The influence of sludge discharge frequency on the performance of a full-scale UASB-type reactor has been assessed. The reactor has eight compartments and treats domestic sewage with an 8 hour hydraulic retention time. In each compartment, a different discharge frequency was employed ranging from 1 to 8 months. During the assessment period, the chemical oxygen demand (COD) removal efficiency, effluent volatile suspended solids (VSS) concentration, and cellular yield coefficient (Y) were evaluated. The results indicated that the operational stability increased with increasing discharge frequency, as shown by a higher COD removal and lower effluent VSS. The values of Y decreased when the sludge discharge frequency increased. In practice, good performance can be achieved when the sludge discharge frequency is between 1 and 4 months; COD removal efficiencies of 60–80%, average VSS effluent concentrations of 25–80 mg L−1, and Y values of 0.10–0.18 kg VSS kg CODremoved−1 were obtained.

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