Like all biological wastewater treatment systems, the UASB reactor produces sludge that accumulates in the reactor. Since the storage capacity is limited, the sludge will eventually be discharged together with the effluent, when the reactor is full of sludge. To avoid the discharge of sludge in the effluent, it is necessary that excess sludge be discharged periodically from the reactor before its storage capacity is exhausted. For minimum operational costs it is preferable to have large discharges with a low frequency. On the other hand the discharges cannot be excessive in order to avoid deterioration of the reactor performance after the discharges.

An experimental investigation was carried out at pilot scale to establish the influence of the magnitude of excess sludge discharge on the performance of UASB reactors, treating municipal sewage. UASB reactors were operated at hydraulic retention times (HRT) of 4 and 8 h, and excess sludge discharges of varying magnitude were applied. The performance and operational stability of these reactors after the discharges were observed and the sludge accumulation and unintentional discharge (wash-out) with the effluent were determined. During periods of steady state without excess sludge discharge, the sludge production was determined from the settleable solids in the effluent.

The data show that for discharges of up to 50% of the sludge mass in reactors with HRT = 4 hours and 60% with HRT = 8 hours, the reactor efficiency and operational stability were affected very little and only during the first few days after the discharge. Discharges of up to 80% of the sludge did not cause instability, but a reduction of the COD removal efficiency was observed during 1 to 2 weeks after the discharge. Independent of the magnitude of sludge discharge, the effluent pH remained in the narrow range of 6.8 to 7.0 and the ratio between alkalinity (average 275 ppm CaCO3) and VFA concentration (average of 28 ppm HAc) was always very high so that there was never a danger of souring of the reactor contents. Based on the obtained experimental data an operational procedure for optimisation of excess sludge discharges was developed.

It is believed that along with the favourable natural conditions during the experimental investigation (average temperature of 27°C), the observed reactor stability can be attributed to a large extent to the design of the used phase separator, which is much more efficient than the conventional one. It is possible that results with a conventional UASB reactor would have been different.

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