Design UASB

Design of an UASB Reactor
Guest article by Dr. Makarand M. Ghangrekar, IIT

UASB Process

Upflow Anaerobic Sludge Blanket (UASB) reactor has been successfully used to treat variety of industrial as well as domestic wastewaters. The UASB reactor can be briefly described as a system in which substrate passes first through an expanded sludge bed containing a high concentration of biomass. The sludge in the reactor may exist in granular or flocculent form, but the granular sludge offers advantages over flocculent sludge. Most of the substrate removal takes place in sludge bed. The remaining portion of the substrate passes through a less dense biomass, called the sludge blanket.

Provision of sufficient volume of sludge blanket is necessary above the sludge bed, which will provide further treatment to the wastewater bye-passed from the sludge bed due to channeling, and will help in maintaining stable effluent quality. Above sludge blanket the reactor has a three-phase separator, Gas-Liquid-Solid (GLS), which separates the solid particles from the liquid and gas, allowing liquid and gas to leave the system.

Proper design of this device is necessary to retain maximum sludge in the reactor to affirm high Solid Retention Time (SRT), about 50 to 100 days or more, and to facilitate treatment with short Hydraulic Retention Time (HRT). In general, the sludge bed occupies 30 to 60% of the total reactor volume, 20 to 30% of the total volume is provided for sludge blanket and GLS separator occupies remaining 15 to 30% of the total volume.

The design of the UASB reactor is carried out taking in to consideration the applicable loading rates, such as, hydraulic loading rate considering limitations of superficial liquid upflow velocity and Organic Loading Rate (OLR). The reactor volume is worked out to suit desired range of loading, and height and plan area are finalised considering upflow velocity. With this approach in design, it may not be always possible to accommodate the desired range of Sludge Loading Rate (SLR) due to limitation of sludge bed volume. Also, it may not be always possible to accommodate the GLS separator device, with dimensions required for providing sufficient area for gas-water interface, for settling, and for aperture at the bottom of GLS device to avoid excessive liquid inlet velocities in the settler. The schematic diagram of UASB reactor is presented in Figure 2.

Figure 2. Schematic diagram of UASB reactor



Under certain situation e.g., for low COD concentration of the wastewater (about 500 mg/L) and under very high COD concentration of the wastewater (10,000 mg/L), the dimensions required for the GLS separator may govern the overall dimensions of the reactor. Hence, it is necessary to give due consideration for the dimensions of the GLS separator required, while designing the UASB reactor. With these objectives this article aims to discuss range of parameters recommended for design of the reactor and to illustrate the design procedure.