processes take place simultaneously in the granular biomass.

Liquid solid separation in a flocculant sludge system occurs by using sedimentation in clarifiers. Due to the fact that flocs have low density, the settling time is much greater than a denser granular biomass, which will achieve full settling in five minutes.

As a result, a conventional plant is designed at 2,500 mg/l – 5,000 mg/l of mixed liquor, compared to an aerobic granular sludge plant at 8,000 mg/l or higher. Aerobic granular sludge displays a greater concentration of specialized micro organisms that are layered. Different types of microorganisms will grow in the granule that can result in a typical particle size of 1.2 mm – 1.5 mm in diameter. Nitrifiers reside on the outer layer of the granule and denitrifying bacteria are present in the deeper layers.

During aeration, oxygen can penetrate about 100 – 200 microns into the granule before it is depleted. This results in the inner part of the granule, nearest the core, being an anaerobic zone and the outside layers being aerobic and anoxic zones. Both the oxygen and substrate are transported into the granule via diffusion.

Phosphate accumulating organisms (PAOs) and glycogen accumulating organisms (GAOs) are present throughout the biomass and have the ability to form extracellular polymeric substance (EPS) that is the building block for granular formation.

Extracellular polymeric substance

Due to its robust structure, the granule has the ability to withstand upset conditions. The extracellular polymeric substance serves as the backbone of the granule and enables the system to withstand fluctuations in chemicals, loading, salinity, pH, toxic shocks, etc. This chemical backbone makes it difficult to interfere with the enhanced settling and stable process characteristics. Process stability, along with settling properties, result in a reliable and robust granular system that is easy to operate.

Granule formation

In an AquaNereda aerobic granular sludge system, selection mechanisms are applied to create dense granules from flocculant biomass. As an alternate, a plant can be seeded with granules from another site. Conventional biomass flocs are selectively wasted by increasing sedimentation stress. The stress continues to increase, thus enlarging the size and density of the flocs, which causes them to settle faster and evolve into granules. Only the bacteria that can adapt due to the selection mechanisms are able to survive in a granule form.

Aqua Nereda cycleCycle structure

Within a single tank, the AGS system creates proper conditions to reliably maintain a stable granule, without the need of a carrier. Due to the layered microbial community within the granule structure, simultaneous processes take place in the granular biomass, including enhanced biological phosphorus reduction, and simultaneous nitrification/denitrification.

Based on the unique characteristics of granular biomass, the AquaNereda system uses an optimized batch cycle structure. There are three main cycles of the process to meet advanced wastewater treatment objectives. Phase durations will be based upon the specific waste characteristics, the flow and the effluent objectives:

Fill/draw. The first phase of the cycle structure is fill/draw, where the influent flow first enters the reactor. The anoxic and anaerobic conditions provide biomass conditioning and phosphorus release. At the same time, treated water is displaced towards the effluent at the top of the AquaNereda AGS reactor.

React. Influent flow is terminated in the second phase of the cycle structure known as the react (aeration) phase. In this phase the biomass is subjected to aerobic and anoxic conditions. The granules perform simultaneous nitrification/denitrification. Concurrently, nitrate is transported by diffusion between outer aerated and inner anoxic layers of the granule, eliminating the need for pumping large recycle flows in the plant. Within the bioreactor, luxury uptake of phosphorus is promoted. The automated control of the process allows energy savings and process optimization.

Settling. Upon completion of the aeration phase, the system proceeds into a settle phase. Here, the influent flow still does not enter the reactor. The granular biomass is separated from the treated water during a very short settling period. Excess sludge is wasted in order to maintain the desired amount of biomass. Finally, the system is ready for a new cycle and influent enters the reactor while the treated water is discharged.

Process and cycle structures are optimized through the use of automated controls. While wastewater flows, loadings and temperatures fluctuate, the process parameters will be adjusted for efficient performance.


Typical applications for an aerobic granular sludge system include: retrofit of existing tanks, increasing treatment capacity, upgrade of existing treatment systems to meet more stringent effluent requirement, or greenfield sites, and enhanced biological nutrient removal.

Nereda aerobic granular sludge plant in Epe, Netherlands

Epe is the location of the first Nereda Aerobic Granular Sludge full-scale municipal system and has been in operation since 2009. It treats an average design flow of 2.1 MGD and a peak hour flow of 9.5 MGD using a three reactor design. Granulation occurred over the winter months, at temperatures below 10°C . The Nereda reactors are followed by tertiary filters, achieving 0.34 gm/l total phosphorus. Overall, the conversion to Nereda AGS reduced energy consumption by 40%. Currently, over 30 full-scale Nereda plants are operational or under construction.

Aqua-Aerobic Systems is currently building a full-scale system at the Rock River Water Reclamation District in Rockford, Illinois, which will demonstrate the system in North America.

Exclusive representation of the AquaNereda system in Canada is through ACG – Envirocan (Eastern Canada) and Waste n’ Watertech (Western Canada).

This article appears in ES&E Magazine’s June 2017 issue.

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