Sludge granulation in UASB digesters treating low strength wastewaters at mesophilic and psychrophilic temperatures

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 degrees C) and psychrophilic (20 degrees 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 a: HRT of 2h and organic loading rate (OLR) of 6 kg COD m(-3) d(-1), the percent COD removals being 95% (30 degrees C) and 92% (20 degrees C), while the percent COD converted to methane reached 67% (30 degrees C) and 48% (20 degrees C). The digester operated at 20 degrees 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 degrees C, while rod or bacilli shaped bacteria fixed to a matrix of extracellular polymer were observed in the granules at 20 degrees C The specific methanogenic activity was 0.44 and 0.35 g CH4-COD gVSS(-1) d(-1) for the 30 degrees C and 20 degrees C granules, respectively. Settling velocities from 25 to 50 m h(-1) were measured for the granules from the 20 degrees C digester.