Alternate: Do I Need to Dry The Cake? An Overview of Sludge Drying Technologies and Drivers for Implementation in the Carolinas

With rising costs and increased pressure on landfilling and land application of biosolids, many utilities across the country and in the Carolina's are evaluating if biosolids drying is the right option for them. In fact, several North Carolina and South Carolina utilities have already implemented drying, are planning to implement drying soon or are currently evaluating drying options. The paper will provide an overview of drying technologies available, summarize experiences and lessons learned in the Carolinas, and highlight business case considerations to help utilities decide if drying is right for them. The presentation will be of interest to utilities and parties considering drying. Drying is a physical separation process aimed at significantly reducing the moisture content of dewatered sludge. Drying is normally the last step in biosolids processing before final disposition, except for when processes such as thermal oxidation, gasification, or pyrolysis are used. More modern biosolids dryers come in several types, all of which operate with the goal of decreasing water content in wastewater sludge. Drying is used in combination with a dewatering process. Dryers are typically fed with dewatered sludge at approximately 15-30% Total Solids (TS) and dry the biosolids to greater than 90% TS. Drying options include thermal drying as well as more passive drying such as solar or biodrying. This presentation will provide an overview of drying options available and how to how a utility may decide if drying and what type of drying is right for them. Thermal drying operates at elevated temperatures and typically requires a fuel or external heat source to operate. Most types of dryers are classified as direct or indirect. Direct dryers employ a heated air stream that makes direct contact with the solids using convective heat transfer, while indirect dryers employ a heat medium that transfers heat to sludge through another intermediate device using conductive heat transfer. Thermal drying is recognized as a process which produces Class A biosolids that can be beneficially used for agriculture or potentially higher end markets. Different types of thermal dryers are used by several utilities in the Carolinas. Rotary Drum dryers are in Winston Salem, NC; Cary, NC (South Plant); Aiken, SC and Sumter, SC and several have been in operation now for over a decade. Belt Dryers are used in Cary, NC (Western Wake); Mooresville, NC; and Kinston, NC. An indirect paddle dryer was recently commissioned in Orangeburg, SC. For most of these facilities, the final biosolids are marketed as Class A and either given away or sold. Pictures of the Orangeburg, SC indirect drying facility are provided in Figure 1 and picture of the Mooresville, NC belt drying facility are provided in Figure 2. Currently most of the drying facilities in the Carolinas operate with natural gas as the main fuel source but it is possible to use the biosolids as the energy source for drying either through biogas produced from anaerobic digestion or using the dried biosolids as a fuel source in a combustion/gasification/pyrolysis system. Using the dried biosolids as a fuel source may also provide ability to destroy PFAS compounds if future regulations limits the market for the dried Class A Biosolids as a soil amendment or agricultural product. Although no combustion/gasification/pyrolysis facilities are currently in operation in the Carolinas, there are a few reference facilities in operation in North America with several more currently in design, construction or start-up. One facility in Buffalo, MN, for example, has been using their dry biosolids as the main fuel source for drying now for over a decade. For utilities with drying or considering drying, being able to add a downstream combustion/gasification/pyrolysis process provides potential future proofing options to deal with biosolids market conditions as well as volatility with future fuel prices. Solar dryers or green house dryers convert radiative solar energy into warm air for biosolids drying. Solar dryers are constructed as greenhouse-type buildings using transparent plastic panels. Dewatered biosolids are spread on the floor and periodically rotated using automatic or manual windrow turners. The solar drying process can dry dewatered biosolids to 75 percent TS, with drying efficiency and viability impacted by local climate conditions. Solar dryers are currently being planned in Summerville, SC to reduce hauling costs and tipping fees and increase compressive strength and stack ability for landfilling as landfill costs have nearly tripled over the past few years. Summerville does not plan to land apply the biosolids because of future potential PFAS and regulatory uncertainty concerns. A biodryer uses the inherent exothermic energy in biosolids to promote drying. Biodryers are similar to in-vessel composting facilities except that they do not use an amendment as required for composting. Biodrying can be used as the last step in biosolids processing, or it can be used in conjunction with a downstream thermal process such as pyrolysis. No Biodryers are currently in operation in the Carolinas, but they could be a suitable option. A facility with pyrolysis is currently in operation in Silicon Valley, CA and other facilities are being planned in California and Pennsylvania. The presentation will provide an overview of the state of the industry for drying and highlight recent innovations in the industry. The presentation will summarize the current drying experience in the Carolinas and innovation experiences from other areas that could be applicable to Carolina facilities. The presentation will also include business case considerations for implementing drying. Figure 3 provides an example case study comparing drying to Class A and Class B digestion and how drying becomes preferred as hauling and tipping fees increase.