Calcutta: Colour-coded bins have arrived at several Metro stations as part of a waste management campaign, but ignorance appears to have foiled intent in the absence of signage telling people which litter to dump where.

Bins meant for degradable waste are green in colour while the ones for non-degradable litter are in blue. At Park Street station this Monday, a green bin and a blue one stood side by side on the platform for Tollygunge-bound trains. But there was no signage to help passengers make the right choice.

A middle-aged man was spotted throwing an empty water bottle into the green bin before taking a train. The bin was already filled with empty plastic - mostly mineral water and soft drink bottles - that should have been in the other bin.

The words "biodegradable" and "non-biodegradable" were written on the bins, but that was apparently not informative enough for most people.

Officials at the station said the bins were provided a couple of months ago, but there was no accompanying directive to put up display boards listing the items that should be dumped in each bin. "We empty the contents of the bins into a designated spot on the road every morning. The civic body collects the waste from there," an official said.

Degradable waste - kitchen, plant and human waste - can be broken down naturally by water, oxygen, the sun's rays or micro organisms. The matter decomposes and eventually blends with the soil.

Items that cannot be broken down or decomposed by natural agents are labelled non-degradable. They are the major contributors to air, water and soil pollution. Plastics, polythene bags, synthetic fibres and aluminium cans comprise non-degradable waste.

Segregation is the crucial part of waste management because that helps in recycling. If waste is not segregated properly, it gets mixed up in landfills. Some of these can contaminate ground water and release methane gas.

In the London Underground and other mass transit systems in developed countries, there are posters, stickers, boards or other signage either stuck on the bins or near them, informing commuters about which bin to dump waste in.

Between April and December last year, 117 people were fined Rs 500 each for spitting inside stations. More than 4,500 people were fined Rs 250 each for littering and violating the photography ban.