What is a Activated sludge?
Activated sludge: Suspended-growth biological treatment
Activated sludge is the biological treatment process used by most mechanical wastewater treatment plants: wastewater is mixed with a suspension of bacteria in aerated basins, the bacteria consume the dissolved organic matter, and the mixture flows to clarifiers where the biomass settles and is returned to the basins while the clear effluent moves on to disinfection. Klarifi identifies 3,818 US publicly owned plants with clarifiers or a suspended-growth process across 50 states, and 1,496 with oxidation ditches, a variant of the process; aeration is the largest energy cost at these plants and 204 aeration and blower projects appear in the capital plans Klarifi tracks.
How the process works
Screened and settled wastewater enters an aeration basin where blowers and diffusers, or surface aerators, supply oxygen to a dense culture of bacteria called mixed liquor. After several hours the mixed liquor flows to a secondary clarifier; the settled biomass, return activated sludge, is pumped back to keep the population high, and a small fraction, waste activated sludge, is removed to the solids-handling side. Sludge age, oxygen and the return rate are the operator’s main controls.
Variants
Conventional plug-flow and complete-mix plants serve cities; extended aeration and oxidation ditches serve smaller towns with long sludge ages and no primary clarifiers; sequencing batch reactors do aeration and settling in one tank on a timed cycle; membrane bioreactors replace the clarifier with membranes for a very high-quality effluent. Biological nutrient removal adds anoxic and anaerobic zones to the same basins to remove nitrogen and phosphorus.
What gets bought and replaced
Blowers and diffusers wear out and are the first target of energy-efficiency projects; clarifier mechanisms, return sludge pumps, dissolved-oxygen instrumentation and control systems follow. Plants converting from lagoons to activated sludge, or adding nutrient removal, are the largest projects in the category.