Activated Sludge Process Operations

Microbial Growth Factors in Activated-Sludge Systems

• Growth and treatment efficiency depend on nutritional and physical variables.
  • Nutritional: availability of substrate (food) and macro/micronutrients (N, P, trace metals).
  • Physical: $\text{pH}$, temperature, availability of free molecular oxygen, mixing/shear, SRT.

pH

• Most treatment plants operate best at a near-neutral range $6.8\;\text{–}\;7.2$.
• General bacterial limits: no growth below $4$ or above $9.5$; growth sharply declines at $\pm 1$ unit around the optimum.
• Low \text{pH}<6.8 ➜
  • ↓ enzymatic activity.
  • ↑ $\text{H}_2\text{S}$ formation (odor & corrosion).
  • Inhibits nitrification; flocs fail to knit; filamentous fungi & Nocardia out-compete.
• High \text{pH}>7.2 ➜
  • ↓ enzymatic activity.
  • ↑ free $\text{NH}_3$ toxicity.
  • Nitrification inhibition & floc disruption.

pH Classifications

pH Shift by Biological Activity

• Denitrifiers release $\text{OH}^-$ ➜ raise pH.
• Fermenters produce fatty acids ➜ lower pH (anaerobic digesters).
• Methanogens use fatty acids ➜ raise pH.
• Nitrifiers destroy alkalinity ➜ lower pH in aeration tanks.
• Organotrophs form $\text{H}<em>2\text{CO}</em>3$ ➜ lower pH.

Temperature

• Impacts:
  1. Diffusion rate of substrates/nutrients into cells.
  2. Enzymatic reaction velocity.
• Optimum varies by population (≈ $20\;–\;35\,^{\circ}\text{C}$ for most municipal STPs).

Oxygen Regimes & Bacterial Types

• Aerobes: require $\text{O}_2$; e.g., Zoogloea ramigera, Nitrosomonas, Nitrobacter.
• Anaerobes: exclude $\text{O}_2$; e.g., sulfate-reducing & methane-formers.
• Facultative anaerobes: flexible; e.g., Bacillus, Escherichia, Pseudomonas.

Bacterial Functional Groups in Activated Sludge

• Filamentous bacteria (≈ 30 species): provide backbone but excess ➜ bulking.
  • Key taxa: Sphaerotilus natans, Haliscomenobacter hydrossis, Microthrix parvicella, Beggiatoa, Thiothrix, types 0041, 0092, 0675, 1701, 021N, 0914, 1851.
• Floc-forming bacteria: initiate & maintain biofloc; e.g., Achromobacter, Pseudomonas, Zoogloea.
• Denitrifiers: facultative anaerobes reducing $\text{NO}<em>3^- \rightarrow \text{N}</em>2,\, \text{N}_2\text{O}$; cause clumping and digester foaming.
• Nitrifiers (strict aerobes):
  • Stage 1: Nitrosomonas, Nitrosospira convert $\text{NH}<em>4^+ \rightarrow \text{NO}</em>2^-$.
  • Stage 2: Nitrobacter, Nitrospira convert $\text{NO}<em>2^- \rightarrow \text{NO}</em>3^-$.
• Poly-P / PAO: luxury uptake of orthophosphate under anaerobic⇆aerobic cycling; genera include Acinetobacter, Klebsiella.
• Sulfur oxidizers: add $\text{O}_2$ to reduced S compounds; non-filamentous (Thiobacillus, Thiospirillopsis, Thiovulum) & filamentous (Beggiatoa, Thiothrix).
• Sulfur reducers: anaerobic use of sulfate; Desulfovibrio, Desulfotomaculum.
• Saprophytes: degrade dead biomass; many are floc formers.
• Sheathed bacteria: chains enclosed by sheath; when sheath breaks, motile swarmers released (S. natans, H. hydrossis).

Key Operational Parameter – Oxygen Uptake

• OUR (Oxygen Uptake Rate): mg $\text{O}_2$ consumed per L per min (or hr).
  • Reflects biological activity & loading.
• SOUR (Specific OUR): $\text{OUR}/\text{MLVSS}$ ➜ mg $\text{O}_2$ g$^{-1}$ MLVSS hr$^{-1}$.
  • High SOUR ➜ young sludge / high F:M.
  • Low SOUR (<1.5 mg/L hr in aerobic digesters) ➜ stabilized sludge.
  • ≈ Toxicants ➜ $\text{SOUR}\rightarrow0$.

OUR / SOUR Test Procedure

1. Grab fresh mixed liquor sample.
2. Shake in closed, partially filled container to air-saturate.
3. Transfer to BOD bottle; insert DO probe.
4. Record DO drop for $10\,\text{min}$.
5. $\text{OUR}= \dfrac{\text{DO}<em>{0}-\text{DO}</em>{10}}{10}\times60$ (mg/L·hr).
6. $\text{SOUR}= \dfrac{\text{OUR}}{\text{MLSS (g/L)}}$.

Floc Formation – Success Indicator

• Occurs only in aerobic suspended-growth; absent in anaerobic digesters.
• Initiated by floc formers as MCRT (SRT) increases.
• Essential cellular materials:
  1. Pili/fibrils (protein micro-hairs).
  2. Extracellular polysaccharides.
  3. Poly-β-hydroxybutyrate (PHB) or starch inclusions.
• Filaments provide internal backbone, giving strength against shear.
• Balanced ratio filaments:floc-formers ➜ dense, compact, shear-resistant flocs that settle well.

Operational Problems & Troubleshooting

Bulking Sludge

• Definition: MLSS exhibits poor settling & compaction, elevating effluent TSS.
• Primary causative factors: low DO, unfavorable F:M, nutrient deficiency, pH <$6.5$, high organic load.
• Two main types: ### A. Filamentous Bulking
  • Predominant form.
  • Filamentous organisms protrude from flocs preventing compaction.
  • Favored by: low DO (<$0.5\,\text{mg/L}$), low F:M (high SRT, complete-mix CMAS), nutrient limits, septicity, sulfide/VFA, reduced S compounds.
  • Species–condition links:
    • S. natans, H. hydrossis ➜ low DO.
    • Microthrix parvicella, Types 0041/0092/0675 ➜ low F:M.
    • Beggiatoa, Thiothrix ➜ sulfide/VFA, reduced S.
  • Control workflow:
    • Microscopic ID (phase contrast ≥1000×).
    • Rapid, non-specific: raise RAS rate; selective chlorination/hydrogen peroxide dosing in RAS, ML, or sidestream.
    • Slow, specific: create aerobic/anoxic/anaerobic selectors; shift feed point; adjust aeration, nutrients, F:M.
    ### B. Viscous (Hydrous) Bulking
  • Excess extracellular biopolymers (zooglea) ➜ slimy, jelly-like sludge retaining water.
  • Typical drivers: nutrient limitation, very high organic load (high COD, VFA), low DO, toxic metals (Cr, sulfide).
  • Indicators: thick slime layers, hydrophilic floc, foaming under intense aeration.
  • Controls: ozone oxidation (≈ $1\,\text{g}\,\text{O}_3\,/\,\text{kg-day}\,\text{VSS}$), cationic polymers/minerals, restore N/P balance, adjust loading.

Rising Sludge (Denitrification in Clarifier)

• Mechanism: $\text{NO}<em>3^-$ → $\text{N}</em>2$ gas inside sludge blanket (critical $6!–!8\,\text{mg NO}_3\text{-N/L}$ @ $20^{\circ}\text{C}$).
• Symptoms: blanket lifts, large clumps burst at surface.
• Mitigation: perform anoxic denitrification upstream, increase RAS withdrawal, cut aeration liquor flow, shorten SRT, speed scraper.

Pin-Point Floc / Ashing / Straggler Floc

• Pin-point: old, over-oxidised sludge; very small dense particles; turbid effluent, low SVI.
• Ashing: gray-white fine solids overflow due to floc shear (over-aeration) & early denitrification.
• Straggler: light fluffy rising particles; young sludge (low MLSS, low SRT).

Foaming / Frothing

• Persistent surface foam in aeration tank; depth up to $2!–!6\,\text{ft}$.
• Contributors: surfactants, low nutrients, recycled solids, over-aeration, polymer overdose, but most problematic from hydrophobic filamentous actinomycetes:
  • Nocardia (Gordona amarae, Rhodococcus, Tsukamurella) – short filaments embedded in floc; favored by high MCRT >$10$ days, low F:M (<$0.05$), high oil/grease, high $\text{pH}>8$ or low <6.5, low DO.
  • Microthrix parvicella – thin protruding filaments, thrives on long-chain fatty acids.
• Foam appearance diagnosis:
  1. Fresh crisp white: normal, no action.
  2. White billowing (soap-suds): very young sludge – decrease wasting.
  3. Thick greasy dark tan: old sludge – increase wasting, lower RAS.
• Control methods:
  • Avoid trapping/recycling foam; skim & waste.
  • Reduce air rate during low flow.
  • Adjust SRT/MLSS; keep F:M moderate.
  • Spray chlorine/calcium hypochlorite or dose polymer antifoams.
  • Pre-treat high FOG discharges; impose grease-trap maintenance.

Testing & Monitoring Tools

• Settleometer (2000 mL, graduated mL/L):
  • Grab sample below scum; fill to $1000$ mL; record settled volume every $5$ min for $30$ min.
  • Parallel 50 % effluent-diluted test discriminates filamentous bulking (both settle the same if filaments dominate).
  • Observe four settling phases: flocculation → blanket → settling → compaction.
• Microscopy (phase contrast ×1000): essential for identifying filament types, zooglea, actinomycetes, nitrifiers.
• Routine parameters: DO (>$2$ mg/L), SVI, F:M, MCRT, temperature, pH, nutrients, SOUR.

Control Strategy Checklist

1. Verify influent characteristics – nutrients, toxicants, FOG, VFA, sulfide.
2. Maintain aeration to keep DO ≥$2$ mg/L$(aerobic zones).$
3. Balance SRT and F:M; avoid very low (<$0.05$) or very high ratios.
4. Configure reactors in series (anaerobic → anoxic → aerobic selectors) to favor floc formers.
5. Adjust RAS/WAS rates; prevent clarifier solids hold-up.
6. Monitor clarifier operation – blanket depth, scraper speed, detention time.
7. Apply chemical/biological aids (chlorine, $\text{H}<em>2\text{O}</em>2$, polymers, ozone) only after root-cause analysis.
8. Establish preventive maintenance for grease control and load equalisation.

These bullet-point notes encapsulate all major and minor concepts, biological mechanisms, operational metrics, examples, and remedial actions discussed in the provided transcript for thorough exam preparation on wastewater activated-sludge process operations.