5.3 Activated Sludge Fundamentals: Aerobic Microbiology, DO Control & Biomass Growth

Activated Sludge Fundamentals: Aerobic Microbiology, DO Control & Biomass Growth

The activated sludge process is the global standard for secondary biological wastewater treatment. Originally developed by Edward Arden and W.T. Lockett in Manchester, England in 1914, it utilizes an engineered, aerobic, suspended-growth culture of microorganisms (mixed liquor) to metabolize dissolved, colloidal, and non-settleable organic pollutants. The active biomass converts biochemical oxygen demand (BOD) into stable inorganic end-products ($CO_2$, $H_2O$) and high-density, bio-flocculated cellular masses that settle readily in secondary clarifiers.

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1. Biological Oxidation and Heterotrophic Metabolism

Biological treatment transforms organic carbon into harmless end-products through two concurrent metabolic pathways catalyzed by heterotrophic bacteria:

                                    +------------------------------------------------+
                                    |       HETEROTROPHIC BACTERIAL METABOLISM       |
                                    +------------------------------------------------+
                                                            |
                            +-------------------------------+-------------------------------+
                            |                                                               |
                            v                                                               v
           [ 1. CATABOLISM (Energy Production) ]                           [ 2. ANABOLISM (Cell Synthesis) ]
           Organics (BOD) + O2                                             Organics (BOD) + NH3 + H2PO4- + O2 + Energy
                  v                                                               v
           CO2 + H2O + Energy (ATP)                                        New Bacterial Cell Tissue (C5H7NO2) + H2O

Stoichiometry and Cell Empirical Formula

Bacterial cellular protoplasm is empirically represented by the chemical formula $\text{C}_5\text{H}_7\text{NO}_2$ (or $\text{C}_5\text{H}_7\text{NO}2\text{P}{0.074}$ when phosphorus is included):

$\text{Organics (BOD)} + \text{O}_2 + \text{Nutrients (N, P)} \xrightarrow{\text{Bacteria}} \text{C}_5\text{H}_7\text{NO}_2 + \text{CO}_2 + \text{H}_2\text{O}$

1. Catabolism (Respiration): Heterotrophic bacteria oxidize approximately 60% of incoming soluble BOD to generate ATP energy, releasing carbon dioxide and water.
2. Anabolism (Synthesis): Heterotrophs utilize the remaining 40% of metabolized BOD along with inorganic nitrogen (ammonia / $NH_4^+$) and phosphorus (orthophosphate / $H_2PO_4^-$) to synthesize new microbial cells (sludge yield).
3. Nutrient Stoichiometry: To sustain unrestricted aerobic synthesis without triggering nutrient-deficient bulking, wastewater must maintain a minimum stoichiometric nutrient ratio:

$\text{BOD}_5 : \text{Nitrogen (N)} : \text{Phosphorus (P)} = 100 : 5 : 1$

If raw wastewater is deficient in nitrogen or phosphorus (common in industrial or food processing discharges), bacterial synthesis is crippled, leading to the overproduction of non-filamentous zoogloeal slime or severe filamentous overgrowth.

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2. Microscopic Evaluation and Indicator Organisms

While bacteria perform the vast majority of biochemical oxidation, they are too small ($0.5\text{ to }2.0\ \mu\text{m}$) to identify individually under standard phase-contrast brightfield microscopy (100x to 400x). Fortunately, single-celled protozoa ($10\text{ to }200\ \mu\text{m}$) and multicellular metazoa ($0.1\text{ to }1.0\text{ mm}$) act as living barometers of process health, sludge age, and environmental stability.

+---------------------------------------------------------------------------------------------+
|                  MICROBIOLOGICAL SUCCESSION ACROSS INCREASING SLUDGE AGE                    |
+---------------------------------------------------------------------------------------------+
| Organism Class   | Representative Genera      | Sludge Condition & MCRT | Effluent Clarity  |
+------------------+----------------------------+-------------------------+-------------------+
| Amoebae          | Arcella, Chaos, Amoeba     | Very Young (MCRT < 2 d) | Turbid, high      |
|                  |                            | High F/M, low DO, shock | dispersed solids  |
| Flagellates      | Bodo, Euglena, Monas       | Young (MCRT 1 to 3 d)   | Cloudy, straggler |
|                  |                            | High organic loading    | floc carryover    |
| Free-Swimming    | Paramecium, Colpidium,     | Transitional (MCRT 3-6d)| Moderate clarity, |
| Ciliates         | Lionotus, Chilodonella     | Moderate F/M            | active grazing    |
| Stalked Ciliates | Vorticella, Carchesium,    | Optimal (MCRT 6 to 15 d)| Crystal-clear,    |
|                  | Opercularia, Epistylis     | Mature, healthy floc    | tight compact floc|
| Rotifers &       | Philodina, Rotaria,        | Old Sludge (MCRT > 15 d)| Fast settling,    |
| Nematodes        | Nematode worms, Tardigrades| Low F/M, high MCRT      | pin floc, ashing  |
+---------------------------------------------------------------------------------------------+

Indicator Roles and Ecological Functions

1. Amoebae and Flagellates: Possess primitive motility and feed on dissolved organic matter and free-swimming dispersed bacteria. They dominate during plant startups, organic shock overloads, or recovery from toxic spills.
2. Free-Swimming and Crawling Ciliates: Cilia propel the organism or allow it to crawl over floc surfaces. They actively consume single, un-flocculated bacteria, clarifying the water column.
3. Stalked Ciliates: Anchor their contractile stalks directly into the floc matrix, creating localized water vortices with head cilia to ingest free-swimming bacteria. Their dominance indicates a stable, well-flocculated, mature mixed liquor with excellent settling characteristics.
4. Rotifers and Nematodes: Multicellular organisms with specialized digestive tracts and grinding mastax organs. They ingest large, dense floc fragments and mineral debris. Their presence signifies an old sludge age (long MCRT, low F/M) and complete nitrification.

Floc Structure and Extracellular Polymeric Substances (EPS)

A healthy activated sludge floc consists of thousands of bacterial colonies held together by a natural polymer matrix known as Extracellular Polymeric Substances (EPS) (composed of polysaccharides, glycoproteins, lipids, and extracellular DNA). EPS gives floc its sticky, cohesive property (bio-sorption), allowing it to entrap colloidal particles in seconds. Filamentous bacteria act as an internal structural backbone; however, excessive filament extension causes bulking.

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3. The Bacterial Growth Curve

When a bacterial population is introduced to organic substrate in a batch or plug-flow biological reactor, growth progresses through four classical physiological phases:

   Biomass Concentration & Substrate Dynamics:
   
   Biomass Mass (X)  ^                 /---\ [ 3. Declining Growth ] (Target for Conventional)
                     |                /     \ 
                     |      [ 2. Log]/       \------\ [ 4. Endogenous Respiration ]
                     |        Growth/                  \ (Target for Extended Aeration)
                     |   [1. Lag]  /                    \
                     +-----------+------------------------------------> Time
                     | Substrate (S) \_ _ _ _
                     |                       \_ _ _ _ _ _ _ _ _ _ _ _

1. Lag Phase: Microorganisms acclimate to new environmental conditions, synthesizing required catabolic enzymes without significant cell division.
2. Log (Exponential) Growth Phase: Food is in vast excess ($F/M \gg 1.0$). Cell division occurs at the maximum specific growth rate ($\mu_{\text{max}}$). Bacteria remain as dispersed individual cells; EPS production is low, resulting in zero bio-flocculation and non-settleable mixed liquor.
3. Declining Growth Phase: Available food becomes rate-limiting ($F/M = 0.2\text{ to }0.5$). Growth slows as bacteria compete for substrate. Microbes secrete dense EPS sticky sheaths, initiating massive bio-flocculation. Conventional activated sludge facilities operate in this phase to achieve maximum BOD removal and excellent settling.
4. Endogenous Respiration Phase: Food is virtually exhausted ($F/M < 0.1$). Bacteria catabolize their own internal cellular reserves and lyse dead cell mass for energy (auto-oxidation). Sludge yield is minimal, and settling is rapid, though over-aeration can produce sheared pin floc. Extended aeration plants and oxidation ditches operate in this phase.

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4. Dissolved Oxygen (DO) Control Targets

Dissolved oxygen is the terminal electron acceptor that drives aerobic heterotrophic respiration and autotrophic nitrification.

+-------------------------------------------------------------------------------------+
|                   AERATION BASIN DISSOLVED OXYGEN (DO) TARGETS                      |
|                                                                                     |
|  Conventional Carbonaceous BOD Removal:     1.5 to 2.5 mg/L                         |
|  Biological Nitrification Systems:          2.0 to 3.0 mg/L                         |
|  Critical Minimum Threshold (Basin Outlet): 1.5 mg/L                                |
|  Upper Operational Limit (Over-Aeration):   > 3.5 to 4.0 mg/L                       |
+-------------------------------------------------------------------------------------+

Operational Consequences of DO Extremes

• Under-Aeration ($DO < 1.0\text{ mg/L}$): Severely retards metabolic rates; halts autotrophic nitrification; creates anoxic micro-zones that favor low-DO filamentous organisms (Sphaerotilus natans, Type 1701, Haliscomenobacter hydrossis); turns mixed liquor dark brown/black with putrid sulfide odors.
• Over-Aeration ($DO > 4.0\text{ mg/L}$): Wastes immense electrical blower energy (which accounts for 50% to 65% of a municipal plant's total power budget); creates excessive hydraulic turbulence that physically shears fragile floc into non-settleable pin floc; carries high DO into secondary clarifiers, inhibiting denitrification and risking rising sludge.

Monitoring Respiration: OUR and SOUR

• Oxygen Uptake Rate (OUR): Quantifies the volume of oxygen consumed by mixed liquor over time, expressed in $\text{mg } O_2/\text{L/hour}$.
• Specific Oxygen Uptake Rate (SOUR / Respiration Rate): Normalizes OUR to the volatile biomass concentration:

$\text{SOUR} = \frac{\text{OUR (mg } O_2/\text{L/hr}) \times 1,000}{\text{MLVSS (mg/L)}} = \text{mg } O_2/\text{g MLVSS/hour}$

Healthy conventional activated sludge exhibits a SOUR between 12 and 20 mg $O_2$/g MLVSS/hr. A SOUR $< 8$ indicates old/endogenous sludge or toxic inhibition; a SOUR $> 30$ indicates young sludge with high organic loading.

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5. Aeration Equipment and Oxygen Transfer Efficiency

Aeration systems must supply adequate molecular oxygen while providing sufficient basin mixing velocity ($> 1.0\text{ ft/s}$) to keep mixed liquor solids suspended.

+-------------------------------------------------------------------------------------+
|                       AERATION SYSTEM EFFICIENCY COMPARISON                         |
+-------------------------------------------------------------------------------------+
| Aerator Class       | Bubble Size / Action      | SOTE (Clean Water) | Wire Transfer|
+---------------------+---------------------------+--------------------+--------------+
| Fine Bubble Membrane| 1 to 2 mm fine bubbles    | 20% to 35%         | 6.0 to 8.5   |
| (EPDM / Ceramic)    | from floor grid           |                    | lb O2/hp-hr  |
| Coarse Bubble       | 6 to 12 mm large bubbles  | 8% to 15%          | 2.0 to 3.5   |
| Diffusers           | from open orifice tubes   |                    | lb O2/hp-hr  |
| Mechanical Surface  | High-speed / low-speed    | 1.5 to 2.5 lb O2   | 2.0 to 3.2   |
| Aerators            | surface impeller splash   | per hp-hr (field)  | lb O2/hp-hr  |
+-------------------------------------------------------------------------------------+

• Fine Bubble Diffusers: Maximum surface-area-to-volume ratio creates high Standard Oxygen Transfer Efficiency (SOTE). However, diffusers are subject to fouling by calcium carbonate scaling, biological slime, or iron precipitates, requiring periodic cleaning with formic acid gas injection or liquid acid washing.
• Coarse Bubble Diffusers: Provide high turbulence and non-clogging reliability but require double the air volume to deliver equivalent dissolved oxygen mass.

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6. Mixed Liquor Parameters: MLSS and MLVSS

The total biological inventory within the aeration basin is monitored via gravimetric laboratory testing:

• Mixed Liquor Suspended Solids (MLSS): The total concentration of all suspended material in the aeration tank liquid, encompassing living bacteria, dead cell debris, inert mineral silts, and non-biodegradable particles. Conventional activated sludge operates at 1,500 to 3,500 mg/L (extended aeration: 3,000 to 5,000 mg/L; membrane bioreactors: 8,000 to 12,000 mg/L).
• Mixed Liquor Volatile Suspended Solids (MLVSS): The fraction of MLSS that volatilizes (burns off) when ignited in a muffle furnace at 550°C ± 50°C. MLVSS represents the active organic biological fraction and typically constitutes 70% to 85% of MLSS in healthy municipal plants.
• Fixed Solids (Inert Fraction): The remaining 15% to 30% inorganic ash. If MLVSS drops below 65%, it indicates excessive accumulation of inert fine grit, silt, or chemical precipitates requiring increased sludge wasting.