8.2 Activated Sludge Process Fundamentals

Activated sludge in one sentence

Activated sludge is a suspended-growth biological process: wastewater is mixed with a concentrated culture of microorganisms (the mixed liquor) in an aeration tank, then solids are separated in a secondary clarifier. Settled solids return as return activated sludge (RAS) to keep biomass in the system; excess solids leave as waste activated sludge (WAS).

Quick Answer: Aeration tank + secondary clarifier = the core activated sludge plant. MLSS/MLVSS measure biomass inventory. RAS recycles settled sludge; WAS controls solids inventory and sludge age. Process modes change how food, oxygen, and solids move through the basins.

Why this dominates Texas municipal plants

Most medium and large Texas municipal plants—and many package plants—use some form of activated sludge because it can meet tight TPDES BOD, TSS, and often ammonia limits in a relatively small footprint compared with ponds. That is also why 30 TAC §30.342(c) treats activated sludge facilities specially for Class D renewal (covered in Section 8.3): the process needs more operator judgment than a simple lagoon.

The aeration tank: where the work happens

In the aeration tank, bacteria and other microbes consume soluble organic matter (measured as BOD or COD), converting it into new cells, CO₂, and water (and, when conditions allow, oxidizing ammonia). Air or pure oxygen is supplied by:

• Diffused aeration (fine or coarse bubble) — high oxygen transfer efficiency when diffusers are clean
• Mechanical aeration (surface aerators, brushes, rotors) — common on oxidation ditches and some package plants

Aeration has two jobs: oxygen transfer and mixing. Mixing keeps MLSS in suspension so microbes contact the food. If mixing fails, solids settle in the basin, dead zones go septic, and effluent quality collapses even if the blower is "on."

Oxygen transfer depends on diffuser condition, air flow, basin DO setpoint, temperature, and wastewater characteristics (surfactants and high solids can reduce transfer efficiency). Hot Texas summers lower oxygen solubility—more air may be needed to hold the same DO. Cold water holds more oxygen but slows biology, so DO may look high while ammonia removal lags.

Secondary clarifier: separation and thickening

The secondary clarifier (final clarifier) must:

1. Produce a clear effluent over the weirs (low TSS)
2. Thicken settled sludge for RAS/WAS withdrawal
3. Provide enough solids storage during diurnal peaks without blanket washout

If the clarifier is overloaded with solids (high MLSS, high sludge volume index, high flow), the sludge blanket rises and solids escape—often looking like a sudden "plant failure" that is really a solids-balance problem.

MLSS and MLVSS

Mixed liquor suspended solids (MLSS) is the concentration of suspended solids in the aeration tank, typically expressed in mg/L. It includes active biomass plus inert and inorganic solids.

Mixed liquor volatile suspended solids (MLVSS) is the volatile fraction of MLSS, often about 70–80% of MLSS in healthy municipal plants, and is a better estimate of organic biomass for F/M calculations.

Rough inventory math uses the pounds formula:

[ \text{lb solids} = \text{MG} \times \text{mg/L} \times 8.34 ]

Worked example — MLSS inventory. An aeration basin volume is 0.50 MG at 2,800 mg/L MLSS. Solids under aeration = 0.50 × 2,800 × 8.34 = 11,676 lb. If MLVSS is 75% of MLSS, MLVSS ≈ 2,100 mg/L and volatile solids ≈ 0.50 × 2,100 × 8.34 = 8,757 lb.

RAS and WAS — the two sludge valves that run the plant

RAS rate is often expressed as a percentage of influent flow (for example, 50–100% depending on mode and settleability). WAS is usually a much smaller continuous or intermittent flow; small WAS changes have large effects over days because they change the entire solids inventory.

Common process modes (know the names)

Conventional activated sludge (often plug flow): Long aeration tanks where concentration gradients exist from inlet to outlet. First zone sees high food (high oxygen demand); later zones polish. Sensitive to shock loads at the head end.

Complete mix: Influent is quickly dispersed through the basin so conditions are more uniform. Better shock-load buffering; local F/M is lower at the feed point than in plug flow.

Plug flow: Emphasizes the longitudinal gradient (related to conventional). Can favor good settling floc when designed and loaded correctly.

Contact stabilization: Influent contacts return sludge in a small contact tank, then mixed liquor settles; returned sludge is aerated/stabilized separately. Useful when aeration volume is limited or loads vary.

Extended aeration: Long aeration detention and long sludge age; common in Texas package plants and small cities. Lower F/M, more endogenous respiration, higher oxygen use per pound of BOD, often simpler wasting strategy—but clarifiers must handle the solids.

Sequencing batch reactor (SBR): Fill, react, settle, decant, idle in one tank on a timed cycle. No separate secondary clarifier in the classic sense. Timing replaces continuous RAS/WAS hydraulics.

Oxidation ditch: Racetrack channel with horizontal rotors/brushes providing aeration and circulation. Often operates like extended aeration with anoxic zones possible for denitrification when DO is managed carefully.

Putting the train together

Typical municipal flow:

1. Preliminary (screens/grit) and often primary clarification
2. Aeration (with RAS)
3. Secondary clarification → effluent to disinfection
4. WAS to thickening/digestion

Exam tip: if a question mentions rising sludge blankets, cloudy effluent after a storm, or "we turned up the air but ammonia is still high," separate oxygen supply, biomass inventory, and clarifier hydraulics in your reasoning. Activated sludge fails in those three places more often than from a single mysterious chemical cause.

Master the vocabulary—MLSS, MLVSS, RAS, WAS, and the named modes—before diving into F/M and SVI math in the next section. Fundamentals questions on the TCEQ exam often hide inside troubleshooting stories.