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Long-Lasting Benefits of Densified Activated Sludge

Hazen used retrofits and plant improvements completed in 2015 to inform a later design of secondary treatment facilities, increasing treatment capacity while minimizing costs at Gwinnett County's Crooked Creek WRF.

Project Outcomes and Benefits

  • Since startup, the facility has achieved superior settling with SVIs averaging 60 mL/g.
  • Fast-track design simplified construction so the Biological Reactor Basins were constructed in a single structure, saving time and money.
  • Future upgrades may increase the capacity to 25 mgd (56% increase) without building new bioreactors due to densification, which translates to a potential savings of approximately $30 million.
Ron Latimer, PE

In 2015, Hazen designed retrofits to Crooked Creek’s Oxidation Ditch 3 (OD3) to improve performance and prepare for future plant upgrades. The reconfigured OD3 consisted of an anaerobic zone at the front of the facility followed by an extended aerobic zone. These changes, combined with a high proportion of readily biodegradable carbon in the facility’s influent, provided ideal conditions for phosphorus uptake and fostered a strong population of Phosphorus Accumulating Organisms (PAOs). After the upgrades, Crooked Creek consistently achieved SVIs between 50-80 mL/g, which is in the range described as Densified Activated Sludge (DAS).

Anaerobic and anoxic zones of the new bioreactor basins.

These favorable settling characteristics were then incorporated into design of secondary treatment upgrades to expand Crooked Creek to 16 mgd-max-month capacity. The upgrades to OD3 offered evidence that a staged anaerobic zone to provide high-food-to-microorganism contact, followed by an extended aerobic zone to create feast-famine conditions, results in excellent settleability. These elements were incorporated into Hazen’s design of the Biological Reactor Basins (BRBs) for the upgraded facility.


The new bioreactors at Gwinnett County’s Crooked Creek WRF allowed continuous flow densification to occur in the absence of a physical selector, and, subsequently, improved plant performance. This could potentially aid in rerating the facility from 16 to 20 mgd without any additional infrastructure, translating to more than $30M in potential cost savings.

The BRBs were designed with a long anaerobic zone, an anoxic zone, and an oxic (aerobic) zone, or an “A2O” process. Microscopic analysis and sampling throughout the reactor revealed a strong PAO population, which provided above-average settling and dense, compact floc. Excellent SVIs were observed during special sampling after only five minutes of settling (77-86 mL/g). About 45% of the MLSS flocs are greater than 212 micron, a threshold meeting the definition of “granules.”

Facility upgrades also included two new activated carbon adsorption systems to treat foul air and provide odor control.

Hazen is designing future upgrades to the facility to increase the capacity to 25 mgd. These upgrades may be achieved without building new bioreactors due to densification, potentially avoiding approximately 6.3 million gallons of volume and realizing a possible savings of approximately $30 million.