During what phase is the wastewater usually in the treatment process before being thickened?

The correct answer is clarification phase because during wastewater treatment, clarification is a critical step where suspended solids are allowed to settle out of the liquid. This process results in the separation of wastewater into clearer liquid, known as supernatant, and a more concentrated sludge that contains the solids.

Prior to thickening, wastewater has usually gone through this clarification process, where the aim is to reduce the volume of solids that will subsequently undergo further treatment or disposal. This phase is where much of the particulate matter is removed, making the subsequent thickening phase more efficient, as the remaining solids can be concentrated more effectively from a less turbid liquid.

In contrast, the other phases mentioned do not align with this specific sequence. Raw sewage phase refers to the initial stage before any treatment has occurred. Dewatering phase occurs after thickening, wherein additional moisture is removed from the sludge. Centrate phase involves the liquid that is separated off after thickening and dewatering, which isn't before thickening. Therefore, understanding these phases highlights the clarity and efficiency of the treatment process leading into thickening.