BELGRADE (Reuters) - Ten kilometers north of central Belgrade cattle bones piled in huge heaps rot in the sun and pools of brown sludge bubble and emit a sickening odor in the dilapidated remains of a glue factory and a tannery, abandoned a decade ago.

The site, that covers around 3 square kilometers (742 acres), is testimony to the challenge Serbia faces to bring its environment to the standards required if it is to join the European Union by 2025, as tentatively planned.

A national cleanup is estimated by the government to cost around 15 billion euros ($17 billion).

Serbia says it will not meet EU environmental and climate change demands by the target date and has proposed an 11-year transition period from when it joins the bloc.

The tannery and glue factory by the Vizelj canal went bankrupt some ten years ago. The site was looted and dismantled by illegal scrap metal traders. Chemicals were spilled and the heaps of bones used in glue production were left unburied.

Uncertainty over the site’s ownership and status has complicated its cleanup.

Environmental activists visited the area and warned authorities about the bones and spilled chemicals weeks ago, said Zoran Jankovic, an activist of local Eko Patrola Pancevacki Rit environmental watchdog.

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“This dump of bones and chemicals is located 10 kilometers from the center of the city and 200 meters from fertile, arable land which feeds the city of Belgrade and is owned by ... (United Arab Emirates-based) Al Dahra,” Jankovic said.

Last October, Serbia sold agricultural company Poljoprivredna Korporacija Beograd (PKB) to Al Dahra Agricultural Company for 105.05 million euros ($118.98 million).

Jankovic said inspectors from the environment ministry surveyed the site this week and last and sampled chemicals “to see what they are dealing with ... and to prepare a plan for removal.”

In an emailed statement, the Ministry for Environment said its inspectors investigated the site and found a “small quantity of waste of unknown origin and composition.”

“As soon as the details of this case are established, the appropriate services will react in line with the law,” it added.

The Al Dahra representatives in Serbia could not be reached for comment.

Last week, Serbia’s minister for environment Goran Trivan estimated investments of 2 billion euros would be needed for municipal and toxic waste processing, and another 5 billion euros for waste water processing, part of the total 15 billion euros expenditure.