One of the design features required in some public in-ground pools that is not seen in private pools is a surge tank. The way to tell if a pool has a surge tank is to see if there is a gutter around the perimeter of the pool. A surge tank has a very specific purpose that involves the safety of the swimmers and the cost of maintaining the pool.
A surge tank is a concrete or metal drainage tank located either under a public swimming pool or somewhere on the pool deck. When a public swimming pool is busy, the volume of swimmers displaces an equal volume of water. Rather than allowing the displaced water to spill out onto the pool deck, a gutter is built around the perimeter that allows the water to displace into the surge tank. The water is then pumped back into the pool to replace the swimmers' volume after they have left the pool.
A surge tank reduces the risk of injury by preventing the displaced pool water from persistently soaking the pool deck. If the pool deck has tile on it, that tile can be a slipping hazard if it is kept wet. Concrete decks become slipping hazards when pools of water are allowed to remain. The surge tank keeps pool operation costs down by eliminating the need to continually add water to the pool as it is displaced by swimmers.
Public swimming pool surge tanks are legal requirements in some states. For example, Wisconsin and Illinois have laws mandating the use of surge tanks. These state laws also dictate the size of the surge tanks, which depends on factors such as the size of the pool, the volume of water displaced and the placement of inlet points along the perimeter of the pool that lead to the surge tank.
State laws that dictate the use of a surge tank also include requirements for the water to be filtered before it is put back into the pool. According to the Mermade website, a common form of surge tank filtration is vacuum filtration. The filter media rate is a measurement of how many gallons per minute can be safely filtered through one square foot of filter area, according to the PM Engineer website. The standard for surge tank filters is 50 gallons per minute for every square foot of filter space.