7.25(6). Thus, the total volume of the USB reactor, not including the gas-liquid separator, is the sum of the sludge zone volume and the sludge blanket zone volume.

The use of a gas-solids separation system in the upper portion of a USB reactor is claimed to be an essential feature, regardless of the settlement characteristics of the sludge (Forster 1985). An investigation of the effects of hydraulic and organic loading rates on the solids and design values of a particular plant is necessary to prove this claim. An increase in organic loading results in increased gas production, reduced floc density, and a greater tendency for floc flotation. The net result is a greater probability of solids wash-out (Forster 1985). This condition is exaggerated at high hydraulic loading rates. Hence, an evaluation of whether solids wash-out significantly depletes sludge solids and whether the solids concentration is tolerated in the final effluent is necessary to determine if a gas-liquid separator is required.

Researchers have reported that USB reactor performance is limited by the ability of the gas-liquid separator to retain sludge solids in the system (Hamoda and Van der Berg 1984). They maintain that the amount of sludge solids retained in the reactor increases with an increased gas-liquid volume in the separator. Lettinga et al. (1980) detail design information for a gas-liquid separator and the start up guidelines for the USB reactor.

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