A total of 4,468 metric tons of illegally imported foreign garbage has been shipped out of China from ports in Qingdao, Shandong province, in the past eight months, customs data show.

It has been removed since China launched Operation Green Fence in February, aimed at restricting recyclable waste from entering the country.

During the operation, customs officers cracked 28 cases involving the illegal importation of solid waste worth about 12.26 million yuan ($2.05 million). Two garbage smuggling cases have also been filed for criminal action.

"Defending our eco-frontier is a significant responsibility of customs officers," said a staff member at Huangdao Customs, who refused to disclose his name.

"Garbage containing hazardous materials could cause serious contamination if imported and processed inappropriately."

The law on the control of solid waste bans imports that cannot be used as raw materials or be recycled harmlessly, because of concerns over pollution and epidemics.

But, lured by high profits, a number of trading companies break the law by importing prohibited garbage.

One customs officer said the inspection process is so rigorous that even if they find just one used syringe needle the entire container will be ordered to be shipped out.

Onsite inspectors open every suspicious container after machinery or document checks, but some shipments are cleverly disguised.

On Oct 10, officers inspecting a container declared to contain waste plastic membrane found rubber plugs scattered between membrane covers. They then found the membranes were mixed with drip bags, medicine bottles, surgical uniforms and gloves.

From June 1, all imported solid waste has been transferred to Qianwan port's special examination area.