Water treatment plant design

In short

Water treatment plant (WTP) design is the sizing and arrangement of unit operations - aeration, coagulation, flocculation, sedimentation, filtration, disinfection and special treatment - that convert raw surface or ground water into water meeting IS 10500:2012 drinking water standards. It matters because the quality of raw water decides the treatment train, and every unit must be hydraulically sized for a design flow with allowance for peak factor and maintenance. It is examined in GATE Civil (Environmental Engineering section, usually 6-10 marks), university Environmental Engineering papers, and in AE/JE and ESE recruitment for state PHE/Water Boards. On the job it is used by Jal Jeevan Mission and AMRUT consultants, L&T, VA Tech Wabag, Tata Projects, SUEZ and state PHED/Jal Nigam engineers who design and commission plants.

Explained

Raw water quality, standards and choice of treatment train

• Objective: make water safe (free of pathogens and toxic chemicals), aesthetically acceptable (no colour, taste, odour, turbidity) and non-corrosive/non-scaling.
• IS 10500:2012 (amended since; always quote the current amendment) gives acceptable limit / permissible limit in absence of alternate source: turbidity 1 / 5 NTU, pH 6.5-8.5 (no relaxation), total hardness 200 / 600 mg/L as CaCO₃, iron 1.0 mg/L (the 0.3 mg/L acceptable limit was revised; check current amendment), fluoride 1.0 / 1.5 mg/L, nitrate 45 mg/L, arsenic 0.01 mg/L (no relaxation - the 0.05 mg/L permissible limit was withdrawn by the 2015 amendment), TDS 500 / 2000 mg/L. E. coli and total coliform must be absent in any 100 mL sample.
• Residual chlorine: minimum 0.2 mg/L free residual at the consumer tap (0.5 mg/L recommended during epidemics), per IS 10500 and CPHEEO.
• Design guide: CPHEEO *Manual on Water Supply and Treatment* (Ministry of Housing and Urban Affairs) is the governing Indian practice document; relevant IS codes include IS 10500 (quality), IS 8419 (filtration equipment), IS 3025 (test methods), IS 1172 (water demand), IS 7090/IS 12117 (clarifier and chemical feed practice).
• Selecting the train:

• Design flow: plant capacity is normally the maximum daily demand (average day × 1.8 per CPHEEO). Units are designed so that one unit can be out for cleaning - minimum two sedimentation tanks, at least two (preferably four) filter beds.
• Design period: 15-30 years for civil structures (intake, tanks), 10-15 years for mechanical/electrical, per CPHEEO.

Preliminary treatment: screening, aeration, plain sedimentation

• Screens: coarse screens (bar spacing 50-100 mm) at the intake, fine screens (5-20 mm) before pumps; velocity through the screen kept 0.6-1.0 m/s so that screenings are not pushed through and grit does not settle.
• Aeration: adds O₂ and strips CO₂, H₂S and taste/odour; oxidises Fe²⁺ to insoluble Fe(OH)₃ and Mn²⁺ to MnO₂ for removal by filtration.
• Types: cascade/weir aerator (simple, head loss 1-3 m), spray aerator (nozzles, 0.7-1.0 kg/cm² pressure), tray/coke-bed aerator, diffused-air (compressed air through diffusers), mechanical surface aerator.
• Plain sedimentation (pre-settling): removes coarse silt without chemicals; detention 3-6 h, removes 60-70% of suspended solids but little colloidal matter.
• Stokes' law governs discrete particle settling in the laminar range (Reynolds number < 1): settling velocity ∝ (diameter)² and ∝ (specific gravity − 1). Colloids (0.001-1 µm) have practically zero settling velocity, which is why coagulation is unavoidable.

Coagulation and flocculation

• Colloidal stability: clay colloids carry a negative surface charge; the electrical double layer and zeta potential keep them apart. Coagulation adds trivalent cations that compress the double layer and destabilise the colloid; flocculation then gently stirs to let destabilised particles collide and grow into settleable floc.
• Mechanisms: double-layer compression, charge neutralisation, sweep-floc (enmeshment in hydroxide precipitate), inter-particle bridging (polymers).
• Common coagulants:
  • Alum, Al₂(SO₄)₃·18H₂O - the Indian workhorse; dose 10-50 mg/L typical, effective pH 6.5-8.5; consumes alkalinity, so lime or soda ash is added if natural alkalinity is insufficient.
  • Ferric chloride / ferric sulphate - wider effective pH (5-11), better in cold and coloured water, more corrosive.
  • Ferrous sulphate (copperas) - needs chlorine (chlorinated copperas) to oxidise Fe²⁺.
  • PAC (poly-aluminium chloride) - lower dose, less alkalinity consumption, less sludge; now common in Indian plants.
  • Coagulant aids: lime, soda ash, bentonite clay (for low-turbidity water), activated silica, polyelectrolytes (anionic/cationic/non-ionic, dose 0.1-1 mg/L).
• Alum reaction (in water with bicarbonate alkalinity): aluminium sulphate reacts with calcium bicarbonate to give aluminium hydroxide floc, calcium sulphate and carbon dioxide. With alum taken as Al₂(SO₄)₃·18H₂O (molecular weight ≈ 666), 1 mg/L of alum consumes about 0.45 mg/L of alkalinity as CaCO₃ and releases about 0.40 mg/L of CO₂.
• Jar test: the standard laboratory method to fix optimum dose and pH - six beakers, rapid mix ~1 min at 100-120 rpm, slow mix 20-30 min at 20-30 rpm, settle 30 min, then measure residual turbidity. The lowest dose giving the target turbidity is the optimum.
• Rapid mixing (flash mixer): detention 30-60 s, velocity gradient G = 300-1000 /s, power 10-40 kW per m³/s of flow; devices - mechanical impeller in a square tank (depth:width about 1:1), in-line static mixer, hydraulic jump in a Parshall flume.
• Flocculation: detention 20-40 min, G = 10-75 /s, Gt = 10⁴-10⁵ (dimensionless); peripheral paddle speed 0.2-0.6 m/s; paddle area 15-20% of tank cross-section; tapered flocculation (high G first, low G later) prevents floc break-up.
• Clariflocculator: circular unit with a central flocculation zone and an annular settling zone - the standard compact Indian design for small and medium plants.

Sedimentation tank design (type II settling)

• Purpose: remove the floc formed in flocculation; 80-90% suspended-solids removal typical.
• Governing idea: an ideal basin removes all particles whose settling velocity v₀ ≥ Q/A (surface area, not depth, controls removal). Q/A is the surface overflow rate (SOR) or surface loading.
• Design values (CPHEEO): SOR 20-40 m³/m²/day (say 30 for alum floc); detention 2-4 h (2.5 h usual); depth 3-4.5 m (plus 0.3-0.5 m freeboard and 0.3-0.6 m sludge zone); horizontal velocity 0.5-1.0 cm/s; weir loading 150-300 m³/m/day; length:width about 3:1 to 5:1 for rectangular tanks; floor slope 1 in 12 (circular) or 1% (rectangular).
• Inlet and outlet: baffled or perforated-wall inlet to spread the flow; submerged orifices or launders with V-notch weirs at outlet; short-circuiting and density/wind currents are the main causes of real performance falling below ideal.
• Sludge removal: manual (small plants, periodic drain-down) or mechanical scrapers/suction headers (continuous).
• Tube settlers / plate settlers: inclined (usually 60°) tubes or plates multiply the effective settling area, so SOR of 40-60 m³/m²/day on plan area is possible - used to upgrade existing tanks with no extra footprint.
• Reynolds and Froude check: flow must stay laminar (Re < 2000 based on hydraulic radius) and stable (Fr > 10⁻⁵) to avoid turbulence and short-circuiting.

Filtration

• Function: remove the remaining floc, 0.1-1 NTU product turbidity, plus 98-99% of bacteria; mechanisms - mechanical straining, sedimentation in pores, adsorption, flocculation within the bed and biological action.
• Rapid sand filter (RSF) - the standard unit in Indian WTPs:
  • Rate of filtration 100-200 m³/m²/day (4-8 m/h); typical 5 m/h.
  • Sand: effective size D₁₀ = 0.35-0.55 mm, uniformity coefficient (D₆₀/D₁₀) 1.3-1.7, depth 0.6-0.75 m over 0.45-0.6 m of graded gravel.
  • Bed area per unit normally 25-50 m² (max ~100 m²); length:width 1.25:1 to 1.5:1; depth of box 2.5-3.5 m with 1.0-2.0 m water above the sand.
  • Under-drainage: manifold-and-lateral pipes, or false floor with nozzles; lateral-to-manifold area ratio and orifice ratios fixed by CPHEEO.
  • Backwashing when head loss reaches 2.0-2.5 m or turbidity breaks through; wash rate 15-25 m³/m²/h (0.25-0.4 m/min) for 5-10 min, expanding the bed 30-50%; wash water 1-6% (typically 2-3%) of filtered water. Air scour or surface wash added to break mud balls.
  • Negative head: if head loss exceeds the depth of water above the bed, pressure inside the bed falls below atmospheric, air comes out of solution and causes air binding - avoided by keeping enough water over the sand.
• Slow sand filter (SSF): rate 2.4-5 m³/m²/day (roughly 40 times slower), finer sand (D₁₀ = 0.2-0.3 mm), no coagulation needed, cleaned by scraping the schmutzdecke every 1-3 months. Needs large area; suited to small, low-turbidity rural supplies.
• Pressure filters: sand in a closed steel shell operating under pressure - used in industry, swimming pools and small package plants; no break in pressure head, but the bed cannot be inspected.
• Dual-media / multi-media filters: anthracite over sand (coarse-to-fine in the direction of flow) gives longer runs and higher rates.
• Membrane processes: microfiltration (0.1 µm), ultrafiltration (0.01 µm, removes bacteria and viruses), nanofiltration (softening, organics) and reverse osmosis (desalination, TDS and arsenic removal); need pre-treatment and generate a reject stream.

Disinfection

• Chlorination is the dominant method in India - cheap, effective and leaves a measurable residual.
• Forms: chlorine gas (large plants, tonners/cylinders, through a chlorinator), bleaching powder (calcium hypochlorite, ~30-35% available chlorine, loses strength on storage), sodium hypochlorite (common in Jal Jeevan Mission schemes, dosed by hypochlorinators), chlorine dioxide, chloramines.
• Chlorine chemistry: chlorine hydrolyses to hypochlorous acid (HOCl) and hypochlorite ion (OCl⁻). HOCl is roughly 80-100 times more germicidal, and it dominates below pH 7.5 - so disinfection is far more effective at lower pH.
• Residual and demand: chlorine demand = applied dose − residual. Break-point chlorination: the residual first rises (combined, as chloramines), then falls as chloramines are destroyed, reaching the break point, after which free residual chlorine appears - dosing is always kept beyond the break point.
• Terms: *pre-chlorination* (before other units, controls algae and odour), *post-chlorination* (after filtration, the normal practice), *super-chlorination + dechlorination* (heavy dose then SO₂/activated carbon), *de-chlorination*, *residual chlorine test* (ortho-tolidine or DPD).
• Contact time: minimum 30 min at the plant; CT concept - the product of concentration (mg/L) and contact time (min) determines log-inactivation.
• Disinfection by-products: trihalomethanes (THMs) form when chlorine meets natural organic matter; IS 10500 limits chloroform to 0.2 mg/L. Reducing organics before chlorination is the control.
• Alternatives: ozone (strong oxidant, also removes colour/taste, but no residual and costlier), UV (no residual, needs low turbidity, widely used in domestic and small-community units), boiling (household), SODIS, iodine/silver (emergency).

Special treatment and plant layout

• Softening: removes hardness. *Lime-soda*: lime removes carbonate hardness, soda ash removes non-carbonate hardness; excess lime raises pH to ~9.5-10.5 so recarbonation (CO₂) follows. *Zeolite/ion exchange*: sodium zeolite exchanges Na⁺ for Ca²⁺/Mg²⁺, regenerated with brine; gives near-zero hardness but adds sodium. *Reverse osmosis* for high-TDS water.
• Iron and manganese removal: aeration + contact/oxidation + filtration; or KMnO₄/chlorine oxidation; green-sand or MnO₂-coated media.
• Defluoridation: Nalgonda technique (lime + alum, developed by NEERI - widely deployed in Andhra Pradesh, Telangana, Rajasthan), activated alumina columns, bone char, RO.
• Arsenic removal: oxidation then coagulation-filtration, activated alumina, iron-oxide-coated sand - critical in parts of West Bengal, Bihar, Assam, Uttar Pradesh.
• Taste, odour and colour: powdered/granular activated carbon, aeration, pre-chlorination, copper sulphate for algae in reservoirs (typically 0.3-1.0 mg/L).
• Fluoridation (adding fluoride) is not Indian practice - India's problem is excess fluoride.
• Plant hydraulics and layout: units are arranged in flow sequence so water moves by gravity - intake → raw water pump → aerator/flash mixer → flocculator → clarifier → filters → clear water reservoir (chlorine dosed at inlet for mixing) → treated water pump → service reservoir. Total head loss across the plant is 3-6 m; a hydraulic profile (water-surface elevations through the plant) is a standard design drawing.
• Residuals management: sludge from clarifiers and spent backwash water are now recovered - thickener → sludge drying beds / centrifuge, with supernatant and backwash water recycled (5-10% of flow), which the CPHEEO manual and CPCB expect rather than discharge to a drain.
• Clear water reservoir: 1/3 to 1/2 of one day's plant output is a common provision for contact time and pump balancing.

Key points

• Design flow - maximum daily demand = 1.8 × average daily demand (CPHEEO); units duplicated so one can be shut for cleaning.
• IS 10500:2012 - Indian drinking water standard: turbidity 1 NTU acceptable, pH 6.5-8.5, coliforms absent in 100 mL; residual chlorine ≥ 0.2 mg/L at the tap.
• CPHEEO Manual - the governing design practice document for water supply and treatment in India.
• Coagulation - chemical destabilisation of colloids by trivalent cations (rapid mix, 30-60 s, G = 300-1000 /s).
• Flocculation - gentle stirring to grow floc: 20-40 min, G = 10-75 /s, Gt = 10⁴-10⁵.
• Jar test - the laboratory test that fixes optimum coagulant dose and pH; no WTP design is complete without it.
• Alum - Al₂(SO₄)₃·18H₂O, dose 10-50 mg/L; consumes 0.45 mg/L alkalinity as CaCO₃ and releases 0.40 mg/L CO₂ per mg/L of alum.
• Surface overflow rate - Q/A, 20-40 m³/m²/day for floc; removal depends on tank *area*, not depth.
• Stokes' law - settling velocity varies with the square of particle diameter; valid for Re < 1.
• Rapid sand filter - 100-200 m³/m²/day, D₁₀ 0.35-0.55 mm, UC 1.3-1.7, 0.6-0.75 m bed.
• Backwash - at 2.0-2.5 m head loss, 15-25 m³/m²/h for 5-10 min, consuming 2-3% of output.
• Negative head and air binding - head loss exceeding the depth of water over the bed; prevented by 1-2 m of standing water.
• Slow sand filter - 2.4-5 m³/m²/day, biological schmutzdecke, no coagulation, large area, low-skill O&M.
• Break-point chlorination - dose past the point where chloramines are destroyed, so a free residual appears.
• HOCl vs OCl⁻ - HOCl is the far stronger disinfectant and dominates at pH below 7.5.
• Clariflocculator / tube settler - compact combined or high-rate settling units used to save land or upgrade old plants.
• Nalgonda technique - lime + alum defluoridation, NEERI-developed, used in Telangana, Andhra Pradesh and Rajasthan.
• Residuals - clarifier sludge and backwash water are thickened and recycled, not drained away.

Formulas and examples

Example 1 - Sizing a sedimentation tank. A town needs 10 MLD average demand. Design a rectangular sedimentation tank at SOR = 30 m³/m²/day and detention 2.5 h.

• Design (maximum daily) flow = 1.8 × 10 = 18 MLD = 18,000 m³/day = 0.2083 m³/s.
• Plan area A = 18,000 / 30 = 600 m². Provide two tanks → 300 m² each.
• Volume per tank = (18,000/2) × (2.5/24) = 9,000 × 0.10417 = 937.5 m³.
• Depth = 937.5 / 300 = 3.125 m → provide 3.2 m water depth + 0.5 m freeboard + 0.4 m sludge zone = 4.1 m total.
• With L:B = 4:1, 300 = 4B² → B = 8.66 m, L = 34.64 m. Provide B = 8.7 m, L = 35 m each.
• Check horizontal velocity: vh = (9,000/86,400) / (8.7 × 3.2) = 0.10417/27.84 = 0.00374 m/s = 0.37 cm/s - below the 0.5-1.0 cm/s guidance, which is safe (no scour of settled floc).
• Weir loading, if the full width is used as outlet: 9,000 / 8.7 = 1,034 m³/m/day - too high, so provide launders to give about 30-45 m of weir length (9,000/300 = 30 m gives 300 m³/m/day).

Example 2 - Removal efficiency and particle size. For the tank above (v₀ = 30 m³/m²/day):

• v₀ = 30 / 86,400 = 3.472 × 10⁻⁴ m/s = 0.347 mm/s.
• Particles settling at 0.20 mm/s are removed to η = 0.20/0.347 = 57.6%.
• Smallest particle fully removed, by Stokes with ρs = 2650 kg/m³, water at 20 °C:

d² = 18 µ v / (g (ρs − ρw)) = (18 × 1.002 × 10⁻³ × 3.472 × 10⁻⁴) / (9.81 × 1650) = 6.262 × 10⁻⁶ / 16,186.5 = 3.868 × 10⁻¹⁰ m². d = 1.967 × 10⁻⁵ m ≈ 0.020 mm (20 µm) - fine silt. Anything finer needs coagulation.

• Check Stokes validity: Re = v d ρw / µ = (3.472 × 10⁻⁴ × 1.967 × 10⁻⁵ × 998) / (1.002 × 10⁻³) = 6.8 × 10⁻³ ≪ 1. Valid.

Example 3 - Alum dose, alkalinity and cost. An 18 MLD plant uses alum at 28 mg/L. Raw water alkalinity is 10 mg/L as CaCO₃.

• Alum needed = 18 × 28 = 504 kg/day (≈ 15.1 tonnes/month).
• Alkalinity required = 0.45 × 28 = 12.6 mg/L as CaCO₃; available only 10 mg/L → deficit 2.6 mg/L as CaCO₃.
• Lime (as CaO, equivalent weight 28, against CaCO₃ equivalent weight 50) = 2.6 × 28/50 = 1.46 mg/L → 18 × 1.46 = 26.2 kg/day of pure CaO; with 70% purity quicklime, 26.2/0.70 ≈ 37 kg/day.
• CO₂ released = 0.40 × 28 = 11.2 mg/L → corrosive, so pH is checked and adjusted after coagulation.
• Chemical cost at ₹22/kg for alum: 504 × 22 = ₹11,088/day ≈ ₹40.5 lakh/year on alum alone.

Example 4 - Flocculator design (G, Gt and paddle power). Design a flocculation tank for 18 MLD with t = 30 min and G = 40 /s; µ = 1.002 × 10⁻³ N·s/m².

• Volume V = (18,000/86,400) × (30 × 60) = 0.2083 × 1800 = 375 m³.
• Power P = G² µ V = 40² × 1.002 × 10⁻³ × 375 = 1600 × 0.3758 = 601 W ≈ 0.60 kW (water power; motor rating ≈ 0.60/0.7 ≈ 0.9 kW, say 1.1 kW).
• Camp number Gt = 40 × 1800 = 72,000 - inside the 10⁴-10⁵ range.
• Paddle velocity from P = CD A ρ vr³/2 with CD = 1.8, paddle area A = 8 m², ρ = 998:

vr³ = 2P/(CD A ρ) = (2 × 601)/(1.8 × 8 × 998) = 1202/14,371 = 0.0836 → vr = 0.437 m/s. Peripheral paddle speed vp = vr/0.75 = 0.58 m/s (within the 0.2-0.6 m/s guide); for a paddle of radius 1.5 m, N = 0.58/(2π × 1.5) = 0.062 rev/s ≈ 3.7 rpm.

Example 5 - Rapid sand filters and backwash. Same 18 MLD plant; filtration rate 5,000 L/m²/h (= 5 m/h = 120 m³/m²/day).

• Hourly flow = 18,000/24 = 750 m³/h. Area needed = 750/5 = 150 m².
• Provide beds of 37.5 m² (7.5 m × 5.0 m, L:B = 1.5): 150/37.5 = 4 working beds + 1 standby = 5 beds.
• With one bed under backwash, rate on the other 4 = 750/150 = 5 m/h (as designed); if a second bed is down, rate rises to 750/112.5 = 6.67 m/h - still inside the 4-8 m/h limit.
• Backwash at 18 m³/m²/h for 7 min per bed: 18 × 37.5 × (7/60) = 78.75 m³ per bed per wash.
• Washing each of 4 working beds once a day = 315 m³/day = 315/18,000 = 1.75% of output - acceptable (2-3% typical).

Where it is used

• Municipal WTPs under AMRUT and the Jal Jeevan Mission: state PHED/Jal Nigam and Water Board engineers design, tender and supervise plants from 1 MLD rural schemes to the ~1,900 MLD Bhandup complex in Mumbai and the 635 MLD Sonia Vihar plant in Delhi.
• EPC and O&M contractors: L&T Water, VA Tech Wabag, Tata Projects, SUEZ India, Thermax, Ion Exchange - design-build-operate contracts for WTPs, package plants and desalination (Nemmeli and Minjur, Chennai).
• Industrial water treatment: clarifier-filter-DM plant trains for thermal power stations (NTPC), refineries, steel plants and pharma, where feed water must be far purer than drinking water.
• Consultancy and design roles: design engineer / proposal engineer preparing DPRs, hydraulic profiles, chemical-dosing calculations and GA drawings; graduate roles typically ₹4-8 lakh/annum, higher in EPC majors.
• Regulation and testing: CPCB and state PCBs, BIS laboratories, NEERI and municipal water-testing labs running jar tests, residual chlorine and bacteriological checks.
• Rural and household products: community defluoridation and arsenic-removal plants, iron-removal units in Assam and Bihar, and the domestic RO/UV market (Eureka Forbes, Kent, HUL Pureit).

Interview questions

• Q: Why can't colloidal turbidity be removed by plain sedimentation? A: Colloids are 0.001-1 µm with a settling velocity near zero and a negative surface charge that keeps them mutually repelled, so they would need impractically long detention; coagulation destabilises them into settleable floc.
• Q: Does the depth of a sedimentation tank affect removal efficiency? A: In ideal settling theory no - removal depends on surface overflow rate Q/A. Depth matters practically: it provides detention for flocculation, storage for sludge and protection against scour and wind currents.
• Q: What does the jar test tell you and how is it run? A: It gives the optimum coagulant dose and pH - six beakers at graded doses, rapid mix about 1 min, slow mix 20-30 min, settle 30 min, then compare residual turbidity; the lowest dose meeting the target is chosen.
• Q: What is break-point chlorination? A: Applying chlorine beyond the dose at which all ammonia-derived chloramines are destroyed, so that free residual chlorine appears and is available for continued disinfection.
• Q: Why is chlorination more effective at lower pH? A: Below pH 7.5 chlorine exists mainly as hypochlorous acid, which is about 80-100 times more germicidal than the hypochlorite ion that dominates at higher pH.
• Q: What is negative head in a filter and how is it prevented? A: When head loss through the bed exceeds the depth of water standing above it, pressure within the bed falls below atmospheric and dissolved air releases, causing air binding; it is prevented by keeping 1-2 m of water over the sand and backwashing before head loss exceeds 2-2.5 m.
• Q: Why is the uniformity coefficient of filter sand kept low? A: A narrow size range (UC 1.3-1.7) stops fine grains from filling the pores between coarse ones, which would cause rapid head loss, and gives predictable expansion during backwash.
• Q: Alum versus PAC - which would you choose? A: PAC needs a lower dose, consumes less alkalinity, works over a wider pH and produces less sludge, so it suits low-alkalinity or cold water; alum is cheaper and universally available, so it stays the default where water has adequate alkalinity.
• Q: A plant's product turbidity rises in the monsoon. How do you diagnose it? A: Check raw water turbidity against design, re-run the jar test for dose and pH, verify flash-mix and flocculator operation, check clarifier sludge build-up and short-circuiting, and confirm filter runs and backwash adequacy - overloaded SOR is the usual culprit.
• Q: Why design a clariflocculator or tube settler instead of separate tanks? A: To save land and cost - a clariflocculator combines flocculation and settling in one circular structure, and tube settlers multiply effective settling area so an existing tank can take 1.5-2 times the flow without expansion.
• Q: What is the difference between coagulation and flocculation? A: Coagulation is the chemical destabilisation of colloids during a brief, intense mix; flocculation is the subsequent slow physical process in which destabilised particles collide and grow into settleable floc.
• Q: How much residual chlorine must reach the consumer and why? A: At least 0.2 mg/L free residual (IS 10500/CPHEEO), as a safeguard against re-contamination from leaks, intermittent supply and back-siphonage in the distribution network.

Common mistakes

• Designing units for average daily demand instead of maximum daily demand, and forgetting to add a standby filter bed and a second clarifier.
• Treating detention time as the design criterion for a clarifier - surface overflow rate governs removal; detention and depth follow from it.
• Mixing units in SOR calculations - m³/m²/day against m/h or L/m²/min; always convert to one system before dividing.
• Forgetting that alum consumes alkalinity, so no lime is provided for low-alkalinity water and coagulation fails in practice.
• Using chlorine dose instead of chlorine demand (or ignoring the break point) when sizing chlorination.
• Swapping slow and rapid sand filter parameters - the rates differ by a factor of about 40, and only the slow filter works without coagulation.
• Quoting IS 10500 limits from memory without noting the distinction between the acceptable limit and the permissible limit in the absence of an alternate source, or ignoring later amendments.

Related topics

Water supply engineering · Wastewater engineering · Sewer network design · Rainwater harvesting and water conservation · Groundwater hydraulics · Environmental impact assessment