The major physical components of a UASB reactor that require careful consideration are the feed inlet distribution, outlet and gas collector. Some important design criteria of UASB reactors treating sewage are presented in Table 7. The required good contact between influent wastewater and sludge is achieved by the even feed distribution at the bottom of the reactor, the turbulence brought about by the natural biogas production and high upward velocity. The in-built Gas Solids Separator (GSS) installed at the top of the reactor takes care of separation/collection of the biogas. A number of important guidelines regarding the design of the GSS are given in Table 8.
Min. average HRT
4 hrs
height
4-5 m
Feed inlet points
1 inlet per 1 to 4 m2
Feed distribution
Each inlet pipe from a separate compartment
Static pressure in feed inlet box
Up to 50 cm
Upflow velocity in aperture
Average daily 4 m/hr
During 2-4 hrs 8 m/hr
Upflow velocity
0.5 m/hr
Table 7: Some design criteria of UASB reactors treating sewage (see fig. 2)
The slope of the settler bottom (i.e. the inclined wall of the gas collector) should be between 45-60 °.
The surface area of the apertures between the gas collectors should be 15-20% of the reactor surface area.
The height of the gas collector should be between 1.5-2 m at reactor heights of 5-7 m.
To facilitate the release and collection of gas bubbles and to combat scum layer formation, a liquid-gas interface should be maintained in the gas collector.
To avoid up-flowing gas bubbles to enter the settler compartment, the overlap of the baffles installed beneath the apertures should be 10-20 cm.
Generally, scum layer baffles should be installed in front of the effluent weirs.
The diameter of the gas exhaust pipes should be sufficient to guarantee the easy removal of the biogas from the gas collection cap, particularly in case of foaming.
In the upper part of the gas cap, anti-foam spray nozzles should be installed in the case the treatment of the waste water is accompanied with heavy foaming.
Table 8: Summary of tentative guidelines for the design of the gas-solids-separator device. (Adapted from Lettinga and Hulshoff Pol, 1991)
Organic loading rate and upflow velocity
Organic loading rate and upflow velocity