Which is a solids treatment thickening process?

The correct response is flotation, as it is a widely recognized method utilized in wastewater treatment to thicken sludge. This process operates on the principle of separating solid particles from liquids through the introduction of air bubbles. When the bubbles attach themselves to the solid particles, they become buoyant and rise to the surface, forming a layer of concentrated solids known as "float." This layer can then be skimmed off, effectively reducing the overall volume of the sludge for further treatment or disposal.

While belt presses, composting, and heat treatment can also relate to solids management or processing, they do not primarily serve the purpose of thickening solids in the way flotation does. Belt presses primarily focus on dewatering solids, composting involves the biological degradation of organic materials rather than thickening, and heat treatment can serve various purposes but is not a standard method for thickening sludge. Hence, flotation is the most appropriate answer for a solids treatment thickening process.