Activated Sludge Process Operations

Vocabulary flashcards summarizing key terms, bacterial groups, operational parameters, and problem conditions related to activated-sludge wastewater treatment, based on IntEnSE Wastewater Engineering module notes.

Activated Sludge Process

Suspended-growth biological treatment system in which microorganisms (mixed liquor) degrade wastewater contaminants and are settled and recycled.

Substrate (Wastewater Biology)

The organic or inorganic food source consumed by microorganisms during treatment.

Optimum pH for Most Bacteria

Near neutral, typically 6.8 – 7.2; growth is inhibited below 4 or above 9.5.

Acidophiles

Bacteria that thrive at pH < 5.4 (e.g., Thiobacillus, Sulfolobus).

Neutrophiles

Bacteria growing best at pH 5.4 – 8.5; most organisms in sewage treatment plants.

Alkalinophiles

Bacteria preferring pH 7 – 11.5 (e.g., Nitrosomonas, Nitrobacter).

Aerobes

Microorganisms that require free molecular oxygen to degrade substrates.

Anaerobes

Microorganisms that degrade substrates without free molecular oxygen; often use sulfate or CO₂ as electron acceptors.

Facultative Anaerobes

Bacteria that can use oxygen when present or switch to alternate electron acceptors when oxygen is absent (e.g., Bacillus, Escherichia).

Floc-Forming Bacteria

Organisms that produce pili, polysaccharides, and PHB enabling them to agglutinate and form compact flocs.

Filamentous Bacteria

Thread-like organisms providing a backbone to flocs; excessive growth causes bulking (e.g., Sphaerotilus natans, Thiothrix).

Gliding Bacteria

Motile filamentous organisms that slide along surfaces, impacting settleability (e.g., Beggiatoa).

Sheath Bacteria

Chains of cells enclosed in a transparent tube; produce motile swarmer cells when sheath breaks (e.g., Haliscomenobacter hydrossis).

Denitrifying Bacteria

Facultative organisms that use nitrate as oxygen source under anoxic conditions, converting NO₃⁻ to N₂ and N₂O.

Nitrifying Bacteria

Strict aerobes that oxidize NH₄⁺ → NO₂⁻ (Nitrosomonas/Nitrosospira) and NO₂⁻ → NO₃⁻ (Nitrobacter/Nitrospira).

Poly-P (PAO) Bacteria

Phosphorus-accumulating organisms that remove excess orthophosphate when cycled through anaerobic and aerobic zones.

Saprophytic Bacteria

Organotrophs feeding on dead organic matter; many aid floc formation.

Sulfur-Oxidizing Bacteria

Organisms that gain energy by adding oxygen to reduced sulfur compounds (e.g., Thiobacillus, Beggiatoa).

Sulfur-Reducing Bacteria

Anaerobes that use sulfate to degrade substrates, producing sulfide (e.g., Desulfovibrio).

Oxygen Uptake Rate (OUR)

Rate at which mixed liquor consumes oxygen, expressed as mg O₂ / L-min; indicates biological activity and loading.

Specific Oxygen Uptake Rate (SOUR)

OUR normalized to biomass (mg O₂ / g MLVSS-hr); used to judge sludge age, toxicity, and process health.

Mixed Liquor Suspended Solids (MLSS)

Concentration of total suspended solids in aeration tank mixed liquor.

Mixed Liquor Volatile Suspended Solids (MLVSS)

Organic fraction of MLSS representing active biomass.

Food-to-Microorganism Ratio (F/M)

Mass of influent BOD load divided by mass of biomass; influences growth, floc quality, and filament dominance.

Mean Cell Residence Time (MCRT or SRT)

Average time biomass remains in the system; governs sludge age and nitrification capability.

Return Activated Sludge (RAS)

Settled biomass returned from secondary clarifier to aeration tank to maintain concentration.

Waste Activated Sludge (WAS)

Portion of settled biomass intentionally removed to control sludge age and prevent bulking.

Floc Formation

Natural aggregation of bacteria into settleable particles; relies on floc-formers and limited filament support.

Bulking Sludge

MLSS with poor settling/compaction leading to high effluent solids; caused by filamentous or viscous mechanisms.

Filamentous Bulking

Predominant bulking type due to excess filamentous growth under low DO, low F/M, nutrient deficiency, or low pH.

Viscous Bulking

Bulking from excess extracellular biopolymer producing slimy, water-retentive sludge; linked to nutrient limits or high COD.

Settleometer

2-L graduated cylinder used to measure sludge settleability and calculate sludge volume index.

Sludge Volume Index (SVI)

Volume in mL occupied by 1 g MLSS after 30 min settling; indicator of sludge settleability.

Rising Sludge

Settled sludge that floats due to gas (typically N₂ from denitrification) formed in clarifier blanket.

Pin-Point Floc

Small, dense particles from over-aged sludge leading to turbid effluent and low SVI.

Ashing

Fine gray-white particles on clarifier surface caused by over-oxidized mixed liquor shearing flocs.

Straggler Floc

Light, fluffy, buoyant solids indicating very young sludge and low MLSS.

Selector (Process)

Aerobic, anoxic, or anaerobic zones placed before aeration basin to favor floc-formers over filamentous organisms.

Nocardia Foam

Thick, dark brown stable foam formed by hydrophobic Nocardia filaments at high MCRT and low F/M.

Microthrix parvicella

Filamentous bacterium with hydrophobic surface causing extensive foaming; thin filaments extend from flocs.

Foaming/Frothing

Accumulation of stable foam on aeration tank surface due to surfactants, high MLSS, or filamentous bacteria.

Selector Toxicants (Filament Control)

Agents such as chlorine or H₂O₂ dosed to RAS or aeration zones to suppress filamentous bacteria.

Volatile Fatty Acids (VFA)

≤ C6 fatty acids produced anaerobically; excess in influent can promote Thiothrix filament growth.

Ozonation (Viscous Bulking Control)

Addition of ozone (~1 g O₃/kg-day VSS) to collapse zoogleal slime and restore compact flocs.

Hydrogen Sulfide (H₂S)

Toxic, odorous gas whose production increases under low pH, reducing enzymatic activity and causing bulking.

Sludge Age Classifications

Young sludge: high SOUR, low MLSS; Old sludge: low SOUR, high MLSS, possible pin-floc or ashing.