1. What is the Streeter-Phelps Model primarily used for?
It is a **mathematical model** used to predict the profile of **Dissolved Oxygen (DO)** concentration in a river over distance, typically after a point source of organic pollution is introduced.
2. What is BOD and why is it important in this model?
**BOD (Biochemical Oxygen Demand)** is the amount of oxygen consumed by microorganisms (bacteria) while they decompose biodegradable organic matter. It represents the pollution load that causes the DO sag.
3. What are the two main opposing processes described by the model?
The two processes are **Deoxygenation** (oxygen consumption due to BOD decay) and **Reoxygenation** (oxygen transfer from the atmosphere into the water).
4. What is the Critical Point (X_c) on the sag curve?
The Critical Point is the location in the river where the **Dissolved Oxygen (DO) concentration reaches its minimum value**, and the oxygen deficit is at its maximum.
5. What does the Deoxygenation Rate Constant (K) represent?
**K** (or **K_d**) is the rate at which organic matter is decomposed by bacteria, which directly controls the rate of oxygen depletion in the water. It is typically expressed in **day⁻¹**.
6. What does the Reoxygenation Rate Constant (R) represent?
**R** (or **K_r**) is the rate at which oxygen is naturally absorbed from the atmosphere back into the river water. It is influenced by turbulence and surface area.
7. What is the Self-Purification Factor (F_s)?
The Self-Purification Factor is the ratio of the Reoxygenation rate to the Deoxygenation rate (**R/K**). A higher value indicates a greater capacity for the river to recover quickly.
8. How does an increase in temperature affect the sag curve?
Higher temperatures **increase both K and R** but usually increase K more significantly, leading to a faster and lower critical DO point (the minimum DO will be lower and occur sooner/closer to the discharge point).
9. What is DO Saturation (C_s)?
DO Saturation is the **maximum concentration of Dissolved Oxygen** that water can hold at a given temperature and pressure. Colder water can hold more oxygen.
10. What is the Initial Deficit (D₀)?
**D₀** is the difference between the Saturation DO (**C_s**) and the DO of the river mixture (**DO_mix**) immediately after the pollution discharge: **D₀ = C_s – DO_mix**.
11. What is Ultimate BOD (L₀)?
Ultimate BOD (**L₀**) represents the **total amount of oxygen** that will eventually be consumed by the organic matter in the river mixture over an infinite period.
12. Why is the critical DO value important?
The critical DO is the **lowest oxygen level** and is used to determine the necessary level of wastewater treatment to protect aquatic life, as this is the point of greatest stress.
13. What DO concentration is generally considered required for sensitive cold-water fish?
Sensitive fish species, like trout, generally require DO levels of **6.5 mg/L or higher** to thrive without stress.
14. What is Hypoxia in a river context?
Hypoxia is a condition of **severe oxygen depletion**, usually defined by DO concentrations below **2-3 mg/L**, which can lead to large-scale fish kills and the death of aquatic organisms.
15. How does a faster stream velocity affect the critical distance (X_c)?
Higher velocity means the water travels faster, so for the same critical time (**t_c**), the critical distance (**X_c**) will be **longer** (further downstream).
16. How does increasing the pollution’s BOD concentration affect the sag curve?
It increases the initial ultimate BOD (**L₀**) of the mix, leading to a **greater maximum deficit (D_c)** and a **lower critical DO** concentration.
17. What happens if the Reoxygenation rate (R) equals the Deoxygenation rate (K)?
If **R = K**, the equation for **t_c** is undefined. Mathematically, the critical deficit occurs at infinite time, meaning the initial deficit (**D₀**) would be maintained until the BOD is exhausted.
18. What is the relationship between Critical Time (t_c) and Critical Distance (X_c)?
The critical distance is the product of the average stream velocity (**v**) and the critical time (**t_c**): **X_c = v x t_c**.
19. How do we account for the changing temperature of the river water in the model?
The constants **K** and **R** are adjusted from their standard 20°C values using temperature correction factors ($\theta_K$ and $\theta_R$) and the river’s mixed temperature (**T_mix**).
20. What factors influence the Reoxygenation Rate (R)?
**R** is influenced by **stream turbulence**, **water depth**, and **stream velocity**. Shallower, faster, and more turbulent streams have higher **R** values.
21. What is the Water Quality Index (WQI) and what does a score close to zero or negative indicate?
The **WQI** is a composite score reflecting river health based on parameters like Critical DO and BOD. A score of **zero or negative** indicates a river in a **severe or critical** health state, suffering from extreme oxygen depletion and high pollution stress, making it acutely hazardous for aquatic life.
22. What does an Initial DO Saturation of 72.6% imply about the water quality at the mixing point?
An initial saturation level of **72.6%** means the mixed river water is immediately **under-saturated** with oxygen. This is a crucial starting deficit that makes the river more vulnerable to the pollution’s deoxygenation effect, potentially leading to a lower critical DO minimum.
23. What is the Oxygen Deficit (27.4%) and how is it calculated?
The Oxygen Deficit is the **shortfall of oxygen** relative to the saturation limit. A value of **27.4%** means the water is carrying a debt equivalent to 27.4% of its saturation capacity, calculated as $$(C_s – DO_{mix}) / C_s$$.
24. What does a Pollution Load of 23.29 kg/s tell a water manager?
The Pollution Load quantifies the **mass rate of ultimate BOD** entering the river per unit of time (**23.29 kg/s**). This very high value is a direct measurement of the **total oxygen consumption potential** being introduced, demanding strict control measures.
25. What is the practical meaning of a Dilution Ratio of 2.50:1?
A Dilution Ratio of **2.50:1** means that for every **one part** of pollutant discharge, there are only **2.50 parts** of clean river water available for mixing. This low ratio suggests the river has **limited capacity** to dilute the pollutant effectively, contributing to high mixed BOD.
26. What does an N/A value for the Recovery Distance signify in a pollution analysis?
An **N/A** (Not Applicable) value for the Recovery Distance (the distance to reach 95% DO saturation) typically signifies that the river **never reaches that recovery goal** within the calculated distance. This usually happens when the critical DO is extremely low, and the river is severely impaired for the entire reach being modeled.