Enhanced Cake Dryness and Granulation via Efficient Energy Evaporation of Surface Moisture using Combined Centrifugal Dewatering and Spray Drying Techniques

A decanter centrifuge discharges dewatered cake solids at a speed in the range of 100 miles per hour. In 1992, a theory was developed whereby the particles are extremely fine (less than 1 mm) and discreet as they are discharged and that if these small particles were allowed to fly freely additional surface moisture would be evaporated. The theory was tested over a period of 20+ years and resulted in a technology combining a decanter centrifuge with a spray drying system. This paper presents the theory and results of full scale testing conducted at locations in Germany, Denmark, China, and the UK. The initial tests utilized a modified decanter centrifuge with an oversized solids collection chamber. Traditional decanter centrifuges collect solids in a compact chamber to minimize space. However as the particles are thrown against the walls of the chamber, they collect and agglomerate into large nuggets or balls. These balls have a relatively small surface to volume ratio and don't allow for efficient evaporation of moisture in downstream processes. One can easily observe this effect on a hot, dry day as surface moisture evaporates from a cake pile, but the moisture remains below the crust. (photo) After the initial tests proved the particle size theory, additional tests were conducted using high speed cameras to document how the particles move within the chamber. (photo) Ultimately, full scale units were built and side-by-side tests conducted. (Photo from Randers) The tests included sampling of feed, cake, centrate and polymer on a traditional decanter centrifuge (GEA model CF4000) and with a GEA granulator 4000. The tests were conducted with only ambient temperature air flow, as well as, progressively warmer air (60 - 130 deg C). Charts and graphics to be provided. The relationship of energy input and moisture evaporated proved to be related as one would expect. Sludge dewaterability analysis uses sludge drying curves to determine which type of moisture a traditional mechanical dewatering device can remove. This same type of analysis shows the amount of energy required to evaporate the surface water. (photo of types of water A, B, C and D). Another unexpected result was the impact on fecal coliform regrowth. In addition to meeting the expected dryness and capacity requirements the system proved to provide solids meeting the British Assurance Scheme (BAS) in regards to e-coli requirements during storage. Hundreds of successful samples were analyzed and documented the rate of regrowth was within the limits. As disposal costs increase due to environmental concerns such as PFAS and as sustainable practices continue to advance, the efficient re-use of energy can be applied to the air chamber. Waste heat sources from digester gas, sludge heat transfer, or other means can provide the energy. The paper details the economics of using the waste heat in comparison to hauling or the cost to use additional energy sources. (charts). Re-using the energy of the flying particle and blowing hot air onto the surface before it impacts the wall, dissipates its energy, and grows in size is the basis of the technology advancement. The paper and presentation steps the reader through the process. Interested readers and attendees will be those involved in dewatering, dewatering optimization, drying, sludge hauling, fecal coliform regrowth, PFAS, beneficial re-use, energy efficiency and sustainability.