The Greener Solution to Biosolids Incineration: Cincinnati's Switch from Multiple Hearth to Fluidized Bed Incineration

Recently there has been a resurgence of interest in the incineration of biosolids, especially with regard to fluidized bed incineration. The recent installation of a fluidized bed incineration facility at the Mill Creek wastewater treatment plant (WWTP) in Cincinnati, Ohio is a good example of what makes this technology so attractive. As part of the plant's current solids handling and processing improvement plan, three new fluidized bed incinerators were recently installed to replace six aging multiple hearth incinerators. Each of the new incinerators is rated to process 96 dry tons per day of biosolids, providing the Metropolitan Sewer District of Greater Cincinnati (MSDGC) with enough firm capacity to process all biosolids produced at the Mill Creek plant plus up to 82 dry tons per day of additional biosolids from other plants within the District.This paper presents the key factors that led to the decision to install the new incinerators. It also presents actual operational data for comparison between the old multiple hearth incinerators and the new fluidized bed incinerators. This paper will be of value to those who are considering fluidized bed incineration as part of their biosolids management approach. In particular this paper will be of interest to incinerator operators considering the fate of their aging multiple hearth incinerators in light of the current emphasis on reducing fossil fuel use, the desire to be “greener”, and the need to meet stricter emissions control requirements proposed by the USEPA through reclassification of biosolids as solid waste.The Mill Creek WWTP has an average flow of 130 million gallons per day and serves the City of Cincinnati and central Hamilton County. The plant began operation in 1959 and was originally equipped with multiple hearth incinerators. After two major upgrades over the years, the units had reached the end of their useful service life. In addition to increasingly high maintenance and operation costs, it was recognized that the multiple hearth incinerators were no longer meeting the expectations of the community with respect to air emissions and fuel efficiency.The District considered several alternatives for biosolids disposal before ultimately selecting fluidized bed incineration as a replacement for the multiple hearth incineration process. Basedon a life cycle cost analysis and the evaluation of non-economic criteria such as reliability and community awareness, fluidized bed incineration was shown to be the best fit for the District as compared to other biosolids management approaches. As part of this selection, energy recovery from the incinerator exhaust gas for power generation was also evaluated. Given the large capital and operational investment required for waste heat boilers, steam systems, and steam turbine equipment, and the relatively low electric utility rates of this region, the District elected to defer the addition of power generation at that time.Now that the incinerators are operational, actual data for operations costs, emission test results, and carbon footprint have become available for evaluation. Recent test results demonstrate that the new incinerators produce emissions that are in compliance with their air permits and, in some aspects, are significantly less than required limits. Total filterable particulates (PM), for example, have been reduced by over 92 percent compared to previous emissions. The District is also mindful of the potential for changes in regulations, and has included provisions for the addition of mercury removal from exhaust gas using carbon adsorption.With properly dewatered feed material, which is currently achieved by centrifuge dewatering at this plant, the new fluidized bed incinerators can operate autogenously, substantially reducing fossil fuel usage compared to the multiple hearth incinerators that they replaced. Data comparing fuel consumption and carbon footprints for both technologies will also be presented in this paper.The investment made by the District in its new solids processing facility is a testament to the reliability, efficiency, and environmental friendliness of fluidized bed incineration.