Methods and systems are described for treating a fluid that includes a particulate fraction and a soluble fraction, such as
wastewater fluid including
biosolids. The treatment includes biochemically transforming solids in the particulate fraction of the fluid in a
biochemical process while simultaneously subjecting the fluid to a
vacuum pressure, and evaporating off at least a portion of the soluble fraction of the fluid and thereby thickening a remaining portion of the fluid. A
residence time of the particulate fraction can be controlled to be at least 25% greater than a
residence time of the soluble fraction, for example. A solids content of the particulate fraction can be controlled to be in a range of from 2% to 99%, for example.