ammonia evaporation (ammonia volatilisation)
Also known as: ammonia loss from digestate
The loss of dissolved ammonia (NH₃) from liquid digestate or manure as a gas when pH rises, temperature increases, or material is surface-applied to soil without incorporation.
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What is ammonia evaporation?
Ammonia evaporation (also called ammonia volatilisation) is the loss of nitrogen from liquid digestate, slurry, or surface-applied manure as gaseous ammonia (NH₃) escaping to the atmosphere. The process is driven by the chemical equilibrium NH₄⁺ ⇌ NH₃ + H⁺, which shifts toward gaseous NH₃ as pH rises above 7.5, temperature rises above 25°C, and surface area increases through spraying or spreading. For Indian CBG plants, ammonia evaporation is the single largest mechanism of fertilizer-value destruction in liquid digestate.
Typical loss rates are large. Surface-broadcast liquid digestate on alkaline Indian soils (pH 8.0–8.5, common across Punjab, Haryana, parts of Maharashtra and Gujarat) loses 20–40% of its ammoniacal nitrogen within 48 hours, and 50–70% within 7 days if not incorporated. Sprinkler-fertigated digestate loses 10–30% of N to in-flight volatilisation from droplets, particularly above 30°C ambient. Open lagoon storage of raw digestate loses 0.5–2% of stored N per day in the first month, falling to 0.1–0.3% per day after a crust forms. Cumulative losses from a poorly managed CBG plant can exceed 60% of the original digester output before the nitrogen reaches a crop.
Mitigation has four levers, each with different cost-benefit. Acidification with citric or sulphuric acid to pH 5.5–6.0 cuts free ammonia by more than 95% but costs ₹150–400 per tonne of digestate plus storage tank corrosion risk. Soil incorporation within 4 hours of surface application cuts losses by 70–90% but requires farmer cooperation and dual passes. Sub-surface injection via slurry tankers cuts losses to under 5% but capex on injection equipment is ₹15–40 lakh and not all soils are suitable. Covered storage with floating membranes cuts pre-application losses by 60–80% at ₹8–15 per m³ of tank volume. Indian CBG plants now routinely combine covered storage plus incorporation guidelines on product labels as the cost-optimal package.
- Loss of ammoniacal N as gaseous NH₃, driven by pH above 7.5, temperature above 25°C, and surface area.
- Surface-broadcast loses 20–40% in 48 hours, 50–70% in 7 days on alkaline soils.
- Mitigation levers: acidification, soil incorporation, sub-surface injection, covered storage.
- Cost-optimal stack: covered storage plus farmer-side incorporation cuts losses to under 15%.
Common questions about ammonia evaporation
Plain-English answers to what people most often ask.
Why does ammonia evaporate from digestate and how can it be prevented?
How much nitrogen is lost through ammonia evaporation from digestate?
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