Sizing Water Filters for Hotels, Restaurants and Offices

Use three calculations in order. Daily demand: Qd = N × q × f ÷ 1,000, where N is rooms, seats or people, q is litres per unit per day and f is occupancy or utilisation — typically 0.65–0.80 for a city hotel and 0.85–0.95 for a school. Peak flow: Qpeak = Qd × PF ÷ T, where PF is the peak factor of 1.5–2.5 and T is operating hours. Treatment flow: QRO = Qd × 1.15 ÷ R, where R is refill hours and 1.15 is a fouling margin.

Peak flow sizes pipes, softeners and pretreatment; treatment flow sizes the RO membrane, because the membrane refills storage over hours rather than serving the tap directly. For hotels, restaurants and offices, storage should hold 30–50% of daily demand so the plant runs at steady output through the busiest period; schools are sized on the break, not the daily average. Do not size the RO on the instantaneous peak: a 120-room hotel may draw 3 m³/h at 08:00 but only needs 0.5 m³/h of drinking water when a 1 m³ tank sits between the membrane and the tap.

At 300 L per room and 75% occupancy, daily demand is 120 × 300 × 0.75 ÷ 1,000 = 27 m³/day. With a peaking factor of 1.8 over 18 operating hours, peak flow is 27 × 1.8 ÷ 18 = 2.7 m³/h. A 3 m³/h softener covers the hot water, laundry and kitchen load with a small margin. Drinking water is a separate stream: at 3 L per room per day, the hotel needs 0.36 m³/day, so a 0.5 m³/h RO with a 1 m³ tank is adequate.

Keep the two duties separate in the quotation. The softener protects calorifiers, dishwashers and laundry equipment and is sized on peak flow and daily hardness load. The RO supplies guest drinking water, ice and coffee, and is sized on consumption plus storage. Combining them into one oversized RO raises capital cost and wastewater without improving hot water performance.

At 20 L per seat, daily demand is 90 × 20 ÷ 1,000 = 1.8 m³/day. A peaking factor of 2.0 over 9 operating hours gives 1.8 × 2.0 ÷ 9 = 0.40 m³/h. Add 15% for fouling and future load, and the design flow is 0.46 m³/h; a 0.5 m³/h RO is the correct size, not a 2 m³/h unit. Store 0.5–1.0 m³ to cover the lunch and dinner rush while the membrane refills.

If hardness is 200 ppm as CaCO₃, the daily hardness load is 1.8 m³ × 200 g/m³ = 360 g. At a resin capacity of 50 g/L and a three-day regeneration interval, the softener needs roughly 25–50 L of resin, plus margin for peak days. That is a small softener, but it protects the dishwasher, combi oven and steam table, which are the most expensive items in the kitchen.

Use 10–20 L per person per day, then count the people who actually drink on site. A 150-person office at 15 L uses 150 × 15 ÷ 1,000 = 2.25 m³/day. With a peaking factor of 2.2 over 8 working hours, peak flow is 2.25 × 2.2 ÷ 8 = 0.62 m³/h. A 1 m³/h RO with a 1–1.5 m³ tank covers the morning arrival and lunch peaks and leaves capacity for growth.

Offices are better served by a central plant and a circulation loop than by a cooler on every floor. A loop keeps water moving, reduces stagnation and gives one point for filter and UV maintenance. If the building has a canteen, gym or public café, meter or estimate those loads separately; they can add 20–40% to the drinking demand without appearing in the staff count.

Four conditions move the numbers most: hardness, iron and manganese, TDS, and water temperature. Hardness above 120 ppm as CaCO₃ usually justifies softening before RO; iron above 0.3 ppm or manganese above 0.05 ppm needs oxidation and filtration, or the membrane and resin will foul. TDS above 1,000 ppm raises osmotic pressure and lowers recovery. Cold feed water reduces RO output by roughly 3% per °C below 25 °C, so winter design must use the lowest expected temperature.

A quotation is only as good as its inputs. Collect the following before asking for a price, and state which values are measured and which are estimated. The difference matters when the plant is later tested against its design flow.