Thickening 101

Thickening is commonly used for concentrating solid streams at a water resource recovery facility (WRRF). Thickening removes excess water from the solids, which reduces the size and scale of downstream processes. Typically, the water removed from the solids during thickening is recycled to the headworks of the WRRF.
The ease with which different types of solids (e.g., primary solids, waste activated sludge [WAS]) can be thickened varies widely. Much of the variability in thickening performance results from the site-specific characteristics of the solids themselves. These characteristics depend on the process used to treat the wastewater before thickening, the way in which water is bonded to the solid particles and/or flocs, and the concentration of extracellular polymeric substances as well as mono and divalent cations present in wastewater. Solids concentration is typically monitored using total suspended solids and total solids. The degree of solids concentration achieved through thickening processes also depends on solids conditioning with polymer, which can be affected by the above-mentioned factors in addition to solids temperature, pH, volatile and suspended solids concentration, and solids/polymer mixing type and intensity. This Thickening 101 Fact Sheet dives into the questions of why to thicken solids, where to thicken solids, how solids are thickened and what technologies are used to thicken solids.