Every three months a helicopter flies over Hong Kong‘‘s restricted border region near Shenzhen, the special economic zone in China‘‘s southern Guangdong province. Its purpose is not to survey the dramatic landscape of the closed area - which after five decades of enforced isolation has evolved into one of south China‘‘s most bio-diverse regions.

The Financial Times reports that from their bird‘‘s-eye helicopter perch, engineers from Swire Sita Waste Services instead examine the North East New Territories landfill, one of three facilities that receive the 9,377 tonnes of municipal waste generated by Hong Kong each day. Officials in Hong Kong say that the territory is rapidly accumulating more waste than it can handle. Sarah Liao, the territory‘‘s secretary for the environment, transport and works, recently warned that, in the absence of "decisive and immediate measures to reduce waste", the three landfills could be filled in as little as four years. With a combined capacity of 96m cubic metres - about 30m of which have been filled - the two facilities managed by Swire Sita at North East New Territories and West New Territories are among the world‘‘s largest landfills. Together with Hong Kong‘‘s third landfill, the South East New Territories facility, in the aptly named Junk Bay, the facilities are all situated in valleys whose "void space" is being rapidly filled with Hong Kong‘‘s detritus.

Hong Kong‘‘s population of 7m is not far off Greater London‘‘s 7.5m. But, with a land area of 1,042 sq km, it is 35 per cent smaller than the UK capital. Adding to the pressure on Hong Kong, 40 per cent of its area is protected natural parkland and it has no hinterland to which it can export non-recyclable waste. The helicopter surveys are needed to monitor the dump‘‘s constantly shifting landforms. The engineers look out not at a vast expanse of rubbish, but instead at a relatively tidy landscape of green plastic sheeting weighted down with thousands of tyres. In addition to environmental considerations such as the need to reduce both vermin and stench, the sheeting traps methane gas, allowing it to be collected by a network of well-heads and rubber tubing. The gas is then used to power the landfill‘‘s auxiliary facilities, with the excess piped 19km to Hong Kong and China Gas, a local gas provider.

James Tam, Swire Sita director and general manager, describes the landfill‘‘s management as a "very complex engineering process that evolves over decades". In Hong Kong‘‘s tropical climate - the city is on the same latitude as Havana - organic matter rots quickly, dispersing methane gas and causing the landfill to subside in places.

"Waste is very dynamic," adds Dominic Leung, a Swire Sita project manager. "You can‘‘t predict what‘‘s coming in and we have to deal with problems immediately. Waste doesn‘‘t wait for us."

As a landfill rots away, it can be re-filled. On the other hand, the more "inert" or "non-putrescent" matter the landfill contains - such as furniture or construction waste - the more stable it will be and the quicker it will fill. Hong Kong‘‘s landfills have previously doubled as burying grounds for millions of chickens culled during two separate avian flu outbreaks. Public health concerns also dictate that the 1,200 tonnes of waste produced every month by Hong Kong‘‘s 100-odd chicken farms cannot be used as compost, but have to be buried. Such organic additions to the landfills, Swire Sita engineers joke, are "bad for stability but good for gas". Covering the landfill also helps reduce one of the site‘‘s costliest problems - water seepage, which must be treated through an