What a rule of thumb is for
A rule of thumb answers "is this proposal the right order of magnitude?" in thirty seconds, before the meeting ends and before the quote goes out. It is a sanity check, not a design: every rule below has a stated domain, and the linked calculators (and behind them, datasheets and a proper water analysis) do the real work. With that said, the working set:
Filtration
- Media vessels size by velocity, not volume: bed area (m²) = flow (m³/h) ÷ design velocity (m/h). Sand and glass 10 – 25 m/h; catalytic media 10 – 12 m/h; carbon 5 – 15 m/h with contact time checked. A 30" vessel has ~0.46 m², so ~9 m³/h of sand duty at 20 m/h.
- Freeboard: keep 40 – 50% of vessel height above the settled bed for backwash expansion.
- Cartridges flow by length: take the datasheet flow per 10" length and scale. Change sediment cartridges at about 1 bar ΔP, which is a change-out figure and not the data sheet's structural maximum.
- Stage ratings 5:1 to 10:1 apart (50 → 10 → 1 µm), coarsest stage cleanable.
Softening and scale
- Convert everything to mg/L as CaCO₃ first (mg/L Ca × 2.5, mg/L Mg × 4.1).
- Resin litres ≈ daily use (m³) × hardness (mg/L) × days between regenerations ÷ 55. Then check peak flow stays under ~30 bed volumes/hour, and the bigger answer wins.
- Heated equipment starts paying for hardness above roughly 120 mg/L as CaCO₃.
RO
- Osmotic pressure ≈ 0.7 bar per 1 000 mg/L TDS; feed pressure must comfortably exceed it at the concentrate end.
- Permeate TDS ≈ 1 – 4% of feed for healthy brackish systems; concentrate concentration factor = 1 ÷ (1 − recovery): 50% recovery doubles, 75% quadruples. Run the LSI on that water.
- Feed conditions: SDI < 5 (design < 3), free chlorine effectively zero, Fe < 0.05 mg/L.
- Flux ~20 – 30 L/m²·h on clean brackish feeds; production moves ~3% per °C.
Disinfection
- UV dose: 30 mJ/cm² drinking duty, 40 validated, at the real peak flow, the real UVT, end of lamp life; 5 µm prefilter always; lamps on the maker's stated life, normally annually.
- Chlorine residual: 0.2 – 0.5 mg/L free at the furthest tap; CT 20 – 30 minutes; efficacy halves by pH 8.
Dosing
- Pump L/h = water m³/h × dose mg/L ÷ chemical g/L, then land the duty at 20 – 80% of pump capacity, diluting the chemical if needed.
- Pressure at the quill, not at the pump; suction lift under 1.5 m or flooded.
Conversions worth memorising
1 m³/h = 16.7 L/min · 1 bar = 10.2 m head = 14.5 psi · 1 US gallon = 3.785 L · TDS ≈ 0.5 – 0.7 × EC (µS/cm).
The caveat that makes them safe
Every number above assumes ordinary water and ordinary duty. The rules fail exactly where waters are unusual (high silica, high organics, extreme temperatures, reuse duty), and they never override a datasheet. Use them to catch the order-of-magnitude error in the meeting; use the calculators for the quote; use the full analysis and the datasheets for the design.
Rule of thumb about rules of thumb: if a proposal survives these checks, it is worth engineering properly; if it fails one, no amount of engineering detail downstream will rescue the sizing. Check the arithmetic before the brochure.