Harnessing the Power of Dried Biosolids - A Decade of Experience

Introduction: In December of 2008, the City of Buffalo, MN began to beneficially use their biosolids as a fuel source at the Buffalo Wastewater Treatment Facility (WWTF). The biosolids treatment system at Buffalo consists of both thermal drying and thermal oxidation processes to reduce the amount of material leaving the site while minimizing overall fossil fuel consumption. The objective of this paper is to discuss the process, project background and to provide an update after more than a decade of operation. The Buffalo system is one of the only North American WWTFs that uses dried biosolids onsite for energy recovery and has a long successful operational track record. With ever increasing interest in reducing fossil fuel energy consumption and concerns related to emerging contaminants, particularly perflorinated compounds such as PFAS, there is a renewed interest in thermal destruction processes such as the one operated by the City of Buffalo. Project Background: Buffalo, MN is located about 30 miles west of Minneapolis and the City of Buffalo's WWTF consists of a conventional extended aeration activated sludge process designed for hydraulic capacity of 4.3 MGD. Prior to the biosolids system upgrade, the City had a combination of biosolids treatment processes using reed beds and liquid Class B land application. The municipality was receiving complaints associated with hauling biosolids out of the facility and it was increasingly difficult to locate farmers willing to accept the biosolids. Because of the difficulties with the Class B land application program, the city became interested in thermal drying to reduce volume of product and odors at the facility. However, the City was very sensitive to conserving energy, so the typical dryer system energy demand was considered unacceptable. After a review of available technologies including systems in operation in Europe, it was found that belt drying systems were being operated in combination with furnaces at similar capacities required by the City of Buffalo. Biosolids Process Description: The City of Buffalo produces only secondary sludge. The plant contains a large aerated sludge holding tank that provide enough capacity for several weeks of sludge storage. From the aerated sludge storage tanks, biosolids are dewatered to approximately 18-20% total solids (TS) via two belt filter presses. After dewatering, the biosolids are dried to greater than 90% TS in a BioCon belt dryer and then combusted in a reciprocating grate furnace where the fuel value of the dried biosolids is utilized to fuel the drying process by recovering energy from the hot flue gas exiting the furnace. The process is known as the BioCon® energy recovery system (ERS). This process combines a medium temperature belt dryer with a biomass furnace, which uses the dry biosolids as a fuel. The intent is to completely combust all organic material and recover process heat for the belt dryer. This integrated dryer and ERS is depicted in Figure 1. The thermal treatment process consists of several steps which are depicted in Figure 2. Dewatered biosolids are pumped from a storage silo, into the dryer cabinet through oscillating depositors, and onto a slowly moving belt located inside the dryer. Heat is transferred to the biosolids by circulating air between an air to air heat exchanger that recovers energy from the furnace flue gas and the dryer cabinet. Moisture is removed from the drying air by continuously extracting a portion of the air from the dryer, transferring it through a condenser and returning it back to the dryer. A portion of the air from the condenser is removed from the system to maintain a negative pressure on the drying system. The foul air is blended with fresh air and used as combustion air in the burners and furnace eliminating the need for separate odor control systems. Screw conveyers transport dried product from the dryer to a buffer silo that feeds a reciprocating grate furnace. The flue gas from the furnace passes through an air to air heat exchanger, where the energy is recovered for the drying process. From the air to air heat exchanger, the flue gas passes through a dry flue gas treatment system that consists of activated carbon and lime injection and a bag filter before being discharged out the stack. The ash from the furnace is transported via conveyors to a storage hopper. The residuals from the bag filter are transported via conveyors to a two bag packing station. Start-up and Performance Testing: During system start-up several issues were observed. The largest two issues were related to molten ash or clinker formation in the furnace and fouling of the air to air heat exchanger. Optimization of the furnace feed, cycling and temperature control were required to mitigate the clinker issue. The air to air heat exchanger initially had sonic horns meant to prevent ash fouling but this was found to be ineffective and heat transfer efficiency would drop after just a few days of operation. Ultimately, the sonic horns were replaced with pneumatic firetube soot blasters which prevented heat exchanger fouling allowing for long term reliable operation. Other issues that were noted during start-up included modifying the level sensor in the dried biosolids feed bin and replacing the packing on the progressing cavity pumps to a mechanical seal to reduce leakage. The performance testing was conducted in two parts. The first was related to energy consumption and natural gas savings by using the ERS system. The guarantee was based on dewatered total solids and that the natural gas consumption would be 574 British thermal unit per pound water evaporated (Btu/lb H2O evap) at 18% TS feed and 164 Btu/lb H2O evap at 22% TS feed. The energy consumption is compared to 1,400 to 1,600 Btu/lb H2O evap which is expected for a typical dryer system. The performance test which was conducted with 18% TS feed showed approximately 200 to 300 Btu/lb H2O evap was required during steady state operation as depicted in Figure 3. The second part of the performance test was the emission testing. The permit limits, summarized in Table 2, were based on volatile organic compounds (VOC's), particulate matter, and mercury. The performance during the stack testing exceeded minimum requirements and it should be noted that the mercury limits were lower than the limits required for new fluid bed incinerators as shown in Table 2. Operation and Lessons Learned: Since completing start-up, the system has now been in operation for more than a decade. Because of the large volume of storage available onsite and equipment capacity, the plant currently runs the system four to six times per year for extended periods of time which is desired for this process due to the high temperature in the furnace as numerous start and stops would reduce life off the equipment. During start-up, the furnace temperature increases at a rate of 80 degrees per hour up to 1,500 F to limit wear on the refractory. Operation temperatures higher than 1,600 F creates clinkers at the bottom of furnace, resulting in bottom conveyors having difficulties with both the size and hardness of ash chunks that need to be removed. The dryer starts on natural gas but at steady state operation it is supported by heat recovered from the furnace exhaust. Improvements to the system dewatering ultimately reduced fuel requirements as initially determined during system performance testing. The operational experience has shown the system is reliable, but several improvements have been made over the years. Some modifications to the system include: - Replacing mild steel conveyors and aluminum duct with stainless steel, - Replacing ERS grates that had worn air holes that allowed too much short circuiting. Additional modifications were made to have all grates moving in synchronization for better fuel agitation, - Adding a shredder after the rotary valve to improve uniformity of dry product from dryer which improved fuel use in the furnace, - Replaced sprinkler system inside the dryer with stainless steel and replaced solenoid valve with ball valve, - Added mixers to the sludge storage tanks to homogenize liquid sludge prior to dewatering, - Modified polymer system to improve dewatering performance. These improvements and lessons learned from the decade of operation will be discussed in the detail in this paper. Conclusion: The Buffalo ERS system is an example where dried biosolids have been successfully used as a fuel source for energy recovery for over a decade. The process provides many benefits and reduces future risks associated with fuel price volatility and biosolids beneficial use reliability. As the industry continues to focus on reduced fossil fuel consumption and concerns associated with emerging contaminants, particularly PFAS, interest in these types of thermal process is anticipated to increase.