6.1 Water Sources, Water Quality and Water Demand

6.1 Water Sources, Water Quality and Water Demand

Introduction to Water Supply Engineering

The provision of safe, adequate, and reliable water supply is a fundamental prerequisite for public health, sanitation, and socio-economic development. Water supply engineering involves the holistic management of water—from its origin as a raw source, through treatment to meet health standards, to its distribution based on projected demand. This unit systematically examines the characteristics of different water sources, the nature of impurities they contain, the parameters that define water quality, associated health risks, and the methodologies for estimating the community's water needs. Mastery of these concepts is essential for designing sustainable and resilient water supply systems.

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1. Sources of Water

Water sources are broadly classified based on their origin and location relative to the earth's surface. The selection of a source involves a complex evaluation of quantity, quality, reliability, and economic feasibility.

1.1 Surface Water

Water that collects on the ground or in streams, rivers, lakes, and reservoirs.

1. Types:

   • Rivers and Streams: Flowing water. Highly variable in flow (seasonal), easily contaminated, usually contain silt.

   • Lakes and Ponds: Impounded still water. Generally clearer due to natural sedimentation, but susceptible to algal growth and stratification.

   • Impounding Reservoirs: Artificially created by constructing dams across rivers. Provide flow regulation, storage, and often better quality due to long detention times.

   • Oceans and Seas: Saline water, require expensive desalination for potable use.

2. Characteristics:

   • Quality: Variable; generally contains suspended solids, microorganisms, and organic matter. Soft water.

   • Quantity: Seasonal fluctuation is high (monsoon vs. dry season).

   • Accessibility: Usually easier to tap and convey.

   • Vulnerability: Prone to pollution from surface runoff.

1.2 Groundwater

Water present in the saturated zone beneath the water table, stored in and flowing through aquifers.

1. Types of Aquifers:

   • Unconfined Aquifer: Has a water table that is free to rise and fall. Recharged directly by percolation from the surface.

   • Confined Aquifer (Artesian): Sandwiched between impermeable layers (aquitards). Water is under pressure. Recharge occurs in distant outcrop areas.

2. Sources:

   • Wells: Vertical excavations.

     • Dug Wells: Shallow, large diameter. Tap unconfined aquifers.

     • Tube Wells/Bore Wells: Deep, small diameter. Can tap confined aquifers. Include hand pumps and power-driven pumps.

   • Infiltration Galleries: Horizontal tunnels constructed along the banks of rivers to collect filtered groundwater.

   • Springs: Natural outflows of groundwater where the water table intersects the ground surface.

3. Characteristics:

   • Quality: Generally clear (free from suspended impurities due to natural filtration), but often contains dissolved minerals (hardness, iron, salts). Usually free from pathogens if the aquifer is deep and protected.

   • Quantity: Relatively constant, less seasonal fluctuation. Yield depends on aquifer characteristics.

   • Accessibility: Requires drilling/pumping equipment, higher initial cost.

   • Reliability: A drought-resistant source if the aquifer is extensive.

1.3 Selection of Water Source

The choice depends on a detailed comparison of multiple factors:

1. Quantity (Adequacy):

   • Source yield must meet the present and future design demand of the community.

   • Analysis of minimum dry-weather flow for rivers, safe yield for aquifers.

2. Quality:

   • Assessment of raw water quality against standards.

   • Groundwater may require treatment for hardness/iron.

   • Surface water almost always requires complete treatment (coagulation, filtration, disinfection).

   • Cost of treatment is a major deciding factor.

3. Reliability and Permanence:

   • Source should be dependable year-round and over the project's design life (50-100 years).

   • Consider climate change impacts on rainfall patterns and recharge.

4. Proximity to the Community:

   • Shorter conveyance distance reduces pipeline and pumping costs.

5. Elevation:

   • A source at a higher elevation than the city allows for gravity flow, eliminating pumping costs.

6. Legal and Environmental Constraints:

   • Water rights, riparian rights.

   • Environmental Impact Assessment (EIA) for dams and large withdrawals.

7. Economic Feasibility:

   • Capital cost (dams, wells, intake structures).

   • Operation and Maintenance cost (treatment, pumping).

   • General Preference Order (where possible): Protected Springs > Deep Groundwater > Impounded Surface Water > Rivers.

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2. Impurities in Water

Natural water is never chemically pure. Impurities are categorized based on their particle size and state.

2.1 Suspended Impurities

1. Size: > 10⁻⁴ mm (100 microns). Visible to the naked eye or under a microscope.

2. Nature: Sand, silt, clay, organic debris, algae, bacteria.

3. Behavior: Do not dissolve. Settle down or float based on density. Cause turbidity and color.

4. Removal Methods: Plain sedimentation (if heavy), coagulation followed by sedimentation, filtration.

2.2 Colloidal Impurities

1. Size: 10⁻⁶ mm to 10⁻⁴ mm (1 nm to 100 nm). Not visible under ordinary microscope.

2. Nature: Fine clay particles, silica, humic acids, viruses, some bacteria.

3. Behavior: Do not settle on standing due to Brownian motion and electrostatic repulsion (negative charge). Cause persistent turbidity and color.

4. Removal Methods: Coagulation (using Alum, Fe salts) to neutralize charge and form flocs, followed by sedimentation and filtration.

2.3 Dissolved Impurities

1. Size: < 10⁻⁶ mm. Exist as ions or molecules.

2. Nature:

   • Inorganic Salts: Calcium, magnesium, sodium bicarbonates, chlorides, sulphates (cause hardness, alkalinity, taste).

   • Gases: Carbon dioxide ($CO_2$), hydrogen sulphide ($H_2S$ - causes rotten egg smell), oxygen.

   • Organic Matter: From decay of vegetation.

   • Toxic Metals: Lead, arsenic, mercury, fluoride (in excess).

3. Behavior: Cannot be removed by physical means. Impart taste, odor, hardness.

4. Removal Methods:

   • Hardness: Lime-Soda process, ion exchange, reverse osmosis.

   • Gases: Aeration, degasification.

   • Dissolved Solids: Distillation, membrane processes (RO, Electrodialysis).

   • Iron/Manganese: Aeration followed by filtration.

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3. Hardness and Alkalinity

3.1 Hardness of Water

1. Definition: The soap-destroying capacity of water, primarily caused by dissolved salts of calcium (Ca²⁺) and magnesium (Mg²⁺). It prevents lather formation and causes scale in pipes and boilers.

2. Types:

   • Carbonate (Temporary) Hardness: Caused by bicarbonates of Ca and Mg. Can be removed by boiling. $\text{Ca(HCO}_3)_2 \xrightarrow{\text{Heat}} \text{CaCO}_3 \downarrow + \text{CO}_2 \uparrow + \text{H}_2\text{O}$

   • Non-Carbonate (Permanent) Hardness: Caused by chlorides and sulphates of Ca and Mg. Cannot be removed by boiling; requires chemical treatment.

3. Units of Measurement:

   • mg/L as CaCO₃: Standard unit. Conversion: $\text{mg/L as CaCO}_3 = \text{Ion conc. (mg/L)} \times \frac{50}{\text{Eq. Wt. of Ion}}$

   • Parts per million (ppm): Numerically equivalent to mg/L for dilute solutions.

   • Degree of Hardness:

     • Clark's degree: 1 part CaCO₃ per 70,000 parts water.

     • French degree: 1 part CaCO₃ per 100,000 parts water.

4. Classification:

   • Soft: 0-60 mg/L as CaCO₃

   • Moderately Hard: 61-120 mg/L

   • Hard: 121-180 mg/L

   • Very Hard: >180 mg/L

3.2 Alkalinity of Water

1. Definition: The capacity of water to neutralize acids. It is a measure of the water's buffering capacity and is caused by hydroxides (OH⁻), carbonates (CO₃²⁻), and bicarbonates (HCO₃⁻) of calcium, magnesium, sodium, and potassium.

2. Significance:

   • Influences coagulant dose in treatment (e.g., Alum requires alkalinity to form flocs).

   • Protects against corrosion in pipes (by forming a protective carbonate layer).

   • Essential for biological processes in wastewater treatment.

3. Measurement: By titration with standard acid (N/50 H₂SO₄) using phenolphthalein and methyl orange indicators.

   • Phenolphthalein Alkalinity (P): Measures OH⁻ + ½ CO₃²⁻.

   • Total Alkalinity (M): Measures OH⁻ + CO₃²⁻ + HCO₃⁻.

4. Relationship with Hardness:

   • If Alkalinity < Total Hardness: Water contains both carbonate and non-carbonate hardness.

   • If Alkalinity = Total Hardness: All hardness is carbonate hardness.

   • If Alkalinity > Total Hardness: Water has carbonate hardness + sodium alkalinity (no non-carbonate hardness).

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4. Living Organisms in Water

Water can harbor a variety of biological entities, many of which are health hazards.

1. Pathogenic (Disease-causing) Microorganisms:

   • Bacteria: Vibrio cholerae (Cholera), Salmonella typhi (Typhoid), E. coli (indicative of fecal contamination).

   • Viruses: Hepatitis A & E, Poliovirus, Rotavirus.

   • Protozoa: Entamoeba histolytica (Amoebic dysentery), Giardia lamblia (Giardiasis). Form cysts resistant to chlorine.

   • Helminths (Worms): Ascaris (roundworm), tapeworms. Their eggs are resistant.

2. Non-pathogenic Microorganisms:

   • Algae: Cause taste, odor, and filter clogging. Some produce toxins (e.g., Blue-green algae).

   • Iron and Sulphur Bacteria: Cause clogging and aesthetic problems (slime, odors).

3. Indicator Organisms:

   • Testing for every specific pathogen is impractical.

   • Coliform Bacteria (especially E. coli) are used as indicator organisms. Their presence signifies recent fecal contamination and the potential presence of pathogens.

   • WHO/NSDWQ Standard: Zero E. coli per 100 ml of drinking water.

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5. Water-Related Diseases and Prevention Measures

5.1 Classification of Water-Related Diseases

1. Water-Borne Diseases:

   • Caused by: Ingestion of water contaminated with pathogenic microorganisms.

   • Examples: Cholera, Typhoid, Bacillary Dysentery, Hepatitis A, Polio.

   • Primary Prevention: Disinfection of drinking water (Chlorination, UV, Ozonation). Protection of source from fecal contamination.

2. Water-Washed (Water-Scarce) Diseases:

   • Caused by: Lack of adequate water for personal hygiene.

   • Examples: Scabies, Trachoma, Conjunctivitis.

   • Primary Prevention: Provision of sufficient water quantity for washing and bathing.

3. Water-Based Diseases:

   • Caused by: Parasites that spend part of their life cycle in an aquatic intermediate host (snail) and infect humans through skin contact.

   • Example: Schistosomiasis (Bilharzia).

   • Primary Prevention: Snail control, preventing human contact with infected water.

4. Water-Related Vector Diseases:

   • Caused by: Insects that breed in or near water.

   • Examples: Malaria (Anopheles mosquito), Dengue (Aedes mosquito), Filariasis.

   • Primary Prevention: Vector Control (larviciding, eliminating stagnant water, using insecticide-treated nets).

5.2 Prevention and Control Measures

1. Engineering (Water Supply) Interventions:

   • Source Protection: Sanitary surveys, fencing, restricted access.

   • Water Treatment: Coagulation, sedimentation, filtration, and most critically, disinfection.

   • Protected Distribution: Maintaining positive pressure, preventing cross-connections, and ensuring sanitary construction.

2. Sanitation and Hygiene (WASH) Interventions:

   • Provision of adequate sanitation facilities (toilets) to prevent source contamination.

   • Hygiene Education: Promoting handwashing with soap, safe water handling, and storage.

3. Surveillance and Monitoring:

   • Regular testing of water quality for bacteriological (coliforms) and chemical parameters.

   • Disease surveillance and outbreak investigation.

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6. Drinking Water Quality Standards

To ensure public health, drinking water must conform to prescribed standards. In Nepal, this is governed by the National Drinking Water Quality Standards (NDWQS).

1. Purpose: To define the maximum permissible limits (MPL) of physical, chemical, bacteriological, and radiological constituents in drinking water.

2. Key Parameters:

   • Physical/Aesthetic:

     • Turbidity: < 1 NTU (Nephelometric Turbidity Unit). Preferably < 0.5 NTU for effective disinfection.

     • Color: < 15 TCU (True Color Unit).

     • Taste & Odor: Inoffensive to consumer.

   • Chemical:

     • pH: 6.5 - 8.5 (Protects pipes, ensures effective disinfection).

     • Total Hardness: < 200 mg/L as CaCO₃.

     • Total Dissolved Solids (TDS): < 500 mg/L (Excellent), < 1000 mg/L (Permissible).

     • Toxic Substances: Strict limits for Arsenic (< 10 µg/L), Fluoride (< 1.5 mg/L), Lead (< 10 µg/L), Nitrate (< 50 mg/L).

   • Bacteriological:

     • E. coli or Thermotolerant Coliforms: Must be 0 per 100 ml of sample.

     • Total Coliforms: Should be 0 per 100 ml. If detected, investigation is required.

3. Compliance: Water supply systems must meet these standards at the point of delivery to the consumer (tap).

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7. Water Demand Estimation

Accurate estimation of water demand is the first and most critical step in designing the capacity of all components of a water supply system: source, treatment plant, pumps, and pipes.

7.1 Types of Water Demand

1. Domestic Demand: Water for household activities (drinking, cooking, bathing, washing, sanitation). This is the core component.

2. Commercial and Institutional Demand: For offices, hotels, schools, hospitals.

3. Industrial Demand: For manufacturing and processing.

4. Public and Civic Demand: For street washing, firefighting, public parks, fountains.

5. Losses and Wastage: Unavoidable leakage from pipes and unauthorized use. A well-maintained system should keep this below 15%.

7.2 Factors Affecting Per Capita Demand

1. Climatic Conditions: Hotter climates increase demand.

2. Standard of Living: Higher income leads to higher consumption (more appliances, gardens).

3. Industry and Commerce: Extent of industrial activity.

4. Pressure in the Distribution System: Higher pressure can increase leakage and consumption.

5. System of Supply:

   • Continuous Supply (24x7): Lower per capita demand as people don't store water.

   • Intermittent Supply (few hours/day): Higher per capita demand due to storage and associated wastage.

6. Cost of Water: Higher tariffs tend to reduce consumption.

7. Policy and Conservation Measures: Public awareness, use of water-efficient fixtures.

7.3 Estimation Methods and Design Period

1. Design Period: The number of years into the future for which the system is designed (typically 20-30 years). Demand is projected to the end of this period.

2. Per Capita Consumption (q):

   • Expressed in Liters per Capita per Day (LPCD).

   • Nepal/India Guidelines:

     • Without Sewerage (Water only): 135 - 150 LPCD.

     • With Sewerage (Full flushing system): 200 - 220 LPCD.

     • For specific city master plans, detailed surveys are conducted.

3. Calculating Average Daily Demand (ADD): $\text{ADD} = \text{Population at design year} \times \text{Per Capita Demand (LPCD)}$

4. Variations in Demand:

   • Water demand is not constant. It varies seasonally, daily, and hourly.

   • Peak Factors are multipliers applied to the ADD to size pipes, pumps, and treatment plants for maximum expected flow.

5. Key Demand Rates:

   • Average Daily Demand (ADD): Basis for annual water volume and source yield.

   • Maximum Daily Demand (MDD): The highest daily demand in a year. $\text{MDD} = 1.8 \times \text{ADD}$

   • Peak Hourly Demand (PHD): The maximum hourly demand on the day of maximum usage. $\text{PHD} = 1.5 \times \text{MDD} = (1.5 \times 1.8) \times \text{ADD} = 2.7 \times \text{ADD}$

6. Fire Demand: Additional water required for firefighting. Estimated using formulas like Kuichling's Formula: $Q = 3182 \sqrt{P}$ (where Q is in L/min, P is population in thousands).

Conclusion: The science of water supply begins with understanding the origin and nature of the raw resource, defining the quality parameters necessary for health, and accurately forecasting the quantity required by the community. These three pillars—source, quality, and demand—form the essential foundation upon which all subsequent engineering decisions for treatment, storage, and distribution are built.