Generally tap water is drinkable everywhere in the mountains of Bulgaria. Currently there are two huts that take their water supply from nearby lakes (Tevno Ezero in Pirin Mountains and The old 7-lakes hut in Rila Mountains) – practically that doesn’t lead to any problems for the visitors but if you are worried of the potential risk take water purifying tablets with you. It is not advisable to drink (at least without purifying) water from open sources like lakes, streams and rivers because of the possible animal and human inhabitation above that may compromise the water.
Depending on your route and the season carrying 1-2 litres of water should be sufficient. There is lack of water sources along some of the high mountain routes that follow the ridges mainly in Pirin Mountains (especially the main Karst ridge), less in the central parts of Balkan Mountains and Rila (the name “Rila” comes from an old Tracian word for water). This can be a problem in hot weather, so before taking a specific route ask about water supplies on the way and take the necessary quantity with you. There is no lack of water supply in Rhodope Mountains where an old tradition says that one should make something good for the society at least ones in his life – building a tap or place to rest on the road for example.