Pool vs. Water-Feature Treatment
Waterfeatures and swimming pools share many of the same water-treatment principles, but their operating environments can be remarkably different. Drawing on experience in both industries, waterfeature and swimming-pool engineer Miroslaw “Miro” Grudniewski examines how filtration, circulation, contamination and dosing interact, and why successful treatment begins with understanding the water a system will actually encounter.
By Miroslaw “Miro” Grudniewski
My professional experience has been built across two closely related but often separate industries: the swimming-pool industry and the water-feature industry.
Over many years, I have had the opportunity to work extensively within both. This has given me a perspective that I believe is relatively unusual, not simply seeing water features through the eyes of a pool professional, or swimming pools through the eyes of a water-feature engineer but also being able to compare the two from practical experience.
That comparison has led me to some conclusions that may seem obvious once stated but are surprisingly easy to overlook in practice. The first is perhaps the most important: Water features are not small swimming pools.
There are obvious similarities. Both systems circulate water, require filtration and rely on some form of water treatment to maintain acceptable water quality. It is therefore understandable that many of the principles developed for swimming pools are also applied to water features. But the similarities can also be misleading. A swimming pool is designed around a relatively well-understood environment: known water volume, defined circulation patterns, an expected bather population and established methods of managing the resulting contamination load.
Water features can be very different. They may be decorative, interactive, open to the environment, heavily aerated, exposed to weather and airborne contamination, or subject to intense but unpredictable human interaction. Their hydraulic arrangements can be highly specialised, and their actual contamination load may have very little relationship to the apparent size or volume of the feature.
No bather load does not mean no contamination load.
Intended Use Versus Actual Use
Now picture a hot summer day in a city centre. A beautifully designed dry plaza. Benches around the perimeter. Parents sitting in the shade while children run between jets of water erupting from the paving.They try to predict the next sequence. Which jet will fire next? Where will the biggest one appear? Who is going to get soaked?
It is a familiar scene. But was this how the water feature was intended to be used? Perhaps not. Does that necessarily mean it was not predictable? This is where the distinction between intended and actual use becomes important.
Guidance from the UK-based Pool Water Treatment Advisory Group (PWTAG) on interactive water features requires risk assessment to consider both intended and non-intended use. It also recognises that decorative and plaza features may attract human interaction and therefore need to be considered in terms of their microbiological risks.
PWTAG Technical Note 67 (TN67) makes a similar point from the perspective of decorative water features, recognizing that people may interact with features that were not necessarily designed for that purpose. The question is therefore not simply whether people are supposed to interact with the water. The question is whether the treatment regime reflects the way the feature is actually being used.
Once children are running through the jets, the distinction between a decorative water feature and an interactive one becomes rather less useful from a water-treatment perspective.
Design Intent Versus Reality
Other European countries have sometimes approached the same distinction differently. In France, for example, different forms of public water use are treated separately within the regulatory framework, including artificial bathing installations and, in other regulatory contexts, water-play uses and decorative fountains.
The terminology can therefore carry regulatory consequences as well as describe design intent. The water does not know what was written on the drawing. It only knows what is happening around it. This is where experience from both industries becomes particularly useful.
What may appear to be a relatively simple water-treatment problem can sometimes be the consequence of decisions made much earlier, in the hydraulic design, circulation arrangement, filtration, mixing, dosing location or even in the original classification of the feature itself. The key is not to replace established swimming-pool guidance, but to explore how some of its underlying principles can be understood and applied when dealing with water features.
PWTAG provides an important framework. Our question is what happens when that framework meets the very different hydraulic and environmental reality of a water feature? There is guidance, experience and standards. But there is surprisingly little discussion about what happens when all of that meets the engineering reality of water features.
This is therefore intended as a practical contribution to that wider discussion, drawing on experience from both industries and exploring what happens when sound water-treatment thinking meets a very different kind of system. Because water has an inconvenient habit of ignoring design intent.
What Role Does Filtration Play?
In a swimming pool, the answer may appear relatively straightforward. In water features, it may be more complicated.
Filtration, for example, is often viewed primarily as a process of removing suspended solids. In water features, however, its relationship with circulation, chemical treatment and microbiological control may deserve closer consideration.
The first challenge is not necessarily filtration capacity. It is what we are asking the filter to receive. A filter collects what the water brings into it. In a water feature, that can be a very different proposition from a swimming pool. Leaves, pollen, airborne debris, bird contamination and other environmental material can enter the system directly. Interactive features may introduce additional contamination, while heavily aerated features can create further exposure to the surrounding environment.
A swimming pool has a relatively predictable primary contamination source: people. A water feature can have multiple uncontrolled sources. This changes the question. Instead of asking only whether the filter is large enough for the design flow and water volume, perhaps we should also ask whether the water reaching the filter is in the condition that the filtration system was actually designed to handle.
Filtration should not be assessed solely against what the designer intended the system to receive. It should be considered in the context of the environment, the actual use of the feature and the contamination load the system is likely to encounter.
This is not simply a theoretical concern. The Government of Western Australia Department of Health, in its guidance for Water Spray Grounds and Interactive Water Features, specifically requires consideration of environmental influences such as nearby water-bird habitats, windblown organic material, sand and dust, storm-water runoff and insect pests when assessing the location of a feature.
The implication is straightforward: the treatment system has to deal not only with intended users, but with the environment in which the feature actually operates.
Before the Filter
This is where pre-filtration becomes important. In a swimming pool, the skimmers and pump strainers are generally sufficient to protect the circulation equipment from larger debris. The contamination load reaching the main filtration system is relatively predictable.
Modern water-feature systems should consider effective pre-filtration as part of the treatment design — removable pre-filter baskets, leaf traps, strainers, cyclonic separation or other appropriate forms of pre-filtration, depending on the application. Older systems may not have been designed with this provision, and retrofitting it is not always practical. Where it can be incorporated, pre-filtration should be treated as part of the treatment design, rather than as an afterthought.
Where pre-filtration is incorporated, its purpose is not simply to keep the primary filter cleaner. It is to preserve the filtration and circulation conditions on which the wider treatment process depends.
If the primary filter is being asked to deal with the environment, perhaps the first question should be: why are we asking it to? Pre-filtration does not replace the primary filtration stage. It protects it from a contamination load that the primary filter may not have been designed to receive continuously.
Changing Conditions
Okay, the filter is loaded. Flow is falling. What does that actually do to the treatment process? A filter problem can become a treatment problem long before anyone notices a water-quality problem. Even a correctly specified filter depends on the hydraulic and contamination conditions under which it actually operates.
What exactly are we specifying, the equipment or its performance? The technology is not the interesting part. The principle is. A filter does not know what was written in the specification. It only knows what arrives at its inlet. That, perhaps, is one of the simplest ways of describing the engineering reality of water features.
PWTAG’s own guidance on pool filtration recognizes that filter performance and circulation cannot be separated. TN49 considers flow requirements as filters become dirty, while TN50 addresses filtration rate, differential pressure and flow monitoring as fundamental aspects of filter operation. A filter does not simply become “dirty.” Its condition changes the hydraulic conditions under which the wider treatment system operates.
What happens inside the filter?
As particulate and organic material accumulate, the filter becomes a progressively different environment from the clean filter described in the original design calculations. The filter is not necessarily just a passive device removing suspended solids. Its condition can influence the wider hydraulic and treatment process.
And that brings us back to flow.
A Treatment Problem
In systems where automatic dosing depends on a minimum flow condition, a progressively loaded filter may first reveal itself not through visibly poor water quality, but through a low-flow or no-flow alarm. A low-flow or time-out alarm should not automatically be treated as a dosing-system fault. It may be the first indication that something further upstream in the circulation system is no longer operating as intended.
In a water feature, that distinction matters. The dosing system may simply be reporting the consequence of a deteriorating hydraulic condition rather than causing it. At that point, the problem is no longer simply a dirty filter. If sampling and dosing are interrupted, the disinfectant residual can begin to fall, potentially without anyone having noticed a water-quality problem yet.
PWTAG’s guidance reinforces the importance of maintaining circulation, filtration and disinfection as an integrated treatment process. TN13, for example, addresses continuous circulation, filtration and disinfection in interactive water features, while also requiring consideration of the risks associated with intended and non-intended use.
The implication is important. The filter is not operating in isolation. Its condition can influence the hydraulic conditions on which sampling, control and dosing depend.
Beyond the Filter
Perhaps this is where the original question begins to change.
I started by asking: What role does filtration actually play in water-feature water treatment? The answer is clearly more complicated than simply removing suspended solids.
Filtration affects flow. Flow affects the operation of sampling and dosing. And the condition of the filtration system can therefore influence the wider treatment process. The chemistry may be correct. The dosing equipment may be correctly specified. The filter may be correctly sized. Yet the treatment process can still be compromised if the system cannot maintain the conditions required for all three to work together.
A specification describes what a system is expected to do. It does not necessarily describe what the system will actually encounter. Perhaps the real question is whether the filtration system has been designed for the water feature it is actually being asked to treat. And that takes us beyond the filter itself. Because filtration does not operate independently of circulation, circulation does not operate independently of dosing, and water treatment does not operate independently of the environment in which the feature exists.
So perhaps the more useful question is not simply: Is the system correctly specified? Is it capable of treating the water it will actually encounter?
Miroslaw “Miro” Grudniewski is a senior waterfeature and swimming pool engineer. He can be reached at miro.engineer.uk@gmail.com.
Photo by Jule Berlin | Shutterstock.