Which solids treatment method requires significant labor but utilizes evaporation?

The solids treatment method that utilizes evaporation and requires significant labor is indeed sand drying beds. In this process, sludge is placed on sand beds where moisture is evaporated, leading to a reduction in the volume of the solids. The labor-intensive nature of this method arises from the need to regularly turn the sludge to facilitate drying and to manage the removal of dried solids, which often requires manual handling.

Additionally, because this method relies heavily on environmental conditions like temperature and humidity for effective evaporation, the operational management can be laborious. Other treatment methods listed, such as composting or anaerobic digestion, involve different biological or chemical processes that do not inherently require evaporation as a primary mechanism for solids reduction, while vacuum filtration focuses more on mechanical removal rather than evaporation. Therefore, sand drying beds effectively meet the criteria of necessitating significant labor while utilizing evaporation for solids treatment.