11.1 Secondary Treatment: Activated Sludge Process Control & Variations

4.3 Secondary Treatment: Activated Sludge Process Control & Variations

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Biological Principles & Microbial Ecology

In the activated sludge reactor, aerobic heterotrophic bacteria serve as the primary biochemical workhorses. They synthesize extracellular polymeric substances (EPS)—primarily polysaccharides, proteins, and lipids—that form the structural glue binding bacteria, colloidal particles, and non-settleable solids into dense, settleable biological flocs.

Protozoan & Metazoan Succession as Sludge Age Indicators

While bacteria perform the bulk of organic oxidation, microscopic examination of higher-order protozoa and metazoa provides operators with an immediate, visual diagnosis of biological health, sludge age, and effluent quality:

+-----------------------------------------------------------------------------+
|                   ACTIVATED SLUDGE MICROBIAL SUCCESSION                     |
+-----------------------------------------------------------------------------+
|  YOUNG SLUDGE / HIGH F/M     TRANSITIONAL PHASE     OLD SLUDGE / LOW F/M    |
|  - Low MCRT (< 3 days)       - MCRT: 4 - 8 days     - High MCRT (> 15 days) |
|  - Dispersed Bacteria        - Free-Swimming        - Stalked Ciliates      |
|  - Flagellates dominate        Ciliates dominate      & Suctorians          |
|  - Amoebas present           - Floc forming         - Rotifers abundant     |
|  - High Effluent Turbidity   - Moderate Turbidity   - Nematodes present     |
|                                                     - Pin Floc / Ashing     |
|  =========================> TIME / SLUDGE AGE ===========================>  |
+-----------------------------------------------------------------------------+

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Aeration Basin Operation & Dissolved Oxygen (DO)

Aerobic heterotrophs and autotrophic nitrifiers require continuous molecular oxygen to sustain metabolic respiration and cell synthesis:

• Target Dissolved Oxygen Range: $1.5 \text{ to } 2.5 \text{ mg/L}$ ($2.0 \text{ mg/L}$ optimal setpoint throughout the aeration basin).
• Consequences of Low DO ($< 1.0 \text{ mg/L}$): Low oxygen environments favor the proliferation of filamentous bacteria (such as Sphaerotilus natans and Type 021N), which possess higher surface-area-to-volume ratios and outcompete floc-forming bacteria for limited oxygen. This triggers severe filamentous sludge bulking. Incomplete BOD oxidation occurs, and autotrophic nitrification ceases entirely.
• Consequences of Excessive DO ($> 4.0 \text{ mg/L}$): Over-aeration wastes substantial electrical blower power. High shear forces from aggressive bubbling physically tear fragile flocs apart into non-settleable micro-flocs (pin floc), increasing effluent turbidity. Furthermore, high dissolved oxygen carried into anoxic zones via return streams suppresses biological denitrification.
• Aeration Technologies:
  • Fine-Bubble Diffusers: Submerged flexible EPDM membranes or porous ceramic domes. High Standard Oxygen Transfer Efficiency ($\text{SOTE} = 25\% \text{ to } 35\%$ in clean water). Deliver tiny bubbles ($1-3 \text{ mm}$) with high contact surface area.
  • Mechanical Surface Aerators: Rotating impellers or brush rotors splashing at the liquid surface. Low capital cost and resilient against clogging, but lower transfer efficiency ($\text{SOTE} = 8\% \text{ to } 12\%$) and vulnerable to aerosol dispersion and freezing in cold climates.

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Mixed Liquor Suspended Solids: MLSS vs. MLVSS

• Mixed Liquor Suspended Solids (MLSS): The total concentration of all filterable solids suspended within the aeration basin, comprising active biomass, dead microbial debris, inert mineral grit, and non-biodegradable suspended solids. Measured in $\text{mg/L}$. Typical conventional range: $1500 \text{ to } 3500 \text{ mg/L}$.
• Mixed Liquor Volatile Suspended Solids (MLVSS): The organic, combustible fraction of MLSS determined by igniting the dried sample filter in a muffle furnace at $550^\circ\text{C}$. Only the volatile fraction represents active, living biological biomass:

$\text{Volatile Fraction } = \frac{\text{MLVSS (mg/L)}}{\text{MLSS (mg/L)}} \approx 0.70 - 0.80 \quad (70\% \text{ to } 80\% \text{ in healthy plants})$

If the volatile fraction drops below $65\%$, it indicates excessive inert mineral silt, sand, or chemical precipitate accumulation in the biological system.

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Process Control Metrics & Formulas

Operators maintain biological process equilibrium through four foundational mathematical tools: $F/M$ ratio, MCRT, SVI, and solids mass balances.

1. Food-to-Microorganism ($F/M$) Ratio

The $F/M$ ratio quantifies the organic loading applied per unit mass of active biological solids per day:

$F/M = \frac{\text{lbs } \text{BOD}_5 \text{ added per day}}{\text{lbs MLVSS present under aeration}}$

$F/M = \frac{Q (\text{MGD}) \times \text{Influent BOD}_5 (\text{mg/L}) \times 8.34 \text{ lbs/gal}}{V_{\text{aer}} (\text{MG}) \times \text{MLVSS} (\text{mg/L}) \times 8.34 \text{ lbs/gal}}$

• Operational Ranges:
  • Conventional Plug-Flow / Complete Mix: $0.2 \text{ to } 0.5 \text{ lb BOD}_5 / \text{lb MLVSS}\cdot\text{day}$
  • Extended Aeration / Oxidation Ditch: $0.05 \text{ to } 0.15 \text{ lb BOD}_5 / \text{lb MLVSS}\cdot\text{day}$

2. Mean Cell Residence Time (MCRT) / Solids Retention Time (SRT) / Sludge Age

MCRT represents the average duration, in days, that a biological cell remains within the treatment system before being removed via wasting or effluent loss:

$\text{MCRT } (\text{days}) = \frac{\text{Total lbs MLSS in the aeration basin (or system)}}{\text{lbs WAS TSS wasted/day} + \text{lbs Effluent TSS lost/day}}$

$\text{MCRT} = \frac{V_{\text{aer}} (\text{MG}) \times \text{MLSS} (\text{mg/L}) \times 8.34}{(Q_{\text{WAS}} (\text{MGD}) \times \text{WAS}_{\text{TSS}} (\text{mg/L}) \times 8.34) + (Q_{\text{eff}} (\text{MGD}) \times \text{Eff}_{\text{TSS}} (\text{mg/L}) \times 8.34)}$

• Operational Ranges:
  • Conventional Activated Sludge: $5 \text{ to } 15 \text{ days}$
  • Extended Aeration Systems: $20 \text{ to } 30+ \text{ days}$

3. Sludge Volume Index (SVI)

SVI is an empirical settling parameter that reflects the compaction and settling characteristics of mixed liquor in a secondary clarifier. It is determined by conducting a 30-minute settled sludge volume test ($\text{SV}_{30}$) in a $1000 \text{ mL}$ graduated cylinder or wide-mouth Settlometer:

$\text{SVI } (\text{mL/g}) = \frac{\text{Settled Sludge Volume after 30 min } (\text{SV}_{30}, \text{ mL/L}) \times 1000}{\text{Aeration Basin MLSS } (\text{mg/L})}$

+-----------------------------------------------------------------------------+
|                        SVI OPERATIONAL INTERPRETATION                       |
+-----------------------------------------------------------------------------+
|  SVI < 80 mL/g        | Dense, granular, old sludge; rapid settling; leaves |
|                       | tiny pin flocs in supernatant (turbid effluent).    |
|-----------------------+-----------------------------------------------------|
|  SVI = 80 - 150 mL/g  | IDEAL SETTLEABILITY: Rapid, uniform sludge blanket  |
|                       | descent with crystal clear supernatant liquor.      |
|-----------------------+-----------------------------------------------------|
|  SVI > 150 - 200 mL/g | BULKING SLUDGE: Slow, fluffy settling; high blanket |
|                       | in clarifier; severe danger of solids carryover.    |
+-----------------------------------------------------------------------------+

4. Return Activated Sludge (RAS) & Waste Activated Sludge (WAS) Control

• Return Activated Sludge (RAS): Concentrated settled biological solids recycled from the bottom of secondary clarifiers back to the aeration basin inlet to maintain target MLSS inventory. Flow rate typically ranges from $25\% \text{ to } 100\%$ of plant influent flow. Mass balance equation:

$Q_{\text{RAS}} = \frac{Q \times \text{MLSS}}{\text{RAS}_{\text{TSS}} - \text{MLSS}}$

• Waste Activated Sludge (WAS): The mass of excess biomass purged daily from the system to maintain a constant target MCRT and $F/M$ ratio. WAS wasting is the primary process control handle operated by wastewater technicians.

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Activated Sludge Process Variations

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Operational Troubleshooting: Bulking, Foaming & Rising Sludge

Distinguishing between different clarifier and biological upsets is essential for choosing the proper corrective action.

+-----------------------------------------------------------------------------+
|                 DIAGNOSTIC MATRIX: SECONDARY CLARIFIER UPSETS               |
+-----------------------+-----------------------------+-----------------------+
| FILAMENTOUS BULKING   |       RISING SLUDGE         |      PIN FLOC         |
| - High SVI (> 150)    | - Normal SVI (80 - 120)     | - Low SVI (< 80)      |
| - Settles very slowly | - Settles well in cylinder, | - Settles rapidly     |
| - Blanket floats      |   floats later with gas     | - Turbid supernatant  |
| - Interlocking fibers | - N2 gas bubbles present    | - Microscopic spheres |
| - Cause: Low DO/F:M   | - Cause: Denitrification    | - Cause: Over-aerated |
+-----------------------+-----------------------------+-----------------------+

1. Filamentous Bulking

• Mechanism: Excessive proliferation of filamentous bacteria (Microthrix parvicella, Sphaerotilus natans, Type 021N, Nostocoida limicola) that extend outward from the biological floc, physically preventing floc particles from consolidating and compacting.
• Diagnostic Signs: Extremely high SVI ($> 150-250 \text{ mL/g}$); slow, uniform sludge blanket settling; clear supernatant in settlometer test; secondary clarifier blanket expands upward until overflowing weirs.
• Root Causes: Basin DO $< 1.5 \text{ mg/L}$; very low $F/M$; septic influent high in sulfides ($\text{H}_2\text{S}$); or nutrient deficiency (ratio of $\text{BOD}_5 : \text{N} : \text{P}$ below $100 : 5 : 1$).
• Emergency Control: Carefully dosed chlorination of the RAS stream at $2 \text{ to } 3 \text{ lbs } \text{Cl}_2 / 1000 \text{ lbs MLSS}\cdot\text{day}$. Chlorine selectively attacks the extended filamentous sheaths protruding into the bulk liquid while sparing the protected bacteria inside the dense floc core.

2. Rising Sludge (Clarifier Denitrification)

• Mechanism: Biological denitrification occurring in the secondary clarifier sludge blanket. When mixed liquor containing elevated nitrate ($\text{NO}_3^-$) is held under anoxic conditions in a deep sludge blanket ($> 2-4 \text{ hours}$), facultative heterotrophs strip oxygen from nitrate, releasing nitrogen gas ($\text{N}_2$):

$2\text{NO}_3^- + 10e^- + 12\text{H}^+ \rightarrow \text{N}_2\uparrow + 6\text{H}_2\text{O}$

• The insoluble $\text{N}_2$ gas bubbles attach to settled sludge flocs, floating large, consolidated sheets or clods of sludge to the surface.
• Crucial Diagnostic Distinction: The SVI test reveals normal, rapid settling ($80 \text{ to } 120 \text{ mL/g}$) and clear supernatant during the first 15–20 minutes. However, after 30 to 60 minutes of sitting in the cylinder, the entire settled sludge mass floats to the top buoyed by gas bubbles.
• Remedy: Increase the Return Activated Sludge (RAS) pumping rate to lower the clarifier blanket depth and reduce solids retention time in the clarifier; reduce aeration basin SRT if nitrification is not permit-mandated.

3. Biological Foaming Types

• Viscous Brown Nocardia Foam: Thick, greasy, chocolate-brown foam accumulating on aeration basins and secondary clarifier surfaces. Caused by actinomycetes (Nocardia, Gordonia) and Microthrix parvicella, which possess hydrophobic mycolic acid cell walls that trap air bubbles. Triggered by high MCRT ($> 10-15 \text{ days}$), warm water temperatures, and elevated influent Fats, Oils, and Grease (FOG). Controlled by aggressively lowering MCRT (increasing WAS wasting), spraying cationic polymer/chlorine sprays on the surface, and never returning skimmed foam to the plant headworks.
• White Frothy Foam: Billowy, white, soapy foam covering aeration basins. Caused by very young sludge (low MCRT, very high $F/M > 0.6$) or synthetic non-biodegradable surfactants. Controlled by decreasing WAS wasting to build mixed liquor solids inventory.