Understanding Water Hardness

  • Objective: Find total hardness of water through two methods: milliequivalents per liter and milligrams per liter as calcium carbonate.

Calculating Equivalent Weight

  • Find the equivalent weight of calcium and magnesium ions.
    • Equivalent weight is determined using the formula:
      \text{Equivalent weight} = \frac{\text{Molecular weight}}{\text{Charge}}
    • For calcium (Ca):
    • Molecular weight = 40 g/mol
    • Charge = 2 (Ca$^{2+}$)
    • Equivalent weight = \frac{40}{2} = 20 \, \text{mg/meq}
    • For magnesium (Mg):
    • Molecular weight = 24.3 g/mol
    • Charge = 2 (Mg$^{2+}$)
    • Equivalent weight = \frac{24.3}{2} = 12.15 \, \text{mg/meq}

Converting Concentrations

  • Calcium ions:

    • Given concentration: 150 mg/L
    • Convert to milliequivalents per liter (meq/L):
      \text{Calcium ions in meq/L} = \frac{150 \, \text{mg/L}}{20 \, \text{mg/meq}} = 7.5 \, \text{meq/L}

  • Magnesium ions:

    • Given concentration: 60 mg/L
    • Convert to milliequivalents per liter:
      \text{Magnesium ions in meq/L} = \frac{60 \, \text{mg/L}}{12.15 \, \text{mg/meq}} = 4.92 \, \text{meq/L}

Total Hardness Calculation (Method 1)

  • Total hardness in meq/L:
    • Total = Calcium + Magnesium
      7.5 + 4.92 = 12.42 \, \text{meq/L}

Conversion to Calcium Carbonate

  • Method 2: Convert total hardness to mg/L as calcium carbonate.

    • Calcium to mg/L as CaCO₃:
    • 150 \, \text{mg/L} \times \frac{50 \, \text{mg as CaCO₃}}{20 \, \text{mg}} = 375 \, \text{mg/L as CaCO₃}
    • Magnesium to mg/L as CaCO₃:
    • 60 \, \text{mg/L} \times \frac{50 \, \text{mg as CaCO₃}}{12.15 \, \text{mg}} = 245.9 \, \text{mg/L as CaCO₃}

  • Total Hardness in mg/L as CaCO₃:
    375 + 245.9 = 620.9 \, \text{mg/L as CaCO₃}

Clarifier Design Problem

  • Context: Wastewater treatment plan with a circular clarifier receiving 5,000,000 gallons per day.
  • Parameters:
    • Clarifier radius: 43 ft
    • Depth: 10 ft
  • Objectives: Calculate detention time and overflow rate.

Detention Time Calculation

  • Formula for volume of circular clarifier: V = h \cdot \pi r^2
    • Where V = volume, h = depth, r = radius
  • Convert flow rate: 5,000,000 gallons/day
    • Using conversion factor: 7.48 gallons/cubic foot:
      \text{Flow rate} = \frac{5,000,000 \, \text{gal}}{7.48 \, \text{gal/cubic ft}} = 668,449 \, \text{cubic ft/day}
  • Detention Time (θ):
    • Formula:
      \theta = \frac{V}{\text{Flow rate}}
    • \theta = \frac{10 \cdot \pi \cdot (43^2)}{668,449} = 0.869 \, \text{days} = 2.06 \, \text{hours}

Overflow Rate (Critical Speed)

  • Formula: V_c = \frac{\text{Depth}}{\text{Detention Time}}
    • V_c = \frac{10 \, \text{ft}}{2.06 \, \text{hours}} = 4.85 \, \text{ft/hour}

Hazardous Waste Legislation

Definition and Significance

  • Regulation: Resource Conservation Recovery Act (RCRA) defines hazardous waste as material that:
    • Causes or contributes to increased mortality or serious illness (carcinogens, damaging chemicals).
    • Poses a substantial hazard to human health and the environment when improperly managed.
  • Key Focus: Proper management throughout its entire life cycle: generation, storage, transport, treatment, and disposal.

Identification of Hazardous Waste

  • EPA Lists:
    • F and K lists: Waste from industrial processes (solvents, manufacturing waste).
    • P and U lists: Pure and commercial-grade unused chemicals (arsenic, cyanide, toluene).
  • Characteristics of Hazardous Waste:
    • Ignitability: Readily catches fire.
    • Corrosivity: pH < 2 or >= 12.5.
    • Reactivity: Unstable, reactive with water.
    • Toxicity: Leaches harmful substances into groundwater.

Regulatory Statistics

  • Waste Generation: Over 23 million tons of hazardous waste disposed annually.

Waste Management and Disposal

  • Cradle to Grave Management: Critical to handle hazardous waste correctly at every stage.
  • Transportation: Must be accompanied by a manifest detailing the waste.
  • Disposal Facilities: Must be permitted to handle specific types of waste. Special routes for hazardous materials.
    • Labeling required for hazardous waste using the globally harmonized system.

Treatment and Disposal Techniques

Treatment Approaches

  • Physical Treatments:
    • Sedimentation: Heavier particles settle, lighter particles skimmed.
    • Absorption: Use materials (e.g., activated carbon) to trap contaminants.
    • Aeration: Introduces oxygen to remove volatile organic compounds (VOCs).
    • Reverse Osmosis: Membrane filtration technique to separate contaminants.

Chemical Treatments

  • Neutralization: Adjust pH levels to mitigate corrosivity.
    • Acidic waste can use lime to increase pH.
    • Alkaline waste can use carbon dioxide for neutralization.
  • Chemical Precipitation: Heavy metals can be removed by forming hydroxides.
  • Thermal Destruction: Effective for organic waste (incineration).

Incineration Techniques

  • Types of Incinerators:
    • Liquid Injection: Suitable for thin liquids; sprays through nozzles.
    • Rotary Kiln: Handles sludge; rotates for mixing.
  • Combustion Optimization Factors:
    • Combustion Temperature, Duration in Chamber, and Oxygen Supply.
  • Efficiency Standards: RCRA requires destruction removal efficiency (DRE) of 99.99% for most organics.

Disposal Practices

Land Disposal Types

  • Landfills: Must segregate compatible wastes to avoid hazardous reactions.
    • Double liner systems for leaks and leachate collection systems.
    • Groundwater monitoring is mandatory.

Injection Wells and Surface Impoundments

  • Injection Wells: Must extend deep enough to prevent contamination of drinking water.
  • Surface Impoundments: Temporary storage for liquid hazardous waste, with strict EPA guidelines.

Non-Regulated Hazardous Wastes

  • Household hazardous waste (e.g., cleaning agents, batteries) is often not regulated by the EPA.
    • Agricultural wastes and fossil fuel combustion wastes can also be considered unregulated.
  • These unregulated materials can still contain hazardous ingredients but lack federal oversight.

Energy Balances

Basics of Energy Balances

  • First Law of Thermodynamics: Energy cannot be created or destroyed.
  • Energy Types: Kinetic, potential, internal.
  • Specific Heat Capacity (c): Amount of energy required to change temperature.
    • Units: \text{kJ/kg°C} or \text{BTU/lb°F} .

Energy Balance Equation

  • Conservation Equation:
    \Delta E = m c \Delta T
  • This equation considers mass (m), heat capacity (c), and change in temperature (ΔT).

Example Problem on Energy Balance

  • Heating Water in a Tank:
    • Given: 40 gallons, temperature change from 50°F to 140°F while heating rate is 5 kilowatts.
    • Calculate total energy required using energy balance equations.
  • Convert gallons to pounds for specific heat calculations.

Power and Efficiency

  • Power (P): Rate of energy flow, measured in watts (1 watt = 1 joule/sec).
  • Efficiency (η): Ratio of useful output energy to input energy.
  • Example: Coal-fired power plants show varying efficiencies based on age and design.

Final Insights

Summary

  • Discussed how to calculate water hardness and detention times in clarifiers and explored the complexities of hazardous waste regulations, energy balances, and disposal methods. Emphasized the importance of precision in calculations and adherence to regulations for both environmental safety and engineering practices.