2.4 Biological Wastewater Treatment & Kinetics

Biological Wastewater Treatment & Kinetics

Biological treatment harnesses dense cultures of microorganisms to consume organic pollutants and nutrients from wastewater. The primary goal is the reduction of oxygen-demanding substances to protect receiving waters from oxygen depletion.

Biochemical Oxygen Demand (BOD) Kinetics

BOD is the standard measure of organic pollution. It quantifies the amount of dissolved oxygen consumed by microorganisms while stabilizing organic matter. The standard test is run for 5 days at $20^\circ C$ ($BOD_5$).

The reaction follows first-order kinetics:

$BOD_t = L_0 (1 - e^{-kt})$

Where:

• $BOD_t$ = Oxygen consumed at time $t$ (mg/L)
• $L_0$ = Ultimate carbonaceous BOD (total oxygen demand, mg/L)
• $k$ = BOD reaction rate constant ($day^{-1}$, base $e$)
• $t$ = Time (days)

While BOD measures biodegradable organics, Chemical Oxygen Demand (COD) uses a strong chemical oxidant (dichromate) to measure almost all organic matter. Therefore, COD is always greater than or equal to BOD.

Monod Substrate Utilization Kinetics

The growth of biomass in a reactor is limited by the availability of food (substrate). This is modeled by the Monod equation:

$\mu = \frac{\mu_{max} S}{K_s + S}$

Where:

• $\mu$ = Specific growth rate of the biomass ($day^{-1}$)
• $\mu_{max}$ = Maximum specific growth rate
• $S$ = Substrate concentration (mg/L of BOD)
• $K_s$ = Half-velocity constant (substrate concentration at which $\mu = 0.5 \mu_{max}$)

This demonstrates that at high substrate concentrations ($S \gg K_s$), growth is zero-order (maximum rate). At low concentrations ($S \ll K_s$), growth is first-order with respect to the substrate.

The Activated Sludge Process

Activated sludge is a suspended-growth process. Wastewater enters an aeration basin where mixed liquor (biomass + wastewater) is continuously aerated. The mixture then flows to a secondary clarifier, where the biomass settles. Most of the settled sludge is returned to the aeration basin (Return Activated Sludge, RAS) to maintain a high active biomass concentration, known as Mixed Liquor Suspended Solids (MLSS). A fraction is wasted (Waste Activated Sludge, WAS) to control the system age.

Key Operational Parameters

1. Mean Cell Residence Time ($\theta_c$ or SRT): Also known as Sludge Age. It is the average time a microbe spends in the system. $SRT = \frac{\text{Mass of MLSS in Aeration Basin}}{\text{Mass of Solids Wasted per Day}} = \frac{V \cdot X}{Q_w \cdot X_r + Q_{eff} \cdot X_e}$

2. Food-to-Microorganism Ratio (F/M): Represents the daily organic loading per unit of biomass. $F/M = \frac{Q \cdot S_0}{V \cdot X}$ Where $Q$ is inflow, $S_0$ is influent BOD, $V$ is basin volume, and $X$ is MLSS concentration. A low F/M ratio corresponds to a high SRT (starvation conditions, good settling). A high F/M ratio corresponds to a low SRT (rapid growth, poor settling).

3. Hydraulic Retention Time (HRT): The average time water spends in the aeration basin. $HRT = V / Q$.

Attached Growth and Advanced Technologies

• Trickling Filters: Fixed-film systems where wastewater is distributed over a bed of rocks or plastic media. Microbes form a biofilm on the media. Oxygen is supplied by natural draft.
• Moving Bed Biofilm Reactors (MBBR): Suspended plastic carriers provide surface area for biofilm growth within an aerated tank. It combines the benefits of activated sludge and fixed-film systems.
• Membrane Bioreactors (MBR): Replaces the secondary clarifier of the activated sludge process with microfiltration or ultrafiltration membranes immersed directly in the aeration basin. This allows for extremely high MLSS concentrations and a very small physical footprint, producing high-quality effluent.

Biological Nutrient Removal (BNR)

Conventional activated sludge removes carbon but not nitrogen or phosphorus. BNR requires specific environmental zones (aerobic, anoxic, and anaerobic).

Nitrogen Removal

1. Nitrification: An aerobic process where autotrophic bacteria (Nitrosomonas and Nitrobacter) oxidize toxic ammonia to nitrite, then to nitrate. $NH_4^+ + 2O_2 \rightarrow NO_3^- + 2H^+ + H_2O$ This process requires immense amounts of oxygen and destroys alkalinity, lowering the pH.

2. Denitrification: An anoxic process (no dissolved oxygen, but nitrate is present). Heterotrophic bacteria strip the oxygen from nitrate, releasing harmless nitrogen gas. $NO_3^- + \text{Organics} \rightarrow N_2(g) + CO_2 + H_2O$

Enhanced Biological Phosphorus Removal (EBPR)

EBPR involves cycling biomass through an anaerobic zone (no DO, no nitrate) followed by an aerobic zone. Under anaerobic stress, Phosphorus Accumulating Organisms (PAOs) release phosphorus to consume volatile fatty acids. When subsequently aerated, they undergo "luxury uptake," consuming much more phosphorus than they released. The phosphorus is permanently removed when the sludge is wasted.