Where water supply is a genuine design constraint
Rainwater harvesting becomes relevant when one or more of the following conditions apply: municipal water costs are high, supply is seasonal or unreliable, environmental certification is being sought, or local regulation limits mains consumption for non-essential use.
This covers a wide range of geographies — the Mediterranean basin, the Middle East, parts of Australia, sub-Saharan Africa, drought-prone regions of North America and the Caribbean. In these contexts, specifying a collection and storage system is not an ecological gesture. It is a practical response to site conditions.
For an infinity or overflow pool, the hydraulic programme already includes a surge tank with significant water volume. In rainfall-rich environments, that surge tank can be partially supplied by harvested rainwater, provided the system is designed to handle the water quality requirements of the pool circuit.
What a correctly engineered system involves
Catchment and storage sizing
The starting point is a water balance: pool volume, evaporation rate for the climate, anticipated splash loss, backwash volumes and frequency, and local rainfall data by month. These figures determine the storage capacity required and whether the available roof catchment area is sufficient to meet it.
A critical point: harvested rainwater is rarely sufficient to fill a pool entirely. A medium-sized pool holds approximately 50m³ — collecting that volume requires a large catchment area and sustained rainfall. Where harvesting earns its place is in ongoing level management: compensating for evaporation, splash loss and backwash over a season. Sized correctly for that purpose, a system is both achievable and cost-effective.
Under-sizing the storage tank is the most common failure mode in retrofitted systems — the tank fills in the first heavy rainfall and the overflow goes to waste, while the pool continues drawing from mains through summer. Correct sizing requires the full annual rainfall distribution, not an average figure.
Pre-filtration and water quality
Harvested rainwater carries particulates, organic matter and, depending on roof materials and surrounding vegetation, varying levels of contamination. It is also naturally soft and mildly acidic, which affects pH management in the pool circuit. Before entering the hydraulic system, it requires a dedicated pre-filtration stage — leaf screens, first-flush diverters and, depending on quality targets, fine filtration or UV treatment.
The treatment specification depends on the intended use: top-up supply to the surge tank requires a different treatment level than direct connection to the pool circuit. Both are viable; the engineering differs.
Hydraulic integration
In an infinity pool or a zero-edge pool, the harvested water feed point, the float valve controlling mains backup supply, and the surge tank overflow all need to be coordinated. Pressure management, backflow prevention and automatic switchover between sources are not complications — they are standard engineering decisions that need to be made once, correctly, at the design stage.
Regulatory compliance
Several jurisdictions require physical separation between harvested rainwater circuits and mains supply — anti-pollution valves, labelled pipework, separate storage. Some restrict rainwater use to specific applications. These constraints affect both the system architecture and the technical room layout and must be established before design begins.
The most effective water conservation measure is not losing it in the first place
Rainwater harvesting addresses supply. It is equally worth addressing demand — specifically, the water losses that are built into a poorly specified filtration system.
Filter selection and backwash volume
Sand and glass media filters require regular backwash to remain effective. Each backwash cycle discharges a significant volume of water to waste — typically 300 to 500 litres for a residential pool filter, more for larger installations.
A filter specified with an appropriate media-to-flow ratio, and cleaned at correct intervals rather than on a fixed timer, reduces this loss considerably. Cartridge filters eliminate backwash water loss entirely and are now available for high-volume installations, not just residential pools.es.
Cyclonic pre-filtration
A cyclonic pre-filter installed upstream of the main filter removes the bulk of suspended particulates by centrifugal separation, with no filter media and no backwash requirement. It extends the service intervals of the main filter significantly, reducing both backwash frequency and the associated water loss. In pools with high organic load — surrounding vegetation, high bather numbers — it is a sound specification decision.
Surge tank sizing in overflow pools
In an overflow pool, an undersized surge tank and a filtration system not programmed to run during rainfall are enough to lose water to waste every time it rains. When the pool level rises, if the surge tank cannot absorb the additional volume and the filtration is not drawing water back into the circuit, the excess goes directly to waste — on a project that may simultaneously have invested in rainwater harvesting. Correct surge tank sizing combined with a properly programmed filtration cycle eliminates this loss entirely.
Morana's advice
Water management is a design parameter, not a feature.
If your project is in a region where supply is a constraint — whether for cost, availability or environmental reasons — that conversation belongs at the brief stage. We specify rainwater harvesting integration as part of the hydraulic design when site conditions and client priorities call for it. The system then disappears into the architecture entirely.
What changes when it is designed in from the start
When rainwater harvesting is part of the original brief, the storage tank can be positioned optimally — often underground, gravity-fed where the topography allows, with access integrated into the pool surrounds. The technical room accommodates the additional equipment without compromise. The hydraulic diagram reflects the full system from the outset.
When it is added later, every one of those decisions has already been made for a different brief. Tank location is constrained by existing structure, pipework requires re-routing, and the technical room rarely has the space or the correct pressure zoning. The result is a system that works partially and complicates maintenance indefinitely.
Frequently Asked Questions — Rainwater Harvesting for Pools
Can you fill a swimming pool entirely with rainwater?
In most climates, no — at least not reliably. A medium-sized pool holds approximately 50m³; collecting that volume requires a large catchment area and sustained rainfall that few sites can guarantee. Where harvesting is genuinely effective is in ongoing level management: compensating for evaporation, splash loss and backwash over a season. Sized correctly for that purpose, a harvesting system is both achievable and worthwhile, with mains supply as automatic backup during dry periods.
Is rainwater safe to use in a swimming pool?
With appropriate pre-filtration and treatment, yes. Harvested rainwater is naturally soft and mildly acidic, which requires pH adjustment before it enters the pool circuit. The treatment specification depends on local air quality, roof materials and the point at which harvested water enters the system, and is defined as part of the hydraulic design.
Does an overflow pool require a different approach to rainwater harvesting?
Yes. In an overflow pool, the surge tank is the logical point of entry for harvested water. The system needs to account for surge tank capacity, the interaction with the overflow weir level, and backflow prevention on all supply connections. These are standard hydraulic engineering decisions specific to the overflow pool typology and must be resolved at the design stage.