Anaerobic digestion plants allow to reuse manure and other agricultural waste for energy production, both in the form of heat and electricity. This energy is valued as renewable and therefore takes incentives in many European countries.
Still an open question is what to do with digestate produced by these plants, which normally has a COD load in the 50.000 mg/l range and an ammonia load in the 4.000 mg/l range. While it has been recognized that digestate has value as fertilizer, there are limitations in its practical use mainly referring to:
- high volumes, which make big storage necessary for winter accumulation;
- too high nitrogen content for nitrogen sensitive areas.
Cross flow membrane based processes have been developed to tackle these limitations.
Coarse solids components are normally separated from digestate by means f screw separators and centrifuges, because they can be reused for green house farming and other agricultural activities.Liquid that is left normally has 2-5% residual solids concentration, with approximately 1% being soluble salts. Liquid digestate treatment may have different goals:
- Partial nitrogen removal
- Complete purification for water discharge
- Nitrogen and other minerals separation for reuse as concentrated fertilizer.
Best option for any situation must be determined on a case by case basis.
Treatment technology is a combination of different phisical steps, while use of chemicals is normally minimized to allow complete reuse of separated materials in the agricultural cycle. A deep understanding of the application is needed to ensure good technical and economical results.
The following is a list of most used treatment steps:
- Solid removal with screw filter and centrifugation
- Biological nitrification/denitrification
- Ultrafiltration for fine solids removal
- Reverse osmosis for salt concentration
- Evaporation for further salt concentration
It is important to point out that should biological nitrification and denitrification steps be implemented, the approach is very different form that used in standard municipal waste water treatment. Digestate, as a matter of fact, is already coming from a biological treatment that has consumed most of the organic load, while it is heavily loaded with solids and ammonia.
Anaerobic digestion is a promising technology for renewable energy production and to increase efficiency of farming operations. Still, digestate treatment is an open question and it cannot be approached on a standard basis.
Different technologies must be used and membrane based processes can play an important role, above all for higher volume operations where digestate must be purified or where concentrated salts can be produced for use as high value biological fertilizers.
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