Final Report: NH4 Removal and Reactive Nitrogen RecoveryEPA Grant Number: SU836766
Title: NH4 Removal and Reactive Nitrogen Recovery
Investigators: Grimberg, Stefan J.
Institution: Clarkson University
EPA Project Officer: Page, Angela
Project Period: September 1, 2016 through August 31, 2017
Project Amount: $15,000
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2016) RFA Text | Recipients Lists
Research Category: Sustainability , P3 Awards , 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 commonly is 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 also is important because ammonia is toxic to animals and a potent greenhouse gas.
The final solution is technically effective:
- The ammonia flux achieved through the Nafion membrane was 10.9 mol/m2d from the digestate into the draw solution.
- An average stripping rate of 100mg NH3/L hr was achieved at the lab scale using a 2 LPM air flow.
- Compost mixed with monopotassium phosphate had the best adsorption capacity of the media tested. On average, it had a breakthrough time of 3 min/g media when challenged with 0.4 LPM of ammonia gas at a concentration of 2.03 mg NH3/L.
- Preliminary full scale design of the ammonia recovery process results in a system that has a positive net present value ($64,000 (30-y, i=0.06) and a relatively low payback period (6 years)
Removal of ammonium from digestate using a cation transfer membrane is very effective and results in minimal changes to the biochemistry of the fluid. Maintaining digestate conditions is important because microflora in anaerobic digesters are very sensitive to chemical and physical properties including pH and temperature. Microbes can acclimate to conditions in anaerobic digesters, but changes in these conditions may lead to lower gas production.
After ammonium ions are transferred across the membrane to the draw solution, they are converted into ammonia, which is removed via air stripping and then recovered via gas sorption onto compost media. Since the draw solution contains primarily potassium and sulfate ions, it has a very low buffering capacity so only a relatively small amount of base is required to raise its pH so that ammonium cations can be converted to volatile aqueous ammonia. To allow for rapid liquid-to-gas phase transfer, a minimum surface area to volume ratio of five is required. To achieve this ratio, a moderate to high air flow rate through diffusers was used to create fine bubbles.
Once ammonia gas is removed from solution, it is recovered in a sorption column. Several media were tested to determine their ammonia sorption capacity, including compost, soil, and monopotassium phosphate enriched compost and soil. Monopotassium phosphate enriched compost was found to be the most effective medium for gas adsorption. The addition of ammonia to the compost provides a fertilizer with a better nitrogen:phosphate:potassium (NPK) ratio than regular compost.
The final solution provides benefits through the environmental and economic benefits of reducing food waste disposed in a landfill, creating more biogas that displaces fossil fuels for heating and displacing commercial fertilizer production.