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.
