An activated sludge plant is a farm. The crop is a mixed community of bacteria and protozoa, and the operator decides what grows by controlling food, oxygen and time.
Who does the work
Heterotrophic bacteria use organic carbon (BOD) for energy and growth. They are the majority of the biomass and remove most BOD. Many can use either oxygen or nitrate (facultative), which makes denitrification possible.
Bacterial shapes. Cocci (singles, pairs, chains, clusters, tetrads), bacilli (rods) and spirals at 1000x. What this indicates: Shape alone does not identify a bacterium, but it is the first step in reading a Gram stain, and tetrads in large numbers can point to high organic acids.Original illustration for this site, not to scale.
Autotrophic nitrifiers use ammonia and nitrite for energy and carbon dioxide for carbon. They grow slowly and are sensitive. See Nitrification.
Phosphorus accumulating organisms (PAOs) store phosphorus when cycled between anaerobic and aerobic conditions. See BNR.
Protozoa and small animals (ciliates, amoebae, flagellates, rotifers, nematodes) graze on bacteria. They clarify the effluent by eating dispersed cells and serve as indicators of sludge condition. See the microscopy library.
How bacteria grow
Aerobic respiration. Bacteria use food (organics) and oxygen to make carbon dioxide, water, new cells and energy. What this indicates: This is why oxygen demand rises with load, and why the new cells must be wasted to hold a steady sludge age.Original illustration for this site, not to scale.
Bacteria convert part of the food into new cells and part into energy (carbon dioxide and water). The observed yield, the net solids produced per pound of BOD removed, is often 0.6 to 1.0 lb TSS per lb BOD, depending on SRT and how much inert material enters. White River's design uses 0.8.
At longer SRT, more of the biomass is consumed by endogenous respiration (cells using their own stored material), so net sludge production drops and oxygen use rises.
Floc formation
Floc is what makes activated sludge work. Bacteria produce sticky extracellular polymers (EPS) that bind cells, organic particles and inorganic fines into clumps. A few short filaments form a backbone that gives floc strength and size.
Floc formation: dispersed bacteria clump together with EPS around a backbone of short filaments into settleable floc.Inside a healthy floc. Floc forming rods held in sticky EPS around a short filament backbone, with trapped particles and stalked ciliates on the edge. What this indicates: Firm, irregular floc with a few internal filaments and a clear background settles well and leaves a clear supernatant.Original illustration for this site, not to scale.
Good floc is medium to large (150 to 500 microns or more), firm and irregular, with a clear background. It settles well and filters fine particles as it settles.
Floc formation depends on:
SRT: too short gives weak, dispersed floc; very long can give small, dense pin floc.
DO: low DO weakens floc and favors filaments.
Food gradient: a high food zone (selector) favors floc formers.
Nutrients: deficiency causes slime (viscous bulking) or filaments.
Toxins: can break up floc.
Shear: excess turbulence breaks floc.
Cations: calcium and magnesium help bridge EPS; a high sodium to divalent cation ratio can weaken floc.
Filaments and bulking
Filamentous bacteria grow in long chains. A few help; too many hold flocs apart, so the sludge settles slowly and compacts poorly. That is filamentous bulking.
Healthy floc compared with filamentous bulking, where filaments bridge between flocs and hold them apart.
Filaments win when conditions favor organisms with a high surface to volume ratio: low DO, low substrate, or low nutrients. Others (Thiothrix, Type 021N) use reduced sulfur from septic wastewater. See the filamentous bulking guide.
Viscous bulking is caused by excess EPS slime rather than filaments, often from nutrient deficiency or a very high F/M with readily degradable food.
Measuring activity
The oxygen uptake rate (OUR) and specific OUR (SOUR, per gram of VSS) show how active the biomass is. A sudden drop suggests toxicity; a high SOUR suggests young, highly loaded sludge. Typical SOUR in a conventional plant is roughly 8 to 20 mg O2/g VSS/h at 20 C, but each plant should learn its own normal.
What the operator controls
Food: you cannot control influent much, but you can control return streams, septage and hauled waste.
Oxygen: DO setpoints and air distribution.
Time: SRT through wasting.
Environment: pH, alkalinity, nutrients, temperature (indirectly).
Change those, and the community changes. That is why the microscope, SVI and SRT trend belong on the same page of your log.
