In this paper, a mathematical model has been derived to predict the granulation time of anaerobic sludge in UASB reactors. In the proposed model, some physical, chemical and biological parameters affecting the granulation phenomena have been considered. To validate the model, 12 pilotscale experiments in 4 UASB reactors are carried out and the results are discussed here. The reactors are started up at different environmental conditions and the granulation time in each experiment is determined. The results show that the model is able to explain different mechanisms involved in the granulation process.

A model describing the main processes occurring in the UASB reactor was developed; in order to maintain simplicity and applicability of the model, only the fundamental aspects were considered. In the model, the reactor is divided in several well-stirred reactors coupled in series and it comprises substrate degradation, biomass growth and the reactions that take place within the granules. The important contribution of the paper is the development of a model taking into account the mass transfer through the film around the granules, the intra-particle diffusion, and the degradation reaction. The model enables the determination of the removal efficiency of the substrate and the increase of both the height of the sludge bed and the granule size with time. The simulated results of an experimental UASB reactor treating sugar-cane mill wastewater were found to be in good agreement with the performance of the reactor. The sensitivity analysis shows that the performance of the reactor is determined by several parameters. The most important parameters are: the bioconversion rate, the mass transfer coefficient in the film, the intra-particle diffusivity, the volumetric fraction of biomass in the reactor, and the number of CSTR considered. These parameters should therefore be carefully determined. The model could be a useful tool in the optimization and development of UASB reactors.

This paper presents the results of an investigation into the treatment of Tehran Refinery effluents in UASB reactors. Four pilot-scale UASB reactors were built with similar dimensions, each with a volume of 45.78 l, and operated in parallel at 37 ºC. The sludge seed was prepared from the waste activated sludge of the wastewater treatment unit of the refinery. The organic loading rates were gradually increased from 0.05-0.1 kg COD/m3.d to about 2, 1.5, 0.5 and 1.5 kg COD/m3.d for reactors 1 to 4, respectively, at an influent COD of about 220 mg/l. This is in accordance with hydraulic retention times of 2.5, 4.5 and 8.5 hours for reactors 1 to 3 and 4.5 hours for reactor 4. Methanol was initially added to the reactors in order to increase the microbial activity of the sludge, except for the last one, in order to compare the effect of methanol on start-up. The  addition of methanol was stopped after 37 days and the reactors were operated for another 30 days. At this stage, the influent COD was varied between 50-300 mg/l. The results show that the COD removal efficiencies of the reactors are around 30-50%. However, when the influent COD increases above 200 mg/l, the COD removal efficiencies increase up to 70%. It is found that methanol has no significant effect in shortening the start-up period in this case. Scanning Electron Microscopy (SEM) examination and image analysis of the granules of sludge were also performed in order to investigate the structure of granules and the size distribution. Analysis of the biogas shows more than 90% methane content. The results obtained in this research are promising for the anaerobic treatment of refinery wastewaters in UASB reactors.

Direct discharge of high-strength industrial effluent often upsets municipal wastewater treatment plant processes. This pilot study was undertaken to evaluate the performance of a UASB system in pre-treating effluent from a medium-size traditional slaughterhouse. Experiments were conducted in a continuous flow 500-1 pilot plant initially inoculated with 2001 municipal anaerobic digested sludge. With an influent COD concentration of 3000-5000 mg/I. the system was started with a loading of 1.8 kg COD/m3.d (F/M of 0.24 kg COD/kg VSS.d) at 25°C. The granules formed after 4 months of operation were dark brownish with a diameter of 1-4 mm and a settling velocity of 20 m/hr. Once granules were formed, up flow velocity was gradually increased to 0.8-1 m/hr (HRT of 2.5 hr) and VSS concentration reached 25 g/I. It was possible to increase the loading up to 14 kg COD/m3.d (F/M of 1.4 kg COD/VSS.d) at 29°C with total COD removal efficiencies of 85-90%. Under these conditions 250-350 I gas (75% methane) was generated for each kg COD removed.