Abstract
The anaerobic biodegradability of domestic sewage for four Egyptian villages and four Egyptian cities was determined in batch experiments. The results showed that the biodegradability of the Egyptian-villages sewage (73%) was higher than that of the cities (66%). The higher biodegradability of the soluble COD of village sewage (69%) as compared to that of the cities (46%) was the reason for the higher biodegradability of the total COD of the villages sewage. The biodegradability of suspended COD was similar for the sewage of both villages and cities (73-74%). The results of a mathematical-model, developed based on anaerobic digestion model number 1, indicate that at applying a UASB reactor for the treatment of Egyptian villages and cities sewage, an optimum HRT of, respectively, 16 and 8 h is required. At these HRTs, a total COD removal and a conversion to methane of, respectively, 62-70% and 59-64% can be achieved for the sewage of cities and, respectively, 71-77% and 67-69% for the villages sewage. The model results also show that in the treatment of villages sewage in a two-step (anaerobic filter + UASB reactor) system a higher total COD removal can be obtained (77-81%) at a short HRT of 10 h (4 + 6 h). However, the excess sludge from the first-step of the two-step system will be less stabilized.