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CHAPTER 8

Water Treatment — Conventional Processes

Water Treatment (Conventional)

Specifies conventional surface water treatment — screening, coagulation, flocculation, sedimentation, filtration (rapid sand filter, slow sand filter), chlorination. Covers design parameters, sizing, hydraulic retention times, chemical doses.

⚗ Water TreatmentManual on Water Supply and Treatment3rd Edition (1999) with 2024 revision updates

Key formulas

  • Overflow rate (sedimentation tank): SLR = Q/A (m³/m²/hr or m/hr) — typically 1.0-2.5 m/hr for conventional settling.
  • Surface loading rate on filter: HLR = Q/A (m³/m²/hr) — typical 5-7 m/hr for rapid sand filter.
  • Retention time: T = V/Q — typical 3-4 hours for conventional sedimentation tank.
  • Weir loading: WLR = Q/L (m³/m/day) — typically < 300 m³/m/day for launder/weir.
  • Chemical dose (mg/L to kg/day): Dose (kg/day) = Q (MLD) × Dose (mg/L) × 1.0 — i.e., dose in kg/day = flow in MLD × dose in mg/L.
  • Velocity gradient G (flocculation): G (s⁻¹) = √(P/μV), where P = power input, μ = dynamic viscosity, V = tank volume.

Key values & thresholds

alum dose mg per L range
10 - 80
typical alum dose mg per L
20 - 40
pac dose mg per L
5 - 30 (more efficient than alum)
lime dose for pH correction mg per L
20 - 50
flash mix G value per sec
300 - 500
flash mix retention seconds
30 - 60
flocculation G value per sec
30 - 80
flocculation retention minutes
20 - 40
sedimentation SLR m per hr
1.0 - 2.5
sedimentation retention hours
3 - 4
sedimentation weir loading m3 per m per day
< 300
RSF HLR m per hr
5 - 7
RSF sand depth mm
600 - 750
RSF sand effective size mm
0.4 - 0.7
RSF backwash rate m per hr
36 - 48
RSF backwash duration minutes
5 - 10
SSF HLR m per hr
0.1 - 0.3
SSF sand depth mm
800 - 1000
chlorine contact time minutes
30
chlorine residual outlet mg per L
0.5

Clause-level requirements

  • Conventional treatment train: screening → aeration (if needed for Fe/Mn) → coagulation/flash mix → flocculation → sedimentation → filtration → disinfection → clear water reservoir.
  • Coagulation chemical dose determined by jar test (laboratory simulation); alum dose 20-40 mg/L typical for surface water with turbidity 50-200 NTU. PAC (polyaluminium chloride) more efficient per unit.
  • Flash mixer: G-value 300-500 s⁻¹, retention 30-60 seconds. Disperses coagulant uniformly.
  • Flocculation: G-value 30-80 s⁻¹ (tapered, decreasing from 60 to 30 through tank), retention 20-40 minutes. Forms larger settleable flocs.
  • Sedimentation tank: SLR 1.0-2.5 m/hr, retention 3-4 hours, weir loading < 300 m³/m/day. Inlet and outlet baffles for uniform flow distribution.
  • Rapid Sand Filter: HLR 5-7 m/hr, sand 600-750 mm deep (effective size 0.4-0.7 mm), gravel bed 300-500 mm, underdrain system. Backwash rate 36-48 m/hr for 5-10 minutes when head loss reaches 2-3 m.
  • Slow Sand Filter: HLR 0.1-0.3 m/hr (1/20 of RSF), sand 800-1000 mm deep (finer), no chemical pretreatment. Used for smaller communities (< 50,000 population).

Practitioner notes — what goes wrong in the field

  • Jar test before design: determine optimum coagulant type, dose, pH range, mixing time. Skipping jar test leads to over/under-dosing in field — 20-30% chemical cost wasted or poor treatment.
  • Alum dose varies seasonally: monsoon turbid water needs 40-80 mg/L; dry season clean water needs 10-20 mg/L. Adjust based on raw water turbidity.
  • PAC (polyaluminium chloride) vs alum: PAC 30-50% less dose required, produces less sludge, works at lower temperatures — preferred for modern WTPs.
  • Sedimentation tank with inclined plates (lamella): reduces footprint 30-50% for same throughput. Used in space-constrained urban WTPs.
  • Rapid sand filter is workhorse — 80% of Indian urban WTPs use RSF. Filter run 24-48 hours between backwash (longer = less backwash water, more efficient).
  • Slow sand filter (SSF): no chemicals needed, low operator skill required, 99%+ bacteriological removal. Disadvantage: large land area (20× RSF). Suitable for rural/small communities.
  • Pressure filters: encased in pressure vessel; higher HLR (8-12 m/hr), smaller footprint. Cost 2× RSF but space advantage in urban areas. Used at Mumbai Bhandup, Delhi Sonia Vihar.
  • Dual media filter (sand + anthracite): higher filter run, better removal of fine flocs. 20-30% higher capacity than sand alone.
  • Filter backwash: 3-5% of treated water used for backwash. Backwash waste treated (sedimentation) and recycled to head of plant — water savings.
  • Chemical storage: 30-45 days bulk storage; weekly dosing; automatic dose control via turbidity sensor + PLC. Modern WTPs have SCADA integration.
  • Residual chlorine: 0.5 mg/L at treatment plant outlet ensures protection during distribution. Higher doses (1-2 mg/L) for long mains or high ambient temperature.

FAQs

What is typical alum dose?
20-40 mg/L for surface water with turbidity 50-200 NTU. Range 10-80 mg/L depending on season. Determine optimum by jar test. PAC (polyaluminium chloride) typically 30-50% less dose required.
What is Surface Loading Rate (SLR) for sedimentation?
1.0-2.5 m/hr for conventional sedimentation. Higher SLR = smaller tank but poor settling. Conservative SLR 1.5 m/hr typical for Indian WTP design, giving 3-4 hour retention.
What is HLR for Rapid Sand Filter?
5-7 m/hr (hydraulic loading rate) — volume of water per unit filter area per hour. Higher HLR causes premature breakthrough; lower HLR wastes filter area. 5 m/hr conservative design.
What is the difference between RSF and SSF?
Rapid Sand Filter (RSF): HLR 5-7 m/hr, requires chemical pretreatment, 24-48 hr filter run, backwashed with water. Slow Sand Filter (SSF): HLR 0.1-0.3 m/hr, no chemicals, schmutzdecke layer does biological removal, scraped top layer every month.
What G-value for flash mixer?
300-500 s⁻¹ for flash mixer, retention 30-60 sec — high turbulence disperses coagulant. For flocculator, lower G-value 30-80 s⁻¹ (tapered from 60 to 30), retention 20-40 min — forms larger settleable flocs.
What is weir loading limit?
< 300 m³/m/day for sedimentation tank launder/weir. Above this, floc is drawn into effluent, reducing removal efficiency. Typical design 200 m³/m/day.

Calculator

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WTP Dosing & Unit Sizing (Alum / Sedimentation / RSF)

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Size coagulant dosing, sedimentation tanks, and rapid sand filters for a given WTP capacity. Based on CPHEEO Chapter 8 design parameters.

Inputs
Plant capacityMLD
Alum dosemg/L
Jar-test-determined, 20-40 mg/L typical for 50-200 NTU raw water
Sedimentation SLRm³/m²/hr
Sedimentation retentionhr
RSF hydraulic loadingm/hr
Area per filter unitm²
Standard filter unit size, 30-80 m² typical
Outputs
Alum consumption
300.0kg/day
Mass = MLD × dose(mg/L) [since 1 MLD × 1 mg/L = 1 kg/day]
Sedimentation tank area
77.2m²
A = Q / SLR = (MLD × 10⁶ / 86400) / SLR
Sedimentation tank volume
1,250m³
V = Q × retention = (MLD × 10⁶ / 86400) × t × 3600
Total RSF area required
19.3m²
A = Q / HLR
Number of filter units
2units
N = ⌈Total area / unit area⌉ + 1 standby
↪ Round up; add one for standby during backwash
CPHEEO Reference Values
Alum dose typical20 – 40 mg/L
Sedimentation SLR1.0 – 2.5 m³/m²/hr
Retention time3 – 4 hr
RSF HLR5 – 7 m/hr
Sand depth600 – 750 mm (eff. size 0.4–0.7 mm)
Download the Excel version to keep a local copy with live formulas — change inputs in the sheet and outputs recompute automatically.

Cross-references

IS 13405 WTP designCPHEEO WS Chapter 8 (Disinfection)IS 10500

Tags

water treatmentcoagulationflocculationsedimentationrapid sand filterslow sand filteralumjar testcpheeo
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Manual on Water Supply and Treatment · 3rd Edition (1999) with 2024 revision updates · Central Public Health and Environmental Engineering Organisation (CPHEEO), Ministry of Housing and Urban Affairs, Government of India.
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