The UASB reactor consists of the following components: influent and distribution system, reactor tank body, and three-phase separator.

The most critical component of the UASB reactor is the three-phase separator, which is installed at the top of the reactor and divides it into a lower reaction zone and an upper settling zone. To achieve satisfactory settling of sludge flocs/particles in the upflow within the settling zone, the primary purpose of the three-phase separator is to effectively separate biogas generated from the sludge bed/layer—particularly under high loading conditions. The reflection plate beneath the gas collection chamber prevents biogas from escaping through gaps between collection chambers into the settling zone. Additionally, the baffle helps reduce liquid turbulence caused by high biogas production in the reaction zone. The reactor should be designed so that as long as the sludge layer does not expand into the settler, sludge particles or flocculent sludge can slide back into the reaction zone.

It should be recognized that occasional expansion of the sludge layer into the settler is not necessarily detrimental. On the contrary, an expanded sludge layer within the settler can capture dispersed sludge particles/flocs and provide some removal of biodegradable soluble COD. However, it is important to maintain sufficient free space for sludge layer expansion to prevent loss of dense sludge during temporary organic or hydraulic loading shocks. Both hydraulic and organic (biogas production) loading rates affect the expansion of the sludge layer and sludge bed. The fundamental principle of the UASB system is based on the formation of well-settling sludge flocs, combined with an internal sludge settling system that achieves separation of gas, liquid, and solid phases. The formation and maintenance of well-settling sludge—whether flocculent or granular—is the cornerstone of successful UASB operation.