Water Guide - Understanding Ammonium and Ammonia in Water Systems
- Gu Zhouying
- May 28
- 2 min read
Updated: Jun 27

Article contents:
What Are Ammonium and Ammonia?
Ammonia (NH₃) is a pungent, toxic, water-soluble gas with a molecular weight of 17 g/mol. In water, it undergoes a chemical reaction forming ammonium ions (NH₄⁺) with a molecular weight of 18 g/mol. This balance between ammonia and ammonium is heavily pH-dependent:
High pH (alkaline): favors ammonia (NH₃)
Low pH (acidic): favors ammonium (NH₄⁺)
Understanding this equilibrium is key for both environmental monitoring and water treatment technologies.
How Is Ammonia Made?
Industrially, ammonia is synthesized using the Haber-Bosch process, combining nitrogen and hydrogen under specific conditions:
N₂:H₂ ratio = 1:3
Pressure = 250–350 bar
Temperature = 450–550°C
Catalyst = α–iron
Reaction equation: N₂ + 3 H₂ → 2 NH₃
This process is vital for producing ammonia used in fertilizers, chemicals, and water treatment agents.
Ammonia and Ammonium in Nature
In natural environments, ammonia and ammonium arise from biological processes:
Nitrogen fixation: Microorganisms like cyanobacteria and rhizobium fix atmospheric nitrogen into ammonia.
Decomposition: Organic proteins decompose into ammonium through microbial activity.
Nitrification: In oxygen-rich water, bacteria oxidize ammonium → nitrite, → nitrate.
While helpful in nutrient cycling, excessive ammonium leads to oxygen depletion and toxic nitrate buildup, which damages aquatic ecosystems.
Why Ammonia Is Dangerous to Aquatic Life
Ammonia is highly toxic to fish. As pH increases, more ammonia (NH₃) is present, raising toxicity levels. The danger threshold is:
0.5 to 1 mg/L ammonium concentration
Fish kills can occur due to direct toxicity or due to oxygen consumption from nitrification.
Membrane Technology Challenges with Ammonia
In processes like reverse osmosis (RO), this balance causes complications:
Ammonium ions (NH₄⁺) are retained by the membrane
Ammonia gas (NH₃) passes through, contaminating the permeate
This raises conductivity in the treated water, affecting quality.
Best Practices to Mitigate This Issue
Pre-treatment pH adjustment: Lower pH to favor NH₄⁺ formation
Ion removal: Use ion exchange or chemical precipitation to remove ammonium
Post-treatment pH restoration: Raise pH to meet final water specs
This approach ensures both effective filtration and compliance with water quality standards.
Conclusion
Ammonia/ammonium equilibrium is pH-sensitive
Toxicity and oxygen demand increase with ammonium overload
Water treatment systems must adapt pH levels to retain ammonium and block ammonia gas
By mastering this balance, engineers and environmental professionals can improve water safety and filtration efficiency.
FAQs
1. Is ammonia in water always harmful?
Not always, but it becomes dangerous when pH is high and ammonia (NH₃) becomes more dominant, which is toxic to aquatic life.
2. Can ammonium be removed through reverse osmosis?
Partially. Ammonium ions are retained, but ammonia gas can pass through the membrane if the pH is too high.
3. What is the ideal pH to minimize ammonia presence in RO systems?
Lowering the pH to below 7 helps shift the balance toward ammonium, which membranes can better retain.
4. Does nitrification remove ammonium naturally in lakes or rivers?
Yes, but it consumes oxygen and produces nitrate, which can also be problematic in excess.
5. Why is ammonia dangerous in aquaculture?
It interferes with fish respiration and can be fatal even at low concentrations.
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