Struvite mainly recovers phosphorus, while OTAR mainly recovers nitrogen. For most high-TAN streams, the two technologies complement each other rather than compete. However, understanding the difference prevents expensive mistakes in technology selection.
Many operators of wastewater and digestate plants consider struvite precipitation for nutrient recovery. However, choosing between ammonia recovery vs struvite precipitation depends on what each technology actually removes. This article explains when struvite is the right choice and when OTAR is preferable. It also shows how the two can work together on complex streams.
What Is Struvite Precipitation?
Struvite is magnesium ammonium phosphate (MgNH₄PO₄·6H₂O), a slow-release fertiliser mineral. It crystallises when magnesium, ammonium and phosphate meet in the right proportions at elevated pH. Precipitation works best on phosphorus-rich streams, typically sewage sludge digestion liquors and food processing effluents. In addition, the product has strong agronomic properties and good market acceptance.
Where Struvite Has Limits
Struvite precipitation is primarily a phosphorus recovery technology. In a typical stream, it removes only 10 to 30% of ammoniacal nitrogen. The reason is stoichiometry, because the reaction needs equimolar phosphate.
However, most high-TAN streams carry far more nitrogen than phosphorus. Examples include digestate reject water, landfill leachate and food processing effluents. As a result, substantial ammonia remains after precipitation and needs further treatment.
When OTAR Is the Right Choice
OTAR suits high-TAN streams regardless of their phosphorus concentration. It fits best where the main challenge is ammonia compliance or nitrogen value recovery. Moreover, OTAR delivers 85 to 99% nitrogen removal efficiency, far above struvite. It also does so at comparable or lower cost per tonne of nitrogen recovered. In addition, OTAR’s modular variants adapt to the energy and product market available at each site.
Combined Approaches
Some streams, such as certain sewage sludge liquors, carry both high TAN and high phosphorus. For these streams, a combined approach is often best. First, struvite precipitation recovers phosphorus and a fraction of the nitrogen. Then, OTAR removes the remaining ammonia load. Organics Group can design integrated treatment trains for complex stream compositions. Therefore, the key is sequencing the technologies to tackle both nutrients efficiently, rather than choosing one.
Should I use ammonia stripping or struvite precipitation for nutrient recovery?
Choose struvite precipitation when phosphorus recovery is the main goal. In contrast, OTAR thermal ammonia stripping suits streams where ammonia compliance or nitrogen recovery matters most. Finally, for streams with high TAN and high phosphorus, use struvite first and OTAR second.
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The OTAR Approach
OTAR (Organics Thermal Ammonia Recovery)
OTAR is a modular platform with over 20 years of operational experience and proven installations in Asia. For operators currently spending on biological nitrogen removal, the most commercially attractive configuration is Variant 2. Which is the water absorption that converts a treatment cost into ammonium hydroxide revenue with no acid supply chain, no salt production and no hazardous chemical handling. Where waste heat is unavailable, Variant 4’s heat recycling system achieves a coefficient of performance exceeding 15, making thermal stripping viable even from cold electricity.
