Running the pump
Pump run time is an important factor in water clarity because pool water must be circulated (turnover) in order to obtain any filtration. Therefore, the amount of time per day the pool operator runs the circulation system directly affects the pool’s daily turnover rate. The turnover rate is measured by the length of time it takes to achieve one turnover.
Inadequate pump run times will not apply enough filtration to sufficiently remove the contaminants contributing to cloudy water. As a general rule of thumb, during the season its recommended pool owners operate the pump at least 10 to 12 hours per day. Running the pump less will limit the amount of water that is filtered and can lead to persistent cloudy water.
According to the Gage-Bidwell law of purification by consecutive dilution, operating the pump longer than this will achieve better results; however, a trade-off exists for residential installations due to increased energy consumption.
The law states that total removal of dirt and debris from pool water cannot be achieved by one turnover, but rather a series of consecutive turnovers is required. The reason for this is pool systems operate by principles of continuous dilution. Purified water that is returned to the pool constantly mixes with the bulk reservoir, continually diluting the suspended particulate in the water. Therefore, a portion of the water at the circulation system intake (skimmers and drain) has already flowed through and been purified by the filtration system.
Gage and Bidwell’s study showed approximately 42 per cent of the dirt in an open pool, under constant exposure to contamination, will be removed by the end of the first turnover. Purification from subsequent turnovers will yield 84, 95, 98 and 99.3 per cent overall dirt removal after the second, third, fourth and fifth turnovers, respectively. To achieve 99.99 per cent removal, Gage and Bidwell found a total of 10 turnovers are required. Therefore, more contaminants are removed, as more turnovers are achieved.
Dirty filter media
Another physical cause of cloudy water can be attributed to dirty filter media. As the media removes particles, its porosity is reduced and the space available for removing further debris gets used up. This reduction in porosity provides a temporary benefit, as it allows smaller particles to be removed as the diameter of the void channels decrease. However, this also places additional resistance to the system, increases the filter’s operating pressure and decreases flow rate. As a result, over time, the number of turnovers achieved is reduced.
To avoid this, proper filter maintenance must be performed to ensure adequate filtration. Typically, filters should be backwashed or rinsed (according to media requirements) once normal operating pressure increases by 70 kPa (10 psi). In addition to routine backwash or rinse filter maintenance, the filter media should be cleaned periodically with chemicals.
Worn filter media
Aside from being dirty, the life of the filtration media can also negatively impact water clarity. In sand filters, for example, the rigid surface of the media aids in filtration by increasing the surface area available to remove microscopic particles from the water. Over time, however, the surface of the sand grain tends to smooth out through natural processes of erosion. And, as a result, the surface area for filtration decreases and filtration performance is affected. Alternatively, cartridge filters can also deteriorate over time. Therefore, these media should be replaced periodically.
Channelling of sand filter beds is another issue. This occurs when there is an uneven flow of water above the filter bed’s surface. Essentially, water flows faster at certain points, which causes a channel to develop in the filtration media. These channels provide the least resistance to flow and thus become the modes of water transport through the filter. Once established, filtration is significantly diminished, as water flowing through the channels circulates through the system unfiltered. The only way to correct a channelling problem is to replace and regenerate the filter bed.
This makes it important for pool operators to ensure there is enough headspace above the filter’s sand bed to make sure a uniform flow of water goes through the filter. This is primarily an issue with sand filters, but can also be problematic with cartridge or diatomaceous earth (DE) filtration media. For instance, a tear in the fabric of the cartridge or a broken grid will also provide a channel for water to travel through the media without being filtered.
