William-Douglas

Swimming Pools 101    

Swimming pools can be a disproportionate expense to an association and in many instances the single largest budget expenditure to an association.  The membership usually does not realize the enormous impact a swimming pool has on the association’s annual budget and many times, ironically, the swimming pool is the least utilized of all the common elements by the membership as a whole.  Being such a large expense, understanding the ongoing maintenance requirements and just the basics of a swimming pool can aid a board in their decision making process.

Swimming pools comes in standard and custom sizes and shapes; but the largest standard size is an Olympic-size swimming pool.  Pools are built either above or below ground, and construction methods for pools vary greatly. The main types of in-ground pools are gunite Shotcrete, concrete, vinyl-lined, and one-piece fiberglass shells.

No matter the shape or size, all pools work in the same basic way.  Any pool with more than 175 cubic feet of water normally uses a combination of filtration and chemical treatment to clean continually by recirculation of a large volume of water. A typical swimming pool needs four major components:

 

  • Tank (basin, shell)
  • Circulation system: pumps, inlets & outlets, pipe work
  • Filtration
  • Chemical treatment system

 

A swimming pool works by continually pumping water, in a cycle, from the pool via filtration and chemical treatment and back to the pool again. By this circulation, the water in the pool is kept relatively free of dirt, debris and microorganisms (bacteria and viruses). Also, it must be possible to make up water lost by evaporation, backwashing the filters and bathers carrying/splashing water out of the pool etc.

The main difference between pools is how the tank/shell is constructed. There are several pool styles, each with their own advantages and disadvantages. While these pool construction methods are quite different, they all rely on similar basic circulation and treatment systems.

Concrete Pools

Over the last 30 years or so the design and installation of PFS pools has developed substantially.

Pre-formed Sectional (PFS) Pools

They can be made from a range of materials including welded stainless steel plate, stainless steel, galvanized panels and glass reinforced plastic panels that are bolted together. Water retention is achieved with a heavy-duty uPVC membrane.  PFS pools can be prefabricated offsite and assembled quickly onsite. The vinyl-lined pool has no structural strength to speak of, as it is watertight only because of its uPVC flexible liner. The structure’s wall assembly can be blockwork, preformed plastic, composite or galvanized tin panels or even timber. The floor is porous to allow the ground water pressure to move the liner and push into the pool, thus preventing the pool assembly from being distorted. The factory-made vinyl lining may be secured just at the top, under the paving slabs, or the onsite vinyl lining secured to the structural wall.

Pre-formed (one-piece) Pools

These are made from fiberglass-reinforced plastic or ceramic, molded in one piece into a tank shape, with the necessary plumbing fitted according to the manufacturer’s instructions. To install the pool, an appropriately sized hole is prepared and the preformed pool lowered into the hole and a surround constructed.

Full & Empty

A concrete in-ground swimming pool may seem like a solid, immovable structure, but it is actually rather like a boat: it can float where the surrounding ground water table is high. Unless the tank has been designed to withstand uplift when the pool is empty, the ground water pressure can actually push the pool up out of the ground.  Partly in order to resist this pressure, in-ground pools are generally kept full all year round. Many in-ground pools have a special hydrostatic relief valve near the main drain to guard against damage from ground water pressure. Basically, if the ground water pressure is great enough, it will push up on a small float, which opens the valve. When the valve opens, ground water flows into the main drain and equalizes the pressure. The seal on this type of valve needs regular careful maintenance to keep it free of grit and silt and stop pool water leaking into the surrounding ground. This approach to equalizing the pressures will restrict the opportunity for maintenance to times when the water table is at it’s lowest, which unfortunately is likely to be midsummer, when the pool is most in demand. Most publicly owned pools, and an increasing number of private ones, are constructed and designed by a structural engineer to resist the uplift from ground water pressure. Care must be taken emptying or filling pools.

Leaks

The commonest leakage problem in small pools is when pipework and fittings (skimmers, underwater lights, main drain etc.) run through the walls and floor of the pool tank. So great care needs to be taken to ensure a good watertight joint at these interfaces.  During normal operation, pool water is removed at the bottom of the pool through two or more main drain outlets and from the surface perimeter of the pool into deck-level transfer channels, skimmers or overflow channels. The bottom outlets are usually on the lowest point in the pool, so that the entire pool floor surface slopes toward them. Most of the dirt and debris, heavier than water, will sink and leave the pool through these outlets. Some debris will usually have to be vacuumed up.  The surface water draw off system works the same way as the main outlets, but they should take water only from the very top of the pool. As a result, any pollutants that more or less floats, skin cells, hair etc., leaves the pool from the surface. Today most public pools work on the deck-level principle: up to 80% of water removal is actively drawn from the surface of the pool, overflowing via transfer channels round the pool.

Inlets and outlets also need to be designed with a safe water flow rate. This design specifies the number and needed to achieve the recommended flow rates.  To keep people from getting their hair or limbs caught in the pipework, the outlets are covered with grilles or special covers. Water is drawn out of the pool by circulation pumps through the pool outlets, and is returned to the pool via the filtration and treatment systems, through the pool inlets.

 

Pumps

The heart of the pool system is the water circulation pump. In a typical pool one or more electric pumps will draw water from the pool, pull it through a strainer and push it through the filtration system and back into the pool. At appropriate points chemical treatments are introduced into the pool water.

Filtration

Sand filters consist of a large circular tank, usually made of glass fiber, reinforced plastic or steel, containing a bed of special, even-graded sand, which has sharp edges.  During filtration, dirty water from the pool is pumped down through the filter bed and the tiny sand particles capture the many insoluble pollutants that can be filtered. At the bottom of the tank, the filtered water flows through the collection system and out of the outlet pipe. Over time, the debris collected in the sand slows down the water flow. If gauges show an increase in pressure on the inlet pipe compared to the outlet pipe, the operator knows there’s a lot of collected debris in the sand. This means it’s time to backwash (cleanse) the filter. To backwash, the operator (usually having to switch off the circulation pump first) adjusts a number of valves to reverse the water flow, so water from the pump pushes up through the sand, dislodging the debris. Automatic backwash systems are also available. At the top of the filter tank, the dirty water flows out through the inlet pipe and into the sewer.

Domestic and small commercial pools normally have a single-handle ‘multiport’ valve to operate the various filter functions. Larger pools have a four or five-valve frontal system Diatomaceous earth filters use filter membranes (or septa) coated with this fine powder. When it gets dirty, it has to be removed, everything cleaned and the membranes recoated with fresh diatomaceous earth. In a cartridge filter, the polyester cloth (usually) or (in some domestic pools and spas) corrugated paper filter cartridges are removed, soaked in a bucket of cleaning chemical, hosed off and dried. So at least two sets of filter elements are needed. After up to a year, the filter element is replaced. For a filter to do its job, all the water in the pool (or more accurately, the equivalent volume) must pass through the filter in a certain amount of time– typically between 30 minutes and six hours –depending on the type of pool (spas, 6-15 minutes). That time is called the turnover period, and has to be related to the bathing load (number of bathers) the pool can take. The pool and filter system is usually connected (via a break tank) to the mains so fresh water can be added to the pool. This is necessary to replace water lost to evaporation, backwashing and the water that is carried out on people’s bodies and swim suits. Fresh water is added also to dilute the pool water in order to remove some of the chemical contaminants that accumulate.

 

Chemical Disinfection

The pool’s filter system does the hard work in keeping the water clean, but it takes chemistry to make sure pool water is safe and suitable to swim in. Again, this subject is dealt with in detail elsewhere but in summary it’s important to control the pool’s chemistry for four reasons.

 

  • Some disease-causing microorganisms – e.g. bacteria and viruses – thrive in untreated water. Without disinfection, there can be cross-infection among bathers.

 

  • pH values can affect the activity and efficiency of disinfectants.

 

  • Low free chlorine and/or high pH values can result in cloudy water; this is unpleasant and can be dangerous if a swimmer gets into trouble and can’t be seen.

 

  • Failure to maintain appropriate chemical concentrations in the pool water can affect users’health and damage parts of the pool and its associated equipment.

 

Disinfectants

The most popular pool disinfectant is the element chlorine, usually in the form of a chemical compound such as calcium hypochlorite (a solid) or sodium hypochlorite (a liquid). When the disinfectant is dosed into the water, it reacts to form various chemicals, most notably hypochlorous acid, the prime disinfectant. It must be monitored, in milligrams per liter (mg/l) or parts per million (ppm) – exact equivalent measures of what is called free chlorine. Depending on the pool and its clientele, the free chlorine residual might be anything from 1.0-7.0 ppm, in order to keep the microorganisms under control.

Because hypochlorous acid is an oxidant as well as a disinfectant, it tends to combine with pollution from bathers to form byproducts that don’t disinfect and can irritate eyes, noses and chests. In particular, ammonia from sweat and urine combines with chlorine to form chloramines. This is the irritant smell associated with swimming pools – not chlorine (which is virtually odorless).  Pool operators have to ensure through an effective system of engineering and control that the level of chloramines is as low as possible.  They must in any case be less than half those of the free chlorine value measured in the pool. It is also important to minimize the ammonia and other pollution going into the pool by encouraging bathers to use toilets and showers before swimming. Chlorine is normally introduced into the pool water in the plant room, usually just before or after the filter. The whole process can be automated so that the pool water is monitored and dosed appropriately and continuously.  One problem with hypochlorous acid is that it can degrade rapidly when exposed to ultraviolet light from the sun. Some organic disinfectants include a stabilizing agent, usually cyanuric acid, that reacts with the chlorine to form a more stable compound that does not degrade as easily when exposed to sunlight. Or cyanuric acid can be added separately alongside an inorganic disinfectant (hypochlorite) to achieve the same effect.

 

Water Balance

Pool water chemical values must be right for disinfection, safe for swimming and good for pool materials. That means attention to pH, alkalinity, hardness and dissolved solids. They need to be measured regularly (and of course the pool water has to be tested for microbes. The pH value is a measure of how acid or alkaline (strictly, basic) the water is. Neutral water is pH value 7; lower than that the water is increasingly acid, above it basic. Depending somewhat on the disinfectant used, the pool’s pH value should be maintained (by adding alkalis or acids as necessary) at 7.2-7.8, ideally. Below that and the water may be irritant to bathers and corrosive to pool materials; higher and disinfection won’t be so effective (so more chlorine is needed). Total alkalinity is a measure mainly of alkaline bicarbonates and carbonates in the water. It should be more than 80 parts per million (but less than 200), to act as a buffer that helps keep the pH value relatively stable. Calcium hardness is controlled so that the pool water is neither corrosive to pool materials nor scale producing. The ideal range is widely debated and varies from 80 to as high as 600 80-200 parts per million. Total dissolved solids (TDS) should be no more than 1,000 parts per million above the TDS of the mains water. Dilution with mains water is likely to be the only way to reduce it.

 

The Cost

Some people believe putting in a swimming pool is the only consideration when it comes to the expense. The continual operating costs, such as labor, chemicals, water, energy, maintenance and periodic replacement of equipment is where the real expense comes into play.     WDPM

 

Copyright – William Douglas Management, Inc. 2016