Live to Dry Another Day: Low Energy Residuals Drying

Potential restrictions on application of biosolids to land related to PFAS and emergence in advanced thermal treatment processes have driven renewed interest in residuals drying technologies, that can reduce mass and produced a dehydrated material from which energy can be recovered. However, every pound of moisture removed from the residuals via direct heating imposes a theoretical latent heat of vaporization penalty of 973 btu. Figure 1 summarizes the additional energy required to remove remaining moisture via direct heating as percentage of dry solids changes, versus a range of potential energy content in residuals, which may reflect a typical range across digested and undigested primary sludge and waste activated sludges. At a high level, according to Figure 1, the typical range of energy content in these materials corresponds to the theoretical energy required for additional moisture evaporation from between 15% and 35% total solids. As such, the amount of theoretical energy required to dry the solids via direct heating may be similar to or even exceed the thermochemical potential contained in the dry solids, such that an energy benefit cannot be realized. This is compounded by conventional drying technologies that typically consume 1,000 to 1,500 btu per dry pound, resulting in drying being an energy intensive barrier to supporting the aforementioned drivers. Moisture must be removed from the biosolids through more efficient methods to maximize the amount of energy that can be usefully recovered. Typically energy requirements for technologies that remove moisture are summarized in Table 1, along with the range of total solids over which the technology is applicable, and the energy requirements for moisture removal on a dry mass basis. Information is provided by existing vendors based on state-of-the-art of each technology at full-scale. Table 1 compares the performance of conventional technologies against the performance of new lower-energy drying technologies that have recently been made commercially available in the USA. These systems, such as low-temperature heat pump augmented dryers and systems that recovery enthalpy of vaporization, can vastly reduce the energy demand required for drying. An example of energy demand for moisture removal from a 5% thickened sludge using conventional technologies versus performance opportunity provided using these newer technologies is provided below. In each case, 19 pounds of moisture must be removed to produce 1 pound of dry solids: - Using centrifugation followed by conventional dryer: Centrifuges can remove 16 pounds of moisture for 160 btu and produce dewatered cake with 25% total solids. If the remaining three pounds of moisture were removed from the centrifuged cake via conventional drying, the total energy required to produce dry solids would be more than 3,000 btu. Sensible heating of the dewatered residuals to combustion temperature requires an additional approximately 1,100 btu, for a total overall energy requirement of 4,100 btu before energy conversion occurs. - Centrifugation is used to produce a 15% pumpable slurry for ~130 btu and then fed to an evaporator with enthalpy recovery, which can remove moisture to 85% total solids for approximately 1,360 btu. The remaining 0.2 pounds of moisture will be removed via direct drying in the downstream process, for 170 btu. As such, the amount of energy required to remove 19 pounds of moisture and produce one pound of dry solids is approximately 1,660 btu. This improved efficiency of moisture removal provides a positive net energy balance compared to the available energy in dried residuals and helps to make drying a positive part of an alternative energy future that can also reduce risk related to cost and allowability of land application and landfilling. This presentation will describe the working principles behind these new technologies, provide case studies of recent deployment in the US and the implications for the near future of residuals drying.