China Focus: How does China's urine recycling system work in space?

Source: Xinhua| 2021-06-22 20:57:03|Editor: huaxia

BEIJING, June 22 (Xinhua) -- Storing basic life support materials like water, food and oxygen aboard a space station can be challenging -- so much so that some are generated aboard rather than delivered to space.

Chinese scientists have devised a system to recycle water from the urine, breath and sweat produced by astronauts in space, which could save up to 100 million yuan (about 15.5 million U.S. dollars) over a period of six months with three astronauts in orbit.

The system has been installed in China's newly launched core space station module Tianhe, meaning urine can be processed into distilled water, some of which will be used for toilet flushing. The remainder, together with collected breath condensate, can be purified further for electrolytic oxygen generation and experiments, said Cui Guangzhi, one of the designers of the urine treatment system.

On June 17, China successfully launched the crewed spacecraft Shenzhou-12, sending three astronauts to Tianhe for a three-month mission. The urine treatment system, a sub-system of the life support system, is undergoing full verification in space for the first time.

On Earth, distilling water is a simple process. Contaminated water is boiled and the rising steam is then cooled or condensed to form pure water.

The water in urine evaporates at 30-40 degrees Celsius. But in space, this process does not occur as usual.

Cui, a researcher with the Second Institute of the China Aerospace Science and Industry Corporation Limited (CASIC), told reporters that distillation in space is performed at a reduced pressure since the boiling points of liquids are depressed at reduced pressure. The water in urine evaporates at a lower temperature.

The lack of gravity in space prevents the separation of the vapor from the contaminated water. During the distillation process, the equipment has to rotate.

Cui explained that rotation creates a centrifugal force that can separate water from air due to their different densities. In the form of vapor, the water in urine will stick to the walls of the evaporation box and can then be extracted.

On Earth, the vapor naturally turns to liquid as soon as the temperature drops. But in space, the vapor needs to be refrigerated in order to cool, which is quite expensive.

Cui said the vapor will convert into water in a pressurized tank as high pressure can make steam liquefy without cooling.

The liquefaction process also releases heat, another product that scientists find valuable. The heat will be recovered and reused.

In addition to urine, the water exhaled by astronauts will also be recycled in the system. With three astronauts in orbit, nearly 2 liters of this wastewater will be recycled every day.

The recycling system can extract 5 liters of distilled water from 6 liters of urine in one working cycle, with the maximum water production rate at 2.5 liters per hour.

Some of the distilled water will be used for toilet flushing and the remainder, together with the collected sweat and breath condensate, will go through an ion-exchange process to remove contaminant ions. The processed water can then be used for oxygen production or space experiments.

Cui noted that Chinese astronauts are scheduled to stay in the space station for periods of three to six months, and there will be high oxygen demand.

The oxygen generation system uses a process called electrolysis to split water into its hydrogen and oxygen component atoms, decreasing the need to deliver oxygen cylinders to space.

Maintaining a long-term human presence in the space station requires a well-organized life support system, for which wastewater recycling and oxygen generation both play essential roles.

The energy and power consumption requirements are relatively high for a complex orbiting station, and some systems work in shifts during peak hours, Cui noted. "The recovered energy is quite valuable for the space station."

China sent the Tianhe module into space on April 29, kicking off a series of key launch missions that aims to complete the construction of the station by the end of next year.

On May 29, China launched the cargo craft Tianzhou-2, and it successfully docked with Tianhe on May 30, delivering supplies, equipment and propellant. The Shenzhou-12 spaceship conducted a fast autonomous rendezvous and docking with Tianhe about 6.5 hours after the launch on June 17, forming a three-module complex with Tianzhou-2. The three astronauts -- Nie Haisheng, Liu Boming and Tang Hongbo -- then entered the Tianhe module, starting their three-month mission in the space station.

The Tianzhou-3 cargo craft and the Shenzhou-13 manned spaceship will also be launched later this year to dock with Tianhe, and another three astronauts will then begin their six-month stays in orbit. Enditem