Restaurant Water Filter Sizing by Seats and Peak Flow

At 20 L per seat, daily demand is 90 × 20 ÷ 1,000 = 1.8 m³/day. With a peak factor of 2.0 over 9 operating hours, peak flow is 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; a 1 or 2 m³/h unit would cycle on and off, waste concentrate and cost more without improving service.

Storage is what makes the small RO work. A 0.75–1.0 m³ tank holds 40–55% of daily demand, so the membrane can refill at a steady 0.5 m³/h through the afternoon and still cover the dinner rush. Put the tank after the RO, keep it pressurised with a booster set, and return the loop through UV so the water at the tap is fresh. If the kitchen is on the same line, size the RO for the combined ice, coffee and kitchen demand, not for drinking water alone.

One incoming line cannot serve every outlet well. Ice and drinking water need low TDS and no chlorine; coffee needs stable alkalinity; dishwashers and steamers need soft water; hand-wash taps need nothing beyond basic filtration. Splitting the line costs a little more pipework but avoids treating all water to the highest standard or compromising the most sensitive outlet.

Size the softener on the daily hardness load rather than on the whole restaurant demand. If the kitchen uses 1.8 m³/day at 200 ppm as CaCO₃, the load is 1.8 × 200 = 360 g/day. At a resin capacity of 50 g/L and a three-day regeneration interval, the softener needs 21.6 L of resin; a 25–50 L charge gives margin for peak days and for hardness above the design value. Salt use is 100–150 g per litre of resin, or 2.5–7.5 kg per regeneration at this size.

Check the peak rinse flow as well. A dishwasher may draw 0.3–0.6 m³/h during fill and rinse, and a combi oven adds its own fill. A 25 L bed at 30 bed volumes per hour can pass only 0.75 m³/h, so confirm the simultaneous demand before choosing the smallest resin charge. If the kitchen also has a reverse osmosis unit, soften the RO feed and take the dishwasher from the same softener outlet.

Plan storage at 40–55% of daily demand: 0.75–1.0 m³ for the 90-seat example. The tank does three jobs: it lets the RO run at a steady rate, it covers the short lunch peak when the ice machine, coffee brewer and dishwasher start together, and it keeps water available during backwash or cartridge changes. Without storage, the RO must match the instantaneous peak, which doubles or triples the membrane size for a demand that lasts 30–60 minutes.

Place the tank indoors, out of direct sun, with a drain and a lid. Use a level switch to start and stop the RO, and a booster pump to hold 2–3 bar at the farthest outlet. If the tank is larger than one day of demand, add a loop and UV; still water in a warm kitchen is a microbiological risk, not a reserve.

Different formats shift both the daily figure and the peak shape. A coffee shop sells few covers but runs the espresso machine and ice bin for ten hours, so its peak is flatter and its daily volume lower. A bar peaks late and uses little dishwashing water but a lot of ice. A hotel kitchen serves breakfast, lunch and dinner with banquet spikes, and usually shares a softener with the laundry.

Peak shape matters more than the daily total when selecting storage. A café may draw 0.15 m³/h for ten hours, while a 90-seat restaurant draws 0.4 m³/h for two hours. The first needs a smaller RO and a smaller tank; the second needs a larger tank relative to the RO. Plot the hourly demand before fixing tank volume, and confirm whether the ice machine or the dishwasher starts first.

Restaurant projects move quickly, so the data collection should be short and specific. The five items below are enough to size a line and to avoid the two common errors: an RO that is too large for the drinking load and a softener that is too small for the dishwasher.