6.5 Treatment and Disposal of Wastewater

6.5 Treatment and Disposal of Wastewater

Introduction to Wastewater Management

The ultimate goal of wastewater engineering is to collect, treat, and dispose of sewage in a manner that protects public health and the environment. After wastewater is conveyed by sewers, it undergoes a series of treatment processes to remove contaminants before its final disposal or reuse. This unit explores the nature of wastewater, the physical, chemical, and biological processes used for its purification, and the methods for safe disposal of both the treated effluent and the resulting sludge. Understanding these processes is key to mitigating water pollution and promoting sustainable water cycles.

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1. Characteristics and Examination of Sewage

1.1 Physical Characteristics

1. Color: Fresh sewage is greyish; stale sewage turns black due to anaerobic decomposition.

2. Odor: Fresh sewage has a musty smell; stale sewage has a foul, offensive odor of hydrogen sulphide ($H_2S$).

3. Temperature: Slightly warmer than groundwater (typically 10-20°C), affecting biological activity.

4. Turbidity: Caused by suspended solids.

1.2 Chemical Characteristics

1. pH: Typically neutral to slightly alkaline (pH 7-8). Turns acidic during septic conditions.

2. Total Solids:

   • Total Suspended Solids (TSS): Solids retained on a filter (paper). A key parameter.

   • Total Dissolved Solids (TDS): Solids passing through the filter.

3. Nitrogen Content: Present as organic nitrogen, ammonia ($NH_3$), nitrites ($NO_2^-$), and nitrates ($NO_3^-$). Indicates nutrient pollution (eutrophication).

4. Phosphorus Content: Another key nutrient from detergents and waste.

5. Chlorides: High levels indicate infiltration of seawater or industrial waste.

6. Toxic Compounds: Heavy metals, pesticides from industrial or agricultural sources.

1.3 Biological Characteristics

1. Pathogenic Organisms: Bacteria (e.g., E. coli), viruses, protozoa, and helminths causing diseases.

2. Indicator Organisms: Coliform bacteria (especially E. coli) are tested as indicators of fecal pollution.

3. BOD and COD: The most important parameters defining the strength of sewage and its polluting potential.

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2. Decomposition of Wastewater, BOD and COD

2.1 Decomposition Processes

1. Aerobic Decomposition: In the presence of dissolved oxygen (DO), aerobic bacteria convert organic matter into stable end products. $\text{Organic Matter} + O_2 \xrightarrow{\text{Aerobic Bacteria}} CO_2 + H_2O + \text{Energy}$ This process is odorless.

2. Anaerobic Decomposition: In the absence of DO, anaerobic bacteria break down organic matter. $\text{Organic Matter} \xrightarrow{\text{Anaerobic Bacteria}} CH_4 + CO_2 + H_2S + \text{other intermediates}$ This process is slow and produces foul-smelling gases.

2.2 Biochemical Oxygen Demand (BOD)

• Definition: The amount of dissolved oxygen required by aerobic microorganisms to decompose the biodegradable organic matter in wastewater at a specified temperature (20°C) over a specific period (5 days). BOD₅ is the standard.

• Significance: Measures the organic strength of wastewater. High BOD means high organic pollution, which can deplete DO in receiving waters, killing aquatic life.

• Typical Values:

  • Strong Sewage: > 300 mg/L

  • Medium Sewage: 200-300 mg/L

  • Weak Sewage: < 200 mg/L

• The BOD Curve: The oxygen demand increases with time and eventually reaches an ultimate value (Ultimate BOD, BOD_u) when all organic matter is oxidized. The BOD exertion follows a first-order reaction: $BOD_t = BOD_u (1 - e^{-kt})$ where k is the deoxygenation rate constant.

2.3 Chemical Oxygen Demand (COD)

• Definition: The amount of oxygen required to chemically oxidize the organic and inorganic matter in wastewater using a strong chemical oxidant (potassium dichromate, $K_2Cr_2O_7$).

• Significance: Measures the total oxidizable matter (biodegradable + non-biodegradable). COD value is always higher than BOD. The BOD/COD ratio indicates biodegradability (typically 0.4-0.8 for domestic sewage).

• Advantage: Test takes only 2-3 hours, unlike the 5-day BOD test.

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3. Primary Treatment Processes and Design of Grit Chamber

Primary Treatment involves the removal of settleable organic and inorganic solids by physical operations (screening, sedimentation).

3.1 Screens

• Purpose: Remove large floating solids (rags, sticks, plastics).

• Types: Coarse bar screens (25-50 mm spacing) followed by fine screens (5-10 mm).

3.2 Grit Chamber

• Purpose: To remove heavy inorganic solids like sand, gravel, grit, and eggshells that could cause abrasion and wear in pumps and pipelines.

• Principle: Sedimentation at constant velocity. Designed to settle only high-specific-gravity solids (≈2.65) while allowing lighter organic solids to remain in suspension.

• Design Criteria:

  • Horizontal Flow Velocity: Maintained at 0.2-0.3 m/s.

  • Detention Time: 30-90 seconds.

  • Removal: Particles > 0.2 mm diameter.

• Types:

  • Horizontal Flow (Rectangular): Most common.

  • Aerated Grit Chamber: Air is introduced to create a spiral flow pattern; grit settles while organics are kept in suspension.

3.3 Primary Sedimentation Tank (Clarifier)

• Purpose: To remove settleable organic solids (feces, food particles) by gravity, reducing the BOD load on secondary treatment.

• Design Parameters (for rectangular tanks):

  • Detention Time: 1.5 - 2.5 hours.

  • Surface Overflow Rate (SOR): 30 - 50 m³/day/m².

  • Weir Loading Rate: < 150 m³/day/m.

• Efficiency: Removes 50-70% of TSS and 25-40% of BOD.

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4. Secondary or Biological Treatment Process

Secondary Treatment aims to remove dissolved and colloidal organic matter using aerobic biological processes.

4.1 Sewage Filtration (Trickling Filter)

• Process: Wastewater is sprinkled over a bed of coarse media (stone or plastic). A microbial slime layer (biofilm) develops on the media. As sewage trickles down, organic matter is adsorbed and oxidized by microorganisms in the biofilm.

• Components:

  • Filter Bed: 1-2.5 m deep.

  • Distribution System: Rotating arms with nozzles.

  • Underdrain System: Collects treated effluent and sludge.

• Secondary Clarifier: Essential to separate the sloughed-off biofilm (humus) from the final effluent.

• Recirculation: Treated effluent is often recycled to dilute incoming sewage and maintain wetting of the filter.

4.2 Activated Sludge Process (ASP)

• Process: The most common secondary treatment. Aerated sewage is mixed with a biologically active mass of microorganisms called activated sludge in an aeration tank. Microorganisms consume organic matter.

• Components:

  1. Aeration Tank: Air is supplied by diffusers or mechanical aerators to maintain DO > 2 mg/L.

  2. Secondary Clarifier: Settles the activated sludge biomass.

  3. Return Activated Sludge (RAS): A portion of settled sludge is recycled to the aeration tank to maintain the microbial population.

  4. Waste Activated Sludge (WAS): Excess sludge is wasted for further treatment.

• Key Parameters:

  • Food to Microorganism Ratio (F/M Ratio): Critical control parameter. Typical range: 0.2 - 0.5 kg BOD/kg MLSS/day.

  • Mixed Liquor Suspended Solids (MLSS): Concentration of microorganisms in the aeration tank (2000-4000 mg/L).

  • Mean Cell Residence Time (MCRT or Sludge Age): 5-15 days.

• Efficiency: Removes 85-95% of BOD.

4.3 Oxidation Ponds (Stabilization Ponds)

• Process: Large, shallow earthen basins where wastewater is treated by natural processes involving algae and bacteria in a symbiotic relationship.

  • Bacteria oxidize organic matter, producing $CO_2$ and nutrients.

  • Algae use $CO_2$ and nutrients for photosynthesis, releasing oxygen.

• Types:

  • Aerobic Ponds: Shallow (0.5-1 m), fully aerobic, high algal content.

  • Facultative Ponds (Most Common): 1-2 m deep. Upper aerobic zone, lower anaerobic zone.

  • Anaerobic Ponds: Deep (2.5-5 m), for high-strength wastewater, precede facultative ponds.

• Advantages: Simple, low cost, low energy, effective pathogen removal.

• Disadvantages: Large land area, climate-dependent, potential for odor.

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5. Wastewater Disposal by Dilution: Oxygen Sag Curve, Streeter-Phelps Equation

5.1 Disposal by Dilution

• The discharge of treated or partially treated wastewater into a large body of water (river, sea) where it is diluted and purified by natural processes (aeration, sedimentation, bacterial action).

• Prerequisites: Adequate flow in the receiving stream to provide dilution and self-purification capacity.

5.2 The Oxygen Sag Curve

• Concept: When wastewater with high BOD is discharged into a river, it depletes the Dissolved Oxygen (DO) downstream. The DO level drops to a minimum (Critical DO Deficit, D_c) and then gradually recovers due to re-aeration from the atmosphere. The plot of DO vs. distance/time downstream is called the Oxygen Sag Curve.

5.3 Streeter-Phelps Equation

• Purpose: To model and predict the DO sag curve in a stream.

• Assumptions: One-dimensional flow, constant deoxygenation and reaeration rates.

• The Equation (for the DO deficit, D): $\frac{dD}{dt} = K_1 L - K_2 D$ Integrated form: $D_t = \frac{K_1 L_0}{K_2 - K_1} \left( e^{-K_1 t} - e^{-K_2 t} \right) + D_0 e^{-K_2 t}$ Where:

  • $D$ = DO deficit at time t (mg/L) = Saturation DO - Actual DO.

  • $L_0$ = Ultimate BOD of the river-wastewater mix at the point of discharge.

  • $K_1$ = Deoxygenation rate constant (day⁻¹).

  • $K_2$ = Re-aeration rate constant (day⁻¹).

  • $D_0$ = Initial DO deficit at t=0.

• Critical Deficit (D_c) and Critical Time (t_c): $t_c = \frac{1}{K_2 - K_1} \ln \left[ \frac{K_2}{K_1} \left(1 - \frac{D_0 (K_2 - K_1)}{K_1 L_0} \right) \right]$ $D_c = \frac{K_1}{K_2} L_0 e^{-K_1 t_c}$

• Significance: Used to assess the impact of a wastewater discharge on a river and to set effluent standards to ensure the river's DO never falls below a minimum (e.g., 4 mg/L for aquatic life).

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6. Wastewater Disposal by Land Treatment

• Principle: Application of wastewater to land, where treatment occurs through physical filtration, chemical adsorption, and biological action in the soil matrix.

• Methods:

  1. Slow Rate (SR) Systems: Wastewater is applied by sprinkling or surface flooding at a rate that does not cause surface runoff. Water is lost via evapotranspiration and percolation (groundwater recharge). Most common.

  2. Rapid Infiltration (RI) Basins: Intermittent flooding in permeable soils (sand, loamy sand). High hydraulic loading. Primary goal is groundwater recharge.

  3. Overland Flow (OF): Wastewater is applied to gently sloping, vegetated terraces. Treatment occurs as water flows over the surface (biofilm on vegetation). Collected at the bottom for discharge.

• Benefits: Reuse of water and nutrients (fertilizer value), low energy.

• Constraints: Requires large land area, careful management to prevent groundwater pollution.

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7. Sludge and Solid Waste Disposal Methods

Sludge is the semi-solid residue generated during wastewater treatment (primary and secondary).

7.1 Sludge Treatment

1. Thickening: Increases solids content by gravity settling (gravity thickener) or flotation.

2. Stabilization: To reduce pathogens and eliminate odor. Methods:

   • Anaerobic Digestion (Most Common): Sludge is heated and mixed in a sealed tank (digester) for 15-30 days. Produces biogas (≈65% CH₄) and stabilized sludge (biosolids).

   • Aerobic Digestion: Prolonged aeration of sludge.

   • Lime Stabilization: Adding lime to raise pH.

3. Conditioning: Improves dewaterability. Chemical conditioning with polymers (polyelectrolytes) is common.

4. Dewatering: Reduces volume. Methods: Centrifugation, Belt Filter Press, Filter Press, Drying Beds.

5. Drying and Incineration: Thermal drying or complete combustion to ash (high cost, used where land is scarce).

7.2 Sludge Disposal/Reuse

1. Land Application (Agricultural Use): Treated, stabilized biosolids are excellent soil conditioners and fertilizers.

2. Landfilling: With municipal solid waste.

3. Incineration Ash: Can be used in construction materials or landfilled.

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8. Latrine and Septic Tank

8.1 Latrines (On-site Sanitation)

Used where sewerage systems are not available.

• Pit Latrine: A simple pit in the ground covered by a slab with a hole. Waste decomposes in the pit.

• Ventilated Improved Pit (VIP) Latrine: Includes a vent pipe to remove odors and flies.

• Pour-Flush Latrine: Uses a small amount of water for flushing into a pit or septic tank.

8.2 Septic Tank

• Purpose: A primary, decentralized treatment unit for individual houses or small communities. It combines sedimentation and anaerobic digestion.

• Process:

  1. Sedimentation: Solids settle to the bottom forming sludge.

  2. Anaerobic Digestion: Sludge is partially stabilized by anaerobic bacteria.

  3. Scum Formation: Light materials (grease, oils) float forming a scum layer.

  4. Effluent: Partially clarified liquid flows out to a soak pit or drain field for further soil percolation.

• Design Features:

  • Typically rectangular, with 24-48 hours detention time.

  • Baffles or a T-shaped outlet prevent scum and sludge from escaping.

  • Requires periodic desludging (every 2-5 years).

• Efficiency: Removes 30-50% of BOD and 60-70% of TSS. Does NOT produce effluent suitable for discharge to surface water.

Conclusion: Wastewater treatment is a multi-stage process designed to mimic and accelerate natural purification. From primary physical separation to secondary biological oxidation and finally to sludge management, each step targets specific pollutants. The choice of treatment and disposal method depends on the wastewater characteristics, available resources, environmental standards, and the intended final reuse or discharge pathway.