Water Supply Exam Review Flashcards 2025

Global Disruption of Water Supplies and Health Impacts

Factors resulting in the disruption of water systems and the spread of v. Cholerae include: * Climate change, cyclones, and drought: These events cause the displacement of populations, which facilitates the spread of alien strains of v. Cholerae. They further disrupt water supplies and lead to the failure of sanitation systems. * Humanitarian crises: Similar to climate events, these crises cause large-scale population displacement, the introduction of alien strains of v. Cholerae, and the collapse of water and sanitation infrastructure. * Complex ongoing emergencies: These situations lead to population displacement and the spread of alien v. Cholerae strains. They often overlay with outbreaks of other diseases, compounding public health challenges. * Delayed supply chains and poor surveillance: These failures mean outbreaks are not controlled in time, allowing community-level transmission to become established. This leads to higher levels of contamination in wastewater, increasing the overall risk of exposure. * Failures in the healthcare system: Lack of timely control allows community-level transmission to proliferate, leading to increased risk from wastewater contamination. * Disruption to vaccinations: When vaccination programs are interrupted, outbreaks cannot be contained effectively, facilitating community spread and raising environmental contamination levels.

Effects of Drought on Water Quality and Scarcity

Drought conditions significantly impact the availability and safety of water through several mechanisms: * Reduced Supply: Low flows and reduced water levels in surface water bodies lead to water scarcity. * Increased Pollutant Concentration: As water levels drop, the concentration of nutrients and pollutants increases. * Pathogen Proliferation: Higher temperatures associated with drought create favorable conditions for waterborne pathogens to increase within the supply system. * Recourse to Unsafe Sources: Lowered groundwater tables and reduced surface flows force consumers to use potentially unsafe or contaminated water sources. * Cyanobacterial Blooms: Elevated temperatures can trigger blooms of cyanobacteria, which increases the levels of natural organic matter and the risk of harmful cyanotoxins in the water. * Hygiene Risks: Reduced water availability for essential activities such as washing, cooking, and general hygiene increases the population's exposure to waterborne contamination.

WHO Guidelines for Drinking-water Quality

The Guidelines support risk management strategies to ensure water safety by controlling hazardous constituents.
Key elements include: * Scientific Basis: Guidelines provide the scientific foundation for developing national or regional water quality standards. * Minimum Requirements: They describe the reasonable minimum requirements of safe practice to protect public health. * Numerical Guideline Values: The Guidelines derive numerical values for water constituents or indicators, but they do explicitly not set up global normative (mandatory) values.
Risk-Based Approach vs. Mandatory Limits: * The earlier approach relied on establishing mandatory limits for all potential contaminants. However, this failed to account for local or national environmental, economic, social, and cultural contexts. * The current risk-based approach allows the Guidelines to be integrated into broader health protection strategies, including sanitation and food contamination management. * Prevention Framework: The Guidelines promote an integrated preventive management framework applied from the catchment area all the way to the consumer. This results in robust management interventions and keeps treatment costs at appropriate levels.

Conventional and Modern Water Treatment Processes

Conventional Process Stages: * Intake Screening: Debris, plants, and fish are screened out at the intake to prevent them from entering the treatment plant. * Aeration: This process controls substances causing taste and odors, prevents corrosion, removes offensive gases like hydrogen sulphide ( $H_2S$ ), and prevents interference with treatment chemicals. * Coagulation and Flocculation: Alum and other chemicals are mixed into the water, causing small particles to stick together into larger particles known as "floc." * Sedimentation: Water passes into a sedimentation basin where the flow slows down, allowing the floc particles to settle at the bottom. * Filtration: Water flows through layers of sand and gravel to remove remaining particles, including bacteria. Carbon may be used to remove residual color, taste, and odor. * Disinfection: Chlorine is added to destroy surviving organisms and provide a residual to ensure the water remains bacteriologically safe during distribution.
Modern Treatment Technologies: * Improved coagulation control mechanisms. * Dissolved air flotation (DAF). * Advanced clarifiers, including lamella separators and advanced sludge-blanket systems. * Ozonation ( $O_3$ ). * Granular activated carbon (GAC) adsorption. * Membrane-based filtration processes. * Air stripping of volatile organic chemicals (VOCs). * Alternative disinfection: Ultraviolet (UV) light, ozonation, and chlorine dioxide ( $ClO_2$ ).

Colloid Destabilisation Mechanisms

Double Layer Compression: This involves adding an electrolyte to the water to increase ion concentration. This decreases the thickness of the electrical double layer surrounding colloidal particles, allowing them to move close enough for attractive forces to overcome repulsive electrical forces.
Charge Neutralisation: Ions with a charge opposite to the colloidal particles are added. These ions adsorb onto the colloid, reducing surface charge and allowing agglomeration.
Entrapment in a Precipitate: Soluble aluminium or iron salts are added at an appropriate pH to form hydroxide flocs. These flocs precipitate using the colloids as nuclei, physically entrapping other particles as they settle.
Particle Bridging: Large organic molecules with multiple charges (anionic or cationic polymers) act as coagulants by forming physical bridges between particles.
Inorganic Coagulants: Compounds like Aluminium Sulphate ( $Al_2(SO_4)_3$ ) and Ferric Sulphate ( $Fe_2(SO_4)_3$ ) facilitate destabilisation through the formation of hydroxide flocs or carbonate precipitates (often reacting with natural alkalinity like calcium bicarbonate ( $Ca(HCO_3)_2$ )).

Transport and Attachment in Granular Filtration

Contact Mechanisms: 1. Interception: A particle moving uniformly collides with a grain of filter media (primarily affects large particles). 2. Sedimentation: Gravitational forces cause particles to settle onto the media (primarily affects large particles). 3. Diffusion: Driven by Brownian motion (random movement due to collisions with fluid molecules). This significantly affects only very small particles. 4. Hydrodynamic Action: Arises from velocity gradients near the media grains. A passing particle is rotated by the gradient, causing pressure differences that move it toward the media.
Removal Efficiency Characteristics: * Size Factors: Efficiency for interception ( $d_p / d_m$ ) increases as media size decreases or particle size increases. For sedimentation, efficiency depends on the ratio ( $v_s / v$ ), where settling velocity ( $v_s$ ) is proportional to the diameter squared ( $d_p^2$ ). * Temperature Factors: The number of collisions in diffusion is proportional to $(\frac{T}{d_p d_m v})^{0.67}$ ; thus, efficiency increases at higher temperatures and for smaller particles/media. * Summary by Size: Very small particles are removed by diffusion; large particles are removed by straining; particles approximately $1\,\mu m$ in size are removed mainly via interception and sedimentation.

Water Treatment Calculations: Surface Overflow Rate

Surface Overflow Rate Calculation: * $v_0 = \frac{Q}{A} = \frac{0.5}{21 \times 5} = 0.0048\,m/sec$
Particle Removal Efficiency: * Since the overflow rate ( $v_0$ ) is greater than the settling velocity ( $v_t$ ) of the target particle, they will not be completely removed. * Particles with settling velocity $v_t < v_0$ are removed according to the ratio $v_t / v_0$ . * $\text{Percent removed} = (\frac{v_t}{v_0}) \times 100 = (\frac{0.003}{0.0048}) \times 100 = 63\%$

Hydraulic Engineering and Pump Power Calculations

System Equations: * Energy Equation: $71 = h_{f1} + h_{f2} + h_{f3} + h_{f4} + h_{f5} + h_{en} + h_v + h_{ex}$ * Friction Loss ( $h_f$ ): $h_f = \frac{LQ^2}{12.1 D^5} = k Q^2$ * Minor Losses: $h_L = k_L \frac{v^2}{2g} = k_L \frac{Q^2}{2g A^2} = S Q^2$ * System Total: $71 = (k_1 + k_2 + k_3 + k_4 + k_5) Q^2 + (S_{en} + S_{ex} + S_M) Q^2$
Line 1 Calculation: * Constants: $k_1 = 25.42$ , $k_2 = 33.90$ , $k_3 = 25.42$ , $k_4 = 13.56$ , $k_7 = 76.27$ . * Loss Coefficients: $S_v = 1.61$ , $S_{ex} = 3.23$ , $S_M = 19.37$ . * $71 = 174.57 Q^2 + 24.21 Q^2 \Rightarrow Q_1 = 0.598\,m^3/s$
Line 2 Calculation: * Constants: $k_6 = 2043.77$ , $k_7 = 355.44$ , $k_8 = 710.88$ . * Loss Coefficients: $S_{en} = 10.58$ , $S_v = 169.26$ , $S_{ex} = 21.16$ . * $71 = 3110.08 Q^2 + 200.99 Q^2 \Rightarrow Q_2 = 0.146\,m^3/s$
Total Flow Rate ( $Q_{total}$ ): * $Q = 0.598 + 0.146 = 0.744\,m^3/s$
Pipe Characteristics for $D = 0.500\,m$ : * Velocity ( $v$ ): $3.79\,m/s$ * Reynolds Number ( $Re$ ): $1.89 \times 10^5$ * Relative Roughness ( $k_s / D$ ): $0.0004 \Rightarrow \lambda = 0.0165$
Pump Power Requirements: * Friction Head ( $h_f$ ): $35.99\,m$ * Pump Head ( $h_p$ ): $H_p = 71 + 35.99 = 106.99\,m$ * Output Power: $P_{output} = \rho g H_p Q = 998 \times 9.81 \times 106.99 \times 0.6 = 628,483.65\,Watts \text{ (628.48 kW)}$ * Input Power ( $\eta = 0.75$ ): $P_{input} = \frac{628.48}{0.75} = 837.97\,kW$