"People think that in countries so bright, so rich, they don't have this kind of problem," Sascha Gabizon, the head of the non-governmental organisation Women in Europe for a Common Future and one of 2,500 water and sanitation experts attending the forum, told AFP.
"The situation is not widely known among politicians in Brussels," she said.
Countries from the former Eastern bloc which recently joined the EU are those most concerned but there are also isolated locations in western Europe, she said, citing France and Ireland as examples.
In Bulgaria, 42 percent of the population lives in rural zones where only two percent of households are connected to a sewage system.
In Romania, 10 million people live without access to pipes, and in the countryside, only 15 percent of residents have running water.
"In some schools in the rural areas, children don't want to go to the toilets because they are too dirty," Diana Iskreva of the Bulgarian organisation Earth Forever said.
And girls often prefer not to go to school when they are menstruating because of the poor conditions.
"It's so dirty, and they have no intimacy," she said.
In these rural areas, toilets are often just a hole in the ground that is never emptied or cleaned.
The consequences on public health are enormous, with the accumulated excrement ultimately infiltrating the soil and polluting wells and water sources.
That water is then used by inhabitants in sewageless zones for drinking, cooking and washing.
"This leads to diseases like Hepatitis A and blue baby disease, which is due to a high level of nitrates in drinking water," Iskreva said.
The authorised level of nitrates is 50 milligrams per litre of water, but in some areas in Romania the level is as high as 500 mg per litre.
The subject is difficult to raise with local officials.
"It's taboo. They are ashamed (to talk about toilets) and they don't really know how bad the situation is in their poorest areas," said Gabizon.
The problem is not a lack of funds.
The EU has allocated 336 billion euros (499 billion dollars) over the next five years for its member states most in need. Of this sum, 18 billion euros are earmarked for improving sanitation.
According to Gabizon, less than 480 million would be necessary to implement immediate solutions.
In cities however the situation is quite different.
A 1991 EU directive requires towns with more than 10,000 inhabitants to have fully functioning sanitation systems.
Local officials prefer therefore to invest in infrastructures in towns and cities in order to abide by the directive.
"Corruption is everywhere in Bulgaria and blocks the money that should go to the poorest people in the rural areas," Iskreva lamented.
Building a sewage system would cost 80 euros per year per person to maintain, "or the equivalent of two months' salary" for inhabitants in these rural areas who are often retirees and who therefore don't want to make the financial sacrifice.
As installing sewage systems in rural areas is not a priority, aid organisations are trying to find other solutions.
In some parts of Bulgaria and Slovakia, some aid organisations have begun installing "dry toilets", a system used in the Scandinavian countryside that avoids odours and where the waste is reused as fertilizer for farming.