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, 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 — and 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 interesting — 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.