Technology Strengths, Weaknesses and Critical Indicators
Ammonia-N can be harvested from manure and digestate using proven chemical/thermal processes:
- Produces a concentrated source of nitrogen from a renewable source
- Requires pre-treatment to remove coarse and fine solids
- Pairs well with anaerobic digesters because digesters convert organic N into ammonium
- Chemical storage required for process chemicals and final product
- Proven technology for nitrogen recovery, odor control, and pathogen reduction
- Only appropriate for digested manure where combination of increased temperature and pH facilitate effective stripping; therefore, all bullets are contingent to this assumption.
- Due to high capital cost, ammonia stripping is appropriate for consideration by farms with a very large number of cows or farms that co-process with poultry manure.
- Pre-treatment to remove suspended solids is required for ammonia stripping; therefore, all bedding materials are acceptable.
- Desirable manures will be high in ammonia-N.
- Ammonia stripping can work in all climates with proper planning.
- Coproduct is dilute liquid manure.
- Optimally, the resulting ammonia product is used to replace commercial fertilizer for on-farm use or sale.
Economic/Return on Investment Considerations
- Capital cost range is in the medium price range compared to other ammonia-N processes (reverse osmosis and biological treatment).
- Estimated annual operating costs are also high due to the chemicals and electricity required.
- No unintended consequences from ammonia-N recovery appear to exist.
- Only a few demonstration systems operating on US dairy farms.
- Only a few technology providers have systems commercially available in the US.
- Ammonia-N stripping is a mature technology that is used extensively in commercial/industrial applications but has seen little uptake in animal agriculture to date.
- Various approaches exist, including air stripping, steam stripping, and membrane, to name a few. Membrane is still experimental and in need of development while air/steam have considerable, practical history within multiple industries.
- Recovery of gaseous ammonia-N from manure for reuse as a commercial fertilizer replacement (gaseous ammonia-N is typically combined with an acid such as sulfuric to create a liquid ammonium sulfate product).
- Odorous emissions from long-term storages are reduced since a portion of the ammonia is removed prior to storage and emitted ammonia is an offensive emission.
- Recovery and reuse of ammonia-N can reduce GHG emissions when process is driven by renewable electrical and heat energy (as compared to conventional nitrogen fertilizer production).
How it Works
- Ammonia stripping is usually operated as a continuous flow but can be operated in batch-mode.
- Solid-free (or nearly solid-free) influent is pumped to a reactor vessel—usually a chemical stripping tower containing plastic media to increase surface area and reaction, although complete-mix and/or plug-flow basin versions also exist.
- Various levels of chemical (a caustic or lime to raise pH), heat (to raise temperature), and/or pre-treatment stripping (to remove carbon dioxide and raise pH) are used to move the aqueous ammonia/gaseous ammonia equilibrium within the liquid manure towards greater gaseous form.
- Air/steam introduced into the vessel is used to strip and transport stripped ammonia-N to a recovery vessel.
- Stripped ammonia-N is recovered in a separate vessel – sulfuric acid or nitric acid is usually used during this process.
- End-product is usually either liquid ammonium sulfate or ammonium nitrate.
- Long-term liquid storage is required until appropriate time for land application.
- Liquid ammonium sulfate can be dried to crystal form.
Pre-treatment and/or Post-treatment Required
- Primary and secondary solid-liquid separation of manure is required to remove suspended solids, especially if a packed-tower is utilized.
- Sand-manure separation is required for influents sourced from sand-bedded stalls.
- Liquid effluent from ammonia stripping can be stored long-term or further processed if significant levels of dissolved phosphorous are present after pre-treatment to remove suspended solids.
- Primary limitations are relatively low amounts of ammonia-N in raw dairy manure, capital and operating cost, storage requirements for recovered ammonia-N product, and lack of agronomic test plots to substantiate benefit to field crops.
- Total manure N recovery rates are estimated to be between 33 and 66% with higher in the range recovery possible with pre-treatment by anaerobic digestion.
- Under the current economic conditions, partnership with poultry operations provide the best opportunity for a successful system due to the comparatively high ammonia-N concentrations.
- Ammonia stripping has no effects on phosphorus or potassium as well as liquid storage volume.
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