Pool water quality comes from turnover, filtration rate and backwash capacity, which are three calculations. This example works them through for a villa pool.
A swimming pool is a water treatment plant with a tiled tank attached. The quality of the water depends on how often the whole volume passes through the filter, how fast the water travels through the filter medium, and whether the backwash arrangement can actually clean the filter. This example calculates each of those for an illustrative villa pool, so you can read your own pool design rather than accepting a package.
The situation
An illustrative villa pool measures 8 metres by 4 metres with an average depth of 1.5 metres. The designer has proposed a sand filter and a single pump. The owner wants to understand what flow rate the pump should deliver, what size the filter needs to be, how much water a backwash consumes and what happens if the turnover period is relaxed to save on plant.
Assumptions (illustrative)
- Pool dimensions: 8 metres by 4 metres, average depth 1.5 metres.
- Target turnover period: 6 hours, as a design decision for a private pool.
- Illustrative design filtration rate through the sand medium: 30 cubic metres per hour per square metre.
- Illustrative backwash rate required to lift and clean the medium: 40 cubic metres per hour per square metre.
- Backwash duration: 4 minutes, as an illustrative figure.
- Illustrative total system head at design flow: 14 metres.
Step 1: Calculate the pool volume
8 metres multiplied by 4 metres gives 32 square metres of water surface. Multiplied by the average depth of 1.5 metres, the volume is 48 cubic metres, or 48,000 litres.
Every later figure follows from this volume. Average depth matters, so a pool with a deep end and a shallow end needs the average rather than the maximum, and a sloping floor is easy to get wrong by a significant margin.
Step 2: Convert the turnover period into a flow rate
48 cubic metres divided by 6 hours gives 8 cubic metres per hour. That is the circulation flow the pump must deliver continuously while the system is running.
Turnover is the design decision that drives everything else. A 4 hour turnover on the same pool would require 12 cubic metres per hour, which is 50 per cent more flow and a larger pump, filter and pipework.
Step 3: Size the filter from the filtration rate
Required filter area is the flow divided by the filtration rate: 8 divided by 30 gives 0.27 square metres. For a circular filter, the diameter follows from the area: the square root of 4 multiplied by 0.27 divided by pi, which is about 0.59 metres. A 600 millimetre filter would therefore be the practical selection.
Filtration rate is the quality lever. Pushing the same flow through a smaller filter raises the velocity through the sand, which reduces how much it actually removes, so an undersized filter produces visibly poorer water at the same turnover.
Step 4: Check the backwash requirement
A 600 millimetre filter has an area of pi multiplied by 0.3 squared, which is 0.283 square metres. At the backwash rate of 40 cubic metres per hour per square metre, backwash flow is 0.283 multiplied by 40, which is 11.3 cubic metres per hour. Over 4 minutes, that is 11.3 multiplied by 4 divided by 60, which is about 0.75 cubic metres of water, or 750 litres.
Backwash needs more flow than filtration, so the pump and the pipework have to deliver that higher rate, and 750 litres has to be replaced with fresh water every time. In a region where water is a real cost, backwash frequency belongs in the operating budget.
Step 5: Express the pump duty and check it against the system
The pump must deliver 8 cubic metres per hour for filtration and about 11.3 cubic metres per hour for backwash, against an illustrative total head of 14 metres made up of the filter resistance, pipework friction and the height to the returns. The selection is therefore made at the higher flow, around 11.3 cubic metres per hour at 14 metres.
A pump chosen only for the filtration flow cannot backwash the filter properly, which means the filter never gets fully cleaned. This is one of the most common faults in small pool installations.
Step 6: Test what a longer turnover would save and cost
Relaxing the turnover to 8 hours reduces the required flow to 48 divided by 8, which is 6 cubic metres per hour, a 25 per cent reduction. The filter area needed falls to 6 divided by 30, which is 0.2 square metres, and the pump is smaller. But the whole volume now passes the filter three times a day instead of four, so contamination introduced by swimmers is removed more slowly.
Turnover is a trade between plant cost and water quality, and the right figure depends on how heavily the pool is used. A lightly used private pool can justify a longer turnover than a pool used by many people, and the decision should be explicit rather than inherited from a package.
What changes the answer
- Average depth: an error here changes the volume and therefore every downstream figure proportionally.
- Turnover period: the single largest lever on plant size, flow and energy consumption.
- Filtration rate: a lower rate through a larger filter improves water clarity at the same turnover.
- Bather load: heavy use introduces more contamination and argues for a shorter turnover and stronger treatment.
- Evaporation and backwash: both consume water continuously in this climate and belong in the running budget.
What the numbers tell you
- Volume, turnover, filtration rate and backwash are four separate calculations, not one package figure.
- The pump must be selected for the backwash flow, not just the filtration flow.
- An undersized filter degrades water quality even when the turnover figure looks correct.
- Backwash and evaporation make a pool a continuing water cost, not only a capital item.
- Ask for these numbers in the pool design; a quotation that names only equipment models has shown you nothing.
Limits of this example
Every figure above is an illustrative assumption used to show the method, not a quote, valuation or market rate. Costs, rates, fees and regulations change and must be confirmed with the relevant authority, lender or supplier before you commit.
Domira builds and supervises projects across Saudi Arabia, and this is the level of detail its site teams work at. Send the drawings or the site, and Domira will tell you what the specification has to say before anyone prices it.
Frequently Asked Questions
What is turnover rate in a swimming pool?
It is the time taken for a volume of water equal to the whole pool to pass through the filtration system. A six hour turnover on a pool means the equivalent of the entire pool volume is filtered four times a day. Shorter turnover gives better water quality and needs more flow, a larger filter and more energy, so it is a design decision.
How do you size a swimming pool filter?
Divide the required circulation flow by the design filtration rate for the medium to get the filter area, then select a filter of at least that area. The filtration rate is the quality lever: pushing the same flow through a smaller area raises the velocity through the medium and reduces how much it removes, so an undersized filter gives cloudy water.
Why does backwashing need more flow than filtering?
Because backwash reverses the flow to lift and separate the filter medium so that trapped dirt washes out, and that requires a higher velocity than normal filtration. A pump sized only for the filtration flow cannot fluidise the medium, so the filter is never properly cleaned. The pump and pipework must therefore be selected at the backwash rate.
How much water does a swimming pool lose in a hot climate?
Evaporation depends on temperature, humidity, wind and whether the pool is covered, and it can be substantial through a Saudi summer, with backwash consumption on top of it. Treat the top up water as an operating cost from the start, and note that a cover reduces evaporation significantly as well as keeping dust out of the water.
Have a Property Question?
Domira can help you navigate property, financing and investment decisions in Saudi Arabia. Speak with our team for an initial consultation.
