Dark limestone walls form a low-lying pool house that runs alongside this chemical-free swimming bath in Edmonton, designed by architecture studio GH3.

Located inside Borden Park, a community park in Edmonton, GH3's bath includes a large natural swimming pool, a children's pool and a pool house for changing and equipment.

All the water in the Borden Park Natural Swimming Pool is filtered using plants, layers of gravel and sand, instead of machinery and chemicals. This makes the project Canada's first chemical-free outdoor swimming pool, according to the studio.

GH3 chose the material for the pool house to draw on this filtering process, creating a gabion wall constructed of dark limestone and steel. It describes the structure as resembling a "filter-like" barrier.

The pool house's slender and low-lying form also draws on the style of an existing mid-century modern building built on the site in the 1950s.

"These extant buildings connect the new with a century-old cultural heritage of architecturally distinctive, civic outdoor bathing pavilions," GH3 said in a project description.

Most of the older structure was demolished but the studio maintained a portion to house electrical equipment, machinery for cleaning sand, as well as pool apparatus like umbrellas and furniture.

The stone walls of the pool house are punctured with tall dark steel doors that lead into several changing bays.

Inside, a stained marine-grade plywood lines the ceilings, covers the floors and forms partitions. This material was chosen for its resistance to moisture and temperature changes.

Outside, a concrete floor runs seamlessly into a sandy beach area and wooden decking around the two pools.

Water is first dechlorinated off site and then filtered using two separate systems for the different pools. The main pool water is pumped through a bed of gravel, sand and zooplankton plants, which remove bacteria and contaminants.

"The filtration system relies on zooplankton plants, which eat harmful bacteria and also give the water its signature greenish colour," GH3 said.

A pair biological water-treatment ponds are used as the filter for the smaller kids' pool.

"The hydrobotanic pool uses shade-giving plants like water lilies to provide a sheltered underwater environment where bacteria-eating zooplankton can flourish, while the adjacent submersive filter pool uses reed plants to help oxygenate the water," the studio added.

A similar system also features in the naturally filtered outdoor swimming pool that well-known firm Herzog & de Meuron completed in Switzerland.

For the final step in the Borden Park Natural Swimming Pool, all the water is sent to a chamber where it is tested before flowing onto the separated pools.

GH3 also strategically placed a series of curved, stainless steel showers to run around the pool decks. All swimmers are required to wash off before bathing to ensure that no contaminants, like sun lotion, tampers with the pool water.

Opposite the pool and building is a recreation area with several patio umbrellas, space for picnicking, and volleyball court.

GH3 is based in Toronto and  led by Pat Hanson, Raymond Chow and Bernard Jin. The studio has also completed a pavilion that resembles a drum in Edmonton's Borden Park, and a glass lake house for a photographer in Ontario.

Photography is by GH3.