2. Material and methods
2.1. Experimental set up
The schematic diagram of the laboratory-scale UASFF reactor used in this study is shown in Fig. 1. The system was designed to behave as tubular flow, therefore height per diameter ratio was set at 20.3. The UASFF reactor was made of a glass column with an internal diameter of 6.5 cm and a height of 132 cm. The total volume of the reactor was 4980 ml and the working volume was 4380 ml (excluding head space). The column consisted of three compartments; bottom, middle and top. The bottom part of the column, with a height of 80 cm was operated as a UASB reactor whereas the middle part of the column with a height of 25 cm was operated as a fixed film reactor. The top part of the bioreactor served as a gas–liquid–solid separator. The middle section of the column was randomly packed with 90 pieces of pall rings with diameter and height of each equal to 18 mm. The specific surface area of the packing material was 341 m2/m3 with a void space of 91.25%. The purpose of the top section (i.e. the gas–liquid–solid separator) of the reactor was to allow separation of the biogas and washed out solids from the liquid phase. An inverted funnel shaped gas separator was used to conduct the biogas to the gas collector tank. A gas sampling port was provided in the connecting tubing for the determination of biogas composition. The UASFF reactor was operated under mesophilic conditions (38 °C) and the temperature was maintained by circulating hot water through the reactor jacket. The circulated flow was from a water bath maintained at the desired temperature to ensure isothermal operation. Six sampling ports (S1–S6) were placed at suitable distances along the height of the reactor (Fig. 1). POME as substrate was continuously fed to the reactor through the base using a peristaltic pump (Cole Parmer, Master flex L/s) and the effluent was collected from the top of the column. In order to distribute the feed uniformly in the reactor, an influent liquid distributor was mounted at the base of the column. The top of the UASFF reactor was connected to a water displacement gas meter to measure the volume of biogas produced.