The final removal efficiency and conversion of organic compounds to methane gas in UASB reactors depend on both physical and biological processes. For sewage, removal of suspended solids occurs by physical processes like settling, adsorption and entrapment. The subsequent Hydrolysis and methanogensesis of the removed solids depends on the process temperature and the prevailing SRT. Zeeman and Lettinga (1999) developed a model for the calculation of the HRT when a certain SRT is a pre-requisite, and this model can be used for preliminary sizing of a UASB reactor. The SRT is determined by the amount of sludge that can be retained in the reactor and the daily excess sludge production. The daily excess sludge production is determined by the biomass yield and the removal and conversion of suspended solids. At a certain temperature the SRT will determine whether Methanogenesis will occur or not. So when the required SRT is known, the corresponding HRT can be calculated provided that the sludge concentration in the reactor (X), the fraction of the influent SS that is removed (R) and the fraction of the removed SS that is hydrolysed (H) are known. The HRT of a UASB reactor can be calculated with the following formulas:

SRT: sludge retention time (days)

H: fraction of removed solids that are hydrolysed

The previous model was used for the calculation of the required HRT for the application of a UASB reactor for sewage treatment Shams City where sewage temperature is 15 ºC. The total COD of the domestic sewage produced from Shams City is 1600 mg/L, of which 60% is in the suspended form (particle size bigger than 4.4 um). Calculate the required HRT of a UASB reactor to treat this sewage.

The following input data were taken into consideration:

R = 0.8; around 85% of total suspended solids (TSS) removal efficiency can be achieved in a UASB reactor at an upflow velocity (Vup) of 0.6 m/hr.

SRT = 30 days; expected minimum SRT to achieve methanogenic conditions during winter time (data should be obtained fro literature or from available local data). At these conditions:

H = 0.15; 15% of the TSS can be hydrolysed.

X = 15 g VSS/l = 21 g COD/l

SS= 0.60

C = 1.600 g COD/l

Accordingly, the model calculation reveals that a HRT of 22 hour is required.