Category Archives: Design

Determining Filter Backwash Rate

Concrete construction is expensive, so it is important to be frugal when designing a new facility. This involves the way the pool is laid out for programming, but it also extends to the mechanical room, which most likely houses a backwash pit. Overestimating the size of the backwash pit means wasted concrete, while underestimating its size can lead to flooding. For this reason, accurately calculating the size of the backwash pit, based on the ideal backwash rate, can help save money. More importantly, once pool construction has been completed, backwashing at the proper flow rate can help conserve filter media, water and operational costs. The backwash rate is measured in gallons per minute per square foot (gpm/sf) and varies from pool to pool.
The Model Aquatic Health Code (MAHC) recommends a backwash rate of at least 15 gpm/sf, which is standard for most pools. The reasoning behind this number more than likely comes from manufacturer recommendations for the backwashing filter rate. For example, Neptune Benson recommends backwashing at a rate 3 gpm/sf higher than the filtration rate. Counsilman-Hunsaker designs for filtration rates between 12 and 12.5 gpm/sf, but this can be increased to 13, or even 13.5, depending on the system. While backwashing at a rate between 15 and 17 gpm/sf is a safe bet, it is not necessarily going to be the most efficient as factors, other than just the filtration rate, can play a role in determining the ideal backwash rate. Additionally, the World Health Organization (WHO) recommends a backwash flow rate of 15-17 gpm/sf.
Per Steve Andrews, the President of Nemato Corporation, a backwash rate should be determined by the filter bed expansion fluidization. Filter bed expansion fluidization can be described as the increase in volume that a bed of particulate experiences when a liquid or gas is forced through it. Andrews explained to Counsilman-Hunsaker that the amount of expansion is a function of filter media density, water temperature and, of course, flow rate. He went on to say that the backwash rate should not be dropped below 12 gpm/sf because it will not be effective. The table below, referenced from the Model Aquatic Health Code Annex, compares the percentage of bed expansion verses the backwash flow rate.

Backwash Flow Rate (GPM/SF)                              Bed Expansion (%)

12.2                                                                                 4.8

15.8                                                                                 13.0

17.9                                                                                 17.4

19.9                                                                                 21.8

21.5                                                                                25.6

23.9                                                                               31.2
When creating the MAHC, experiments were done in order to determine the ideal backwashing rate of a filtration system. It was found that fluidization, which allows for debris to flow through the filter sand, occurs between 20-23 gpm/sf. As these flow rates coincided with the percentages shown in the chart above, the MAHC recommends a minimum of 20% bed expansion. It is important to note that if the water being used to backwash the system greatly exceeds 68 degrees Fahrenheit, the backwash rate will need to be increased to account for the subsequent change in water viscosity. Counsilman-Hunsaker contacted a representative from Neptune Benson about the language used in the MAHC who confirmed that sizing a filtration system for 35% bed expansion with a target bed expansion of only 20% would allow for adequate backwash and prevent filter media from washing out at the completion of the process.
For a mechanical system that has not been designed to meet such backwashing requirements, there is a trick that can help improve efficiency. The WHO recommends air-scouring sand filters prior to backwashing with water. This is a practice used in the UK and other parts of the world, but hasn’t been commonly practiced in the United States. Air-scouring involves forcing pressurized air through the filtration system, prior to backwash water, in order dislodge debris particles from the filter media. It is considered more efficient than only backwashing using water and could help improve water quality.
Every mechanical system will slightly differ depending on the filter media material, filter media depth and backwash water temperature. However, because the bed expansion is measured as a percentage, the correlating 20-23 gpm/sf backwash rate should become more commonly practiced. In the end, a more efficient backwash cycle means decreased backwash frequency and thus, increased lifespan of the filtration system.

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System Bonding for Water Slides

Proper pool bonding is incredibly important when it comes to considering the safety of swimmers. Pools require electricity to power certain components that make the pool function. Things like pumps, lights and heaters cannot operate without electricity. When pools and their necessary components are not properly bonded or grounded, some electricity can make its way into the pool and potentially harm swimmers. This is why Counsilman-Hunsaker ensures our standards are up-to-date and in line with National Electric Code (NEC) requirements.

Bonding involves electrically connecting all exposed metallic items not designed to carry electricity as protection from electric shock. What this essentially means is that bonding keeps electricity separate from swimmers. In an improperly-bonded pool, electrical gradients will seek the easiest path for conductivity. Bonded pools make electric currents flow into a grid that disperses them.

The requirement to bond and ground pools and pool equipment is a safety requirement incorporated into the NEC, specifically article 680. Equipotential bonding is intended to reduce the voltage gradients in the area around the pool by use of a common ground bonding grid in accordance with NEC 680.26.

The pool shell reinforcing steel, including at least three-feet of the perimeter deck, and all metal anchors, inserts, fittings, light niches, and equipment in the pool and within five-feet of the pool’s edge, as well as the mechanical equipment in the filter room, must be bonded together per NEC Article 680 to form an equipotential bonding grid. Further, due to the importance of controlling electrical currents in and around the pool, the bonding system should be taken back and connected to a positive, true and adequate ground. The ground should be tested and certified as a condition of acceptance.

The code does not specify with regards to water slide components that are beyond the five-foot perimeter around the pool. Water slides have a variety of components including the fiberglass flume, support structures, and the start tower. The water slide start tower and support structures are typically metal structures and, despite not being in direct contact with the water, must be bonded. The start tower is easily within reach of those entering the water slide start tub, and leaking water slide joints may provide a direct connection to the water slide support structure.  Thus, there is a need to bond these components with the rest of the pool components. Water slide manufacturers typically show these requirements on their engineered documents.

Ultimately, by properly grounding and bonding your pool components, you make your pool safer for everyone to enjoy. Swimmer safety is a top priority for Counsilman-Hunsaker, which is why we take these NEC requirements very seriously and regularly ensure our designs create a safe environment for everyone.

Pool Chemical Room Recommendations

One of the specialty areas of aquatic facilities is the chemical room. These rooms are areas of the facility that are subjected to aggressive materials and more restrictive building code requirements. The International Building Code (IBC) describes a high-hazard occupancy as one “that involves the manufacturing, processing, generation or storage of materials that constitute a physical or health hazard in quantities in excess of those allowed.” High-hazard group occupancy ratings may require sprinkler systems, non-combustible floors, storage containment requirements and fire ratings.


Facilities that include a large body of water or more than one body of water can easily exceed the exempted quantity of chemicals allowed in the code. Designating the pool chemical storage rooms as a high-hazard group (H-2 or H-3) occupancy rating will allow for larger quantities of chemicals to be stored.  Chemical storage requirements are a function of the type of chemical stored. Anticipated pool chemical usage should be reviewed against available storage with a minimum storage quantity covering one week of use.


Common pool chemicals include the following:

  • Sodium Hypochlorite (liquid chlorine) is classified as an irritant with a sodium hypochlorite concentration of less concentration of less than one-percent. It is non-flammable and low in hazard. Some codes limit storage to 500 gallons or 1000 gallons.
  • Calcium Hypochlorite (table chlorine) is classified as a corrosive class three oxidizer. It is flammable and high in hazard. Some codes limit storage from 2 to 200 pounds in a single location. If this maximum quantity is exceeded, this space will need to be classified as a high-hazard group H occupancy.
  • Bromine (BCDMH) is classified as a corrosive, either class one or class two oxidizer. It is not flammable in and of itself, but it may ignite combustible materials in which it comes into contact. As such, it is identified as a hazard. Some codes limit storage to as much as 1000 to 4000 pounds in a single location. Typically, occupancy fire ratings of the room in which it is stored and used is two hours. Some jurisdictions may require that the space be provided with a qualified and approved sprinkler system. Additional storage of Bromine can be provided in a high-hazard group H occupancy room if the building has such a room.
  • Muriatic Acid (hydrochloric acid) is classified as a corrosive. Muriatic acid is highly reactive liquid acid. It must be stored separate from oxidizers and in a well-ventilated space. The IBC allows for up to 500 gallons of a corrosive to be stored and used before needing to reclassify the storage space as high-hazard group H occupancy.
  • Carbon Dioxide (CO2 – carbon dioxide liquid) is a liquefied gas which is colorless and odorless. CO2 must be stored in accordance with all current regulations and standards. The stored space should be well-ventilated.


Every pool chemical room requires mechanical ventilation at a minimum rate of one CFM per square foot of floor area over the storage area, as stated per IBC and IFC or 10 air changes per hour, whichever is more restrictive. Confirm with local codes and regulations if more stringent standards are required. Fumes and vapors shall be vented with exhaust taken per IBC and IFC recommendations. Return inlets for chlorine rooms shall be located low in the space as chlorine vapor is 2.5 times heavier than air and sinks to the floor, and return inlets for muriatic acid shall be high as acid vapors rise. In cold weather climates, heat must be provided to keep the room at a minimum temperature of 40°F to prevent freezing.


Understanding the challenges and provisions associated with pool chemical rooms allows for the proper design and construction of these specialty areas. Although these rooms are a small area of the facility, they are subjected to the harshest conditions. Typically, if a facility has great-looking chemical rooms, you can be sure the rest of the facility looks great too.


pH Buffering

The world of pH buffering is changing. For years, pool operators had to choose between using CO2 or muriatic acid to adjust the pH of their pool. As time went on, some sophisticated operators installed both systems on their pools and manually switched between them to maintain the desired level of total alkalinity in the pool. And that was where the industry remained for many years, until now.

We live in a world where chemical controllers can function as building automation systems for the pool. This functionality has helped cause a major evolution in the world of pH buffering. Owners can now install a dual system of CO2 and muriatic acid that allows the controller to demand either chemical on a “time-based proportional feed system.” This simply means that the owner can control (through the chemical controller) how much CO2 is fed in proportion to acid. This is currently the only way to automatically control total alkalinity, as there are no chemical probes or other ways to digitally monitor it readily available. But, this technology allows for a pool’s total alkalinity to be maintained nearly automatically.

What is Total Alkalinity?

Total alkalinity is the measure of all dissolved bicarbonate (HCO3-), carbonate (CO3-), and hydroxide (OH-) ions.  It acts as a pH “buffer” in pool water to prevent large changes in pH. Low total alkalinity allows for rapid pH change, which makes it more difficult to control pH levels. Ultimately, this can contribute to corrosion in the pool. On the flipside, high total alkalinity makes adjusting the pH difficult, resulting in cloudy water and scaling. It’s important to find that total alkalinity balance to ensure the longevity of your facility.

CO2, as a pH buffer, raises the total alkalinity, while muriatic acid lowers it. If the source water has high total alkalinity (over 70 ppm), muriatic acid is recommended over CO2. Total alkalinity levels should be tested in all source water, along with calcium hardness and pH.

Industry standards typically recommend a total alkalinity between 80 and 120 ppm depending on the type of sanitizer used in the pool water.

Muriatic Acid

Muriatic acid is a 31.5% solution of hydrochloric acid typically used as a pH-buffering chemical. It’s usually automatically injected into the pool recirculation system in small amounts. The acid reacts with the sanitizer, thus counteracting the pH, and raising the effects of the sanitizer. In the pool, it will lower pH and total alkalinity.

Carbon Dioxide (CO2)

CO2 is an alternative pH-balancing chemical that is effective with “soft” source water. CO2 is a liquid, but when vented, it becomes a gas. It can be injected into the pool via a sand stone or mazzi injector. CO2 tends be more user-friendly and safer to handle than muriatic acid.

Carbon Dioxide is a liquefied gas that is both colorless and odorless. There are many regulations and standards surrounding its storage, so it’s important to remain aware of these. Tanks must be properly secured and segregated from any areas of activity, and the stored space should be well ventilated.

Adjusting Total Alkalinity:

Increasing total alkalinity involves adding one of the following chemicals to the pool: sodium bicarbonate, CO2 or baking soda (dry). Decreasing total alkalinity involves adding either sodium bisulfate (dry acid) or muriatic acid (liquid). When lowering total alkalinity, it is common to “slug the acid.” This means adding acid (liquid acid or pre-dissolved dry acid) to the deep end of the pool in a concentrated area.

Do You Need Both Systems?

The short answer: yes. Monitoring and controlling total alkalinity is an important part of maintaining water clarity. It’s something that should be tracked daily, and constantly kept in check. The same goes for pH and sanitizer levels.

The issue that we see with pools that utilize a single system is pool operators having to constantly add sodium bicarbonate or acid manually. As pool operators will tell you, this is no fun task, especially since it allows for inaccuracies. Dual systems eliminate the need to manually add either chemical and ensure levels are accurate.

We have seen some operators create a sodium bicarbonate slurry and setup a chemical feed pump to add it to the pool. However, this is still a manual system controlled by the operator, as the operator decides when to turn the system on and off based on total alkalinity readings. Additionally, this is not seen as an industry standard.


The perceived cost for dual systems is relatively small on a year-round pool. Most year-round pools are built with a chemical controller that could easily have this capability. The relative cost add is typically negligible when understanding the cost owners face buying other chemicals to maintain total alkalinity throughout the life of the facility. The difference: a dual system is part of upfront capital cost, and manually adding chemicals is part of operational costs.

Not Quite Industry Standard

While this idea is starting to take shape, it is not something that has been developed as an industry standard. The first of these systems should be installed in the summer of 2017. After these first systems are installed, it will be critical to collect feedback to help determine the potential for the system to be standardized.

If your chemical controller was installed prior to this timeframe, it will take some manipulation for the controller to be able to have this capability. However, some brands and models just won’t be able to. Most chemical controllers are actually built for a specific project. So if you are looking to add this feature, communicate with your aquatic engineer or chemical controller manufacturer to determine the best way forward.


Backstroke Flags

The sport of swimming continues to grow and with that we are seeing an increasing amount of television coverage. From the Olympic Games to the NCAA Championships, swimming fans can now tune to their local sports station and watch the meet. With that in mind, it is particularly important that we, as pool designers, continue to adapt to ensure that the pools we create lend themselves well to being filmed, while also continuing to provide the fastest and most competitive environments.

For instance, a growing trend involves removing backstroke flags when not in use. This provides a better view for spectators and creates a clear path for television cameras to capture the action. Designing with this trend in mind means altering backstroke flag and post specifications to accommodate cameras. For instance, backstroke flags and posts that are designed to be easily removed or quickly adjusted to prevent sagging might take precedence over another product. Or it may be that flags and posts that are more aesthetically pleasing on camera need to be selected. A designer’s ultimate goal should be to ensure swimmers’ stroke counts don’t change from warmup to race, regardless of the flags being removed or adjusted during the meet.

Backstroke flags come in two types of materials: nylon and vinyl. Nylon flags can be used for both indoor and outdoor installations, and can span up to 60 feet (eight lanes). However, due to their weight, they are not recommended for larger spans, as they can sag and cause deflection in posts. Vinyl flags are generally more durable, but are recommended for indoor use only. They can span lengths of 100+ feet and can be used for 50-meter cross course purposes. Vinyl flags are more attractive, especially when identification and logos are used. Counsilman-Hunsaker makes sure to supply its facilities with the appropriate backstroke flag dependent on pool type, pool configuration, and aesthetic appeal.

While backstroke flags are important, so are the stanchion posts and cable that support them. Counsilman-Hunsaker recommends 8-foot backstroke stanchion posts. These posts are designed to support the flags, while also allowing them to be removed for filming purposes. Anchors are also often used for various course configurations to allow flags to be moved when altering course setup. Counsilman-Hunsaker also recommends utilizing a stainless-steel cable with a take-up reel and ring end fitting on each respective end. This cable allows for the precise tightening of backstroke flags, and provides the necessary support to reduce sagging.

As the sport continues to evolve, we hope to partner with competitive swimming leaders to ensure owners are educated and receive the best products when building new aquatic facilities. As designers, it is important to consider the little details that can make a pool stand out for a growing audience of spectators.