At 7 am it is already broad daylight in the city of Shenzhen and the air is pungent with the smell of animal waste. Wearing his blue uniform, Zeng Hui gets into a small control room, opens a barrier, and begins to release waiting vans.

Zeng is the customs officer of Wenjindu Port, which operates under Shenzhen customs. It is the most important and the only land route pass for agricultural products provided to Hong Kong from the mainland. More than 80 percent of such agricultural products go through this port.

In 2011 pigs delivered through the port made up 95 percent of Hong Kong's market demand and the cows 100 percent, prompting Wenjindu to be called the city's "green lifeline".

Zeng has worked at the port since 1982. The 47-year old from Shaoguan in Guangdong province has witnessed many changes at the port over the past 20 years.

"When I came to customs, a van needed half an hour to finish all the procedures and then it got passed through. Now it takes only five seconds," he said.

He said customs makes great efforts to cut the release time of vans passing through the port. In previous years, agricultural enterprises needed to declare each shipment of their product to customs, but in 1987 a new system was adopted to allow agricultural enterprises to declare all their products on a monthly basis. This measure cut the average waiting time from half an hour to two minutes.

In May 2011, customs adopted a new system to allow agricultural enterprises to declare their products online. If they do this before they reach the port, their vans get automatically released.

"Most of the vans get directly passed within seconds under this system. The system alarms when there are inspection instructions from our risk department and it also sends a few random instructions. Only at these occasions we would stop the van and inspect it," Zeng said.

An incident on Sept 30 last year impressed Zeng. It was raining heavily and the water disrupted the computer system.

"The system failed to work, and the vans were lined up for at least 5 or 6 kilometers that day, and the wind and rain continued. All the staff of our department come out to release them and we worked until midnight," he said. "We have to make sure the products get to Hong Kong, otherwise the agricultural products prices in Hong Kong will soar."

Zeng said in the 1980s many agricultural products were still carried to Hong Kong by the "three express trains" - daily trains which left from Shanghai, Wuhan and Zhengzhou. However, in the 1990s as highway transportation developed, Wenjindu Port replaced the three trains.

But the wheel of history never stops. Wenjindu Port, located in the city center, now faces a new destiny.

The Shenzhen government plans to shift Wenjindu's main function to passenger inspection and transfer the agricultural product inspection to Liantang Port, a new port to be built by 2018.

"By that time I will have retired, but I guess if the port does shift, I will feel it is a bit of a pity and a bit of a loss," he said.

Contact the writer at huangyuli@chinadaily.com.cn