A comparison of two different medium scale MBRs (ultrafiltration and microfiltration) using respirometric methods has been achieved. The ultrafiltration membrane plant (0.034 µm pore size) maintained recirculation sludge flow at seven times the influent flow, and membranes were backwashed every 5 min and chemically cleaned weekly. The microfiltration membrane plant (0.4 µm pore size) maintained recirculation sludge flow at four times the influent flow, membrane-relax was applied after the production phase and membranes were chemically cleaned in the event of high trans-membrane pressure. Both technologies showed a similar performance with regard to heterotrophic kinetic and stoichiometric parameters and organic matter effluent concentrations. The influent was characterized by means of its COD fractions and the average removal percentages for COD concentrations were around 97% for both plants in spite of influent COD fluctuation, temperature variations and sludge retention time (SRT) evolution. Both SRT evolution and temperature affect the heterotrophic yield (YH) and the decay coefficient (bH) in the same range for both plants. YH values of over 0.8 mg COD/mg COD were obtained during the unsteady periods, while under steady state conditions these values fell to less than 0.4 mg COD/mg COD. bH by contrast reached values of less than 0.05 d−1.

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