Water Treatment Principles and Engineering Notes

Introduction to Water Treatment

• The primary purpose of water treatment is to provide potable water that is also palatable.
• Potable Water: Water that is healthy for human consumption, meaning it is free from harmful microorganisms and organic or inorganic compounds.
• Palatable Water: Water that is aesthetically acceptable to drink, characterized by being free from turbidity, color, odor, and taste.
• Primary Drinking Water Standards: These are established to ensure safe, finished water. They cover microbiological organisms, disinfection by-products, radionuclides, turbidity, and various organic and inorganic chemicals.
• Secondary Drinking Water Standards: These standards are not health-related but address aesthetic, cosmetic, or technical effects. These include considerations for color, odor, corrosivity, $pH$, dissolved solids, and inorganic chemicals.

Physical and Inorganic Characteristics of Raw Water

• Turbidity: A measure of optical clarity caused by light scattering and absorbance by suspended particles. It is measured in Nephelometric Turbidity Units (NTU). The acceptable level is $5\,NTU$. Turbidity serves as an indicator for concentrations of microbial water constituents.
• Particle Classification:
  • Suspended Particles: Larger than $1\,\mu m$.
  • Colloidal Particles: Sized between $0.001\,\mu m$ and $1\,\mu m$. These are generally not distinguishable by the unaided eye but may adsorb toxic metals or synthetic chemicals.
  • Dissolved Particles: Smaller than $0.001\,\mu m$.
• Taste and Odor: Derived from biological sources or dissolved natural organic and inorganic constituents.
• Temperature: Affects physical and chemical parameters including density, viscosity, vapor pressure, surface tension, solubility, and reaction rates.
• Inorganic Constituents and Thresholds:
  • Calcium ($Ca^{2+}$) and Magnesium ($Mg^{2+}$): Concentrations above $60\,mg/L$ cause hardness.
  • Chloride: Levels above $250\,mg/L$ impart a salty taste; levels below $50\,mg/L$ can be corrosive.
  • Fluoride: Toxic at concentrations between $250\,mg/L$ and $450\,mg/L$.
  • Iron ($Fe$): Taste threshold is approximately $0.01\,mg/L$. It stains laundry and bathroom fixtures brownish.
  • Manganese ($Mn$): Imparts a dark brown color, unpleasant taste, and stains fixtures at concentrations around $0.4\,mg/L$.
  • Nitrate: Known to cause infant methemoglobinemia.
  • Sulfur: Imparts objectionable odors and is corrosive to concrete and pipes.

Organic and Microbial Constituents of Raw Water

• Natural Organic Matter (NOM): Result of biochemical degradation of vegetation. It is measured as total organic carbon (TOC). NOM consumes treatment chemicals and may lead to corrosion or slime growth in distribution systems when oxidants are present.
• Synthetic Organic Chemicals (SOCs): Derived from anthropogenic activities such as industrial activity, agricultural land use, urban runoff, and municipal effluent. Examples include fuels, cleaning solvents, herbicides, and pesticides.
• Bacterial Pathogens:
  • Vibrio cholera: Causes classic cholera (diarrhea, vomiting, dehydration, muscle cramps, coma).
  • Salmonella: Causes enteric fever, headaches, and abdominal pain.
  • Shigella dysenteriae: Causes bacillary dysentery (abdominal pain, diarrhea, fever).
  • Escherichia coli: Causes diarrhea.
• Viral Pathogens:
  • Polovirus (Types 1, 2, 3): Leads to fever, headache, muscle pain, and skin sensitivity.
  • Human Adenovirus 2: Causes infections in lungs, eyes, urinary tract, and intestines.
  • Rotavirus A: Causes severe diarrhea and dehydration.
• Protozoan Pathogens:
  • Cryptosporidium parvum: Causes severe diarrhea and nausea.
  • Giardia lamblia: Causes sudden diarrhea, bloating, and weight loss.

Hardness Removal Mechanism and Speciation

• Hardness: Defined as the measure of divalent metal content in water.
• Carbonate (Temporary) Hardness: Associated with alkalinity ions ($CO_{3}^{2-}$ and $HCO_{3}^{-}$).
• Noncarbonate (Permanent) Hardness: Associated with non-alkalinity ions ($SO_{4}^{2-}$ and $Cl^{-}$).
• Mathematical Formulas:
  • Total Hardness ($TH$): $TH = \sum \text{divalent cations}$
  • Equivalent Weight ($EW$): $EW = \frac{MW}{n}$
  • Concentration in meq/L ($Cq$): $Cq = \frac{C}{EW}$
  • Total Hardness ($TH$) as the sum of components: $TH = CH + NCH$
• Alkalinity: The buffering capacity of water. $Alk = [HCO_{3}^{-}] + [CO_{3}^{2-}] + [OH^{-}] - [H^{+}]$.
  • $pH < 4.5$: Main substance is $H_{2}CO_{3}$.
  • $pH \approx 8.3$: Main substance is $HCO_{3}^{-}$.
  • $pH > 12.3$: Main substances are $CO_{3}^{2-}$ and $OH^{-}$.
• Hardness Speciation Terms:
  • $CH$: Carbonate Hardness.
  • $NCH$: Noncarbonate Hardness ($TH - CH$).
  • $CCH$: Calcium Carbonate Hardness.
  • $MCH$: Magnesium Carbonate Hardness.
  • $CNCH$: Calcium Noncarbonate Hardness.
  • $MNCH$: Magnesium Noncarbonate Hardness.

Softening Processes: Soda-Lime and Ion Exchange

• Soda-Lime Softening Chemicals Requirements:
  • Carbon Dioxide ($CO_{2}$): Requires Lime.
  • Calcium Carbonate Hardness ($CCH$): Requires Lime.
  • Calcium Noncarbonate Hardness ($CNCH$): Requires both Lime and Soda Ash.
  • Magnesium Carbonate Hardness ($MCH$): Requires Lime (2 equivalents usually implied for Magnesium).
  • Magnesium Noncarbonate Hardness ($MNCH$): Requires Lime and Soda Ash.
• Ion Exchange Softening:
  • Best for water with high noncarbonate hardness and $TH < 350\,mg/L$ as $CaCO_{3}$.
  • Replaces hardness ions with sodium ions: $Ca(HCO_{3})_{2} + 2NaR \rightarrow CaR_{2} + 2NaHCO_{3}$.
  • Percentage Bypass calculation: $\% \text{Bypass} = \frac{\text{Desired Hardness}}{\text{Initial Hardness}} \times 100$.
  • Breakthrough occurs when effluent concentration equals influent concentration.

Coagulation and Flocculation

• Coagulation: A chemical process that alters suspended/colloidal matter to stick together. Mechanisms include charge neutralization and compression of the electrical double layer.
• Coagulant Properties: Must be a trivalent cation, nontoxic, and insoluble in neutral $pH$ ranges.
• Common Coagulants:
  • Inorganic Metallic: Aluminum sulfate (Alum) [$Al_{2}(SO_{4})_{3} \cdot 14H_{2}O$], Ferric chloride [$FeCl_{3}$], Sodium aluminate [$Na_{2}Al_{2}O_{4}$].
  • Prehydrolized: Polyaluminum chloride (PACl).
  • Organic: Ionic and nonionic polymers.
  • Natural: Moringa oleifera, Opuntia spp.
• Jar Test: Used to determine coagulant type and dose. Involves rapid mixing (addition), then slow mixing (floc formation), followed by settling.
• Sludge Generation: $1\,meq/L$ of coagulant consumes $1\,meq/L$ of alkalinity and produces $1\,meq/L$ of chemical sludge. Total sludge is the sum of chemical sludge and removed suspended solids ($SS$).
• Flocculation: A physical process using slow-speed paddles to create differential velocities for particle contact. Mechanisms include interparticle bridging and enmeshment.

Sedimentation Principles

• Sedimentation: Gravity-based removal of particles. Movement is affected by particle size, shape, density, and fluid properties (density and viscosity).
• Stokes' Law (Laminar flow settling velocity): $v_{s} = \frac{g(\rho_{p} - \rho)d_{p}^{2}}{18\mu}$.
• Transition Regime settling velocity: $v_{s} = \sqrt{\frac{4g(\rho_{p} - \rho)d_{p}}{3c_{d}\rho}}$.
• Ideal Settling Tank Assumptions: Uniform flow, even distribution of particles at the entrance, and every particle reaching the bottom is removed.
• Critical Velocity ($v_{c}$) or Overflow Rate ($OR$): $v_{c} = \frac{h_{0}}{\theta} = \frac{Q}{A}$.
• Percentage Removal: For a particle with velocity $v_{s}$, the removal is $\frac{v_{s}}{OR} \times 100\%$.

Filtration and Disinfection

• Filtration: Separating impurities by passage through a porous medium. Used for turbidity and pathogen removal.
• Filtration Rate: Calculated as $v = \frac{Q}{A}$ (e.g., $8 \times 10^{6}\,L/d$ through a $48\,m^{2}$ filter results in $166.7\,m/d$).
• Disinfection Types:
  • Primary Disinfection: Inactivation of microorganisms.
  • Secondary Disinfection: Maintaining a residual in the distribution system.
• Disinfectant Comparison Table:
  • Free Chlorine: Excellent for bacteria and viruses; forms THMs.
  • Ozone: Excellent for bacteria, viruses, protozoa, and endospores; forms Bromate.
  • Chlorine Dioxide: Excellent for bacteria and viruses; forms Chlorite/Chlorate.
  • Ultraviolet (UV) Light: Excellent for protozoa; bad/fair for viruses; no by-products.

Membrane Separation Processes

• Process Mechanism: Water is pumped under pressure through a semipermeable membrane.
• Configuration: Includes submerged membrane modules or hollow-fiber bundles (Feed in -> Permeate out and Retentate out).
• Membrane Hierarchy (from largest to smallest pore size):
  • Microfiltration (MF): Removes sediment, algae, and protozoa.
  • Ultrafiltration (UF): Removes bacteria and small colloids.
  • Nanofiltration (NF): Removes viruses and divalent ions ($Ca^{2+}$, $Mg^{2+}$).
  • Reverse Osmosis (RO): Removes ionic species ($Na^{+}$, $Cl^{-}$) and dissolved organic matter.