Hello,
To be considered a civilization you must have cities with a form of government, a food surplus, and complex architecture. Since the Sumerians were located in Anatolia near the Persian Gulf, they got their water for agriculture there and therefore had a consistent and reliable food surplus. The Sumerians were also known for their complex architecture such as roads and tools which helped in the use of the advancement of the region. And finally, they had cities (like any other civilization). So that's how they met the criteria of a civilization. Hope that helps!