6. Water Supply, Sanitation and Environment (ACiE06)

6.1 Water Sources, Water Quality and Water Demand

Sources of Water

Water sources are broadly categorized into surface water and groundwater, each with distinct characteristics and selection criteria:

• Surface Water:
  • Rivers: Provide continuous flow, but quality can fluctuate significantly with rainfall and pollution. Selection considers flow availability, upstream pollution, and proximity to consumption.
  • Lakes: Offer large storage capacity and often better quality than rivers due to natural sedimentation. Selection depends on volume, depth, watershed protection, and potential for algal blooms.
  • Reservoirs: Man-made impoundments of surface water, providing controlled supply and improved quality through detention. Selection involves topography, geology, evaporation losses, and land acquisition.
• Groundwater:
  • Springs: Natural outflows of groundwater. Offer high quality, often requiring minimal treatment. Selection based on yield, reliability, and protection from contamination.
  • Wells (Dug/Bore): Access groundwater by drilling or digging. Provide reliable, high-quality water, less susceptible to surface pollution. Selection considers aquifer yield, depth, water table fluctuations, and potential for subsidence.
  • Infiltration Galleries: Horizontal perforated pipes laid in permeable beds near rivers or lakes to collect naturally filtered water. Offer good quality but limited yield. Selection based on aquifer characteristics and river/lake bed permeability.

Impurities in Water

Water contains various impurities that affect its quality and suitability for consumption:

• Suspended Impurities: Visible particles like clay, silt, and sand. They cause turbidity and can harbor microorganisms. Removed by sedimentation and filtration.
• Colloidal Impurities: Very fine particles (organic and inorganic) that remain suspended due to electrostatic charges. They cause color and turbidity and are difficult to remove by plain sedimentation. Coagulation and flocculation are required.
• Dissolved Impurities: Minerals (e.g., calcium, magnesium, iron), gases (e.g., CO2, H2S), and organic compounds. They affect taste, odor, hardness, and can be corrosive. Removed by aeration, ion exchange, or softening processes.

Hardness

Hardness is caused by multivalent metallic cations, primarily calcium (Ca²⁺) and magnesium (Mg²⁺). It is typically measured in mg/L as CaCO₃.

• Definition: The property of water that prevents lathering with soap and causes scale formation in pipes and boilers.
• Temporary Hardness (Carbonate Hardness): Caused by bicarbonates of calcium and magnesium. Can be removed by boiling.
• Permanent Hardness (Non-Carbonate Hardness): Caused by sulfates, chlorides, and nitrates of calcium and magnesium. Cannot be removed by boiling and requires chemical softening.
• Effects: Increased soap consumption, scale formation in hot water systems, undesirable taste.
• Measurement: Expressed as equivalent CaCO₃.
  Hardness (mg/L as CaCO3) = (Concentration of ion (mg/L) * Equivalent weight of CaCO3) / Equivalent weight of ion
Equivalent weight of CaCO3 = 50
Equivalent weight of Ca = 20
Equivalent weight of Mg = 12.15

Alkalinity

Alkalinity is the capacity of water to neutralize acids, primarily due to the presence of bicarbonates, carbonates, and hydroxides.

• Carbonate Alkalinity (CO₃²⁻): Present at higher pH values.
• Bicarbonate Alkalinity (HCO₃⁻): Most common form, especially in natural waters.
• Hydroxide Alkalinity (OH⁻): Present at very high pH values.

Living Organisms in Water

Microorganisms can pose significant health risks:

• Bacteria: E.g., E. coli (indicator of fecal contamination), Vibrio cholerae.
• Viruses: E.g., Hepatitis A, Rotavirus.
• Protozoa: E.g., Giardia lamblia, Cryptosporidium parvum (resistant to chlorine).
• Algae: Can cause taste and odor problems, clog filters, and produce toxins.

Water-Related Diseases and Prevention Measures

Contaminated water can transmit various diseases:

• Cholera: Caused by Vibrio cholerae, severe diarrhea.
• Typhoid: Caused by Salmonella typhi, fever, abdominal pain.
• Dysentery: Caused by bacteria (e.g., Shigella) or amoebas, severe diarrhea with blood/mucus.
• Hepatitis A: Viral infection affecting the liver.

Prevention Measures: Safe water sources, proper treatment (disinfection), protected distribution systems, hygiene education, sanitation.

Drinking Water Quality Standards

Standards ensure water is safe for human consumption.

• WHO Guidelines: International reference for drinking water quality.
• Nepal National Standards (e.g., NWQG 2007):
  • pH: 6.5 - 8.5
  • Turbidity: ≤ 5 NTU (Nephelometric Turbidity Units)
  • Total Dissolved Solids (TDS): ≤ 1000 mg/L
  • Coliform (E. coli): 0 per 100 mL (for treated water)
  • Residual Chlorine: 0.1 - 0.2 mg/L at the consumer end

Water Demand Estimation

Accurate demand estimation is crucial for designing water supply systems.

• Per Capita Demand: Average daily water consumption per person (Lpcd). Varies with climate, lifestyle, and economic status.
  Total Daily Demand = Population * Per Capita Demand
• Fire Demand: Water required to extinguish fires. Estimated using empirical formulas.
  Kuichling Formula:
  Q = 1136 * sqrt(P)
  Where, Q = Fire demand in Liters per minute (L/min)
  P = Population in thousands
• Minimum Day Demand: Lowest water demand on any given day.
• Maximum Day Demand: Highest water demand on any given day (typically 1.8 to 2.0 times average daily demand).
  Max Day Demand = Peak Factor * Average Daily Demand

6.2 Intake and Distribution Systems

Types of Intakes

Structures used to divert water from its source:

• River Intake: Located upstream of pollution sources, typically with screens and gates.
• Lake Intake: Submerged pipes extending into the lake, often with a tower for access and screens.
• Reservoir Intake: Similar to lake intakes, often with multiple gates at different depths to draw water of best quality.
• Infiltration Gallery: Horizontal porous pipes laid in river beds or banks to collect naturally filtered water.

Factors Affecting Selection of Intake Location: Water quality, quantity, foundation conditions, navigability, flood levels, ice formation, accessibility, proximity to treatment plant, and cost.

Pipe Materials, Joints, Valves and Fittings

• Pipe Materials:
  • Cast Iron (CI): Durable, strong, but brittle and heavy.
  • Ductile Iron (DI): More flexible and stronger than CI, excellent corrosion resistance.
  • Polyvinyl Chloride (PVC): Lightweight, corrosion-resistant, easy to install, cost-effective for smaller diameters.
  • High-Density Polyethylene (HDPE): Flexible, strong, excellent chemical resistance, suitable for trenchless installation.
  • Galvanized Iron (GI): Steel pipes coated with zinc, prone to corrosion over time, mainly for internal plumbing.
• Joints:
  • Socket and Spigot Joint: Common for CI/DI, uses rubber gasket and cement/lead.
  • Flange Joint: Bolted connection, used for valves and fittings.
  • Compression Joint: Uses a gland and bolts to compress a rubber ring.
  • Welded Joint: For steel and HDPE, provides strong, leak-proof connection.
• Valves and Fittings:
  • Gate Valves: For isolating sections of pipe, fully open or closed.
  • Globe Valves: For throttling flow, precise control.
  • Check Valves: Allow flow in one direction only.
  • Air Valves: Release trapped air or admit air during draining.
  • Pressure Reducing Valves: Control downstream pressure.
  • Fire Hydrants: For firefighting connections.
  • Bends, Tees, Reducers: For changing direction, branching, or changing pipe diameter.

Break Pressure Tanks

Purpose: Used in hilly terrains to break excessive pressure in pipelines, prevent water hammer, and allow for controlled pressure zones. They dissipate excess head and reduce pipe class requirements downstream.

Design: Involves determining the appropriate elevation to limit pressure in the pipe to acceptable levels (e.g., 60-70 m head). The tank size is usually small, acting as an open channel or simple reservoir to ensure atmospheric pressure at that point.

Service Reservoirs

Store treated water before distribution.

• Elevated Service Reservoirs (ESR): Provide gravity flow and maintain pressure in the distribution system.
• Ground Level Service Reservoirs (GLSR): Store large volumes, often requiring pumping for distribution.

Capacity Determination: Based on balancing daily demand fluctuations, fire demand, and emergency storage. Typically designed for:

• Balancing storage (to meet hourly fluctuations): 20-30% of average daily demand.
• Fire storage: As per fire demand for a specified duration.
• Emergency storage (for breakdowns, repairs): 25-50% of average daily demand.

The total capacity is the sum of these components, often expressed as a percentage of the maximum daily demand (e.g., 30-50% of MDD).

Design of Branch Water Distribution System

A tree-like system where water flows in one direction from the main to branches and sub-branches.

• Advantages: Simple to design and construct, less expensive.
• Disadvantages: Water stagnation at dead ends, poor pressure maintenance, large areas affected during pipe breaks.
• Design: Involves sizing pipes based on demand at each node, maintaining minimum residual pressure (e.g., 15-20 m), and checking velocities (0.6-3.0 m/s).

Design of Looped Water Distribution System (Hardy Cross Method)

A network of interconnected pipes forming loops, allowing water to flow in multiple directions.

• Advantages: More reliable, better pressure distribution, less water stagnation, easier to isolate sections for repair.
• Disadvantages: More complex to design and construct.
• Hardy Cross Method: An iterative method for analyzing flows and head losses in pipe networks.
  Principle: Based on two fundamental principles:
  1. The algebraic sum of head losses around any closed loop must be zero (conservation of energy).
  2. The flow entering any junction must equal the flow leaving it (conservation of mass).
  
  The method assumes an initial distribution of flow in each pipe, then calculates the head loss in each pipe using a head loss formula (e.g., Hazen-Williams or Darcy-Weisbach). An adjustment factor (ΔQ) is calculated for each loop to correct the assumed flows until the sum of head losses in each loop approaches zero.
  ΔQ = - (Σ h_f) / (n * Σ (h_f / Q_assumed))
  Where, ΔQ = Flow correction for the loop
  h_f = Head loss in a pipe (e.g., h_f = K * Q^n, where n is typically 1.85 for Hazen-Williams or 2 for Darcy-Weisbach)
  Q_assumed = Assumed flow in the pipe
  K = Resistance coefficient for the pipe
  The adjusted flow for each pipe in the loop becomes Q_new = Q_assumed + ΔQ (with sign convention). This process is repeated until ΔQ becomes negligible.

6.3 Water Treatment Process and Technologies

Screening

The first step in water treatment, removing large floating and suspended matter.

• Coarse Screens (Bar Screens): Spacing of 20-100 mm, remove large debris (logs, rags).
• Fine Screens: Spacing of 3-10 mm, remove smaller suspended solids.

Plain Sedimentation

Removes suspended solids by gravity without chemicals.

• Horizontal Flow Settling Tanks: Rectangular tanks where water flows horizontally.
  Design Criteria:
  • Surface Loading Rate (Overflow Rate, V_s): Velocity at which particles settle. Typical range: 12-18 m³/m²/day for plain sedimentation.
    V_s = Q / A
    Where, Q = Flow rate (m³/day)
    A = Surface area of tank (m²)
  • Detention Time (t_d): Time water stays in the tank. Typical range: 4-8 hours.
    t_d = V / Q
    Where, V = Volume of tank (m³)
    Q = Flow rate (m³/day or m³/hr)
• Circular Settling Tanks: Water enters centrally, flows radially outwards, and overflows at the periphery.

Sedimentation with Coagulation

Enhances sedimentation by adding chemicals to destabilize colloidal particles.

• Coagulants:
  • Alum (Aluminum Sulfate, Al₂(SO₄)₃·14H₂O): Most common, forms aluminum hydroxide floc.
  • Ferric Chloride (FeCl₃): Effective over a wider pH range.
• Jar Test: Laboratory procedure to determine the optimal coagulant dose, pH, and mixing conditions. Involves rapid mixing, slow mixing, and settling in beakers.

Flocculation

Slow mixing process after coagulation to promote collision and growth of destabilized particles into larger, settleable flocs.

• Mechanical Flocculators: Paddles or impellers rotate slowly.
• Hydraulic Flocculators: Baffled channels create velocity gradients.
• Velocity Gradient (G): A measure of mixing intensity.
  G = sqrt(P / (μ * V))
  Where, G = Velocity gradient (s⁻¹)
  P = Power input (W)
  μ = Dynamic viscosity of water (Pa·s)
  V = Volume of flocculation tank (m³)
  Typical G values for flocculation are 20-70 s⁻¹.

Filtration

Removes remaining suspended solids, turbidity, and some microorganisms.

• Slow Sand Filter (SSF):
  • Principle: Biological layer (schmutzdecke) formed on sand surface filters physically and biologically.
  • Design Criteria: Low filtration rate (0.1-0.2 m³/m²/hr), large land area, simple operation, no chemicals.
  • Cleaning: Scraping off the top layer of sand.
• Rapid Sand Filter (RSF):
  • Principle: Mechanical straining and adsorption. Requires pre-treatment (coagulation-flocculation-sedimentation).
  • Design Criteria: High filtration rate (5-10 m³/m²/hr), smaller footprint, more complex operation.
  • Backwashing: Cleaning by reversing flow of water (and sometimes air) to flush out trapped solids.

Disinfection

Kills pathogenic microorganisms.

• Chlorination: Most common method. Chlorine reacts with water to form hypochlorous acid (HOCl) and hypochlorite ion (OCl⁻), which are disinfectants.
  • Breakpoint Chlorination: Adding enough chlorine to oxidize all organic matter and ammonia, leaving a free chlorine residual.
  • Contact Time: Time required for chlorine to effectively kill pathogens (e.g., 20-30 minutes).
  • Residual Chlorine: Small amount of chlorine remaining in water to provide protection against recontamination in the distribution system (e.g., 0.1-0.2 mg/L).
• UV (Ultraviolet) Disinfection: Uses UV light to inactivate microorganisms by damaging their DNA. No chemical residuals.
• Ozonation: Uses ozone (O₃), a powerful oxidant and disinfectant. Effective against viruses and protozoa, also removes taste/odor/color.

Softening

Removes hardness-causing ions (Ca²⁺, Mg²⁺).

• Lime-Soda Process: Adds lime (Ca(OH)₂) and soda ash (Na₂CO₃) to precipitate calcium and magnesium as carbonates and hydroxides.
• Ion Exchange: Water passes through a resin bed that exchanges hardness ions (Ca²⁺, Mg²⁺) for non-hardness ions (Na⁺).

Miscellaneous Treatments

• Aeration: Brings water into contact with air to remove dissolved gases (CO₂, H₂S), oxidize iron (Fe²⁺ to Fe³⁺) and manganese (Mn²⁺ to Mn⁴⁺), and improve taste/odor.
• Removal of Color/Odour/Taste: Can be achieved by activated carbon adsorption, ozonation, or aeration, depending on the cause.

6.4 Design and Construction of Sewers

Estimation of Quantity of Wastewater

Wastewater flow is primarily based on water consumption, typically 70-80% of water supplied.

• Population Forecast: Predicting future population is critical for long-term design.
  • Arithmetic Increase Method: Assumes a constant rate of population increase.
    P_n = P_0 + n * I_a
    Where, P_n = Population after n decades
    P_0 = Present population
    n = Number of decades
    I_a = Average arithmetic increase per decade
  • Geometric Increase Method: Assumes a constant percentage growth rate.
    P_n = P_0 * (1 + r)^n
    Where, r = Average geometric growth rate per decade
  • Incremental Increase Method: Accounts for increasing or decreasing rates of growth.
    P_n = P_0 + n * I_avg + n * (n+1) / 2 * I_inc_avg
    Where, I_avg = Average arithmetic increase
    I_inc_avg = Average of incremental increases
• Per Capita Wastewater Flow: The amount of wastewater generated per person per day (Lpcd).

Sewerage System Types

• Separate System: Carries domestic and industrial wastewater in one set of sewers and stormwater in another.
  • Advantages: Smaller sewers for wastewater, less treatment volume.
  • Disadvantages: Dual piping network, higher initial cost.
• Combined System: Carries both wastewater and stormwater in a single set of sewers.
  • Advantages: Single piping network, lower initial cost.
  • Disadvantages: Larger sewers, higher treatment volume during wet weather, potential for combined sewer overflows (CSOs).
• Partially Combined System: A hybrid where some stormwater (e.g., from roofs) is admitted into sanitary sewers, while street runoff is kept separate.

Design Criteria of Sewers

Sewers are designed for gravity flow, ensuring self-cleansing velocities.

• Minimum Velocity: To prevent solids from settling and causing blockages.
  • 0.6 - 0.75 m/s at partial flow (e.g., 0.3 times full depth) for self-cleansing.
  • Self-cleansing velocity: The minimum velocity required to scour and carry away suspended solids.
• Maximum Velocity: To prevent scour and erosion of the sewer material.
  • 2.5 - 3.0 m/s (depending on pipe material).

Shapes of Sewers

• Circular: Most common, hydraulically efficient for full flow.
• Egg-shaped: Better self-cleansing at low flows, as the smaller invert provides higher velocity.
• Horseshoe: Used for large sewers and tunnels.

Sewer Materials

• Cast Iron (CI) / Ductile Iron (DI): Strong, durable, used for pressure mains or where structural strength is required.
• PVC (Polyvinyl Chloride): Lightweight, corrosion-resistant, smooth, easy to install.
• Concrete (RCC): Strong, durable, common for large diameter sewers.
• Brick Masonry: For large, custom-built sewers, less common now.

Design of Sewers for Separate System and Combined System

Sewer design involves determining pipe diameters and slopes to carry peak flows while maintaining velocities within acceptable limits. Manning's formula is commonly used for open channel flow (gravity sewers).

Manning's Formula at Partial Flow:
V = (1/n) * R^(2/3) * S^(1/2)
Q = A * V
Where, V = Flow velocity (m/s)
n = Manning's roughness coefficient (dimensionless)
R = Hydraulic radius (m) = A / P_w (Area of flow / Wetted perimeter)
S = Slope of the energy grade line (m/m) (for uniform flow, equal to pipe slope)
Q = Flow rate (m³/s)
A = Cross-sectional area of flow (m²)
For partial flow, A and R are functions of the flow depth (d) relative to the pipe diameter (D), often determined using hydraulic elements charts or tables.

Construction of Sewers

• Trenching: Excavating the trench to the required depth and width, ensuring proper bedding.
• Bedding: Laying a stable foundation (e.g., sand, gravel) for the pipe to support it and distribute loads.
• Jointing: Connecting pipe sections to ensure watertight seals.
• Backfilling: Refilling the trench, compacting the soil in layers to prevent settlement.

Sewer Appurtenances

Structures built along sewers to facilitate inspection, maintenance, and operation.

• Manholes: Access points for inspection, cleaning, and ventilation. Spaced at regular intervals (e.g., 50-100 m).
• Drop Manholes: Used when a branch sewer enters a main sewer at a significantly higher elevation, to prevent damage from falling wastewater.
• Lampholes: Small openings for inserting a lamp to check for blockages.
• Flushing Devices: Tanks that periodically release a flush of water to clean sewers with insufficient self-cleansing velocity.
• Inverted Siphons: Sections of sewer that flow under pressure beneath an obstruction (e.g., river, road).

6.5 Treatment and Disposal of Wastewater

Characteristics of Sewage

Sewage (wastewater) contains various impurities:

• Physical: Turbidity, color, odor, temperature, Total Suspended Solids (TSS).
• Chemical: pH, Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), nutrients (nitrogen, phosphorus), chlorides, heavy metals.
• Biological: Pathogenic microorganisms (bacteria, viruses, protozoa), indicator organisms (coliforms).

Decomposition of Wastewater

• Aerobic Decomposition: Occurs in the presence of oxygen. Microorganisms convert organic matter into stable end products (CO₂, H₂O, NO₃⁻). Faster, less odorous.
• Anaerobic Decomposition: Occurs in the absence of oxygen. Microorganisms convert organic matter into methane (CH₄), CO₂, H₂S, and other reduced compounds. Slower, produces foul odors.

BOD (Biochemical Oxygen Demand)

A measure of the amount of oxygen consumed by microorganisms while decomposing organic matter in water.

• 5-day BOD at 20°C (BOD₅): Standard test, measures oxygen consumed over 5 days at 20°C.
• BOD Reaction Rate Constant (k): Rate at which BOD is exerted.
  L_t = L_0 * (1 - e^(-k*t))
  Where, L_t = BOD exerted at time t
  L_0 = Ultimate BOD (total oxygen demand)
  k = BOD reaction rate constant (base e, per day)
  t = Time (days)
• Ultimate BOD (L₀): The total amount of oxygen required for complete decomposition of organic matter.

COD (Chemical Oxygen Demand)

A measure of the oxygen equivalent of the organic matter in water that can be oxidized by a strong chemical oxidant (e.g., potassium dichromate).

• Dichromate Method: Standard laboratory method.
• COD > BOD: COD measures both biodegradable and non-biodegradable organic matter, while BOD measures only biodegradable. Therefore, COD is always greater than or equal to BOD.

Primary Treatment

Physical processes to remove large and settleable solids.

• Screening: Removes large floating debris.
• Grit Removal: Removes heavy inorganic solids (sand, grit) to protect pumps and prevent abrasion.
  Design of Grit Chamber (e.g., horizontal flow):
  • Horizontal Velocity (V_h): Maintained at 0.25-0.3 m/s to allow grit to settle but keep organic matter in suspension.
    V_h = L / t_d
    Where, L = Length of chamber (m)
    t_d = Detention time (s)
  • Detention Time (t_d): Typically 60-90 seconds.
  • Settling Velocity (V_s): Velocity at which grit particles settle. For discrete particles, Stokes' Law can be used:
    V_s = (g * (ρ_p - ρ_w) * d²) / (18 * μ)
    Where, g = Acceleration due to gravity (m/s²)
    ρ_p = Density of particle (kg/m³)
    ρ_w = Density of water (kg/m³)
    d = Diameter of particle (m)
    μ = Dynamic viscosity of water (Pa·s)
• Primary Sedimentation (Primary Clarifier): Removes settleable organic and inorganic solids. Similar design principles to plain sedimentation for water treatment (surface loading, detention time).

Secondary/Biological Treatment

Removes dissolved and colloidal organic matter using microorganisms.

• Trickling Filter: Wastewater is sprayed over a bed of media (rocks, plastic) where a biological film (biofilm) grows. Microorganisms in the film oxidize organic matter.
• Rotating Biological Contactor (RBC): Large, closely spaced discs rotate slowly through wastewater, developing a biofilm that treats the sewage.

Activated Sludge Process (ASP)

A suspended growth biological treatment process.

• Aeration Tank: Wastewater is mixed with activated sludge (floc of microorganisms) and aerated to provide oxygen for biological activity.
• Secondary Settling Tank (Secondary Clarifier): Separates the treated effluent from the activated sludge.
• Return Sludge: A portion of the settled sludge is returned to the aeration tank to maintain a high concentration of active microorganisms.
• Mixed Liquor Suspended Solids (MLSS): Concentration of suspended solids (microorganisms) in the aeration tank.
• Food-to-Microorganism Ratio (F/M Ratio): Ratio of incoming organic load (food) to the mass of microorganisms (F/M). Controls the process efficiency.
  F/M = (Q * S₀) / (V * X)
  Where, Q = Influent flow rate (m³/day)
  S₀ = Influent BOD concentration (mg/L)
  V = Volume of aeration tank (m³)
  X = MLSS concentration (mg/L)
• Sludge Retention Time (SRT) / Mean Cell Residence Time (MCRT): Average time microorganisms remain in the system. Crucial for process stability.
  SRT = (Mass of solids in aeration tank) / (Mass of solids wasted per day)
SRT = (V * X) / (Q_w * X_r + (Q - Q_w) * X_e) (More detailed, considering wasted sludge flow Q_w, return sludge concentration X_r, and effluent suspended solids X_e)

Oxidation Ponds (Stabilization Ponds)

Large, shallow earthen basins for natural wastewater treatment using algae and bacteria.

• Aerobic Ponds: Shallow, oxygen supplied by photosynthesis and surface aeration.
• Facultative Ponds: Most common, aerobic layer at the top, anaerobic layer at the bottom.
• Anaerobic Ponds: Deep, no oxygen, for strong organic wastes.
• Design Criteria: Based on organic loading rate (kg BOD/ha/day), detention time, and depth.

Wastewater Disposal by Dilution

Discharging treated wastewater into a large body of water (river, lake, ocean) where it is diluted and naturally purified.

• Oxygen Sag Curve: Illustrates the depletion and recovery of dissolved oxygen (DO) in a river downstream from a wastewater discharge point.
  Streeter-Phelps Equation: Describes the oxygen sag curve.
  D_t = (k_d * L_a) / (k_a - k_d) * (e^(-k_d * t) - e^(-k_a * t)) + D_0 * e^(-k_a * t)
  Where, D_t = Dissolved oxygen deficit at time t (mg/L)
  D_0 = Initial DO deficit at mixing point (mg/L)
  L_a = Initial ultimate BOD of mixed river water (mg/L)
  k_d = Deoxygenation rate constant (base e, per day)
  k_a = Reaeration rate constant (base e, per day)
  t = Time of flow downstream (days)

Wastewater Disposal by Land Treatment

Applying wastewater to land for treatment and beneficial reuse (e.g., irrigation).

• Slow Rate (SR): Wastewater applied at low rates to vegetated land, treated by soil-plant-microbe system, often used for irrigation.
• Rapid Infiltration (RI): Wastewater applied at higher rates to highly permeable soils, treated as it percolates through the soil to groundwater.
• Overland Flow (OF): Wastewater applied to gently sloping vegetated land, flows as a thin sheet, treated by biological processes as it moves downslope.

Sludge and Solid Waste Disposal Methods

• Sludge Disposal: Digestion (anaerobic/aerobic), dewatering (drying beds, mechanical), composting, incineration, landfilling, agricultural reuse.
• Solid Waste Disposal: Landfilling (sanitary landfills), composting, incineration, recycling.

Latrine and Septic Tank

• Latrine: Basic sanitation facility for human waste disposal, ranging from simple pit latrines to more advanced pour-flush systems.
  Design: Simple pit latrines involve digging a pit for excreta collection. Improved designs include a concrete slab, vent pipe, and water seal.
• Septic Tank: An on-site wastewater treatment system for individual households or small communities.
  Function: Receives raw sewage, allows solids to settle and undergo anaerobic digestion in the tank. Effluent then flows to a soil absorption field (leach field) for further treatment.
  Design: Based on population served, detention time (typically 24-48 hours), sludge storage volume, and liquid depth (e.g., 1.2-1.8 m). Requires two or three compartments and baffles.

6.6 Concept of Environmental Assessment

Environmental Assessment Framework

Tools to predict and evaluate the environmental consequences of proposed projects.

• Basic Environmental Study (BES): A simplified assessment for small-scale projects with minor environmental impacts. Focuses on identifying potential impacts and basic mitigation measures.
• Initial Environmental Examination (IEE): A more detailed screening process for projects with potentially significant but manageable impacts. Determines if a full EIA is required.
  • Process: Project screening, baseline data collection, impact identification, mitigation measures, report preparation, review, and approval.
• Environmental Impact Assessment (EIA): A comprehensive study for projects with significant environmental impacts.
  • Process: Screening, scoping, baseline data collection, impact prediction and evaluation, mitigation measures, alternative analysis, environmental management plan (EMP), public consultation, report preparation, review, decision-making, and monitoring.
  • Matrix: A tool used in EIA to identify and assess interactions between project activities and environmental components.
  • Mitigation: Measures taken to avoid, reduce, or compensate for adverse environmental impacts.

Government Acts, Rules/Regulations/Procedures for BES/IEE/EIA (Nepal)

• Nepal Environment Protection Act, 2019 (2076 BS): The primary legal framework governing environmental protection and management, including provisions for IEE and EIA.
• Environment Protection Rules, 2020 (2077 BS): Provides detailed procedures and guidelines for conducting IEE and EIA, including project categorization and approval processes.
• EIA Guidelines: Sector-specific guidelines issued by the Ministry of Forests and Environment to standardize the EIA process.

Types of Disaster and Mitigation Measures

Natural and man-made disasters pose significant risks, requiring preparedness and mitigation.

• Types of Disaster:
  • Earthquake: Ground shaking, structural damage, landslides, tsunamis.
  • Flood: Overflow of water onto land, causing inundation, erosion, and damage.
  • Landslide: Mass movement of rock, debris, or earth down a slope.
  • Drought: Prolonged period of abnormally low rainfall, leading to water scarcity.
• Mitigation Measures:
  • Earthquake: Building code enforcement (seismic design), retrofitting existing structures, early warning systems, public awareness.
  • Flood: Embankments, dykes, dams, river training works, flood plain zoning, early warning systems, afforestation.
  • Landslide: Bio-engineering (vegetation), retaining walls, slope stabilization techniques, proper drainage, land-use planning.
  • Drought: Water conservation, rainwater harvesting, irrigation efficiency, developing drought-resistant crops, water resource management, inter-basin water transfer.