Digester Cleaning

Digesters can become partially filled with a bottom layer of settled grit and a top layer of floating scum. These accumulations reduce the active volume of the digesters and degrade their performance. When this happens, the digesters must be drained and deposits removed. The cleaning process is usually expensive and time consuming. Therefore, attention should be given to reducing the rate at which grit and scum accumulate, and in making it easy to clean the digesters when it becomes necessary.

Effective removal of grit from wastewater in the headworks of plants is the best preventive approach in reducing the amount of grit entering the digesters. Similarly, separate processing of scum collected from the clarifiers, such as hauling to a rendering plant, can reduce scum accumulation in the digesters. However, grit and scum entering a digester cannot be eliminated completely. Therefore, the design of an efficient mixing system that maintains a homogeneous mixture within the digester and keeps the grit and scum from separating out can practically eliminate the need for digester cleaning.

Providing multiple withdrawal points, as in a waffle bottom digester, can improve the grit removal process. Pipes extending into the upper levels of the digesting sludge can be used to remove floating materials periodically while the digester is in operation, before the materials form a thick mat. One of the advantages of an egg-shaped digester is that because of its small areas at the bottom and top, grit and scum accumulations are minimal, and any amount accumulated can easily be removed. When a digester needs emptying and cleaning, a dedicated and adequately sized drain pump for that purpose can make the cleaning activity faster and easier.

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