Ammonia Removal During Solid Waste Anaerobic Digestion Increasing Energy Generation and Reactive Nitrogen RecoveryEPA Grant Number: SV839352
Title: Ammonia Removal During Solid Waste Anaerobic Digestion Increasing Energy Generation and Reactive Nitrogen Recovery
Investigators: Grimberg, Stefan J.
Institution: Clarkson University
EPA Project Officer: Page, Angela
Project Period: March 1, 2018 through February 29, 2020
Project Amount: $75,000
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet - Phase 2 (2017)
Research Category: P3 Awards , Sustainability , P3 Challenge Area - Water
Anaerobic digestion (AD) is the preferable strategy to reduce the volume of food waste sent to solid waste landfills while simultaneously transforming the degradable organic compounds into valuable and sustainable products. Ammoniacal nitrogen, a product of the metabolism of microbes involved in AD, inhibits biogas production because at high concentrations it is toxic to the microorganisms responsible for producing the gas. To address this concern the overall objective of this project was to develop an ammonia removal and recovery process for food waste digesters to increase the environmental and economic value of the AD system through higher biogas yields and fertilizer production. To achieve this objective, laboratory research and engineering design were used to determine the best method to remove ammoniacal nitrogen from the digester digestate and recover it as a cost effective soil amendment for agriculture systems. Ammonia is commonly used in agriculture as a form of nitrogen fertilizer for plants and mostly comes from the Haber-Bosch process, which is energy intensive and consumes 1-2% of the global electricity supply. Recovery is also important because ammonia is toxic to animals and a potent greenhouse gas.
The overall objective of the Phase II project will be to use the laboratory findings to develop a pilot-scale system that removes and recovers ammonia from the food waste AD slurry, increases the biomethane potential of the digestate, and provides two sources of fertilizer. The current AD at Clarkson University will be modified to operate as two independent systems, one with ammonia removal and recovery and one without, so that the impact of ammonia removal on system performance can be directly determined. At the present, less than 100 pounds of food waste per day is put into the digester. During Phase II, 200 pounds of food waste per day will be added so that there is enough digestate for the two systems to run in parallel. A membrane reactor will be built adjacent to the digester as a separate structure and will be connected to the experimental tank. The membrane reactor will have three channels, each of which will have digestate flowing on one side, and contain draw solution on the other. The channels will be operated in parallel so that two will be removing ammonia from the draw solution at any one time and the other channel will be used for air stripping. The ammonia-rich effluent from air stripping will be pumped into the compost adsorbing bed located outdoors. The bed will be designed so that the flow is distributed throughout the bed allowing adequate retention time for removal by the amended compost.
The efforts made by the Clarkson University team further optimize the lifecycle of an anaerobic digester utilized for food waste management and resource recovery with regards to three sustainability principles: people, prosperity, and the planet.
People: The implementation of efficient anaerobic digestion enhances community awareness of environmental issues while also engaging people who are interested in sustainability and STEM fields. In local New York politics, Governor Cuomo has suggested banning organic material originating from universities from being placed into landfills. Anaerobic digestion is a promising alternative technology to dispose of these organic materials. It meets solid waste regulations, helps people envision a waste free society, and reduces landfill space and aesthetic problems associated with landfills so that communities can grow and develop in other ways. Additionally, the anaerobic digester serves as a facility supporting research of students and faculty. Research generates revenue for universities and provides valuable hands-on experience for future generations.
Prosperity: Ammonia removal and recovery at full capacity will be economically sustainable and result in a 50% increase in net benefits ($64k for 30 year project) without considering the increased value of the biofertilizer produced. The fertilizer produced is of value to members of the local community, as farming is a popular source of inexpensive locally grown food and for some a source of income. Waste is diverted from landfills and saving fuel normally used to transport organic waste to landfills.
Planet: The anaerobic digester embodies the idea of cradle-to-cradle design. Waste is being diverted (at full scale, 600 lbs per day), nutrients are being recycled and recovered, biogas is created and used to fuel a generator, and a useful soil amendment is produced. Additionally, the digester reduces greenhouse gas emissions normally released during landfilling of organic matter, combustion of LP for heat and production of nitrogen fertilizers. Total greenhouse gas reduction is 680 metric tons of carbon dioxide equivalent emissions per year, which is a 150% increase over anaerobic digesters running without TAN removal and recovery. Implementing this P3 project will aid the local community in sustainability efforts while also serving as a model for other universities and communities to follow.
Scale-up experiments will determine optimum operating conditions including digestate and air flow rates. Membrane fouling may be a concern in this system. Therefore, a system will be developed to monitor how the ammonia flux changes with time so that proper cleaning intervals can be determined. Noninvasive approaches to cleaning will be tested including swapping which sides of the membrane are exposed to the draw solution and digestate and using air bubbles for membrane cleaning. Both of these methods would allow for continuous operation of the digester. The data obtained from phase I adsorption experiments indicate that the addition of monobasic phosphate to compost is beneficial for the adsorption process, but the optimum amount has not been determined and will be part of Phase II. Finally, the mathematical model of the system will be updated and an overall life cycle assessment of ammoniacal nitrogen removal and recovery from the anaerobic digester will be conducted to verify the economic and environmental benefits. This assessment will determine the value of a full-scale system where 600 pounds of food waste added per day.