Can you guess what the most surprisingly popular event at Edinburgh Doors Open Day is?
Edinburgh Doors Open Day is a chance for people to explore interesting buildings around Edinburgh and the Lothians completely free.
The idea is to open doors to properties which would usually be closed to the public, or charge an entrance fee.
Edinburgh Doors Open Day is organised by the Cockburn Association of Edinburgh and East Lothian, a 144-year-old charity which campaigns for the conservation of buildings and landscape in the city and the Lothians, frequently opposing development on the Green Belt.
One of the most surprising popular events so far is Seafield sewage treatment centre, which has been so in-demand that tickets have sold out.
The Lothian Bus Depot is also a popular choice, and visitors do not need to book.
“People love to know how things work,” said Ms Gray, adding that the Bus Depot is especially popular with children and families.
“It shows that Open Doors Day is not just about pretty buildings, it’s about functional ones too,” she said.
This is the first year Seafield sewage treatment centre has been involved in Doors Open Day, and it has been ‘surprisingly popular’, Ms Gray said.
So much so that tickets sold out in just 10 days, before September 13.
There will be three tours on each day, bringing over 70 people to the site over the weekend.
“Doors Open Day is all about giving members of the public the chance to see behind the scenes in places they would not ordinarily be able to visit,” said Matthew Pastellas, Communinity and Engagement Coordinator at Veolia, the international firm which manages Seafield on behalf of Scottish Water.
“I think it’s a popular place to visit as it’s a little bit quirky - not everyone can say they’ve walked around a sewage treatment works!”
Mr Pastellas said the sewage plant was ‘delighted’ to be taking part in Doors Open Day.
“Sites like Seafield are a hugely important, but often overlooked, vital part of the nation’s infrastructure,” he said.
“Dealing with the waste that homes and businesses produce will always be a challenge.
“We hope that by opening our doors and welcoming in the public, we can all have a shared understanding of what we can all do to ensure we look after our environment and our planet’s resources.”
Seafield is Scotland’s largest waste water treatment works.
It processes 300 million litres of waste water every day from almost one million people - enough waste to fill 121 Olympic-sized swimming pools.
The site treats and pasteurises the raw sewage to generate biogas energy. It is capable of producing up to 2300 kilowatts of electricity - enough to power up to 600 homes.
Sewage is also turned into ‘sludge cake’ and sold to farmers as organic fertiliser.
The site underwent a multi-million-pound Odour Improvement Plan in 2012.