Ammonia Stripping

Overall Summary

Primary Application

• 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.

Industry Uptake

• Only a few demonstration systems operating on US dairy farms.
• Only a few technology providers have systems commercially available in the US.

Technology Maturity

• 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.

Primary Benefits

• 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).

Secondary Benefits

• 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.

Limitations

• 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.

Other Considerations

• 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.