1.3 Raw Water Characteristics & Quality Changes
Key Takeaways
- True color measures dissolved organic content after filtering out suspended matter, whereas apparent color includes both dissolved and suspended solids.
- Under anaerobic conditions in the reservoir hypolimnion, iron and manganese become chemically reduced and highly soluble in water.
- Reservoirs thermally stratify in summer into the warm epilimnion, the transition metalimnion, and the cold, anaerobic hypolimnion.
- Spring and fall lake turnovers occur when surface water reaches its maximum density at 4°C (39.2°F) and sinks, mixing the reservoir and releasing iron, manganese, and hydrogen sulfide.
Why This Topic Matters for the Exam
Water treatment operators must monitor raw water characteristics to adjust chemical dosages and maintain finished water quality. Exam questions frequently test your knowledge of physical and chemical parameters, particularly the difference between true and apparent color, and how dissolved iron and manganese behave. Crucially, you must understand reservoir thermal stratification—specifically the characteristics of the epilimnion, metalimnion, and hypolimnion—and the chemistry and water quality impacts of spring and fall lake turnovers.
Physical and Chemical Water Quality Parameters
Raw water is evaluated using physical, chemical, and biological parameters. Physical parameters describe properties that affect the appearance or feel of water:
- Turbidity: A measure of light scattering by suspended particles (silt, clay, organic matter) in Nephelometric Turbidity Units (NTU). High turbidity shields pathogens from disinfectants.
- Color: Divided into apparent color and true color. Apparent color is the color of the water containing both dissolved and suspended matter (measured directly). True color is the color remaining after suspended matter is removed by filtration or centrifugation. Color is measured in Pt-Co (Platinum-Cobalt) units.
- Taste and Odor: Often caused by organic compounds like geosmin and 2-methylisoborneol (MIB) produced by cyanobacteria, or by dissolved gases like hydrogen sulfide. The threshold odor number (TON) is used to quantify odors.
Chemical parameters dictate chemical feed requirements:
- pH: The measurement of hydrogen ion concentration. Raw water pH affects coagulation chemistry and corrosion control.
- Alkalinity: The water's capacity to neutralize acids. Adequate alkalinity is necessary for alum coagulation to function properly.
- Hardness: The concentration of calcium and magnesium ions, which causes scale buildup in pipes and prevents soap from lathering.
- Dissolved Oxygen (DO): Crucial for aquatic life. Low DO levels indicate organic decomposition and lead to anaerobic conditions.
- Iron and Manganese: These metals are soluble in their reduced form ($Fe^{2+}$ and $Mn^{2+}$) under anaerobic conditions. When exposed to oxygen, they oxidize to insoluble forms ($Fe^{3+}$ and $Mn^{4+}$), causing orange-brown or black stains and deposits.
Biological Contaminants
Biological quality involves pathogens like bacteria (E. coli), viruses, and protozoa (Cryptosporidium and Giardia). Algae fluctuations depend on sunlight, nutrients, and temperature.
Reservoir Stratification
During summer, solar heating creates thermal stratification, separating a reservoir into three distinct zones based on temperature and density:
- Epilimnion: The top layer. It is warm, well-mixed, and oxygen-rich due to atmospheric contact and photosynthesis.
- Metalimnion (or thermocline): The transition layer, where temperature decreases rapidly with depth (at least 1°C per meter).
- Hypolimnion: The bottom layer. It is cold, dense, and anaerobic (devoid of oxygen) due to organic decomposition. Under anaerobic conditions, iron and manganese dissolve, and hydrogen sulfide gas ($H_2S$) is generated.
Thermal Destratification and Lake Turnover
As seasons change, the temperature differences between the layers disappear, leading to lake turnover.
- Fall Turnover: As autumn approaches, air temperatures drop, cooling the warm epilimnion. When the surface water cools to 4°C (39.2°F), it reaches its maximum density. This heavy water sinks, forcing the lighter, bottom water upward. Winds assist this process, causing the entire reservoir to mix.
- Spring Turnover: In winter, ice forms at 0°C on the surface, while the denser 4°C water sits at the bottom. In spring, as ice melts and the surface water warms to 4°C, it sinks, initiating another full mix of the water column.
Water Quality Impacts of Lake Turnover
Lake turnover mixes the anaerobic, mineral-rich hypolimnion with the rest of the reservoir, causing:
- Oxygen Depletion: Lowers overall DO, potentially causing fish kills.
- Taste and Odor Spikes: Releases hydrogen sulfide gas (rotten-egg odor) and organics.
- Iron and Manganese Surges: Soluble metals disperse throughout the reservoir. When chlorinated, they oxidize and precipitate, causing red or black water.
- Increased Oxidant Demand: Organic matter and reduced metals consume chlorine, requiring increased dosages.
Realistic Exam Scenarios
Scenario A: During late October, a plant experiences musty odors, low dissolved oxygen, and a surge in raw iron and manganese. Recognizing fall turnover, the operator increases potassium permanganate to oxidize the metals before filtration and raises chlorine to meet the higher demand. Scenario B: An operator filters a muddy raw water sample. Measuring the filtrate yields true color (dissolved organic matter), whereas measuring the unfiltered sample yields apparent color (dissolved plus suspended matter).
Which reservoir zone is characterized by being cold, dense, and anaerobic, often leading to the reduction and dissolution of iron and manganese?
What occurs during fall turnover when the surface water of a reservoir cools to 4°C (39.2°F)?