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.

By augmenting the standard drift flux models in CFX, engineers at MMI have established a set of validated and verified models for clarifier performance. These models include settling algorithms and rheological functions for activated sludge mixtures. The models have recently been used at a number of UK sites to optimize final effluent quality for increased load.

MMI Engineering is a wholly owned subsidiary of GeoSyntec Consultants and provides a range of environmental, geotechnical, hydrological and civil engineering services.

Construction costs can exceed $1 million for a new 22m diameter tank at a water treatment plant.

Concentration profiles through a cross section of the clarifier approaching 8000 mg/l solids in the blanket.

A useful post-processing idea is to track stream lines for the solid phase velocity field.