Balance tank / Surge tank 

A surge tank receives the water from a pool perimeter overflow gutter system. Swimmers create waves when they enter a pool, compete, swim laps, exercise or just play. The perimeter gutter system captures this surge of water.
A perimeter gutter and surge tank system also complete the filtration circuit where aesthetic designs for a flush-to-deck water surface or an infinity edge are desirable, and in our tropical monsoon climate it takes care of the overflow from heavy rain.

Surge tanks are typically constructed of concrete and located either under floor of the pump room or under the pool deck next to th pool. Cast-in-place concrete allows the swimming pool designer to size the surge tank as needed to fit the site conditions. The surge tank size depends upon the surface area of the swimming pool. Most local and state health codes require one  gallon (4 litres) of surge capacity for every one  sq ft (0.9 sq. metre) of pool surface area. This generally equates to 10% of the pool volume for small pools up to 200 m2 (or about 280 m³ ) or 15 - 16% for larger pools.

The water from the perimeter gutter to the surge tank can enter at several locations. The number of gutter dropouts depends on the size of the pool. Main re-circulation piping pulls water from the surge tank to the re-circulation pump and filter system before it is chemically treated and returned to the pool. 

A water level controller  in the surge tank monitors the water level and automatically tops up the water level if it drops below design levels. A manhole and cover provides access to the interior of the surge tank for cleaning and routine maintenance.

When surge tanks are constructed under the pool deck next to the pool, they are usually placed at the deep end of the pool. This approach saves on construction costs as one wall is common with the pool.
 Surge tanks are always part of the design of modern pools for Olympic venues, universities, Schools and public pools.