This article was originally published by Citiscope and is republished with permission.
HONG KONG — When it comes to water, Hong Kong is in a paradoxical situation.
This city of more than 7 million people has few reliable sources of drinking water. There’s no major river or natural lake to draw from. Groundwater is scarce and difficult to reach. And the city’s hilly terrain makes it difficult to create reservoirs. Under such a crunch, Hong Kong depends heavily on water imports from China’s Guangdong province.
On the other hand, Hong Kong is one of the wettest cities in Asia. The city regularly sees tropical storms drop heavy rains. The runoff rushes down from the mountains into the city’s dense urban core, causing frequent flooding and waterlogging in low-lying areas where land has been reclaimed from the sea.
All of this makes Hong Kong a place with more than its share of water problems. But it also makes the city a good place to look for interesting solutions.
One of those ideas has been around since the 1950s but remains an anomaly today among the world’s cities. And you can find it in the form of a special water pipe attached to almost every toilet in the city.
Instead of flushing its scarce freshwater supplies down toilets, Hong Kong uses seawater. The water is pumped from 42 treatment plants on the coast through a massive network of pipes carrying water into businesses and residences just for toilet flushing. Using seawater for this purpose reduces the city’s freshwater consumption by 20 percent.
At one of the treatment plants in Wan Chai, a bustling business district jammed with high-rises, engineer Season Chan showed me how it works. Four mammoth pumps pull in water from the sea. The water passes through wiry mesh gates that filter out aquatic creatures, floating plastics and other debris. Then the seawater gets treated — not as intensively as if it were for human consumption, but in a creative way that costs almost nothing.
An electric current is passed through the seawater, triggering a reaction with the salts called “electro-chlorination”. That produces hypochlorite, which not only disinfects the water but also kills the foul seawater smell. “After decades of research and trials, we have made seawater flushing not only cost-efficient, but expanded the supply to nearly all of Hong Kong,” Chan says.
Heavy rains produce frequent flooding in Hong Kong’s low-lying areas. (WingLuk/Wikimedia Commons/cc)
This system got started in 1957, as Hong Kong’s growing population began straining its water supply, and riots over water access became common. Public housing in two newly developing areas was set up with the proper piping. The experiment was seen as a success and inspired city planners to expand the concept to other districts.
“There was no option,” recalls Chung Man Tsang, senior engineer with Hong Kong’s Water Supplies Department. “City planners at that time decided to make the city’s weakness into a strength.”
Some experts think Hong Kong’s seawater model could work in other world cities — but not all of them. Cost is a huge deterrent, as the system requires building out a separate pipeline system for toilet flushing.
“Hong Kong invested early,” says Tze Ling Ng, a professor at Hong Kong University of Science and Technology, who has studied the system. “Now it’s reaping benefits as it has to only add to the pipe network in new housing complexes.”
Ng looked at the potential applicability of such a system in 15 coastal cities around the world. She found the best candidates are cities such as Chennai, Jakarta, Karachi, Mumbai and Shanghai. They’re densely populated places where a little infrastructure can reach a lot of people — and where labour to build such a system remains cheap.
By contrast, retrofitting cities such as Los Angeles, Miami or Sydney with seawater flushing would cost more than alternatives such as recycling used water. However, seawater could potentially work for some kinds of new construction. “Seawater flushing can also be very useful for new coastal cities where dual pipelines can be easily laid,” she says.
There’s another downside to the seawater system. Seawater pipelines are more susceptible to corrosion and bursting than freshwater pipelines. Hong Kong is addressing this problem by substituting pipelines lined with cement or epoxy. PVC pipes are used in some places but they’re not as strong as iron pipes.
Filling up the tank
Hong Kong’s other problem — heavy rains — has pointed the city to a different set of water solutions. And the best place to find them is at a set of sporting fields close to where the city’s lush green mountainsides flatten and disperse into busy streets and tall buildings.
The Happy Valley Recreation Ground has 11 fields, a jogging track, and a horse racing course surrounding the periphery. It’s one of the biggest open spaces in Hong Kong, and it’s popular with people of all ages. One recent afternoon, a group of toddlers was warming up for a rugby match, while other youths played football on an adjacent pitch and a group of old men jogged slowly around the track.
Sporting fields in Hong Kong sit atop a vast stormwater retention tank. (Alok Gupta)
Just below the fields is a massive underground storage tank. During heavy rains, the tank fills with runoff flowing down from the mountains, and temporarily holds it there to prevent it from inundating the city below. The tank can hold 60,000 cubic metres of water, or roughly the equivalent of 24 Olympic-sized swimming pools.
Ellen Nga See Cheng, a senior engineer with the city’s Drainage Services Department, took me inside to see it. The tank feels like an enormous empty underground car parking area — it’s the length of five football fields. It has grey concrete walls and an endless row of concrete pillars holding up a three-metre (ten-foot) high ceiling. The floors were slippery and a few pieces of trash were strewn about, leftovers from when Typhoon Merbok last filled the tank in June.
Cheng likened the tank to a “sponge”, which has become a favourite metaphor in China. “Sponge cities” refers to a range of strategies for controlling flooding in urban areas. Some allow for more rain to soak into the ground, through rain gardens or permeable pavements. Others, like Happy Valley, are more engineered solutions.
“We have no space to store water that is gushing from the hills into the urban areas,” says Cheng, adding that 40 percent of Hong Kong is declared a nature reserve and all the other land is built up. “Going underground is the only way out. And Happy Valley Recreation Ground became a natural choice.”
The water storage scheme cost HKD 1.7 billion (USD 218 million) to build and was completed in March of this year. It’s not the first such system in Hong Kong. The first one, built in 2004 beneath a football and rugby ground, holds almost twice as much water. But the Happy Valley water storage is more advanced technologically.
The stormwater retention tank can hold as much water as 24 Olympic-sized swimming pools. (Alok Gupta)
The system is computer operated and connected to Hong Kong Observatory, the city’s weather department. When sensors signal a heavy rainfall, stormwater is channeled into the tank. The tank drains automatically when rains subside and it’s safe for water to be released into drainage outlets.
Critics have pointed out that not much of the stormwater collected is put to productive use. Only about 10 cubic metres a day is used to maintain the grasses and trees of the recreational fields and parks above; a bit more will soon be used at the racecourse. But most of the water eventually flows out into the sea. Cheng says technology to turn the stormwater into drinking water could be added in the future if it proves cost-effective.
In the meantime, when it comes to flood control, the storage tank seems to be doing its job. Engineers at the Drainage Services Department say the system worked exactly as planned during Typhoon Merbok, and also during a storm last October when heavy rains prompted flood warnings.
“Hills and land reclamation is a deadly combination and a recipe for floods in a high-rainfall area,” Cheng says. “We need underground infrastructure to retain water like a sponge.”
Citiscope is a nonprofit news outlet that covers innovations in cities around the world. More at Citiscope. org.