Vera M A1 and Lavelle P2
1EIMCO Water Technologies USA, 2 EIMCO Water Technologies Ltd, UK


The stabilization of the solids produced by main liquid stream processes in most wastewater treatment plants is done by anaerobic digestion and it is the preferred stabilization process for plants with flows greater than 100,000 PE.

In order to obtain the full benefits of the anaerobic digestion stabilization process it is important that the digester operates at an optimal rate by achieving a uniform solids concentration and distribution, a homogenous temperature and pH, as well as a suppression of scum or foam and a reduction or elimination of grit deposition. All of these process-related parameters are interrelated because they are factors that can and should be controlled throughout efficient digester mixing.

Grit and scum accumulation in mixed digester systems that do not apply adequate amounts of energy, or provide a flow capable of keeping the biomass in active motion, have led to increased treatment works expenses, in the form of downtime, cleaning and disposal fees. Some plants that do not have the extra capacity to take one digester out of operation for cleaning are forced to handle the operational problems caused by underperforming units until they can resolve the economical and logistics issues that permit them to clean the digester.

In mixing digesters, the elimination or control of grit and scum accumulation requires up to ten times as much mixing energy as is required for the maintenance of good operating conditions (i.e. controlled temperature and solids distribution). This assertion was validated by a review of the conditions found in two digesters of similar size after several years of operation. One of the digesters was mixed with a low flow / low energy system and the other one was mixed with a high flow / high energy mixing system. The state of the two digesters will be reviewed and compared. Also, a CFD analysis will be used to model these digesters and a correlation to the results found in real life will be presented together with an investigation of the differences in designs, their performance and their operational and maintenance advantages and challenges.

KEY WORDS Anaerobic Digester, Foam control, Grit Deposition, Mechanical Mixing, Pump Mixing, Sludge.

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