Abstract: An apparatus for covering a water container having an open top and treating a source water therein. The apparatus has one or more dividing walls mountable to an interior space of the water container for partitioning it into multiple water-treatment zones, and a cover mountable to at least a portion of the open top of the water container above the one or more dividing walls. Each dividing wall has or forms one or more openings for directing the source water from one water-treatment zone to the next. The cover has one or more vaulted cover sections each defining a gas chamber above the dividing walls. Each cover section has a condensation surface in the corresponding gas chamber for condensing a steam evaporated from the source water to clean water, and at least one water-collection channel coupled to at least one side of the condensation surface for collecting the condensed clean water.