A hotel can have enough daily RO production and still run short during breakfast service. The calculation must connect three quantities: water produced over the day, water delivered during simultaneous use, and water available in storage when demand rises.
Room count alone cannot connect those quantities. Build the demand profile for the outlets supplied by the proposed RO system, then test the production schedule against it.
Define the demand that actually reaches the RO tank
Start with a list of connected outlets and equipment. Mark each as included, excluded or awaiting confirmation. A beverage zone and a whole-property supply are different design boundaries.
EPA’s hotel water-use guidance identifies several distinct uses, including sanitary facilities, laundry, kitchens, landscaping and heating or cooling. Assign only the connected uses to the proposed RO installation. Use the hotel water-treatment guide to establish the treatment boundaries before calculating capacity.
For an operating property, collect meter readings with the corresponding occupancy and service schedule. EPA’s metering and submetering guidance explains how separate activity measurements and interval records support a clearer view of water use. For a new property, build a provisional schedule from equipment fill volumes, cycle counts and expected overlapping operation.
| Input | Record | Calculation supported |
|---|---|---|
| Connected demand | Outlets supplied by this tank; excluded branches | Correct treatment boundary |
| Daily volume | Normal and busy-day draw, in m³/day | Total production requirement |
| Demand timing | Volume by period and simultaneous peak flow | Storage drawdown and distribution duty |
| Production availability | Effective run hours, interruptions and net tank inflow | Required RO output and refill time |
| Storage | Usable volume, starting level and reserved volume | Minimum inventory through the day |
| Delivery conditions | Required outlet pressure, elevation and piping losses | Distribution pump duty |
Use a recording interval short enough to capture the demand being investigated. An hourly consumption total can conceal a brief equipment-filling peak.
Keep production, delivery and storage separate
Production is the net treated-water flow entering storage during effective running hours. Allow for startup diversion, flushing and unavailable periods when establishing that net output.
Distribution is the flow delivered from storage to the connected outlets, at the required pressure. It can exceed the production rate temporarily.
Usable storage is the volume between the normal high level and the minimum operating level. It excludes headspace and water below that lower level. State how much of this usable volume is reserved for interruptions.
Ask for production projections at the site’s feed conditions. DuPont’s FilmTec technical manual identifies temperature, pressure, salinity and recovery as performance variables. With other conditions unchanged, colder feed reduces permeate output. Specify the temperature range when requesting an equipment proposal.
Worked example with stated assumptions
Consider a defined hotel treated-water zone with 12 m³/day of demand. Assume net RO inflow is constant from 06:00 to 16:00, giving 10 effective production hours. Demand is constant within each period below; no shorter peak exceeds 2.5 m³/h. The model includes no additional tank losses or production interruptions.
The required net production duty is:
12 m³/day ÷ 10 h/day = 1.2 m³/h.
Assume 4.0 m³ of usable storage, containing 3.0 m³ above the minimum operating level at 06:00. These assumptions define the required duty for this example.
| Period | Consumption | RO production | Inventory at period end |
|---|---|---|---|
| 06:00–08:00 | 2.5 m³/h × 2 h = 5.0 m³ | 1.2 m³/h × 2 h = 2.4 m³ | 3.0 + 2.4 − 5.0 = 0.4 m³ |
| 08:00–16:00 | 0.75 m³/h × 8 h = 6.0 m³ | 1.2 m³/h × 8 h = 9.6 m³ | 0.4 + 9.6 − 6.0 = 4.0 m³ |
| 16:00–06:00 next day | 1.0 m³ over 14 h | 0 m³ | 4.0 − 1.0 = 3.0 m³ |
Daily consumption is 5 + 6 + 1 = 12 m³. Production is 2.4 + 9.6 = 12 m³. Ending inventory equals starting inventory, so the balance repeats without progressively emptying or filling the tank.
During the morning peak, storage supplies (2.5 − 1.2) × 2 = 2.6 m³. The distribution duty is nevertheless 2.5 m³/h, with pressure assessed separately. The RO duty remains 1.2 m³/h.
From 08:00, the net refill rate is 1.2 − 0.75 = 0.45 m³/h. Recovering 3.6 m³ therefore takes 3.6 ÷ 0.45 = 8 hours. The complete daily inventory swing is 3.6 m³; the assumed 4.0 m³ usable band leaves 0.4 m³ at the low point. Agree the actual interruption reserve before selecting a tank’s gross capacity.
Calculate feed and concentrate at the RO boundary
For this calculation, assume 70% RO recovery, with recovery defined as permeate flow divided by feed flow in the manual linked above. Also assume all RO permeate enters the tank, so the 12 m³/day tank inflow equals permeate production. Select the actual project recovery using the water analysis and membrane design.
- RO feed volume: 12 ÷ 0.70 = 17.14 m³/day, rounded.
- Concentrate volume: 17.14 − 12 = 5.14 m³/day, rounded.
- While running: approximately 1.714 m³/h feed = 1.200 m³/h permeate + 0.514 m³/h concentrate.
Account for any permeate diverted to flushing or other internal uses when calculating the required RO production. Add pretreatment backwash and other service-water requirements separately when calculating the incoming supply requirement.
Test the schedule before requesting a configuration
Check a busy day, delayed startup and reduced net production. At 1.0 m³/h for 10 hours, this example would produce 10 m³ against 12 m³ of consumption: inventory falls by 2 m³ each day. Extra storage only delays that shortage.
Use the resulting production, pump and storage duties to discuss a commercial RO configuration. Send the connected-outlet list, demand schedule, water analysis, temperature range, usable storage and delivery-pressure requirements. MAKORYN can review the project inputs for configuration and quotation.