3.1 Activated Sludge Microbiology
Key Takeaways
- Activated sludge control is environmental selection: operators manage food, oxygen, solids age, pH, toxicity, and settling conditions rather than individual organisms.
- Heterotrophs drive BOD removal, nitrifiers drive ammonia conversion, filaments influence settling, and protozoa help confirm whether the sludge is young, stable, old, or stressed.
- Aerobic, anoxic, and anaerobic zones are defined by available electron acceptors, not by tank appearance or whether a mixer is running.
- Biology and clarification must be interpreted together because good BOD removal still fails compliance if floc does not settle and return properly.
Why activated sludge microbiology matters
The biological-treatment chapter is where many wastewater operator questions stop being vocabulary and become judgment calls. WPI's current wastewater treatment outline places suspended growth processes, activated sludge, complete mix, extended aeration, nutrient removal zones, and process-control testing inside the treatment process evaluation and adjustment domain. On the exam, that usually means you are not asked to name a microorganism in isolation. You are asked what the biology is doing, what plant signal proves it, and which operator lever is least likely to make the problem worse.
Activated sludge is a suspended-growth process. Wastewater enters an aeration basin that contains mixed liquor, meaning wastewater plus biological solids. Heterotrophic bacteria use soluble and fine organic matter as food, need oxygen in aerobic treatment, and form sticky biological particles called floc. The floc must do two jobs: remove biochemical oxygen demand in the aeration basin and settle in the secondary clarifier so clear effluent can separate from biomass. A plant can remove BOD in the basin and still fail if the floc will not settle.
The organisms operators actually control
Operators do not control individual bacteria one at a time. They control the environment that selects for useful populations.
| Group or condition | What it tells you | Operator meaning |
|---|---|---|
| Heterotrophic bacteria | Fast BOD removal | Respond quickly to food, oxygen, pH, and toxicity changes |
| Floc-forming organisms | Compact settleable biomass | Needed for low effluent TSS and stable clarifier blankets |
| Filamentous organisms | Framework at low levels, bulking at high levels | Excessive growth can raise SVI and carry solids over the weirs |
| Nitrifying bacteria | Ammonia conversion to nitrate | Slow growers; need enough dissolved oxygen, alkalinity, temperature, and sludge age |
| Protozoa and metazoa | Grazing and maturity indicators | Stalked ciliates often fit stable operation; many flagellates can fit young or stressed sludge |
A useful exam habit is to connect each organism to a process result. Heterotrophs explain BOD removal. Nitrifiers explain ammonia removal. Filaments explain poor compaction. Protozoa help interpret whether the sludge is young, stable, old, or shocked, but a microscope observation should be compared with dissolved oxygen, pH, sludge age, mixed liquor suspended solids, and settling tests.
Aerobic, anoxic, and anaerobic biology
Biological treatment is not just "add air." Aerobic zones contain dissolved oxygen and support BOD oxidation and nitrification. Anoxic zones lack free dissolved oxygen but contain nitrate; denitrifying organisms can use nitrate as the electron acceptor and release nitrogen gas. Anaerobic zones lack both free oxygen and nitrate and are used in some biological phosphorus removal configurations.
The trap is to confuse these zones by smell or tank label. A covered tank is not automatically anaerobic, and a basin with mixers is not automatically anoxic. The available electron acceptor defines the zone. If nitrate is present and free dissolved oxygen is absent, think anoxic. If free dissolved oxygen is present, think aerobic even if the basin looks dark.
Reading biological condition from plant evidence
Biology should be diagnosed from trends, not one glance at the basin. A good daily review ties together influent loading, aeration-basin DO, pH, temperature, MLSS or MLVSS, 30-minute settleability, sludge volume index, return activated sludge flow, waste activated sludge rate, secondary clarifier blanket, effluent turbidity, effluent BOD, effluent TSS, and ammonia.
A sudden drop in oxygen uptake, dispersed floc, and worse effluent can indicate toxic shock, especially after an industrial slug or chemical spill. Increasing air alone will not revive a poisoned biomass if the organism activity has been inhibited. Low DO with normal-looking floc may instead point to blower capacity, diffuser fouling, increased organic loading, or nitrification demand. A rising ammonia result should make you check DO, alkalinity, pH, temperature, and sludge age before blaming the laboratory.
Scenario traps
- Do not describe disinfection as part of activated sludge control. Chlorine and ultraviolet treatment act after biological separation unless a site-specific filament-control procedure uses carefully controlled chlorination.
- Do not assume more organisms always mean better treatment. Too much old biomass can increase oxygen demand, foam, and solids handling burden.
- Do not use color alone. Dark sludge, light foam, earthy odor, septic odor, and clear supernatant are clues, not final diagnoses.
- Do not forget the clarifier. Activated sludge is one system: aeration creates floc, the clarifier separates it, RAS returns it, and WAS removes excess biomass.
Mini example
A plant has stable flow, aeration DO near 2 mg/L, good 30-minute settling, low effluent BOD, low effluent TSS, and many stalked ciliates. That points to a mature, stable biomass. If the same plant suddenly shows low DO, cloudy effluent, mostly small flagellates, and a recent high-strength load, the better first interpretation is young or stressed sludge under increased food demand, not a need to disinfect the aeration basin.
Exam focus: living inventory
A helpful way to think about activated sludge is as a living inventory that must be fed, aerated, returned, and intentionally reduced. If the inventory is starved, floc may age and break down. If it is overloaded, food remains in the effluent and oxygen demand rises. If it is poisoned, the solids number may stay high while treatment activity falls. That is why strong operator answers rarely rely on one lab value. They compare biological activity, clarifier separation, and permit-facing results before changing air, RAS, or WAS.
A mixed liquor sample has compact floc, many stalked ciliates, low effluent BOD, low effluent TSS, and a stable secondary blanket. What is the best interpretation?
Why are nitrifying organisms often the first important population lost when sludge age is cut too far?
Which process condition best supports denitrification?