A secondary clarifier is the final treatment stage of a traditional activated sludge sewage works. It separates solid precipitate material from effluent water prior to discharge. Because of recent changes in environmental legislation, many treatment works in the UK are required to carry increased throughput or meet more stringent effluent quality limits. This means that more clarification capacity is needed. But with land in urban areas scarce and construction costs high, there is an increasing need to maximize the performance of existing units rather than build new ones.

The standard technique for designing a final clarifier is mass flux theory. However, this method uses a one-dimensional settling model and cannot account for the ‘density current’ flow typical in a clarifier.