Vice President, Yemi Osinbajo, stood in for Buhari, who had been due to attend the ceremony in the Ogoniland area of Rivers state in what would have been his first visit to the delta as president. Osinbajo pledged the government would reverse the damage and that his government would restore the ecosystems.

In August 2011, the United Nations Environment Program (UNEP) said Ogoniland may require the world's biggest ever clean-up after a succession of oil spills. UNEP's Executive Director, Achim Steiner, has told DW that the restoration will take some time.

DW: What does this clean-up mean for the people of Ogoniland?

Achim Steiner: I think it's a historic turning point after four decades of oil exploration in the Niger Delta, in particular Ogoniland. Today the start of the largest ever clean-up and restoration undertaken by Nigeria means that for the first time, the people in the region have reasons to look forward to the future in a way that does not further compound the extraordinary damage that has been done to oil leaks. The illegal refineries and attacks on oil pipelines, have devastated both the environment and livelihoods of the people. With announcement of a $1 billion (896 million euros) fund for phase one to begin the restoration we have turning point in history. Those were the words of President Buhari, who was represented here by his vice president.

How clean will Ogoniland be after this clean-up operation. Will they have removed all the oil traces?

Well, we are talking about unprecedented efforts of clean-up both in terms of complexity and in terms of the quantity of work. We are looking at least at about 25 years before we can begin to say that conditions are restored, to some extent to what they were before. I think in the near future, the priority will be to address the issues such as drinking water. At the moment, there are communities who are drawing water from water wells where the amount of benzene in the water exceeds 900 fold the levels recommended by the World Health Organization. There are both immediate priorities that have to do with human health and the area. Overtime, what is planned is a massive program of removing soil, freeing it of hydro carbons and then returning it.

How soon can life return to the areas which were once polluted?

I wish the answer was "very soon," but the truth is - they are dealing with four decades of neglect. The damage to the environment was simply huge as the price you had to pay for an oil industry to be able to operate. And I think the truth is that we are talking of years before people can really look at a life that is not damaged by what we see here today. But I think the most important thing is, you have to start from somewhere and so for too long no one took responsibility to start. Now that the UNEP proposal has been accepted by all parties and by oil companies and the government having committed to $1 billion, hope has begun to return to the communities.

In August 2011, a UNEP report said the region may require the world's biggest ever clean-up after a succession of oil spills. What has changed since this report was issued five years down the line?

Unfortunately it is not much. We in UNEP as much as anyone in the Niger Delta and Ogoniland community have been quite frustrated by the lack of progress. It is President Buhari and his government that not only committed to implement the recommendations from our report when he was elected, but he has now initiated that program. We have lost another five years in a very tragic story that has affected hundreds of thousands of people. It is long overdue and it is also a reminder to the oil industry and governments that this neglect is not only an injustice but also a tragedy in human terms.

­Ogoniland has been hit by a series of militant attacks; can the clean-up revive the region?

This is one of the challenges, the clean-up can now begin and will certainly start to make a difference also to the livelihoods of people. But it is quite clear that if the attacks on oil pipelines continue, they will simply make it worse and the clean-up will become much more complicated. This is one of the challenges the Rivers State government and communities face whether this kind of activity can be stopped, because otherwise, we can move one step forward and go back two steps back. It is a longstanding conflict and many people had lost faith in both government and oil companies to actually solve this problem.

Achim Steiner is the executive director of UNEP

Interview: Eunice Wanjiru