Having visited several sewage treatment plants in the past, I didn't expect much when I set out for the Huaifang Water Reclamation Plant.

However, after entering the facility — which is run by the Beijing Drainage Group — in the capital's Fengtai district, I found it to be more like a beautiful garden than a compound for sewage disposal.

I don't think anyone would realize that it contains a facility that can dispose of 600,000 metric tons of domestic sewage every day if they were not told.

As breezes caressed my cheek, instead of a foul stink, I smelled the fragrance of flowers as birds sang incessantly. In a pool with a fountain, there were fish of various colors and sizes, and the reflections of trees and plants moved with the mild ripples. Amid the babble of running water, I was astonished to learn that the pool is actually the plant's water outlet.

In addition to supplementing a 16-hectare wetland park, the reclaimed water from the plant is diverted into the nearby Xiaolong ("Small dragon") River.

The sewage treatment facility, covering about 170,000 square meters, lies 17.45 meters below the park. It's so big that heavy-duty trucks can be driven around it.

The only parts of the facility that can be seen above ground are three 15-meter-high towers, which are chimneys that emit flue gases generated by the treatment process.

However, they have been submerged in nature and wrapped in vines. Through the dense foliage, one can see wooden boxes that have been specially set up for birds.

"Many swifts have built nests in the boxes. These birds are very smart. In summer, they live on the northern side of the tower. In winter, they stay on the south side," said Fu Wei, head of the plant.

Swifts are just one of more than 50 species of bird that have chosen the park as their home, he added, noting that several other animal species can also be observed, including yellow weasels.

When the facility was being built, from 2014 to 2017, the company planted 102 species of plant in the wetland park, including 27 that can bear edible fruit such as apples, peaches and apricots, he said, adding there are also 65 types of nectar-producing plants.

As such, it is little wonder that the plant has become a popular destination since 2017, when it was listed as one of the environmental facilities that would regularly open to the public.

To date, it has received roughly 23,000 visitors in some 1,200 batches, Fu said.

The two major visitor groups come from businesses and institutions in the sewage treatment sector, and from government bodies, including public establishments.

Fu said that one of the reasons the two groups come is that they want to see how the underground sewage treatment plant — which is still a relatively new idea — has addressed the "not in my backyard" phenomenon and become a community-friendly facility.

He compared sewage treatment plants to public toilets in the capital's hutong, or alleys, where limited space means that many homes lack inside toilets.

"People all think it is a must to have public toilets in the hutong, but nobody wants one close to their home. It's a problem that every sewage treatment plant has to face after it is constructed. Underground water treatment plants offer a solution to the problem," he said. He added that the price of housing in a residential community to the south of the plant has risen by 3,000 yuan ($435) per square meter since 2016.

One of the reasons is that the plant has helped rejuvenate the Xiaolong River, which had either dried up or was full of malodorous water. He added that the river has become a popular destination for visitors, with people bringing fishing rods to try their luck at night.