If a sludge cake contains 60% solids, where did it most likely originate from?

The characteristic of a sludge cake containing 60% solids strongly indicates that it most likely originated from a drying lagoon. In drying lagoons, the primary goal is to dewater the sludge through evaporation and drainage, which can lead to high solids content. The environment allows for a significant reduction in water content, resulting in the concentration of solids.

While the activated sludge process generally produces a lower solids content in its sludge due to the nature of aerobic treatment and the need for potential return flows, processes like composting and thickening tanks also yield different compositions. Composting facilities typically return a nutrient-rich product that can vary widely but doesn't usually reach such high solids percentages. Thickening tanks also concentrate sludge, but the solids content there generally does not compare to what is achieved in drying lagoons, where moisture reduction is maximized over time.

Given these factors, the origin of the 60% solids sludge cake points towards drying lagoons as the most plausible source.