📐 Class 4 Water Treatment

Variables

• C = disinfectant residual at end of contact zone (mg/L)

• T₁₀ = time for 10% of water to pass through (minutes)

• T₁₀ = T × Baffling Factor (BF)

• BF ranges from 0.1 (poor baffling) to 1.0 (plug flow)

• Higher CT = more disinfection credit

📊 Worked Example

C = 1.8 mg/L free Cl₂, T = 40 min, BF = 0.7 → T₁₀ = 28 min → CT = 1.8 × 28 = 50.4 mg·min/L

💡 Exam Tip

Class 4 exams test baffling factor application. Know that a fully baffled clearwell has BF = 0.7, and a completely mixed tank has BF = 0.1. CT must be calculated using T₁₀, not total HRT.

Variables

• N₀ = initial pathogen concentration

• N = final pathogen concentration after treatment

• 1-log = 90% removal

• 2-log = 99% removal

• 3-log = 99.9% removal

• 4-log = 99.99% removal

📊 Worked Example

N₀ = 1,000 cysts, N = 1 cyst → Log = log(1000/1) = log(1000) = 3-log inactivation (99.9%)

💡 Exam Tip

Ontario requires: Giardia 3-log, Cryptosporidium 2-log, Viruses 4-log total inactivation + removal. Multiple barriers (filtration + disinfection) contribute to total log credit.

Variables

• Cᵢ = ozone residual at each measurement point (mg/L)

• Δtᵢ = time interval between measurements (min)

• Ozone decays rapidly — residual must be measured at multiple points

• CT is the area under the residual vs. time curve

📊 Worked Example

Residuals: 0.8, 0.5, 0.3 mg/L over 3-min intervals → CT = (0.8+0.5+0.3) × 3 = 4.8 mg·min/L

💡 Exam Tip

Ozone CT values are much lower than chlorine CT. For Giardia 3-log at 20°C pH 7: ozone CT ≈ 0.48 mg·min/L vs chlorine CT ≈ 73 mg·min/L. Ozone does not receive virus credit in Ontario.

Variables

• Irradiance = UV light intensity at the target (mW/cm²)

• Exposure Time = contact time in the UV reactor (seconds)

• UV Dose = mW/cm² × s = mJ/cm²

• Effective dose depends on UVT, lamp output, flow rate

• UV Transmittance (UVT) measured at 254 nm

📊 Worked Example

Irradiance = 5 mW/cm², Exposure = 8 s → UV Dose = 5 × 8 = 40 mJ/cm²

💡 Exam Tip

Class 4 UV credit: Cryptosporidium 2-log at 10 mJ/cm², 3-log at 36 mJ/cm²; Giardia 3-log at 5 mJ/cm². UV does NOT receive virus inactivation credit in Ontario. UVT must be monitored continuously.

Variables

• Q = permeate flow rate (L/h)

• A = membrane surface area (m²)

• Typical MF/UF flux: 20–100 LMH

• Higher flux → faster fouling

• Net flux = gross flux × (1 − backwash fraction)

📊 Worked Example

Q = 500 L/h, A = 10 m² → Flux = 500/10 = 50 LMH

💡 Exam Tip

Transmembrane pressure (TMP) increases as membranes foul. Gradual TMP increase = irreversible fouling requiring CIP. Sudden TMP spike = air binding or integrity breach.

Variables

• Qpermeate = permeate (product) flow rate

• Qfeed = feed flow rate

• Qreject = Qfeed − Qpermeate

• Typical MF/UF recovery: 90–95%

• Typical RO recovery: 75–85%

📊 Worked Example

Feed = 1,000 L/h, Permeate = 900 L/h → Recovery = 900/1,000 × 100 = 90%

💡 Exam Tip

Higher recovery means less reject/brine to dispose of but increases concentration polarization and fouling risk. RO recovery is limited by scaling potential of the reject stream.

Variables

• V_bed = volume of GAC/media bed (m³ or L)

• Q = flow rate through the bed (m³/min or L/min)

• Typical EBCT for taste/odour: 5–15 min

• Typical EBCT for micropollutants: 15–30 min

• Longer EBCT = better removal but higher capital cost

📊 Worked Example

V_bed = 50 m³, Q = 5 m³/min → EBCT = 50/5 = 10 minutes

💡 Exam Tip

EBCT is the key design parameter for GAC adsorbers. It is NOT the same as hydraulic retention time — it uses the bed volume, not the void volume. Breakthrough occurs earlier with shorter EBCT.

Variables

• BrO₃⁻ = bromate concentration (regulated at 10 µg/L in Ontario)

• [O₃] = ozone dose/residual

• [Br⁻] = bromide concentration in source water

• pH = higher pH increases bromate formation

• Control: lower pH, reduce O₃ dose, add NH₃, use UV/H₂O₂ AOP

📊 Worked Example

Source water Br⁻ = 200 µg/L → bromate formation risk is significant; pH should be lowered to 6.5–7.0 during ozonation

💡 Exam Tip

Bromate is a potential carcinogen formed when ozone reacts with bromide. The Ontario MAC is 10 µg/L. Ammonia addition suppresses bromate formation by scavenging hydroxyl radicals.

Variables

• hf = friction head loss (m)

• L = pipe length (m)

• Q = flow rate (m³/s)

• C = Hazen-Williams roughness coefficient (new PVC: 150, old CI: 80–100)

• D = pipe diameter (m)

• Higher C = smoother pipe = less head loss

📊 Worked Example

L = 500 m, D = 0.3 m, Q = 0.05 m³/s, C = 120 → hf ≈ 2.8 m

💡 Exam Tip

Class 4 exams test the concept: doubling flow increases head loss by 3.5× (Q^1.852). Doubling pipe diameter reduces head loss by 28× (D^4.87). Pipe roughness (C) decreases with age.

Variables

• Q = flow rate (varies linearly with speed)

• H = head (varies as square of speed ratio)

• P = power (varies as cube of speed ratio)

• N = rotational speed (RPM)

• VFD (Variable Frequency Drive) uses affinity laws to optimize energy

📊 Worked Example

Speed increases from 1,200 to 1,440 RPM (ratio = 1.2): Q increases 1.2×, H increases 1.44×, P increases 1.73×

💡 Exam Tip

The power law (cube relationship) is why VFDs save energy — reducing speed by 20% reduces power by 49%. This is the most energy-efficient way to reduce pump output.

Variables

• P_atm = atmospheric pressure head (≈10.3 m at sea level)

• P_s = suction pressure head

• h_vapor = vapor pressure head of water

• h_f_suction = friction losses in suction piping

• NPSH_available must exceed NPSH_required to prevent cavitation

📊 Worked Example

NPSH_available = 10.3 − 3.0 (static lift) − 0.5 (friction) − 0.24 (vapor at 20°C) = 6.56 m

💡 Exam Tip

If NPSH_available < NPSH_required, cavitation occurs. Increase NPSH_available by: lowering pump, increasing suction pipe diameter, reducing suction pipe length, or cooling the water.

Variables

• ΔP = pressure surge (Pa)

• ρ = water density (1,000 kg/m³)

• a = wave speed in pipe (typically 900–1,200 m/s for water mains)

• ΔV = change in flow velocity (m/s)

• Surge pressure can be 5–10× normal operating pressure

📊 Worked Example

ΔV = 1.5 m/s, a = 1,000 m/s → ΔP = 1,000 × 1,000 × 1.5 = 1,500,000 Pa = 1,500 kPa

💡 Exam Tip

Water hammer is prevented by: slow-closing valves (close in >2L/a seconds), surge tanks, air chambers, and pressure relief valves. Pump shutdown causes negative pressure wave (column separation).

Variables

• z = elevation above datum (m)

• P = pressure (Pa)

• ρ = water density (1,000 kg/m³)

• g = 9.81 m/s²

• HGL drops along the direction of flow due to friction losses

• HGL must be above pipe to maintain positive pressure

📊 Worked Example

Elevation = 50 m, Pressure = 300 kPa → HGL = 50 + 300,000/(1,000×9.81) = 50 + 30.6 = 80.6 m

💡 Exam Tip

The HGL must always be above the pipe crown to prevent negative pressures (which cause contamination intrusion). Minimum pressure in distribution: 140 kPa (20 psi) residential; 550 kPa (80 psi) maximum.

Variables

• Minimum pressure: 140 kPa (20 psi) at all service connections

• Maximum pressure: 550–700 kPa (80–100 psi) to prevent pipe damage

• PRV set point = maximum allowable downstream pressure

• Booster station required when HGL is insufficient for high-elevation areas

📊 Worked Example

HGL = 120 m, Pipe elevation = 80 m → Pressure = (120−80) × 9.81 = 392 kPa (57 psi)

💡 Exam Tip

Pressure zones are created when elevation differences exceed 40–50 m. PRVs reduce pressure at zone boundaries. Booster stations increase pressure for high zones.

Variables

• Dose = chemical concentration required (mg/L)

• Flow = plant flow rate (m³/day)

• 0.001 = unit conversion (mg/L × m³ = g, ÷1000 = kg)

• For liquid chemicals: Feed Rate (L/d) = kg/d ÷ (Concentration × SG)

📊 Worked Example

Dose = 2.5 mg/L, Flow = 50,000 m³/d → Feed Rate = 2.5 × 50,000 × 0.001 = 125 kg/d

💡 Exam Tip

Always check units. If flow is in ML/d: Feed Rate (kg/d) = Dose (mg/L) × Flow (ML/d) × 1. If using liquid chemical at 12.5% strength (SG 1.2): Volume = 125 ÷ (0.125 × 1.2) = 833 L/d.

Variables

• Mass of solute = chemical dissolved (g or kg)

• Mass of solution = total solution mass (g or kg)

• For liquid: % = (mass solute / volume × SG) × 100

• Sodium hypochlorite: typically 6–15% available chlorine

• Alum: typically 48% Al₂(SO₄)₃ solution

📊 Worked Example

10 kg NaOCl in 90 kg water → % = 10/(10+90) × 100 = 10%

💡 Exam Tip

When diluting: C₁V₁ = C₂V₂. Example: dilute 15% NaOCl to 1%: V₁ = (1% × 100 L) / 15% = 6.67 L of stock + 93.33 L water.

Variables

• Volume = tank/basin volume (m³ or L)

• Flow Rate = plant flow (m³/h or L/min)

• HRT = theoretical time water spends in the tank

• Actual contact time = HRT × Baffling Factor

• T₁₀ = HRT × BF (used for CT calculations)

📊 Worked Example

Clearwell volume = 2,000 m³, Flow = 500 m³/h → HRT = 2,000/500 = 4 hours

💡 Exam Tip

HRT is NOT the same as T₁₀. For CT calculations, T₁₀ = HRT × BF. A clearwell with BF = 0.7 and HRT = 4 h has T₁₀ = 2.8 h = 168 min.

Variables

• Q = flow rate through filter (m³/h)

• A = filter surface area (m²)

• Typical rapid sand filter: 5–15 m/h (120–360 m³/m²/d)

• Typical dual media filter: 10–20 m/h

• High loading → shorter filter run, more backwashing

📊 Worked Example

Q = 500 m³/h, Filter area = 50 m² → Loading = 500/50 = 10 m/h

💡 Exam Tip

Filter loading rate is also called surface loading rate or hydraulic loading rate. Exceeding design loading causes turbidity breakthrough. Reduce flow or take filters offline during high-demand periods.

Variables

• pH = measured pH of finished water

• pHs = saturation pH = (pK₂ − pKs) + p[Ca²⁺] + p[Alk]

• LSI > 0: scale-forming (CaCO₃ deposits) — corrosion protection

• LSI < 0: corrosive (dissolves CaCO₃ scale) — lead/copper release

• LSI = 0: balanced (saturation equilibrium)

• Target: LSI between 0 and +0.5 for corrosion control

📊 Worked Example

pH = 7.6, pHs = 8.1 → LSI = 7.6 − 8.1 = −0.5 (corrosive — adjust pH or alkalinity)

💡 Exam Tip

Class 4 operators must understand corrosion control for lead and copper. LSI < −0.5 requires treatment (pH adjustment, alkalinity addition, orthophosphate). The Lead and Copper Rule targets LSI near 0.

Variables

• Q = flow rate (m³/d)

• L_weir = total weir length (m)

• Typical design WOR: 125–500 m³/m/d

• V-notch weirs are common in clarifiers

• WOR controls velocity near the weir — high WOR causes turbulence and floc carryover

📊 Worked Example

Q = 20,000 m³/d, Weir length = 100 m → WOR = 20,000/100 = 200 m³/m/d

💡 Exam Tip

High weir overflow rates cause floc carryover (turbidity spikes in settled water). Solutions: add more weir length (launder extensions), reduce flow, or improve flocculation.

Variables

• Q = flow rate (m³/d)

• A_surface = clarifier surface area (m²)

• Typical design SOR: 20–60 m³/m²/d for conventional clarifiers

• DAF: 5–15 m³/m²/h (higher than sedimentation)

• SOR = settling velocity of particles that are 100% removed

📊 Worked Example

Q = 15,000 m³/d, Diameter = 20 m → A = π×10² = 314 m² → SOR = 15,000/314 = 47.8 m³/m²/d

💡 Exam Tip

Particles with settling velocity > SOR are 100% removed. Particles with settling velocity < SOR are partially removed. Reducing SOR (larger clarifier or lower flow) improves removal efficiency.

Variables

• C(t) = residual at time t (mg/L)

• C₀ = initial residual (mg/L)

• k = first-order decay rate constant (per hour)

• t = time (hours)

• Higher temperature → higher k → faster decay

• Higher NOM → higher k → faster decay

📊 Worked Example

C₀ = 1.0 mg/L, k = 0.05/h, t = 24 h → C = 1.0 × e^(−0.05×24) = 1.0 × 0.301 = 0.30 mg/L

💡 Exam Tip

Chlorine residual must be maintained throughout the distribution system (minimum 0.05 mg/L free chlorine in Ontario). Decay is faster in warm water, long dead-end mains, and areas with high NOM.

Variables

• [Ca²⁺] = calcium concentration (mg/L)

• [Mg²⁺] = magnesium concentration (mg/L)

• MW: Ca = 40, Mg = 24.3, CaCO₃ = 100

• Soft: < 75 mg/L, Moderate: 75–150, Hard: 150–300, Very Hard: > 300

• Hardness causes scale in pipes and water heaters

📊 Worked Example

Ca²⁺ = 80 mg/L, Mg²⁺ = 18 mg/L → TH = 80×(100/40) + 18×(100/24.3) = 200 + 74 = 274 mg/L as CaCO₃

💡 Exam Tip

Lime softening removes hardness: Ca²⁺ + Ca(OH)₂ + CO₂ → 2CaCO₃↓. Mg²⁺ removal requires excess lime (pH > 10.8). Ion exchange softening uses Na⁺ to replace Ca²⁺ and Mg²⁺.

Variables

• [HCO₃⁻] = bicarbonate (mg/L) — dominant at pH 6–8.3

• [CO₃²⁻] = carbonate (mg/L) — significant at pH > 8.3

• [OH⁻] = hydroxide (mg/L) — significant at pH > 10

• MW equivalents: HCO₃⁻ = 61, CO₃²⁻ = 30, OH⁻ = 17

• Alkalinity buffers pH changes during disinfection

📊 Worked Example

[HCO₃⁻] = 122 mg/L → Alkalinity = 122 × (50/61) = 100 mg/L as CaCO₃

💡 Exam Tip

Alkalinity is consumed by coagulation (alum reduces alkalinity). Minimum alkalinity of 30–40 mg/L as CaCO₃ is needed for effective coagulation. Add lime or soda ash to restore alkalinity.

Variables

• THM = chloroform + bromodichloromethane + dibromochloromethane + bromoform

• MAC in Ontario: 100 µg/L total THMs

• Higher NOM → more THM precursors

• Higher pH → more THM formation

• Higher temperature → faster THM formation

• Control: reduce NOM (enhanced coagulation, GAC), reduce Cl₂ dose, reduce contact time

📊 Worked Example

TOC = 8 mg/L, Cl₂ dose = 3 mg/L, pH 8, 25°C → high THM risk; enhanced coagulation required

💡 Exam Tip

THMs are regulated at 100 µg/L (annual running average). HAAs (haloacetic acids) are regulated at 80 µg/L. Both are DBPs formed by chlorine reacting with NOM. Enhanced coagulation is the primary control strategy.

Variables

• TOC_in = raw water TOC (mg/L)

• TOC_out = treated water TOC (mg/L)

• Ontario requires enhanced coagulation for TOC > 2 mg/L

• Target removal depends on source water alkalinity and TOC

• Low alkalinity + high TOC → highest removal required

📊 Worked Example

TOC_in = 8.5 mg/L, TOC_out = 3.2 mg/L → Removal = (8.5−3.2)/8.5 × 100 = 62.4%

💡 Exam Tip

Enhanced coagulation targets (USEPA Stage 1 DBPR): at TOC 4–8 mg/L, alkalinity < 60 mg/L → 40% TOC removal required. Lower pH (5.5–6.5) and higher coagulant dose improve NOM removal.

Variables

• [Cl⁻] = chloride concentration (mg/L)

• [SO₄²⁻] = sulfate concentration (mg/L)

• CSMR > 0.5 indicates elevated lead/copper corrosion risk

• CSMR > 0.58 triggers lead corrosion concerns

• Chloride promotes pitting corrosion; sulfate promotes uniform corrosion

📊 Worked Example

[Cl⁻] = 60 mg/L, [SO₄²⁻] = 80 mg/L → CSMR = 60/80 = 0.75 (elevated risk)

💡 Exam Tip

CSMR is important for lead service line communities. Switching from chloride-based to sulfate-based coagulants can reduce CSMR. Orthophosphate addition forms protective scale on lead pipes.

Variables

• AOC measures biodegradable fraction of DOC

• AOC < 10 µg/L: biologically stable (no regrowth risk)

• AOC > 100 µg/L: significant regrowth potential

• Ozonation increases AOC (breaks NOM into biodegradable fractions)

• BAC filtration reduces AOC by biological degradation

📊 Worked Example

After ozonation: AOC increases from 20 to 150 µg/L. After BAC: AOC decreases to 15 µg/L (biologically stable)

💡 Exam Tip

AOC is the key parameter for distribution system biological stability. High AOC promotes bacterial regrowth, biofilm formation, and nitrification. BAC filtration after ozonation is the most effective AOC reduction strategy.

Variables

• t = Student's t-value for n−1 degrees of freedom at 99% confidence

• s = standard deviation of replicate measurements at low concentration

• MDL = lowest concentration reliably detected above noise

• Reporting limit (RL) is typically 3–10× MDL

• Results below MDL reported as < MDL

📊 Worked Example

7 replicates at 0.5 µg/L: s = 0.05 µg/L, t = 3.143 → MDL = 3.143 × 0.05 = 0.16 µg/L

💡 Exam Tip

Class 4 operators must understand QA/QC concepts. MDL is determined by the laboratory, not the operator. Regulatory limits must be above the MDL to be enforceable.

Variables

• V_stock = volume of coagulant stock added to jar (mL)

• C_stock = concentration of stock solution (mg/mL)

• V_sample = volume of water sample in jar (mL)

• Standard jar volume: 1,000 mL or 2,000 mL

• Typical stock: 10 g/L alum = 10 mg/mL

📊 Worked Example

1 mL of 10 mg/mL alum stock in 1,000 mL sample → Dose = (1 × 10)/1,000 = 0.01 mg/mL = 10 mg/L

💡 Exam Tip

Jar test determines optimal coagulant dose, pH, and mixing conditions. Evaluate settled water turbidity, colour, and pH. The optimal dose is the lowest dose achieving the target turbidity.

Variables

• Free chlorine = HOCl + OCl⁻ (hypochlorous acid + hypochlorite)

• Total chlorine = free + combined (chloramines)

• Combined chlorine = total − free

• DPD #1 = free chlorine; DPD #3 = total chlorine

• Amperometric titration is more accurate for regulatory compliance

📊 Worked Example

DPD #1 = 0.8 mg/L (free), DPD #3 = 1.2 mg/L (total) → Combined = 1.2 − 0.8 = 0.4 mg/L

💡 Exam Tip

Interference: high free chlorine (> 5 mg/L) can bleach DPD → false low reading. Mn²⁺ and Cr⁶⁺ interfere. For regulatory compliance, use amperometric titration or calibrated colorimeter.

Variables

• Measured at 90° to incident light beam

• Ontario MAC: 1 NTU treated water (0.3 NTU for membrane systems)

• Continuous monitoring required for surface water plants

• Individual filter effluent: < 0.3 NTU 95% of time

• Calibrate with formazin or StablCal standards

📊 Worked Example

Filter effluent turbidity = 0.25 NTU (compliant); spike to 0.8 NTU after backwash (filter ripening — filter to waste)

💡 Exam Tip

Turbidity is the most critical online measurement for surface water plants. Spikes above 0.3 NTU trigger investigation. Continuous turbidity monitoring is required by O. Reg. 170/03.

Variables

• [H⁺] = hydrogen ion activity (mol/L)

• pH 7 = neutral, pH < 7 = acidic, pH > 7 = basic

• Each pH unit = 10× change in [H⁺]

• Calibrate with two buffers (e.g., pH 4 and pH 7, or pH 7 and pH 10)

• Temperature affects pH measurement — use temperature compensation

📊 Worked Example

[H⁺] = 10⁻⁷·⁸ mol/L → pH = 7.8

💡 Exam Tip

pH affects: chlorine speciation (HOCl vs OCl⁻), coagulation efficiency, corrosion, and THM formation. At pH < 7.5, HOCl dominates (stronger disinfectant). At pH > 8, OCl⁻ dominates (weaker). Target pH 7.0–7.5 for optimal disinfection.

Variables

• O. Reg. 170/03 minimum: 0.05 mg/L free chlorine

• Recommended: 0.2–0.5 mg/L throughout distribution

• Maximum: 4 mg/L (aesthetic objective)

• Total chlorine (chloramine systems): ≥ 0.05 mg/L

• Residual must be maintained to prevent regrowth

📊 Worked Example

Distribution system sample: 0.03 mg/L free Cl₂ → AWQI required; investigate and flush

💡 Exam Tip

Any sample below 0.05 mg/L free chlorine is an adverse result requiring immediate investigation and reporting. Chloramine systems maintain total chlorine ≥ 0.05 mg/L.

Variables

• 1 NTU = absolute maximum for treated water

• 0.3 NTU = 95th percentile target (monthly)

• 0.3 NTU = individual filter effluent target

• Membrane systems: ≤ 0.1 NTU

• Turbidity > 1 NTU = AWQI requiring immediate reporting

📊 Worked Example

Monthly turbidity data: 28/30 days ≤ 0.3 NTU = 93.3% compliance (below 95% target — investigate)

💡 Exam Tip

Turbidity is a surrogate for pathogen removal. High turbidity interferes with disinfection (shields pathogens from UV/chlorine). The 0.3 NTU target ensures adequate disinfection efficiency.

Variables

• E. coli = 0 in any treated water sample (zero tolerance)

• Total coliforms = 0 in treated water (< 1 CFU/100 mL)

• Positive E. coli = AWQI requiring immediate reporting to MECP

• Confirm positive with repeat sample and investigate source

• Boil water advisory may be required

📊 Worked Example

Treated water sample: E. coli = 1 CFU/100 mL → AWQI, notify MECP immediately, issue boil water advisory, investigate

💡 Exam Tip

E. coli is the primary indicator of fecal contamination. Any detection in treated water is an adverse result. Total coliform positives also require investigation but are not as immediately serious as E. coli.

Variables

• Immediate = as soon as aware (by telephone to MECP Spills Action Centre)

• Written report = within 7 days of becoming aware

• AWQI triggers: any MAC exceedance, observation of contamination, equipment failure affecting treatment

• Notify: MECP, local Medical Officer of Health, owner

• Document: date/time, nature of incident, corrective actions

📊 Worked Example

Turbidity spike to 1.5 NTU at 14:00 → Call MECP Spills Action Centre immediately → Written report by 14:00 next week

💡 Exam Tip

The MECP Spills Action Centre is available 24/7 at 1-800-268-6060. Do not wait for laboratory confirmation before reporting. Err on the side of caution.

Variables

• Operational Plan: updated annually, reviewed by owner

• Management Review: annual assessment of QMS effectiveness

• Internal Audit: annual audit of all QMS elements

• Corrective Actions: documented and tracked to closure

• Third-party accreditation: required every 3 years

📊 Worked Example

Annual DWQMS cycle: Jan (internal audit) → Mar (management review) → Jun (Operational Plan update) → Sep (third-party audit every 3 years)

💡 Exam Tip

The DWQMS is required under O. Reg. 188/07 for all municipal residential drinking water systems. Non-compliance can result in licence revocation. The 21 elements of the DWQMS cover all aspects of QMS.