Six men responsible for an illegal chemical discharge that led to water supplies being cut off to 30,000 residents in suburban Shanghai received jail sentences ranging from two to five years in a Shanghai court on Wednesday.

It was the ninth judgment of an environmental pollution case in the municipality since 2010, according to Shanghai High People's Court.

"Three cases of environmental pollution, involving six defendants, were adjudicated from 2010 to 2012, and the number has risen to six cases involving 15 defendants so far this year," said Zou Bihua, vice-president of the court.

"It reveals the increasing criminal tendency in this sector as well as a stiffer crackdown on violations of environmental protection," he said.

The leak, caused when a valve being used to control discharge from a chemical tanker to a vessel was left open, happened at Jueshi Port in Shanghai's Jinshan district, home to the municipality's chemical industry park, on Jan 10, according to the prosecution.

More than 52 tons of styrene - a chemical that can damage the intestines, kidneys and respiratory system if ingested - leaked into the river near the port.

The toxic flammable liquid contaminated the port and the Huangpu River, which is at the lower reaches of the port, and caused operations to be suspended at two water plants.

Tens of thousands of homes in Jinshan and the neighboring Songjiang district had to rely on bottled water and clean water dispatched from fire engines for at least three days, after residents started smelling a pungent odor at around 7:40 pm on Jan 10.

Thousands of residents in Zhujing, Jinshan district, which was most affected, were evacuated from the southwestern suburb temporarily.

Some 46 residents became ill after inhaling the chemical, and the economic loss totaled nearly 5 million yuan ($817,000), the court said.

Wang Jinlong, the legal representative of Nanjing Dragon and Phoenix Chemical Co Ltd, who proposed to load the dangerous chemical at the port, which was not qualified to handle dangerous goods, received a jail term of five years.

Liu Agui, manager of the port, was sentenced to three years and six months imprisonment.

Xi Jianping, general manager of Shanghai Jinwei chemical company, who agreed to load the goods at the port after making a deal to sell the chemical to Wang, was sentenced to three years' imprisonment.

The three men who operated the vessel, Liu Langjian, Du Guoxing, and Liu Kang, received jail terms ranging from two to four years.

Liu Langjian told police during interrogations that the leak was caused by his negligence but denied acting intentionally. However, the court ruled that the men's behavior reflected their mindsets of dodging any penalties while dealing with the hazardous substance in a casual way.