Sludge Granulation in UASB Digesters Treating Low Strength Wastewaters at Mesophilic and Psychrophilic Temperatures

Abstract

This work reports a comparative study of the granulation process during the start-up of UASB digesters treating a diluted wastewater (500 mg COD l−1 as sucrose) at mesophilic (30°C) and psychrophilic (20°C) temperatures. The results show that the granulation process follows a similar pattern at both temperatures, complete granulation being achieved between 1 and 2 months from the start-up, at upflow superficial liquid velocities of 0.05 to 0.15 m h−1 and hydraulic retention times (HRT) of 6 to 3h. The UASB systems showed excellent stability and high treatment efficiency at HRT of 2h and organic loading rate (OLR) of 6 kg COD m−3 d−1, the percent COD removals being 95% (30°C) and 92% (20°C), while the percent COD converted to methane reached 67% (30°C) and 48% (20°C). The digester operated at 20°C maintained similar efficiencies when the OLR was increased up to 9 kg COD m−3 d−1, at an HRT of only 1.3h. The developed granules were similar in shape and size (2 to 3mm diameter), although different microbial morphologies were observed. Filamentous Methanothrix -like bacteria were predominant in the sludge cultivated at 30°C, while rod or bacilli shaped bacteria fixed to a matrix of extracellular polymer were observed in the granules at 20°C. The specific methanogenic activity was 0.44 and 0.35 g CH4-COD gVSS−1 d−1 for the 30°C and 20°C granules, respectively. Settling velocities from 25 to 50 m h−1 were measured for the granules from the 20°C digester.