Request nitrogen fractions with an unambiguous reporting basis
Total nitrogen includes different fractions: organic nitrogen, ammonia nitrogen and, where present, nitrite and nitrate. Organic nitrogen bound to particles may follow the solids retained by UF. Dissolved species need a separate assessment. TSS and total COD alone cannot establish permeate ammonium concentration.
Request total nitrogen, total ammonia nitrogen and pH, together with temperature, solids and conductivity. Check whether results are reported as mg N/L or mg NH₄⁺/L: these are different quantities. For ammonium ion, multiplying mg NH₄⁺/L by 14/18 approximately converts it to mg N/L. Use the same basis throughout the mass balance.
A balance shows why clarification is not nitrogen removal
Consider an original example: 10 m³/h of conditioned digestate feeds UF and contains 1,000 mg N/L of dissolved total ammonia nitrogen. Assume complete passage of this fraction, without volatilisation, reaction or accumulation. Water recovery is 75 %, producing 7.5 m³/h of permeate and 2.5 m³/h of concentrate.
In this model, both outlets remain at 1,000 mg N/L. Permeate carries 7.5 kg N/h and concentrate carries 2.5 kg N/h, adding up to the incoming 10 kg N/h. Even if fully retained solids became four times more concentrated, that concentration factor would not apply to a solute passing through the membrane.
| Illustrative stream | Flow | Ammonia nitrogen |
|---|---|---|
| Feed | 10 m³/h | 10 kg N/h |
| UF permeate | 7.5 m³/h | 7.5 kg N/h |
| UF concentrate | 2.5 m³/h | 2.5 kg N/h |
For RO, distinguish NH₄⁺ from NH₃
Dissolved ammonia nitrogen is distributed between ionic ammonium, NH₄⁺, and unionised ammonia, NH₃. Their equilibrium depends on pH and temperature. The neutral species can pass through RO membranes more readily, so a salt rejection figure does not establish the same nitrogen rejection.
Experimental work by van der Wal and colleagues investigates this pH-dependent separation using model solutions and real digestate. It demonstrates why speciation matters rather than establishing a universal digestate operating point. Adjusting pH also requires assessment of scaling, chemicals, materials and all affected streams.
Select the treatment train around the nutrient outlet
Where process water is the objective, evaluate RO permeate against the actual specification. Another RO pass or a targeted polishing step may be needed. Where the aim is a nitrogen product, assess a dedicated recovery process and its integration with solids concentration.
The IEA Bioenergy report describes several recovery approaches. These are engineering routes to compare rather than interchangeable pieces of equipment. The suitability of stripping, ion exchange or other processes depends on composition and the intended product. Holding nutrients in concentrate redistributes the load; it does not remove that load from the site.
What a combined UF and RO trial should demonstrate
Recirculation can reduce an immediate outlet while increasing an internal load. Close the balance around the entire plant and identify the stream that actually exports nitrogen. If the balance does not close, investigate sampling, inventories and possible transformations before attributing removal to the membranes.
- Paired feed, permeate and concentrate samples, with measured flows and consistent nitrogen reporting units.
- Ammonia nitrogen and total nitrogen in each outlet, alongside pH and temperature during load changes.
- Quality at the proposed recovery and after cleaning, rather than only the best initial result.
- Destinations for nutrients, concentrates and wash water, including any return to the digester.
Common questions
Can solids-free UF permeate be reused directly?
Removing solids addresses one treatment objective. Ammonium, salts, dissolved organics and every other parameter required for the intended use still need assessment.
Does a second RO stage always remove more ammonium?
A stage treating concentrate primarily increases water recovery. Treating permeate again requires a second pass. Nitrogen rejection also depends on pH, temperature and membrane selection.