Self-heating of Dried Biosolids: Causes and Mitigation Options for Drying Facilities

Organic-based fertilizer resources, such as compost and pelletized biosolids, have been known to have the potential to self-heat while in storage or transportation. This phenomenon has been reported since the 1920's, though not all products exhibit this characteristic. Self-heating of product stored in silos at production facilities or in bulk at fertilizer blenders or field application sites can lead to odor and other nuisance conditions if the self-heating can progress to a smoldering event. As the industry leader in biosolids drying, Synagro has done considerable work to understand the mechanisms that cause reheating and to identify methods to mitigate this issue. One major mechanism that can cause reheating is related to chemical reactions that can occur based on compounds present in the biosolids feedstock for the drying process. Specifically, un-oxidized iron transitioning to an oxidized form (an exothermic reaction) post-drying has been identified as an initiator of the self-heating processes. A literature review on the topic of reheating of dried biosolids showed several research papers that pointed to a link between iron and instances of reheating. This research showed that the interaction of iron and sulfur has an impact on the duration and time for onset of reheating events [1, 2]. While these studies were based on biosolids that came predominantly from tanneries, further studies had shown similarity between these tannery biosolids and biosolids from predominantly municipal sources with respect to reheating [3]. Synagro has operational experience with reheating of dried product, and our own internal investigations came to similar conclusions on the iron content of the biosolids leading to reheating issues. Table 1 summarizes details from several facilities on iron and sulfur content of their biosolids, chemical addition at the treatment plants they service, and the extent of reheating experienced at the drying facilities. Biological activity in dried biosolids is also a source of reheating concern based on industry knowledge and research on the topic. Dryer systems are designed to destroy biological activity in the biosolids to meet the regulatory requirements for land application, and data from numerous facilities show that drying systems perform exceptionally well with respect to those standards. Even after the drying process, though, there can be residual levels of micro-organisms remaining that may cause reheating. However, the conditions to trigger biological reheating typically occur over a longer period of time and are more of a concern for longer term storage. There are additional compounding factors that can cause both reactions to occur. Specifically, both the chemical and biological reactions are driven by the presence of oxygen and moisture. Research into moisture impact on dried biosolids reheating showed higher moisture content leading to longer durations of reheating with higher maximum temperatures [4]. The availability of oxygen will also impact these reactions, with oxygen in this case coming through interactions with air. A lab scale study on biosolids reheating found that the presence of moisture alone was not able to cause spontaneous reheating and showed that the introduction of air (oxygen) had an immediate impact on reheating events on samples that had been rehydrated [5]. The exposure of the biosolids to air and moisture can be made worse by the physical characteristics of the dried biosolids produced by the drying process. The geometry of the product being stored can impact the amount of void space in the bulk material, and thus many pathways for air to interact with the product. On a smaller scale, the porosity of the surface of the product can also impact the availability of oxygen for these reactions to occur. It is critical to identify the issue in the preliminary stages of the project and plan accordingly. When evaluating whether a potential pelletized product is likely to self-heat there are several characteristics that can be used to identify the level of concern over these issues, including: - Iron and Sulfur Concentrations - Particle Size Distribution - Product Storage Equipment and Management Techniques - Process Additives - Product Dryness Based on the outlook for reheating for a specific biosolids project, there are several options for addressing any potential concerns. Drying facilities are often designed with nitrogen blanketing systems for key pieces of equipment where dried biosolids are stored in order to keep oxygen levels low to prevent these types of reactions. Some systems may also have oxygen monitoring within the process as well to ensure low oxygen levels on a continuous basis. Additional options for monitoring the process for signs of reheating include thermal imaging cameras and Carbon Monoxide sensors. Each drying system is unique, which will inevitably lead to site specific considerations. The Great Lakes Wastewater Authority's drying facility is a great example of a plant that has been able to proactively identify and address potential areas where reheating may occur through projects to optimize the existing process equipment. The key to these projects was the flow of material through the system and eliminating areas where product may become stagnant within the process. The first project was a modification to the facility's recycle bin, which sometimes had product bridging that could lead to hot spots. The lower portion of the recycle bin where the product is discharged was modified to promote better material flow, which has greatly reduced areas of product accumulation. Another project, which was completed in May of 2023, was a redesign of the ductwork section on the outlet of the dryer. The original design had a lengthy horizontal run (10-15 ft) before a short radius 90° elbow connected to a vertical run of ductwork. Dried product suspended in the process air stream would be carried from the dryer through this ductwork to the roof of the facility, where a cyclone separator would separate the product from the air. It was determined that larger product would fall out of suspension in the horizontal ductwork section, causing a buildup of material at the elbow. In early 2023 it was decided to redesign this ductwork section to address the product build up at the elbow. Computational Fluid Dynamics (CFD) modeling was done to look at the airflow in the ductwork and find an alternative design that would increase air velocity to help keep the product suspended in the air stream. Since the redesigned elbow has been installed, the plant staff have observed practically zero buildup of material in the new ductwork.