What causes cooling tower foam?

Question

Is it the bugs that are/were killed?

Answer 1

Basically, it's because an industrial water supply (read: need a large and cheap quantity) is somewhat dirty (compared to potable water).

Persistent foaming can be caused by the concentrations of certain combinations of dissolved solids, or by the contamination of the circulating water with foam-causing compounds.

Algae, protein-films, soil particulates, organophosphates (detergents and otherwise), oils all can cause or contribute to foaming (and subsequent inefficiencies).

Answer 2

it's most likely impurities in the water itself