The feasibility of the up flow anaerobic sludge blanket reactors UASB for adequate sewage treatment has been investigated since 1980 at both pilot and full scale installations [2], but at the moment, it is largely restricted to countries with a relatively cold climate [3].
The anaerobic fluidized bed and the expanded granular sludge bed reactors, with HRTs of about 2–4 h [4] and the UASB reactor, with an HRT of 4–8 h [5] offer good results, while the attached growth process named anaerobic filter needs a longer HRT on assuming constant organic loading rates for all systems.
As domestic wastewater flows are relatively huge in large cities and should be treated at short HRTs to be more feasible and often are at ambient or low temperatures, complex substrates could leave the reactor before being biodegraded. In UASB systems, with relatively adequate HRT, the sludge bed acts as a filter to the SS, thereby increasing their specific residence time. In this way, the UASB reactor may achieve high COD and SS removals at a relatively short HRT if compared to the conventional primary sedimentation tanks. Consequently one of the aims of the study was to study the influence of the low HRT on the reactor performance.
Material and methods
The Experimental work was carried out at El Berka wastewater treatment plant. Using domestic wastewater from the primary sedimentation channel the experimenting set up was started by the UASB reactor.
The reactor consists of a column portion (130 cm) of about 23 L and a gas–solid separator (GSS) portion (20 cm) of about 6.28 L. The reactor total volume is about 25.60 L this volume was used to calculate the organic loading and the hydraulic retention time. The UASB was operated over 100 day under different temperature ranges (15–25) °C.
The up flow velocity was varied according to the hydraulic retention time HRT variation, no recirculation was applied. Effluent recycle was not necessary to fluidize the sludge bed as sufficient contact between the wastewater and sludge is guaranteed even at low organic loading rate in UASB reactor [6]. Fig. 1 shows the reactor setup and the influent water characteristics are shown in Table 1.