Water Treatment: Sedimentation & Filtration
Key Takeaways
- Sedimentation removes settleable solids by gravity; overflow rate (surface loading) governs clarifier sizing.
- Horizontal flow clarifiers use length, width, depth, and settling velocity for removal efficiency.
- Filtration through sand/anthracite removes residual turbidity, floc, and pathogens with proper coagulation.
- Headloss in clean and loaded filters increases until backwash is required; filter run time affects O&M.
- Slow sand filters rely on schmutzdecke biologically; rapid filters are primary for municipal surface water.
Quick Answer: Size clarifiers with overflow rate Q/A and detention time. Filters remove residual turbidity after coagulation-sedimentation — monitor headloss and run length to backwash.
Physical treatment steps follow chemistry in conventional plants. Sedimentation and filtration appear in both water supply and wastewater secondary clarification contexts.
Sedimentation Principles
Particles settle when gravitational force exceeds drag. Stokes' law (laminar spheres):
[ v_s = \frac{g(\rho_p - \rho_w)d^2}{18\mu} ]
Larger, denser particles settle faster. Discrete settling — particles do not interact. Flocculent settling — floc grows during descent (typical in water treatment).
Overflow Rate and Detention
[ v_o = \frac{Q}{A_{surface}} ]
Design v_o less than settling velocity of target particles for high removal. Typical water treatment 2–4 gpm/ft² (verify units in problem).
Detention time (t_d = V/Q) — 2–4 hours conventional sedimentation basins.
Worked example: Clarifier 100 ft × 30 ft; Q = 6 MGD. Convert Q to gpm: 6×694.4 ≈ 4166 gpm.
[ A = 3000 \text{ ft}^2; \quad v_o = 4166/3000 = 1.39 \text{ gpm/ft}^2 ]
Horizontal vs. Vertical Flow
Rectangular horizontal clarifiers: inlet stilling, sludge collection at bottom, effluent launders. Circular clarifiers with rotating scrapers common in wastewater.
Short-circuiting reduces effective detention — baffles improve plug flow.
Plate Settlers and Tube Clarifiers
Inclined plates reduce settling distance → higher effective overflow rate in compact footprint.
Filtration — Rapid Sand Filters
Dual-media: anthracite over sand — coarse top catches solids; sand polishes. Effective size and uniformity coefficient describe gradation.
| Parameter | Role |
|---|---|
| Filtration rate | 2–6 gpm/ft² typical rapid sand |
| Headloss | Rises as pores clog — terminal headloss triggers backwash |
| Schmutzdecke | Surface mat on slow sand filters |
Removal Mechanisms in Filters
Straining, impaction, sedimentation on grains, adsorption — coagulated particles essential for pathogen and turbidity credit.
Backwash
Reverse flow (or air scour + water) lifts media to flush solids. Backwash rate must fluidize bed without media loss. Waste backwash water to settling or reclaim.
Wastewater Secondary Clarification
Activated sludge effluent contains biological floc — clarifier returns settled sludge (RAS) to aeration; excess WAS wasted. Solids loading rate and state point analysis (conceptual) prevent bulking washout.
Exam trap: High SVI (bulking sludge) → poor settling → solids loss over weirs.
Turbidity and Pathogen Log Removal
LT2ESWTR (conceptual) — filtration credits toward Giardia/Cryptosporidium inactivation/removal. Combined coagulation + filtration + disinfection provides multiple barriers.
Performance Metrics
[ \text{Removal %} = \frac{Turbidity_{in} - Turbidity_{out}}{Turbidity_{in}} \times 100 ]
Particle counting and SDI (silt density index) matter for RO pretreatment (advanced treatment chapter).
Design Sequence
- Peak flow Q_peak for clarifier (often daily max × peaking factor).
- Select v_o from regulations or handbook.
- A = Q/v_o; depth sets volume and t_d.
- Filter area from filtration rate × safety factor.
Exam trap: Using weir overflow rate interchangeably with clarifier overflow rate — they are different checks (weir loading gpm/ft of weir).
Sedimentation and filtration translate coagulation success into clear, low-turbidity water ready for disinfection.
Weir Loading and Solids Loading
Weir overflow rate (gpm/ft of weir) prevents hydraulic short-circuiting in circular clarifiers. Solids loading rate = Q × MLSS / area in secondary clarifiers — paired with state point analysis conceptually.
Filter Media Specifications
Effective size d10 and uniformity coefficient Cu = d60/d10 describe gradation — lower Cu means uniform sand, better stratification after backwash.
Conventional Treatment Train
Flash mix → flocculation → sedimentation → filtration → disinfection — know residence times order of magnitude: seconds, minutes, hours, minutes, minutes-contact.
Troubleshooting Turbidity Breakthrough
Sudden filter turbidity spike — check coagulant dose, raw water algae, or mud ball formation from poor backwash.
Plate Clarifiers and Solids Contact Units
Solids contact clarifiers combine flocculation and sedimentation in one unit with sludge recirculation — higher overflow rates possible with stable sludge blanket.
Air Binding in Filters
Air binding from supersaturated water reduces filter run length — degasifiers on groundwater high in CO₂ before filtration. On the FE Environmental exam, confirm units before substituting into handbook equations; distractors often differ only by conversion factors. Practice locating handbook sections by keyword during timed drills so lookup takes under one minute. When a stem gives excess data, identify the governing regulation or equation first, then ignore unrelated values. Document assumptions on scratch paper: stability class, waste classification, exposure pathway, or discount rate as applicable. Cross-check magnitude: TCLP thresholds in milligrams per liter, emissions in pounds per hour, and cancer risk slopes in consistent units. NCEES items may supply all required constants in the stem; do not assume every value appears in the FE Reference Handbook.
Secondary Clarifier Solids Loading
Q = 10 MGD, MLSS = 2500 mg/L, clarifier diameter 80 ft (area 5027 ft²).
[ SLR = \frac{10 \times 8.34 \times 2500}{5027} = \frac{208{,}500}{5027} \approx 41\text{ lb/ft}^2\text{·day} ]
Compare to typical limit 30–40 — borderline; risk of solids washout during peak flow.
Filter Run Time Estimate
Filter area 600 ft², rate 4 gpm/ft², terminal headloss 10 ft reached after 48 h. Total filtered volume = 4 × 600 × 60 × 48 = 6.9 Mgal per run — backwash waste ≈ 2–5% of filtered water depending on air-scour protocol.
Hazen Settling Velocity
For discrete particle settling, overflow rate v_o should be less than particle settling velocity v_s. If v_s = 0.01 ft/s and v_o = 0.008 ft/s, 90%+ discrete removal expected for particles with that v_s (simplified settling column theory).
Quick Answer: Coagulate (charge neutralization) → flocculate (gentle mixing) → settle. Dose alum or ferric salts; watch alkalinity and pH. Lime softens by precipitating CaCO₃ and Mg(OH)₂.
Chemical treatment precedes sedimentation and filtration in most surface water plants. FE Environmental water topics (~14% combined water/wastewater) frequently test coagulation chemistry and dose stoichiometry.
Colloid Stability and Coagulation
Clarifier surface overflow rate is defined as:
Rapid granular filters primarily remove:
Increasing filter headloss during a run indicates:
In activated sludge plants, secondary clarifiers return: