Thick sludge digestion - research design and validation of key process unit operations

Advances in the design of anaerobic digesters have seen a trend towards digestion of thicker sludges in the US, UK and elsewhere. This includes a number of new facilities using bolt on technologies to mesophilic anaerobic digestion such as Cambi Thermal Hydrolysis, Prepasteurisation and Biological Hydrolysis. Cambi Thermal hydrolysis utilises thickenedsludge and the digestion process is fed at high DS%, typically 12-14%DS thereby intensifying the digestion process and reducing reactor volumes.

There is limited experience in the design of digestion systems for such applications including heating, mixing and pumping. As the trend continues towards advanced digestion technology, a greater emphasis will be placed on these basic unit process operations. Monsal have conducted fundamental research into digester heating and mixing technology in collaboration with BHR and Yorkshire water. A number of plants have been designed using design tools generated from the research. This includes the digester mixing systems for a major sludge processing facility in Scotland, which is an advanced digestion facility employing Cambi Thermal Hydrolysis followed by mesophilic anaerobic digestion. This advanced facility treats a design EP of 562,000, processing 16,000 tds/year of hydrolysed sludge utilising two 4000 m3 digesters.

This paper looks at the design of the mixing technology for thick feed sludges for the full scale plant. The background research is examined and reviewed. The conceptual design is examined and reported. The full scale design and testing is also reported and examined being compared with the design stage. The paper looks at the use of fundamental sludge rheologyto design effective systems for thick sludge digestion and reports on the effects of thermal hydrolysis pre-treatment.

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