By Dave Peterson
In most watershapes, we circulate and treat water through use of pumps and filters – and although we still don’t think about it much these days, we do so because fresh water is in precariously short supply and we can’t simply fill and dump it as we please.
Yet even a perfect watershape – that is, one devoid of leaks, never subject to splash-out and never in need of backwashing – occasionally requires the addition of new water if only because evaporation will carry it away, bit by bit. In fact, there’s no way to cut Mother Nature out of her share, or to keep her from meddling with the level of any outdoor pool through precipitation.
To make the most of the water used in the systems we build, we must completely understand all of the ways that water enters and exits them. That may seem simple and obvious, but it’s actually a fairly complicated subject when you dig into it and consider influent and effluent in all of their various forms.
I’m an engineer by training, and I know through education and experience that the water in any watershape is subject to physics and the principles of conservation of mass, energy and momentum.
Let’s take a swimming pool and its treatment system as an example: We know that evaporation transfers a certain mass of water beyond the surface boundary and into the atmosphere, so if we want to maintain the water mass in the pool (and also want to avoid having to do it manually), we must install an automatic filling device to transfer external water across the surface boundary, usually through a three-quarter-inch potable-water line. This is called an influent.
Precipitation is also an influent, either as rain, snow, ice or even wind-blown water from adjacent decks or separate bodies of water. For this reason, we should always include an overflow port called the effluent. So at this point we have a pair of influents – the autofill device and precipitation – as well as a pair of effluents in the forms of evaporation and overflow.
Those are the main players, but there are more. Consider the drain plugs on simple cartridge filters, for example. They are not used much, but they represent potentially significant outflows. Even more significant are the backwash ports of sand and diatomaceous earth filters, which can send hundreds or even thousands of gallons to waste. Of course, effluents of this sort are always balanced on the influent side because autofill devices do what they need to do to maintain watershape volumes in steady states by automatically filling them to desired set points.
It’s important to note that the influent sources of water must be healthy. Obviously, we don’t want the introduced water to contain bacteria, viruses, pollutants, toxins, colors, odors or other contaminants. For this reason, we use potable water sources – that is, water that’s fit to drink.
Potable water can originate from a city’s domestic water supply, a well or some other source. The key requirement is that it must meet generally accepted water-quality parameters for healthy, drinkable water and should require no further treatment.
In truth, potable water is usually pretreated in some way before it arrives at our kitchen sinks. If the source is the local municipality, for example, its water utility filters and chemically treats the water before it moves into extensive networks of water mains and residential laterals and ultimately reaches the tap. To prevent pathogens from developing in city water supplies, it is common for the water to leave the treatment plant with a certain residual of chlorine – one that survives and can be detected at the faucet.
We take the reliability of these municipal and other water systems for granted, but potable water is in short supply these days in many parts of the country. This explains why water conservation is so large a consideration and has led to so much emphasis on the use of reclaimed water for a variety of purposes.
Unfortunately, this reclaimed water is unsuitable for use in almost all watershapes. The water originates in sanitary sewers (that is, it’s the water from toilets). To be sure, it is subjected to several treatment systems that produce reasonably clean water, and while it is definitely not drinkable, it can be used for various other purposes. This is why some cities have developed parallel “purple pipe” networks – reclaimed-water lines that distribute it to irrigate public golf courses, parks and landscaped areas. In some cases, this water is also made available for residential use – but only for irrigation.
Again, this water is not for consumption by humans or animals, which is why these lines are identified with the color purple and places where it is used have signage warning against drinking it. Once this water passes into the soil, there’s essentially no risk, and plants actually benefit from the nitrogen, phosphorus and other contaminants it carries.
With people and pets, it’s a different story. We know that, when swimming, it is almost impossible to prevent water from getting in our mouths, eyes, noses and ears, so use of reclaimed water in pools and spas is definitely out. And if you’re thinking that fountains and other decorative waterfeatures aren’t intended for human contact and might be candidates for reclaimed-water use, think again.
In fact, even if nobody ever interacts directly with these waterfeatures, it is possible (and indeed likely) that even a slight breeze can combine with water vapor, overspray or atomization to produce airborne water – and if the airborne water is contaminated and inhaled, it could be dangerous or even deadly. (Legionnaire’s disease killed 34 people staying at a posh hotel when contaminated water vapor was accidentally distributed through its air conditioning system.)
Is it possible to use reclaimed water in watershapes? Yes, it can be done, but only if the water is strenuously pretreated to meet drinking-water standards can it be recycled into a watershape. Also, there are some watershapes that can use reclaimed water directly, including man-made wetlands – but only if contaminant levels are sufficiently low and/or those contaminants are of types compatible with wetland systems.
With those thin exceptions, use of this water is definitely not indicated with watershapes.
Now we come to the autofill system itself. In my business, we always start with a potable-water supply terminated with a valve so that, if necessary, the supply can be shut off for watershape maintenance.
From a construction-contract standpoint, we generally require the owner to provide a water line and isolation valve of the correct size and get very specific about where and how the connection will be made. That seems simple, but every once in a while you run into a surprise.
We built a pool for a residence a few years ago, for instance, where we knew the only point of connection was via a tee cut into the lateral line between the water meter and the front of the house. What we didn’t know was that the builder had used a new PEX plastic plumbing system that requires special crimped fittings – and it cost us about $700 to hire a certified plumber to make the connection. We didn’t know about the unusual pipe, and because the contract didn’t specifically exclude unusual connections, we ended up eating the plumber’s fee.
After the isolation valve (which may be far from the rest of the autofill equipment), we set up a wye strainer, a backflow preventer (usually of the pressure-reducing-valve type) and a high-quality brass solenoid valve.
The wye strainer prevents debris from entering the components – a lesson we learned after a small pebble jammed open a solenoid valve so that it leaked and made a pool overflow. This happened twice, and it was all because the landscape contractor responsible for extending the water line to the equipment area didn’t bother to flush the system properly before telling us it was ready for our connection (and we didn’t think to flush it either).
The backflow preventer is required by the International Plumbing Code to keep water from the pool (considered “contaminated” by pollutants) from flowing back up the line to the kitchen sink. (In freezing climates, it is important to use backflow preventers designed for use in cold conditions.)
The solenoid valve is the autofill system’s main operating component – a mechanical device we choose with care. We’ve found that the plastic solenoid valves shipped with certain electronic autofill devices can crack under pressure and fail over time, so now we typically use brass valves from Rain Bird (Azusa, Calif.). Beyond pools and spas, we occasionally use mechanical autofill devices for certain waterfeatures. The details are all the same, but solenoid valves are not used and pressure regulators are sometimes added because the float valves perform better at 40 psi.
The last detail to consider in system design is the size of the autofill components. We work with a minimum of three-quarter-inch lines, moving up from there (to two inches in the case of large commercial pools) depending on the surface area of the watershape and anticipated losses to evaporation in the range of a quarter to a half inch per day across the entire surface. We size the system to replace that evaporated volume in 20 minutes so that, theoretically, the day’s water needs can be handled in one shot before the autofill controller shuts off for the day.
Now that we have examined where and how water enters our watershapes, it’s time to consider overflows and the three primary places it can go when it moves out of a watershape: into the landscape, into a storm drain or down a sanitary sewer.
In some projects, more than one of these will come into play depending on what, specifically, is being discharged. In addition, there are some regions in which storm drains and sanitary sewers are combined – a factor that definitely affects system design. As a rule however, you don’t want to send overflows into sanitary sewers.
Generally, sanitary sewers are sized to handle domestic waste from houses and are not large enough to handle storm runoff. In fact, the flow from a good-size storm can easily exceed sewer capacity, which can result in sewer backups that will send hazardous material out onto the street.
Residential pools can have tremendous surface areas to collect precipitation. In the Southeast, for example, rainfall studies show that up to six-and-a-half inches can fall per hour in limited bursts. In Toronto, rain fell at a rate of nearly eight inches an hour on a stormy day in August 2005. That’s a lot of water in a hurry, and we should also plan for a contingency in which the autofill system fails or a pool’s operator leaves a manual filling line open.
Our rule of thumb is that we use a three-inch overflow line for each 1,000 square feet of surface area. Even that will struggle to keep up with a short, high-intensity burst of rain, but in practical terms, it doesn’t make sense to design all systems to accommodate hundred-year storms.
So where do we send all of that rainwater? When it comes to precipitation, it is preferable to discharge the overflows to the landscape whenever possible so that the rain and snow will enter the soil and eventually replenish our diminishing supplies of groundwater. In freeze/thaw regions, these discharges need to occur away from structures, including watershapes. In California, some cities require the overflow to run to daylight in a swale, with any excess runoff not absorbed into the ground to be picked up by an atrium grate and carried through a curb-core to the storm drainage system.
That’s interesting, because, typically, people see pool water as being contaminated with chemicals and think that it should never be allowed to flow to groundwater or into a storm drain. While certainly there are cases where copper-based algaecides discharged in large volumes to a coastal estuary might kill fish and other aquatic life, the vast majority of overflows pose no such threat.
After all, watershapes will overflow only during heavy or prolonged storms, basically because there’s approximately two to three inches of freeboard between the maximum operating water level and the elevation of the overflow port. Peak rates of six-and-a-half or even eight inches seldom last more than a few minutes, so even then the water level is unlikely to exceed the freeboard in one burst.
To be sure, a prolonged storm might eventually exceed the freeboard, but when the pool overflows, the escaping water is predominantly fresh rainwater that has just fallen from the sky and any contaminant content will be quite diluted relative to the huge volume of the pool’s water.
Yes, water overflowing from the pool may carry contaminants, but only in much-diluted forms. And when that effluent mixes with the incalculable volume of rainwater that didn’t fall on the pool’s surface but instead fell on roofs, streets, decks and landscapes, the chemistry is so diluted that it would be hard to detect contamination at any ecologically significant levels.
Furthermore, these overflow incidents are the exception rather than the rule, and the output of common, smaller rainfall incidents will be completely stored within the watershape’s freeboard, at which point evaporation between cloudbursts will continuously create new capacity.
There are, of course, the other effluent sources – namely, filter backwashing and equipment-maintenance drainages.
In the former case, the water flows backward through the filter medium to dislodge debris (and some of the medium itself) and send it to a discharge point. We don’t want to discharge this effluent to a storm drain because it is fouled by chemicals and debris. Indeed, diatomaceous earth should never be sent to storm drains or sanitary sewers, with separator tanks usually being required to keep the media out of these systems.
For large watershapes (or where water is scarce), backwash systems are also sometimes discharged to recovery basins where small, low-rate filters reclaim the water by collecting and concentrating the debris. These small filters are usually backwashed with potable water, but volumes and flow rates are much lower, the net result being reduced water usage and effluent volume. (This is common practice in seawater systems at places such as Sea World, where Shamu’s waste needs to go the sanitary sewer but the seawater does not.)
As for discharges from the small maintenance drains on the underside of some filters, strainers, centrifugal separators, flow cells, filter-vent valves and other equipment, these are mostly present in commercial settings where they’re discharged to floor sinks connected to sanitary sewers.
Few residential devices have these drains, and they are actuated so infrequently that dumping the water onto the equipment pad to evaporate or letting it fall onto the soil is the most common practice. It is very unusual to have a sanitary sewer line anywhere near an equipment pad – unless, of course, the equipment is indoors, in which case the pad will tend to be set up to resemble a commercial installation.
As we all get more ecologically conscious and society’s attitudes about water usage change, we’ll doubtless be challenged to think in new ways about how we deal with pollution, recycling, waste discharge and energy efficiency. Watershapes and their influents and effluents are very much part of this emerging picture.
Many of us are taking steps even now to reduce splash-out and evaporative losses, for example, and we’re using technologies such as variable-speed pumps to conserve energy and reduce water consumption. With a little ingenuity, in fact, I see a time when watershape overflows will be collected and stored for future use – or routinely sent into landscapes to regenerate groundwater supplies and take the pressure off our storm-drainage and flood-control systems. It’s likely in this context that we’ll also need to take a hard look at the chemicals we use to treat our watershapes and get involved with systems that don’t leave residual killing agents behind for discharge into the environment.
For now, we just need to keep our eyes open for opportunities to conserve and protect water and become accustomed to the ways these measures influence our system designs. After all, if we don’t get there first, it’s more than likely local authorities will be only too happy to help us get things straight!
David Peterson is president of Watershape Consulting of Carlsbad, Calif. He’s been part of the watershaping industry since 1994, when he began working for an engineering firm that specialized in large aquariums and marine-mammal exhibits. In 1998, he stepped onto the manufacturing side of things with Polaris Pool Systems, ultimately serving as vice president of engineering there before starting his own firm in 2004 to support industry professionals with design, engineering and construction-management services. He earned a BS degree in civil engineering in 1995 from the California State Polytechnic University at San Luis Obispo and is a registered civil engineer.