Shanghai customs is stepping up its crackdown on the importation of suspected smuggled solid waste material, but concedes locating the offending companies can be difficult.

"In some cases, the exporting enterprises went bankrupt, making it hard to locate the recipients overseas," said Han Dejun, head of material inspection at the outer port of Shanghai customs.

"In other cases, the shipping companies have closed down or exited the Chinese market, making it difficult to return the foreign waste."

In May, Shanghai customs returned more than 7,000 metric tons of suspected smuggled solid waste to the source countries which had been shipped to the eastern port city.

It was the largest return of solid waste by Shanghai customs since July 2017 when the country decided to phase out imported waste with rising public awareness about environmental protection and people's anxiety about pollution.

To prepare for the clearance in May, customs talked to more than 10 shipping companies, urging them to make an effort to return the foreign waste.

Most of the foreign waste returned was undeclared or had not been declared within a specified time limit, customs said. Among the waste, for example, was more than 100 tons of cardboard from Australia that had been piling up at the Shanghai port for seven years, which was finally returned in five containers.

Around 1,800 tons of suspected smuggled solid waste was disposed of in a safe way after the shipping companies involved could not be reached, Han said.

Undeclared solid waste is often stacked at ports or warehouses for long periods, taking up storage facilities at the port, and possibly posing health risks due to pollutants and industrial waste.

So far, China has banned the import of 40 types of waste and another 24 are to be included by the end of this year, according to the Ministry of Ecology and Environment.

Over the past two years, customs authorities across the country have been returning banned waste to the source countries to avoid pollution risks and high disposal costs.

Han said Shanghai will step up efforts to intensify inspections of imported waste.

China started to import solid waste in the 1980s, when it was considered a source of raw materials. Despite its relatively weak capacity for waste treatment, the country was for years the world's largest importer of solid waste.