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How do microorganisms clean wastewater?

Activated sludge does not make pollutants vanish. It couples mass transfer, enzymatic hydrolysis, metabolism, biomass synthesis and solids separation: part of the carbon becomes carbon dioxide and water, part becomes new biomass, and settling plus sludge return/wasting separates that biomass from the treated water.

Direct answer

Direct answer

Microorganisms clean wastewater by transforming soluble and hydrolysable organic matter. Influent mixed with return activated sludge brings colloids and particles into flocs for capture and hydrolysis; soluble substrate diffuses into cells, where heterotrophs use one fraction for respiration and energy and another to synthesize cells and extracellular polymer. Aerobic treatment requires adequate oxygen, mixing, nutrients and contact time. The pollution has destinations: carbon becomes CO₂, waste sludge and a residual in the effluent. The secondary clarifier retains settleable floc, return sludge maintains biomass, and waste sludge controls solids retention time. Air alone cannot compensate for unsuitable loading, SRT, pH/temperature, settling or return-sludge control.

Four conditions must hold together

Biological reaction and solids separation are one system; failure of either degrades effluent.

Substrate must contact and become available

Readily biodegradable solutes enter cells quickly. Colloids, particles and macromolecules must first be captured and hydrolysed. Refractory or toxic material does not disappear just because air is added.

The community needs activity and adequate SRT

Temperature, pH, nutrients, toxicity and solids retention shape the community. Fast heterotrophs and slow nitrifiers require different SRT margins.

Electron acceptor, mixing and transfer must match

Aerobic carbon removal and nitrification need oxygen; denitrification needs nitrate and low DO. Aeration supplies oxygen and mixing, but more is not automatically better.

New biomass must be retained and removed

Clarification, return activated sludge and waste sludge set biomass and SRT. Poor settling or hydraulic washout loses solids even when reactions occur.

1

An aeration basin supplies oxygen and mixing, not cleaning bubbles

Rows of bubbles turn over mixed liquor, online probes and air piping line the basin, and secondary clarifiers are visible downstream—one continuous reaction and sludge-recycle system.

An aeration basin supplies oxygen and mixing, not cleaning bubbles：Influent and return-sludge mixing、Aeration bubbles and mixed liquor、DO and process probes、Downstream secondary clarifier1234

What to identify

  1. 1Influent and return-sludge mixing
  2. 2Aeration bubbles and mixed liquor
  3. 3DO and process probes
  4. 4Downstream secondary clarifier

What the image proves

Bubbles transfer oxygen and suspend floc; biochemical reactions in and around flocs perform the removal. Airflow must match load, oxygen demand and transfer, not surface whiteness.

How to verify on site

Profile DO, ammonia/COD and mixing by zone; verify air distribution and valves, and reconcile energy with load, DO control and effluent results.

2

One activated-sludge floc is a micro-reactor

The close view shows bacteria, protozoa, small particles and water pores held by extracellular polymer; oxygen and solutes diffuse inward and products diffuse outward.

One activated-sludge floc is a micro-reactor：EPS structural matrix、Heterotrophic bacteria、Protozoa and grazers、Organic particles and water pores1234

What to identify

  1. 1EPS structural matrix
  2. 2Heterotrophic bacteria
  3. 3Protozoa and grazers
  4. 4Organic particles and water pores

What the image proves

A floc is a structured community, not one bacterium. Transfer is faster outside and low-oxygen microzones may form inside; capture is followed by hydrolysis, diffusion and metabolism.

How to verify on site

Assess floc size/density, filaments and protozoa with microscopy, then combine with OUR/SOUR, DO, soluble COD and settling—not one image alone.

3

The process closes through reaction, settling, return and wasting

The cutaway shows influent/return mixing at left, an aerated bioreactor in the center, secondary separation at right and bottom piping for returned and wasted sludge.

The process closes through reaction, settling, return and wasting：Influent and RAS mixing、Aerobic biological reaction、Clarifier clear-water zone、Return and waste sludge lines1234

What to identify

  1. 1Influent and RAS mixing
  2. 2Aerobic biological reaction
  3. 3Clarifier clear-water zone
  4. 4Return and waste sludge lines

What the image proves

Biomass remains longer than the hydraulic pass because return sludge creates a longer SRT. Too little wasting causes old/accumulated sludge; too much wasting removes organisms faster than they grow.

How to verify on site

Balance influent/effluent solids, MLSS/MLVSS and RAS/WAS flows and concentrations; calculate SRT and verify blanket and solids flux.

4

The same equipment behaves differently at different load and sludge states

Parallel reactors show dark high-load liquor, more balanced mature floc and low-solids/washed-out conditions; settling cylinders display the corresponding sludge-water interface.

The same equipment behaves differently at different load and sludge states：High-load, high-F/M reactor、Balanced load and mature floc、Low biomass or washout、Settling cylinders and supernatant1234

What to identify

  1. 1High-load, high-F/M reactor
  2. 2Balanced load and mature floc
  3. 3Low biomass or washout
  4. 4Settling cylinders and supernatant

What the image proves

Color and clarity alone do not prove performance. High F/M can cause dispersed growth and oxygen deficit; very low F/M/long SRT can cause aging, while low MLSS may be washout.

How to verify on site

Compare F/M, SRT, MLSS/MLVSS, DO, OUR, SVI/30-minute settling, supernatant turbidity and influent/effluent load together.

5

Diagnosis combines basin observation, settling, chemistry and microscopy

The basin-side station contains a settleometer, mixed-liquor and supernatant samples, color/titration tubes and a microscopic floc/filament image; foam or floating sludge is visible behind.

Diagnosis combines basin observation, settling, chemistry and microscopy：30-minute settleometer、Mixed-liquor and supernatant samples、Colorimetric or titration tests、Floc and filament microscopy1234

What to identify

  1. 130-minute settleometer
  2. 2Mixed-liquor and supernatant samples
  3. 3Colorimetric or titration tests
  4. 4Floc and filament microscopy

What the image proves

Foam, floating sludge, solids loss or turbidity may come from load, DO, SRT, nutrients, filaments, denitrification or hydraulics. No single test spans them all.

How to verify on site

At one timestamp combine basin photos, DO/pH/temperature, COD/BOD/ammonia, MLSS/SVI, blanket, microscopy and operating events, then follow the causal chain.

Six destinations for an organic pollutant in activated sludge

The path explains both aeration demand and waste-sludge production.

  1. 1 Contact and mix

    Influent organics + return sludge

    Bring substrate, floc and electron acceptor together.

  2. 2 Capture

    Particles/colloids → EPS and floc

    Move part of the load from water into floc surfaces and pores.

  3. 3 Hydrolysis and transfer

    Macromolecule → small molecule → cell

    Extracellular enzymes release soluble substrate that diffuses into cells.

  4. 4 Respiration

    Substrate + O₂ → CO₂ + H₂O + energy

    Heterotrophs oxidize part of the carbon and consume oxygen.

  5. 5 Biomass synthesis

    Substrate + N/P → cells and EPS

    Another fraction becomes new sludge.

  6. 6 Settle, return and waste

    Floc → clarifier → RAS/WAS

    Return maintains biomass; wasting removes growth and controls SRT.

What do the members of a floc do?

Functions overlap and shift with operation; these are the major diagnostic roles.

Heterotrophic bacteria

Main function
Use biodegradable carbon for respiration and new biomass
Sensitive conditions
Sensitive to load, DO, pH/temperature, nutrients and toxicity; usually faster-growing than nitrifiers
Field evidence
BOD/soluble-COD removal, OUR/SOUR, F/M and floc activity

Hydrolysers and extracellular enzymes

Main function
Convert particles and polymers to molecules cells can take up
Sensitive conditions
Low temperature, toxicity or insufficient contact leaves slow substrate behind
Field evidence
Particulate/soluble COD fractions, respiration curves and supernatant change

EPS and floc formers

Main function
Build the adhesive matrix, capture particles and create settleable aggregates
Sensitive conditions
High F/M, nutrient imbalance, shear or excessive filaments degrade settling
Field evidence
Floc form, SVI, supernatant turbidity, effluent TSS and microscopy

Protozoa and metazoa

Main function
Graze dispersed bacteria, aid clarification and indicate community maturity
Sensitive conditions
Toxicity, low oxygen, shock load and extreme SRT shift populations
Field evidence
Interpret microscopy with DO, load, SRT and effluent—not alone

Organic removal does not equal complete nitrogen and phosphorus removal. Biomass assimilates some nutrients, but stable total-N and total-P removal needs nitrification, denitrification, biological phosphorus removal, chemical routes or sludge wasting with explicit mass paths.

Track three evidence groups

Load and reaction conditions

Influent COD/BOD and flow, F/M, DO, pH, temperature, ammonia, alkalinity and nutrient balance describe food and environment.

Biomass and SRT

MLSS/MLVSS, RAS/WAS flow and concentration, SRT and OUR/SOUR show how much active sludge is retained and its state.

Settling and effluent

SVI/settling curve, blanket, supernatant turbidity, effluent TSS/COD/BOD/ammonia and microscopy test both reaction and separation.

Where should common abnormalities lead?

Signal combination
Soluble COD/BOD rises while DO is low
First suspicion
Organic shock load, insufficient oxygen/mixing or toxicity
Next action
Check flow/load, airflow and DO profile, OUR, pH/temperature and industrial discharge events
Signal combination
Reactor removal is reasonable but effluent TSS/turbidity rises
First suspicion
Clarifier solids/hydraulic load, dispersed floc, bulking or equipment fault
Next action
Check blanket, SVI/curve, RAS, surface load and microscopy; separate reaction from separation
Signal combination
BOD/COD removal is normal but ammonia rises
First suspicion
Insufficient nitrifier SRT, low temperature, low DO/alkalinity or inhibition
Next action
Check SRT, temperature, zone DO, pH/alkalinity and ammonia/nitrite trends
Signal combination
SVI is high, blanket rises and filaments dominate
First suspicion
Filamentous bulking selected by low DO, low F/M, nutrient deficit or specific substrates
Next action
Identify conditions and filament type before changing selection pressure; do not rely on chemical settling aid alone

Four misconceptions

More air always makes microbes work faster

Beyond oxygen and transfer demand, extra air adds energy and shear and may damage anoxic zones. Control to load and DO.

Higher MLSS always means more capacity

High MLSS may be old, oxygen-transfer-limited and overload the clarifier; activity, SRT, load and separation govern capacity.

A clearer aeration basin is better

An aeration basin should contain mixed-liquor solids; unusual clarity may mean washout, failed return or insufficient biomass.

Microbes make pollution disappear

Carbon becomes CO₂, cells and waste sludge; nitrogen and phosphorus also need explicit destinations and mass balance.

Continue learning

Next guideHow is ammonia converted step by step into nitrate?Biological treatmentWhy is denitrification needed to remove total nitrogen?Biological treatmentHow do microorganisms release and take up phosphorus?