WTP Pro Cal

Q = V × A, A = π D² ÷ 4

This is the starting point for almost every other calculation on this page. The intake or raw water pipe's cross-sectional area is found from its diameter, and multiplying that area by the design velocity gives the volumetric flow rate. A 0.3 m pipe carrying water at 1 m/s has an area of about 0.0707 m² and therefore delivers roughly 0.0707 m³/s — about 254.5 m³/hr, which several of the other tabs on this page use as their default design flow.

Design velocity in raw water mains is usually kept between about 0.6 and 3 m/s: too low and sediment settles inside the pipe, too high and head loss and erosion/water-hammer risk climb sharply.

A = Q ÷ Vo

Sedimentation basins are sized on surface loading, not depth — a particle only settles out in time if the basin's surface area is large enough relative to the flow passing through. Required area is simply the design flow divided by the chosen surface overflow rate (Vo). At 254.5 m³/hr and an overflow rate of 1 m³/m²·hr, a settling basin needs about 254.5 m² of surface area.

Typical Vo values run about 1–1.5 m³/m²·hr for plain sedimentation and up to ~2.5 m³/m²·hr where coagulation/flocculation precedes the basin — using a value outside this range without a basis is one of the most common sizing mistakes.

Mass (kg/day) = Dosage (mg/L) × Flow (m³/day) × 86.4 ÷ 1000

Once a jar test establishes the optimum coagulant dose in mg/L, this scales it to the full plant flow to give the daily mass of coagulant (alum, ferric chloride, PAC, etc.) that needs to be dosed. At a flow of 1000 m³/day and a dose of 10 mg/L, the plant needs about 864 kg of coagulant per day — the figure that feeds directly into the Sludge Production tab.

The optimum dose changes with raw water turbidity, colour, alkalinity and temperature, so it should be re-verified by jar testing whenever raw water quality shifts (e.g. monsoon turbidity spikes) rather than left fixed year-round.

Rate (m³/m²·hr) = Q ÷ Filter Area

Filtration rate is the hydraulic loading each square metre of filter media has to handle. At 254.5 m³/hr through a 50 m² filter bed, the loading works out to about 5.09 m³/m²·hr — comfortably inside the conventional rapid sand filter design band of roughly 5–15 m³/m²·hr (many municipal plants design closer to the 5–7.5 m³/m²·hr end for a safety margin).

Running filters above their design rate shortens filter runs, increases headloss build-up, and risks turbidity breakthrough — always check the manufacturer/media-specific allowable rate for the actual media (sand, dual-media, GAC) in use.

Volume = Filter Media Volume × Porosity × Number of Washes

Backwash water demand is estimated from the volume of the filter media bed, its porosity (the void fraction that has to be flushed of trapped floc and debris), and how many backwash cycles run per day. A 2 m³ media bed with 0.4 porosity, backwashed once a day, needs about 0.8 m³/day of backwash water for that filter.

This is a simplified planning estimate — actual backwash water use also depends on backwash rate (m³/m²·hr), bed expansion, and wash duration, and is normally cross-checked against the filter manufacturer's recommended backwash sequence.

Mass (kg/day) = Flow × Dose × 86.4 ÷ 1000

The same dosing relationship used for coagulant applies to disinfectant chemicals — chlorine, sodium hypochlorite (as available chlorine), or chlorine dioxide. At a flow of 254.5 m³/hr and a target free-chlorine dose of 1 mg/L, the plant needs to feed roughly 22 kg of chlorine per day.

The dose entered here should be based on chlorine demand testing (breakpoint chlorination) on the actual water, not a generic figure — raw water with high organic content or ammonia will consume far more chlorine before a stable free residual is achieved.

Sludge (kg/day) = Coagulant Used × Sludge Factor (0.5–1)

Coagulation and sedimentation don't remove turbidity for free — the material has to go somewhere, and that "somewhere" is sludge. This estimate applies a sludge production factor (typically 0.5–1 kg of wet sludge solids per kg of coagulant dosed, depending on raw water turbidity and coagulant type) to the daily coagulant mass. At 864 kg/day of coagulant and a factor of 0.6, expect around 518 kg/day of sludge, which then drives sludge drying bed or thickener sizing.

This factor is a rough planning multiplier, not a substitute for an actual sludge volume index (SVI) or solids balance for detailed sludge-handling design.

H = Hs + Hf  |  P = ρ × g × Q × H ÷ η

Total dynamic head (H) is the sum of static head (the actual elevation the water must be lifted) and friction head (losses through pipe, fittings and valves). Hydraulic power then follows from density (ρ = 1000 kg/m³ for water), gravity (g = 9.81 m/s²), flow (Q in m³/s), head (H in m) and pump efficiency (η). At Hs = 4 m, Hf = 6 m, Q = 0.0707 m³/s and η = 0.7, the pump needs about 9.9 kW of input power.

This is hydraulic/shaft input power at the pump — for motor sizing, apply a further margin (commonly 15–20%) and round up to the nearest standard motor rating, and remember friction head itself depends on the flow rate, so it should be re-checked at the actual operating point, not just the design point.

Capacity (L) = Flow × Softening Factor × Time

This gives a first-pass estimate of the treated water volume an ion-exchange softener needs to handle across a run, scaling daily flow by a softening factor (reflecting resin capacity/hardness reduction requirement) and the operating time window. At a flow of 100 m³/day, a factor of 10 and a 24-hour run, the estimated capacity works out to 24,000 L.

Actual softener sizing for a resin vessel should ultimately be based on raw water hardness (as CaCO₃), resin exchange capacity (kilograins or equivalents per m³ of resin), and desired regeneration frequency — this tool gives a quick planning figure, not a final resin-bed design.

Recovery R = Permeate ÷ Feed × 100  |  Salt Rejection SR = (Cf − Cp) ÷ Cf × 100

Recovery rate tells you what fraction of the feed water becomes usable permeate — the rest leaves as concentrate/reject. At a feed of 10 m³/hr and permeate of 6 m³/hr, recovery is 60%. Salt rejection tells you how effectively the membrane is removing dissolved solids, comparing feed TDS (Cf) against permeate TDS (Cp).

Brackish water RO systems typically target 60–85% recovery and greater than 95–99% salt rejection for a healthy membrane; a falling rejection percentage over time on the same membrane is usually an early sign of membrane fouling or scaling that needs cleaning-in-place (CIP) attention.

CT = C × T

CT — disinfectant residual concentration (C) multiplied by contact time (T) — is the standard way disinfection adequacy is judged for pathogens like Giardia and viruses. A chlorine residual of 2 mg/L held for 30 minutes gives a CT of 60 mg·min/L.

The CT value your system actually needs depends on the target organism, water pH and temperature, and is specified in regulatory CT tables (such as those under the US EPA Surface Water Treatment Rule, or the applicable national/local drinking water standard) — always check the value your CT result needs to meet or exceed rather than assuming a single universal number.

Storage = Daily Demand × Storage Factor

Clear water / distribution storage tanks are sized with a buffer above raw daily demand to absorb peak-hour draw, firefighting reserve, and any treatment or supply interruption. At a daily demand of 500 m³ and a storage factor of 1.5, the recommended tank volume is 750 m³.

Storage factor choice should reflect local demand-pattern data (peak-hour factor), the level of supply reliability required, and any fire-flow reserve mandated by the applicable local code — 1.2–2.0× daily demand is a common planning range for municipal clear water tanks.