Chapter 10: Water Treatment – Principles and Processes

Water Treatment: Principles and Processes

Introduction to Water Treatment

Water treatment is essential for providing safe and aesthetically pleasing drinking water. This process involves removing contaminants and ensuring water meets regulatory standards for human consumption.

• Principles of water treatment: Removal of physical, chemical, and biological contaminants to produce water that is both palatable and potable.

• Regulations: Drinking water quality is regulated by the Safe Drinking Water Act (SDWA) and amendments. The US EPA sets maximum contaminant levels (MCLs).

• Design and operation: Includes understanding schematics, performing calculations (e.g., retention time, filtration rates), and applying design methods for treatment plants.

Palatable vs. Potable Water

Definitions and Differences

• Palatable water: Water that is aesthetically pleasing (good taste, odor, color, low turbidity) and does not pose a health threat due to chemical content (e.g., chloride, iron, manganese).

• Potable water: Water that is safe to drink, regardless of aesthetic qualities. Potability is affected by the presence of microbials (e.g., Giardia, Cryptosporidium), organic chemicals (e.g., alachlor, chlordane), inorganic chemicals (e.g., lead, mercury), and radionuclides.

Goal: Municipal water treatment aims to provide water that is both palatable and potable.

Sources of Drinking Water

Groundwater and Surface Water

• Groundwater: Sourced from shallow and deep wells.

• Surface water: Sourced from rivers, lakes, and reservoirs.

Categories of Drinking Water Quality

• Physical: Color, turbidity, temperature, taste, odor.

• Chemical: Inorganic and organic chemicals.

• Microbiological: Bacteria, viruses, protozoa, algae.

• Radioactivity: Alpha and beta particles, photon emitters, radionuclides (e.g., 226Ra, 228Ra, U).

Comparison: Groundwater vs. Surface Water

Property	Groundwater	Surface Water
Composition	Constant	Variable
Mineral Content	High	Low
Turbidity	Low	High
Color	Low or none	Colored
Dissolved Oxygen	Low or none	Present
Hardness	High	Low
Other	High Fe, Mn	Taste and odor

Overview of Water Treatment Processes

Surface Water Treatment

Conventional surface water treatment involves several sequential steps to remove contaminants and ensure water safety.

• Screening and rapid mixing with coagulant

• Flocculation basin

• Settling (sedimentation) tanks

• Rapid sand filtration

• Disinfection and storage

Example schematic: Surface water supply → Screen → Rapid mix (coagulant) → Flocculation basin → Sedimentation → Rapid sand filter → Disinfection → Storage → Distribution

Groundwater Treatment

Groundwater treatment often focuses on softening and removal of specific minerals.

• Rapid mix and reaction basin

• Settling tank

• Recarbonation

• Rapid sand filtration

• Disinfection and storage

Key Water Treatment Processes

Coagulation and Flocculation

These processes are used to remove suspended particles and reduce turbidity.

• Coagulation: Addition of chemicals (coagulants) to destabilize colloidal particles by neutralizing their surface charge.

• Flocculation: Gentle mixing to promote aggregation of destabilized particles into larger flocs that can settle by gravity.

Colloids: Small (0.001–1 μm), usually negatively charged particles that remain suspended due to repulsion.

Common Coagulants

• Alum: $\mathrm{Al_2(SO_4)_3 \cdot 14H_2O}$

• Ferric chloride: $\mathrm{FeCl_3}$

• Ferric sulfate: $\mathrm{Fe_2(SO_4)_3}$

• Ferrous sulfate: $\mathrm{FeSO_4}$

• Poly(aluminum hydroxy) chloride (PAX, PACI): $\mathrm{Al_2(OH)_xCl_{6-x}}$ or $\mathrm{Al_n(OH)_mCl_{3n-m}}$

• Polyelectrolytes and coagulant aids (e.g., activated silica, clay)

Alum Reaction and pH Considerations

• Alum hydrolyzes in water: $\mathrm{Al_2(SO_4)_3 \cdot 14H_2O \leftrightarrow 2Al^{3+} + 3SO_4^{2-} + 14H_2O}$

• $2Al^{3+}$ neutralizes colloid surface charge.

• If sufficient alkalinity: $2Al^{3+} + 6HCO_3^- \rightarrow 2Al(OH)_3(s) + 6CO_2$

• If insufficient bicarbonate: $\mathrm{Al_2(SO_4)_3 \cdot 14H_2O \rightarrow 2Al(OH)_3(s) + 3H_2SO_4 + 14H_2O}$

• Optimum pH: 5.5–6.5; Operating pH: 5–8

Rapid Mixing

• Blends chemicals and water; retention time < 30 s.

• Mechanical mixing with impellers and baffles.

• Detention time: $t_o = \frac{V}{Q}$

• Velocity gradient: $G = \sqrt{\frac{P}{\mu V}}$

• Where $P$ = power (W), $V$ = volume (m3), $\mu$ = dynamic viscosity (Pa·s), $G$ = velocity gradient (s-1).

Flocculation

• Paddle units rotate slowly (<1 rpm).

• Water velocity: 0.5–1.5 ft/s (0.15–0.48 m/s).

• Detention time: at least 20–30 min.

• Agitators with variable speed drive; peripheral speed: 0.5–3.0 ft/s (0.15–0.91 m/s).

Sedimentation

• Settling basins allow flocs to settle out (residence time ≥ 4 hours).

• Design equations:

  • Water velocity: $v = \frac{Q}{A_t}$

  • Overflow rate: $V_o = \frac{Q}{A_s}$

  • Detention time: $t_o = \frac{V}{Q}$

  • Weir overflow rate: $WOR = \frac{Q}{\text{weir length}}$

Filtration

• Removes particles too small for sedimentation.

• Types: single media (sand), dual media (anthracite coal and sand), multimedia (anthracite, sand, garnet).

• Effluent turbidity: sedimentation (1–10 JTU), desired after filtration (<0.3 JTU).

• Rapid sand filters require periodic backwashing (15–20 min every 1–2 days).

• Design must accommodate flow with one filter out of service.

Backwashing

• Water (sometimes air) is forced upward through the filter to suspend and remove trapped particles.

Disinfection and Final Treatment

• Disinfection (e.g., chlorination) is performed before distribution.

• Hydrofluosilicic acid may be added for dental health.

• Poly- or orthophosphate may be added to prevent corrosion.

• High-pressure pumps are used for distribution.

Example Calculations

Example 10-1: Coagulation Plant Calculation

Problem: A plant with a flowrate of 0.044 m3/s doses alum at 25.0 mg/L. Raw water suspended solids: 8 mg/L; effluent: 3.0 mg/L. Find total mass of alkalinity consumed and dry solids removed per day.

Example 10-2: Rapid-Mixing Basin Design

• Given: Flow = 7570 m3/d, G = 790 s-1, detention time = 5 s, T = 10°C.

• Volume: $V = (5\,\text{s})(7570\,\text{m}^3/\text{d})\left(\frac{1\,\text{d}}{86400\,\text{s}}\right) = 0.44\,\text{m}^3$

• Power imparted: $P_{\text{water}} = (790\,\text{s}^{-1})^2 (1.307 \times 10^{-3}\,\text{Pa} \cdot \text{s})(0.438\,\text{m}^3) = 357\,\text{W}$

• Motor power: $P_{\text{motor}} = \frac{P_{\text{water}}}{0.8} = 447\,\text{W}$

Example 10-10: Filter Bed Calculation

Problem: Plant flow = 0.044 m3/s, dual media filter beds (3 m × 2 m), design loading rate = 150 m3/day·m2. Find number of filter beds per bank and loading rate with one filter out of service.

Summary Table: Key Water Treatment Steps

Step	Main Purpose	Key Features
Coagulation	Destabilize colloids	Chemical addition, rapid mixing
Flocculation	Aggregate particles	Slow mixing, paddle units
Sedimentation	Settle flocs	Quiescent basins, 4+ hr detention
Filtration	Remove fine particles	Sand/dual/multimedia filters, backwashing
Disinfection	Destroy pathogens	Chlorination, UV, ozone

What You Should Know

• Conceptual understanding of each treatment step

• Ability to perform basic calculations (e.g., retention time, loading rates)

• Familiarity with regulatory standards and water quality categories