Water Supply Water Treatment

WATER TREATMENT – DEFINITION & GOALS

• Water treatment = all processes that make raw water “acceptable” for its intended end-use (drinking, industrial, medical, etc.).
• Ultimate objectives
• Remove / reduce contaminants.
• Produce water that is safe, aesthetically pleasing and regulation-compliant.

WHY TREAT PUBLIC WATER SUPPLIES?

• Health protection – prevent waterborne disease, toxicity, carcinogenicity.
• Aesthetic improvement – taste, odor, color, staining.
• Regulatory compliance – e.g., mandatory filtration of surface sources; corrosion control for $\text{Pb}$ & $\text{Cu}$.

SOURCE-DEPENDENT ISSUES

Groundwater – Advantages vs. Disadvantages

• Advantages
• Little/no bacteria & turbidity.
• Nearly constant temperature & chemistry.
• Disadvantages
• Often hard (high $\text{Ca}^{2+},\;\text{Mg}^{2+}$).
• Possible excess $\text{Fe},\;\text{Mn},\;\text{H}_2\text{S},$ radionuclides.
• Risk of synthetic chemicals / pesticides.

Surface Water – Problem Characteristics

• Always presumed microbiologically contaminated.
• Usually has excess turbidity.
• Temp, minerals, contamination fluctuate widely.

MAJOR WATER-TREATMENT PROCESSES

• Aeration.
• Coagulation–Flocculation–Sedimentation–Filtration (conventional).
• Lime softening.
• Ion exchange.
• Membrane processes (MF, UF, NF, RO).
• Disinfection.
• Adsorption (activated carbon).

BIOLOGICAL GROWTH IN RAW WATER

Algae

• Problems: taste, odor, color, toxins, filter clogging, slime, corrosion, THM formation.
• SHOULD NOT be totally eliminated – necessary part of aquatic ecosystem.

Aquatic Weeds

• Types: floating, submerged, emergent.
• Issues: taste/odor, color, intake clogging, insect breeding.

PRELIMINARY (INTAKE) TREATMENT

• Screening – bar screens or wire-mesh to remove large debris.
• Routine issues: clogging, corrosion.
• Presedimentation – removes grit (gravel, sand, silt).
• Cyclone degritter – centrifugal grit removal.
• Microstrainer sequence

1. Raw water enters drum interior → outward flow through fabric.
2. Drum rotates; debris deposited.
3. High-pressure backwash cleans fabric; debris collected inside trough.

COAGULATION & FLOCCULATION

• Target: non-settleable suspended, colloidal, and dissolved solids.
• Four “conventional filtration” steps

1. Coagulation (rapid mixing + coagulant).
2. Flocculation (gentle mixing).
3. Sedimentation.
4. Filtration.
   • Electro-chemistry
   • Zeta potential – electrostatic repulsion keeping particles apart; coagulant dose reduces it.
   • Van der Waals force – natural attraction helping particles agglomerate.
   • Typical coagulants
   • Alum (Al$<em>2$(SO$</em>4$)$<em>3$·xH$</em>2$O), ferric salts, ferrous sulfate, sodium aluminate.
   • Coagulant aids – activated silica, weighting agents/adsorbents, polyelectrolytes.
   • Alkalinity builders – lime, soda ash, caustic soda.
   • Jar Test – bench-scale tool to select chemical & dose.
   • Temperature effect – low T ↓ coagulation / flocculation efficiency.
   • Operator control tests: jar, pH, turbidity, filterability, zeta potential.
   • Safety – dust inhalation, slippery liquids, exothermic alum + quicklime.

SEDIMENTATION BASINS / CLARIFIERS

• Objective: remove settleable floc ⇒ lighten filter loading.
• Zones
• Inlet – dissipate energy, distribute flow.
• Settling – quiescent zone.
• Outlet – uniform effluent withdrawal.
• Sludge – store solids.
• Detention time $=\dfrac{\text{Basin Volume}}{\text{Flow Rate}}$.
• Surface loading (overflow rate) ≈ $1\text{ gpm/ft}^2$ (for typical designs).
• Short-circuiting causes effective time < design; minimized with good baffling, inlet design, wind protection.

FILTRATION

• Function: remove remaining fine suspended matter.
• Turbidity – measure of suspended material (floc, microbes, silt, precipitates).
• Importance of turbidity removal
• Shields microbes from disinfectant.
• Consumes disinfectant.
• Causes deposits/taste/odor in mains.
• Direct filtration = coag + floc + filtration ONLY (no sedimentation).
• Advantage: cost savings.
• Limitation: only for low raw-water turbidity.

DISINFECTION

• Disinfection = kill pathogens; Sterilization = kill ALL organisms.
• Oxidants usable: chlorine, chloramines, bromine, iodine, ClO$<em>2$, KMnO$</em>4$, O$2$, ozone, PEROXONE. • Chlorine advantages • Strong oxidant, easy feed, reasonable cost, lasting residual. • Chlorine forms: gas, calcium hypochlorite (HTH), sodium hypochlorite. • Dosage equation $\text{Dose} = \text{Demand} + \text{Desired Residual}$. • CT concept $CT = C\;[\text{mg/L}] \times T\;[\text{min}]$ – equal kills achieved by different C–T combinations. • Factors influencing kill: concentration, contact time, water temperature, pH, and interfering substances (turbidity, NH$</em>3$, Fe/Mn, H$<em>2$S, organics). • Chlorine residual types • Free available. • Combined (chloramines). • Chlorine demand = chlorine applied – residual. • Chlorination ancillary uses: pipeline disinfection, oxidation of Fe/Mn/H$</em>2$S, taste/odor control, aid to coagulation & filtration, algae/slime control.
• Gas-chlorine safety
• 150-lb & 1-ton containers.
• Heavier-than-air, lethal ≥$0.1\%$.
• Leak test with ammonia (white smoke).
• Essentials: SCBA, repair kit, floor-level exhaust fan.
• KMnO$<em>4$ must be pre-treatment only – residual MnO$</em>2$ must later be removed.

FLUORIDATION

• Purpose: raise fluoride to caries-protective level.
• No chemical difference between natural vs added fluoride ions.
• Dental fluorosis = overexposure → opaque white to black/brown spots, pitting.
• Chemicals: sodium fluoride, sodium fluorosilicate, fluorosilicic acid.

CORROSION & SCALING CONTROL

• Corrosion = material deterioration via chemical reaction; scaling = precipitation deposits.
• Problems: health (e.g., Pb), aesthetics (rusty water), economics (equipment fail).
• Influencing factors: DO, TDS, pH, alkalinity, temperature, metal type, stray current, bacteria.
• High TDS ↑ conductivity ⇒ faster corrosion.
• Bacteria (e.g., SRB) generate CO$<em>2$, H$</em>2$S, slime – accelerate attack.
• Common scales: CaCO$<em>3$, CaSO$</em>4$, MgCO$<em>3$, MgCl$</em>2$.
• Higher temperature ↓ solubility ⇒ more scale.
• Stabilization techniques
• Adjust pH/alkalinity (target $6.8!\text{–}!7.3$ for CaCO$_3$ film).
• Protective linings.
• Corrosion inhibitors / sequestering agents.

IRON & MANGANESE CONTROL

• Normally exist dissolved (clear).
• Oxidation precipitates → yellow/brown/black water, staining, coffee/tea darkening.
• Aesthetic limits typically $\le 0.3\text{ mg/L Fe},\;0.05\text{ mg/L Mn}$.
• Control methods

1. Oxidation–precipitation–filtration (chemicals: chlorine, ClO$<em>2$, ozone, KMnO$</em>4$).
2. Ion exchange.
3. Sequestration (polyphosphate or sodium silicate) – keeps metals soluble; must precede oxidation exposure.
   • Four-step removal sequence: oxidation → detention → sedimentation → filtration.

LIME SOFTENING

• Hardness mainly from $\text{Ca}^{2+}$ & $\text{Mg}^{2+}$; other divalent ions minor contributors.
• Consumer objection varies; excessive hardness: spots, scale, laundry curd.
• Very soft water can be corrosive.
• Major softening methods: lime–soda ash & ion exchange.
• Key precipitates: CaCO$<em>3$ and Mg(OH)$</em>2$.
• Lime forms: hydrated (Ca(OH)$<em>2$), quicklime (CaO). • Typical detention: flocculation $40!\text{–}!60$ min, sedimentation $2!\text{–}!4$ h. • Six common problems: excess CaCO$</em>3$, Mg(OH)$_2$ scale, after-precipitation, sludge carry-over, unstable water, interference with other processes.
• Sludge dewatering: drying beds, lagoons, thickeners, vacuum filters, centrifuges.

ION EXCHANGE SOFTENING

• Definition: exchange of ions on insoluble resin with ions in water.
• Pros: lower capital, easy operation, safe chemicals, site-specific, controllable finished hardness, simpler waste disposal vs lime sludge.
• Operating cycles: softening → backwash → regeneration → rinse.
• Backwash loosens & cleans resin.
• Rinse removes excess regenerant (salt) before returning to service.

ADSORPTION (ACTIVATED CARBON)

• Adsorption = surface attachment of organics via physical/chemical forces.
• Removes tastes, odors, colors, and toxic/carcinogenic organics (incl. synthetic chemicals in aquifers).
• Activated carbon manufacture: carbonize feedstock → steam/air activation → porous high-surface-area product.
• Forms & use
• PAC – dosed to raw water, removed later by clarification/filtration.
• GAC – fixed beds that water flows through; periodically regenerated or replaced.
• Alternative organics removal: aeration, oxidation, conventional treatment.

AERATION

• Purposes
• Strip gases (CO$<em>2$, H$</em>2$S, CH$<em>4$). • Oxidize soluble metals. • Remove VOCs & radon. • Improve taste/odor, add DO. • Equipment: cascade, cone, slat-and-coke, draft, spray, packed tower (air stripper). • Excess CO$</em>2$ effects: odor, taste change, corrosion.
• Excess DO: ↑ corrosion, floating floc, filter air-binding.
• Ventilation essential – H$<em>2$S (dense) & CH$</em>4$ (explosive) hazards.

MEMBRANE PROCESSES (PRESSURE DRIVEN)

• Microfiltration (MF) > Ultrafiltration (UF) > Nanofiltration (NF) > Reverse osmosis (RO) (smallest pores).
• Osmotic pressure = pressure induced as water migrates through membrane into concentrated solution.
• RO principle: apply P > $\pi$ to concentrated side ⇒ force pure water to dilute side; effectively reverses natural osmosis.

EXAMPLE DESIGN / DOSAGE CALCULATIONS

(Note: all calculations follow the generic dimensional relationships below – values shown came from transcript scribbles.)
• Chemical dose (lb) $= \dfrac{V\,(\text{gal}) \times 8.34 \times \text{desired mg/L}}{1,000,000}$.
• Example Copper-sulfate: $15{,}000{,}000\;\text{gal} \times 8.34 \times 0.125\,\dfrac{\text{mg}}{\text{L}} /1{,}000{,}000 \approx 250\;\text{lb}$.
• Chlorine dosage $= \text{Demand} + \text{Residual}$.
• Sample: $1.7\;\text{mg/L demand} + 0.9\;\text{mg/L residual} = 2.6\;\text{mg/L applied}$.
• CT equivalence: longer contact @ lower C ≅ shorter contact @ higher C, as long as $CT$ product is equal.
• Surface loading (filters) $= \dfrac{\text{gpm}}{\text{filter area } (\text{ft}^2)}$ – example: $1750\,\text{gpm} / 397\,\text{ft}^2 = 4.41\,\text{gpm/ft}^2$.
• Sedimentation detention $t = \dfrac{V}{Q}$ – e.g. $22,000\,\text{ft}^3 / (1100\,\text{gpm} \times 0.1337) \approx 21.4\,\text{min}$.

SAFETY & OPERATIONAL BEST PRACTICES

• Chemical handling – wear PPE, control dust, avoid mixing incompatible chemicals (alum + quicklime ⇒ heat + H$_2$).
• Chlorine – continuous monitoring, SCBA ready, leak test w/ ammonia, maintain negative room pressure.
• Aerators – ensure ventilation to prevent gas build-up & explosion risk.
• Maintain CT, pH, turbidity goals to assure pathogen inactivation & minimize DBPs (e.g., THMs).
• Routine lab tests – pH, alkalinity, hardness, Fe/Mn, turbidity, chlorine residual, fluoride, zeta potential, filter effluent quality.

KEY EQUATIONS & NUMERICAL REFERENCES (LaTeX form)

• Chemical dose (lb) $= \dfrac{V (\text{gal}) \times 8.34 \times C (\text{mg/L})}{1{,}000{,}000}$.
• Overflow rate $= \dfrac{Q}{A<em>s} \; (\text{e.g., }1\,\text{gpm/ft}^2)$. • Detention time $t = \dfrac{V}{Q}$. • CT concept $CT = C \times T$. • pH range for CaCO$</em>3$ film: $6.8 \le \text{pH} \le 7.3$.
• Fe limit $\le 0.3\,\text{mg/L}$; Mn limit $\le 0.05\,\text{mg/L}$.

ETHICAL / PRACTICAL IMPLICATIONS & REAL-WORLD LINKS

• Ecological balance – avoid total algae removal; maintain source ecology.
• Public health – correct fluoridation prevents caries; excessive fluoride yields fluorosis.
• Infrastructure longevity – corrosion control prevents lead/copper leaching & asset loss.
• Sustainability – membrane & adsorption processes address emerging contaminants (pesticides, solvents, PFAS).