This paper shows the response of a UASB reactor subjected to increasing influent COD concentrations.

The study was carried out using a 10.5 1 UASB reactor continuously operated during ten months, and fed with synthetic wastes prepared daily using glucose, ammonium acetate, methanol and nutrient solution. The. flow rate of 16 l.d−1 was held constant throughout the experiment, corresponding to a hydraulic retention time (θ) of 15.6 h. Four intermediate sampling ports besides the influent and effluent ones allowed a study of the reactor behaviour along its vertical profile.

For average values of the initial COD concentrations ranging from 1780 to 9700 mg.l−1, corresponding to volumetric loading rates (BV) of 2.7 to 14.8 kg COD.m−3. d−1, COD removal efficiencies varying from 98 to 80% were achieved.

It could be concluded that the step-increase of the influent COD affected the reactor performance in two different ways. First, the reactor could assimilate the shock loads due to the step-increase of the influent COD and BV beyond the transient periods following changes in the operation parameters. Second, the overall efficiency decreased gradually with the step-increase of the initial COD and BV.

Kinetic studies indicate that the overall kinetic parameters are probably affected by the step-increase of influent COD concentrations.

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