Most RO enquiries start with one number: a capacity in cubic metres per day. That number matters, but on its own it is not enough to design a plant that works well. Recovery, feed water quality, operating hours and storage all change the size and cost of the equipment. This article walks through the figures an engineer needs and why.
Start from demand, not from a round number
Product water capacity should come from how much water is actually used. For a hotel that means guest rooms, kitchens, laundry and landscaping. For a factory it means each process line, cleaning and boiler or cooling make-up. Add the uses together over a typical day and over the busiest day of the year. The busiest day, not the average, sets the plant size.
Then look at the shape of demand through the day. Water use is rarely flat. Mornings in a hotel or shift changes in a factory create peaks. An RO plant prefers to run steadily, so it is usually sized close to the daily volume and paired with storage tanks that absorb the peaks. Trying to size the RO itself for the peak hour leads to an oversized plant that stops and starts.
Operating hours
A plant that runs around the clock needs smaller equipment than one that only runs during the day for the same daily volume. Operating hours also affect how often membranes are flushed and how much standby equipment is sensible. Decide early whether the plant should run continuously or on a schedule.
Recovery: product versus feed
Recovery is the share of the feed water that leaves the plant as product. The rest leaves as concentrate, carrying the salts. Higher recovery means less raw water and less concentrate, which is attractive, but pushing recovery too far concentrates salts until they scale on the membranes.
The safe recovery depends on the feed chemistry. Brackish RO can usually run at a higher recovery than seawater RO, and the limit for a particular water is set by hardness, silica, sulphate and other scaling ions. Antiscalant dosing extends the range, but only so far. Recovery therefore decides the size of the intake, the feed pumps, the pre-treatment and the concentrate line.
Feed TDS and temperature
Total dissolved solids in the feed set the pressure the plant needs and the quality it can reach. Higher TDS means more pressure, more energy and, for the same membranes, slightly saltier permeate. Temperature matters as well: warm water passes membranes more easily but also lets more salt through and encourages biological growth. Gulf sites see warm feed water for much of the year, which has to be built into the design.
What happens when feed quality changes
Wells drift, seasons change and network water varies. A plant sized only for today's analysis may struggle later. Good practice is to agree a design range for TDS and temperature and to select pumps and membranes for the poorer end of it.
Product water quality
The target quality is part of sizing. Irrigation water, boiler feed and drinking water all have different requirements. Some uses can take blended water, mixing RO permeate with filtered raw water, which reduces the RO size. Others need a second pass or polishing. Be clear about what the water is for.
Allowing for growth
If demand is expected to grow, the plant can be designed with space for extra pressure vessels, a larger intake and headroom in the pre-treatment, so capacity is added in steps rather than replaced. Building the full future size on day one often means running an oversized plant at low load for years, which is harder on membranes and less efficient.
A checklist for your enquiry
- Average and peak daily demand, and expected growth
- Operating hours per day
- Water source and a recent analysis
- Target product water quality and use
- Available storage, space and power
- How concentrate will be discharged
These figures let AFG engineers size the plant properly, as described on our process page. Whether you run a factory in Sharjah or a resort elsewhere in the region, the same questions apply. Send what you have through the quote form and we will come back with anything still missing.