Sewage Characteristics: Physical, Chemical, and Biological Parameters (BOD, COD, TOC Kinetics)

Wastewater characterization is essential for designing secondary biological treatment systems and predicting the impact of effluent discharge on receiving water bodies. Sewage contains a complex mixture of organic matter (carbohydrates, proteins, fats) and inorganic salts, characterized through physical, chemical, and biological parameters.

The organic strength of wastewater is primarily quantified using Biochemical Oxygen Demand (BOD), which measures the amount of dissolved oxygen required by aerobic microorganisms to biologically stabilize biodegradable organic matter at a specified temperature (typically $20^\circ\text{C}$). The first-stage carbonaceous BOD kinetics follow a first-order reaction equation:

$$\frac{dL_t}{dt} = -K \cdot L_t$$

Integrating over time $t$ yields the remaining carbonaceous organic matter ($L_t$) and the oxygen exerted ($BOD_t$):

$$L_t = L_0 \cdot e^{-K \cdot t} = L_0 \cdot 10^{-K_D \cdot t}$$

$$BOD_t = L_0 - L_t = L_0 \cdot \left(1 - 10^{-K_D \cdot t}\right)$$

Where $L_0$ is the ultimate carbonaceous BOD ($\text{BOD}_u$), $K$ is the reaction rate constant to base $e$, and $K_D$ is the deoxygenation constant to base $10$ ($K_D = \frac{K}{2.303}$). The value of $K_D$ varies with temperature according to the modified Arrhenius relationship:

$$K_{D(T^\circ\text{C})} = K_{D(20^\circ\text{C})} \cdot \theta^{(T - 20)}$$

Where $\theta = 1.047$ for temperatures between $20^\circ\text{C}$ and $30^\circ\text{C}$. Beyond 5 to 8 days, nitrifying bacteria trigger a second stage known as Nitrogenous BOD (NBOD).

In addition to BOD, organic pollution is evaluated using Chemical Oxygen Demand (COD)—which measures total biodegradable and non-biodegradable organic matter oxidized by a strong chemical oxidant ($\text{K}_2\text{Cr}_2\text{O}_7$) under acidic conditions—and Total Organic Carbon (TOC). The ratio of $\frac{\text{BOD}_5}{\text{COD}}$ serves as a crucial indicator of biodegradability: a ratio $\ge 0.6$ indicates easily treatable municipal wastewater, whereas a ratio $< 0.3$ indicates significant non-biodegradable industrial toxic inputs requiring chemical pretreatment.

Conventional Indian municipal monitoring relied heavily on wet-chemistry $5$-day BOD tests ($BOD_5$ at $20^\circ\text{C}$) and laboratory COD refluxing. These methods involve high analysis latency ($5$ days for BOD), making real-time control of secondary aeration tanks impossible during sudden industrial load dumps or seasonal dilution events.

Under strict CPCB regulations and online monitoring directives, Indian municipal and industrial Sewage Treatment Plants (STPs) are rapidly adopting Real-Time Online Continuous Effluent Monitoring Systems (OCEMS). Modern facilities utilize UV-Vis Spectrophotometric Probes and automated TOC analyzers installed directly at plant intake and outfall channels. By establishing correlated empirical calibration algorithms between optical absorbance and organic loads, utilities receive instant $BOD$, $COD$, and $TOC$ estimations, enabling automated bio-reactor aeration adjustments and emergency diversion to equalization basins before stream discharge.

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