Relief of Undigestion: Optimizing a Biosolids Drying Facility in Response to Major Feedstock Changes

The City of Cumberland, Maryland is a city resting in the Appalachian Mountains on the western side of the state. The City operates the Cumberland Wastewater Treatment Plant (WWTP), an advanced wastewater treatment facility with an annual average design capacity of 15 MGD to meet the needs of its ~20,000 residents. In 2008, the City started the solicitation process for a Heat Drying Facility (HDF) that would process all of the biosolids generated at the WWTP into a Class A EQ product for beneficial reuse. The treatment process at the time included Biological Nutrient Removal (BNR) that was being upgraded to an Enhanced Nutrient Removal (ENR) process, along with an Anaerobic Digester and Lime Stabilization process to treat their biosolids. NEFCO began operations of the HDF in late 2010 and operations went smoothly. In 2013, the City informed NEFCO that there would be an extended shutdown of the Anaerobic Digester for maintenance work. During most of 2013 and 2014, NEFCO processed undigested sludge from the WWTP through the HDF. It is no secret in the biosolids industry that the treatment of undigested biosolids comes with a long list of associated issues. Undigested sludge leads to highly variable feedstock quality that will impact both processing capabilities and product marketing options. The Primary Sludge will be more odorous, will contain a large amount of grit and trash, and has a fibrous nature that will significantly impact the quality of the dried biosolids. Facilities processing undigested will experience finished product with less size uniformity, low product densities caused by high fiber content, stronger odors from both dewatered sludge cake and finished dried product, and higher generation of dust. The undigested sludge will also increase wear on major pieces of equipment and limit throughput of the facility during periods of low density. These issues also coincided with an increased demand on the HDF, as there was no longer any solids destruction from the Digester. This led to a very challenging time for operations with a longer processing schedule, increased staffing, and frequent unplanned shutdowns. The NEFCO team developed approaches to handle these challenges, made it through these issues, and got back to regular operations once the Digester came back online in 2014. In 2020, the City again announced plans to shut down the Digester. However, this time it appeared that it may be a permanent shutdown as the City was struggling with the financial case to repair the Digester for reliable operation. Being a smaller facility, the savings from utilizing the biogas produced by the Digester was not enough to make for an attractive return on investment. Realizing that the approaches used in 2013/14 would not be sustainable for long term operations, NEFCO developed a plan to optimize the drying process to mitigate the issues brought on by an undigested feedstock. The team started by reviewing the data available from the 2013/2014 Digester shutdown and summarizing the processing issues from that period and what was done to address them. The team also consulted with staff in NEFCO's Detroit facility, which also processes undigested sludge, to prepare some lessons learned from their operations that could help with the adjustments in Cumberland. A plan was then developed which focused on a combination of operations and process equipment adjustments, evaluating the major solids handling systems that are part of the drying process and looking for ways to adjust their operation to address the bottlenecks. This included the centrifuge dewatering system, pugmill mixer, product screener, and crusher mill. For each piece of equipment, the team consulted vendors and reviewed O&M manuals from manufacturers to come up with a list of action items to be taken. This list of action items was then assigned to team members and ranked in priority to produce an overall action register to guide the efforts. By far the most important process-related task was to address the issues with the sludge feed to the centrifuge. The shutdown of the Digester had led to excessive amounts of trash being present in the sludge, which was frequently plugging the feed line to the centrifuge. NEFCO worked with their Corporate Leadership to expedite the procurement and installation of a grinder system to deal with this trash problem, which led to a significant improvement in plant operations. One of the greatest benefits of a biosolids drying facility is the final pelletized product, which has significant fertilizer and alternative fuel beneficial uses. There were still several product-related issues, dealing mostly with the poor size distribution and low density of the product. The team continued to work its way through the action register, making incremental changes that eventually began to show improvements. Some of the changes were counter intuitive to what might be expected, such as adjustments to the crusher mill. The crusher is meant to take the oversized portion of the product from the screener and bring them back down in size so that they can be recycled to the front of the process as part of the pugmill mix. The issue that came up in Cumberland was that after the Digester was shut off, the product became more pliable and would get flattened by the crusher instead of breaking apart. These flattened pieces would not mix well or be broken up in the pugmill, leading to an accumulation of them in the recycle bin that would then cause issues for the operators trying to control the level in the bin. The answer here came from the advice from the Detroit facility, which had experienced this in the past. They had increased the gap between the rolls in the crusher to reduce the number of pellets being flattened. It was initially thought that this would cause more issues, allowing larger pellets to make it to the recycle bin untouched. However, this change did have a positive impact on the product being sent to the recycle bin and helped the operators maintain their bin levels. This is just one example of a meticulous process to improve the plant's operations and quality of the final biosolids product. Similar efforts were put into reviewing the operation of the pugmill mixer and screener to try and stop pellets from getting too big in the first place. Figures 1 & 2 below show the significance of these changes, measured by performing a sieve analysis on the product stream before being screened. Throughout the process of working on these tasks, the operators of the plants diligently ran sieve tests on a regular basis, on top of their normal lab work, to understand how the changes they were making were impacting the product quality. The reduction in the amount of material on the #4 and #5 Tyler mesh screens was a major goal of the project that was able to be reached by the hard work of the team. A final achievement of the team was the increased throughput of the facility. Much of the equipment works more efficiently and effectively when operated at its design throughput, so through all the work put in to address the issues from the change to an undigested feedstock the processing capacity of the plant was pushed to a higher level than it ever had been. This has led to much more efficient utilities usage rates and other consumables on a per ton processed basis. The plant also now operates on a similar schedule to when it was processing digested sludge, a welcome relief to the dedicated operating staff at the HDF. The effort to address the change in the sludge in Cumberland has turned into a Continuous Improvement activity for the plant, and the team still regularly meets to work on improving operations at the facility. €ƒ Figure 1: Pellet Size Distribution from before Improvements, shown in % by weight retained on sieve screens measured in Tyler Mesh # Figure 2: Pellet Size Distribution (based on Tyler Mesh #) from after Improvements, shown in % by weight retained on sieve screens measured in Tyler Mesh #