Many sludge processing plants employ drying units to produce a final granular product that is safe, easily handled and an acceptable material for agricultural and horticultural use. The structure of the final product varies considerably in size, shape, and structure depending on a range of factors, mainly relating to the type of dryer used. Some dryers are capable of producing a granulate which is stable and suitable for further use, but other systems require the addition of mechanical devices such as pellet mills or pelletisers.
Studies by the animal feed industry have highlighted optimum designs for successful pelletisation, which have been extremely useful to the Biosolids industry. Researchers have suggested that the size of the pellet has significant influence on the wear and durability of the product but have identified formulation as the greatest influence. Moisture content, and a composition of over 1% fats or oils are two of the factors affecting binding properties and stability. The higher quality of pellets are stronger, less susceptible to breaking and dust generation, in short making handling and distribution easier. Numerous tests are available to determine durability using the Pellet Durability Index (PDI), hardness, fibre content, and moisture content of pellets in order to ascertain quality. These factors are examined in the paper and highlight the need for further research into the understanding of the mechanisms at work in the pelletisation of sewage sludge.
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