Comparison of Different Methods for Evaluating the Hydraulics of a Pilot-Scale Upflow Anaerobic Sludge Blanket Reactor

Four methods commonly used in the literature for evaluating the hydraulics of Upflow Anaerobic Sludge Blanket (UASB) reactors were compared in a pilot-scale UASB reactor (518L). The methods are based on the analysis of the tracer concentration curve in the collection points by using ionic lithium (Li+) as a tracer. The tracer was collected at two points: in the effluent and in the sludge blanket. The methods evaluated were: (i) Qualitative analysis of the Residence Time Distribution (RTD) normalized curve (i.e., dimensionless with respect to concentration and time); (ii) empirical qualitative relations between experimental time and theoretical time by using the time-concentration tracer curve; (iii) dispersion number by using traditional equations; and (iv) dimensional axial dispersion model. The study was performed at theoretical hydraulic residence times (t(0)) of 600, 480, 300, 240 and 180min. The results obtained show that the lack of criteria for defining the flow inside the reactor (plug flow, dispersed-flow or complete-mix flow) yields inconsistencies in methods (ii) and (iii). In contrast, methods (i) and (iv) are consistent with each other and can be considered as complementary. For these last two methods, it was found that there is a dispersed flow in the reactor (effluent), and a complete-mix flow in the region of the sludge bed and sludge blanket. However, method (i) is qualitative and does not relate the flow pattern with the dispersion number or Peclet number that measures the mixing magnitude or the axial dispersion in biological reactors.