Aerobic granular sludge has proven its value in sequencing batch reactors, forming dense, fast-settling granules that enhance nutrient removal, reduce tank volumes, lower capital costs, and simplify expansions. Until now, AGS has been limited to SBRs, but that’s changing. AGS is now a viable option for continuous flow treatment plants.
This breakthrough is powered by a combination of metabolic, kinetic and physical selection techniques. Controlling environmental and operational conditions can promote dense, spherical granules while removing the lighter, floc-forming bacteria.
Whether enhanced nutrient removal or increased plant capacity is needed, granular sludge can serve as the workhorse that powers the plant. Due to its compact form, the mixed liquor settles rapidly, allowing operation at higher MLSS concentrations. This translates into new plants with smaller footprints, lower construction costs and capacity increases — all while enhancing nutrient removal through simultaneous reactions in the mixed liquor and within each granule.
The compact granules from Granite AGS range from 200 to 1,000 microns. Whether called granular or densified sludge, maintaining a good particle size distribution is critical for success. Early AGS systems celebrated particles up to 3 millimeters, but diffusion limitations soon revealed that overly large granules limited oxygen, phosphorus, nitrogen and carbon transfer — creating inactive zones. Many in the industry now refer to “densified sludge” for smaller particles in the 200-800 micron range, which still qualify as granular.
The science
Granular sludge forms in plants configured for biological nutrient removal with distinct anaerobic, anoxic and aerobic zones. Deeply anaerobic selector basins at the head of the process stimulate the formation of extracellular polymeric substances, giving granules their structure. These zones also select for denitrifying phosphorus-accumulating organisms (dnPAOs) — nutrient-removal powerhouses that can generate volatile fatty acids, denitrify and uptake phosphorus under both anoxic and aerobic conditions. The anaerobic conditions promoting granule formation also favor dnPAO growth. Downstream, the process follows a familiar BNR layout — anoxic zones with internal recycle, followed by aerobic zones.
Physical selection controls granular particle size. In the final aeration zone, aeration and mixing are decoupled, creating slight solids stratification. A weir/actuator combination allows surface wasting to remove only lightweight, flocculent biomass while retaining the heavier granules. The result is dense microbial colonies with each granule acting as a miniature treatment plant with internal aerobic, anoxic and anaerobic layers for simultaneous carbon, nitrogen and phosphorus removal.
Operational advantages
Granular sludge is denser and settles faster than conventional activated sludge, with Sludge Volume Index values of 50 to 80 mL/g, allowing MLSS concentrations of 5,000 to 8,000 mg/L. This means higher throughput in the same footprint — ideal for doubling plant capacity without new tank construction. While more aeration is required to manage the added load, it remains more cost-effective than building new tankage.
Operating an AGS or densification system is similar to any BNR plant. Basins are sized using the same kinetics and design criteria, and the biological reactions are the same. The advantage is that with metabolic and physical selection, the granular particles perform reactions simultaneously, yielding lower effluent nitrogen and phosphorus and a compact sludge.
Existing facilities can be reconfigured for AGS operation. Oxidation ditches and three-, four- or five-stage BNR systems can all be converted into AGS processes.
Parkson is a leading provider of equipment and advanced solutions in water and wastewater treatment.
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