Once ammonia is stripped from wastewater, you must decide what to do with the ammoniated air. A proposal for a 450 m³/day stripper compared thermal destruction, water scrubbing and acid scrubbing, and chose water scrubbing to produce 15% ammonium hydroxide. It suits sites with waste heat and a use for the product.
The challenge
The design handled centrate at about 21,400 mg/l ammonium, aiming at below 600 mg/l, an 85% reduction target in the brief. That removes about 9,400 kg of ammonia a day. The stripper needs about 5 MW of waste heat and 150 kW of installed power.
Choosing the stripping method
Biological treatment needs land, a carbon source and specialised bacteria. pH-driven stripping needs large chemical use. Breakpoint chlorination suits only lower concentrations. Steam stripping works but has a more complex, costlier train. A thermally driven stripper was chosen.
Three ways to handle the off-gas
| Route | Advantages | Drawbacks |
|---|---|---|
| Thermal destruction | Ammonia destroyed, no chemicals, one unit can raise heat and destroy | Needs fuel, so only practical with landfill gas, biogas or syngas |
| Water scrubbing | Recovers ammonium hydroxide or ammonia gas, minimal emissions as stripping air recycles | By-product to store and remove, more heat needed |
| Acid scrubbing | Recovers ammonium salts, heat load cut by up to about 40%, exothermic reaction | Acid handling and salt product |
Water scrubbing was selected here. Ammonia dissolves in the scrubber water until the target concentration is reached, then is withdrawn. If ammonia gas is wanted, a separator uses heat and pressure to drive it off and condense it as a liquid.
Scope and spares
The supply covered a feed heater, stripping column with packing cleaning, heat recovery from outgoing liquid, a waste heat boiler, air blower, water scrubber with chiller and an optional ammonia recovery loop. The spares included an economiser and exhaust fan, the feed heater and one of each pump. Installation guidance was 16 weeks of a mechanical engineer and 4 weeks of an electrical engineer.
What to consider for your own project
- Match the off-gas route to your fuel and market.
- Do not ignore storage of the by-product.
- Quantify waste heat first.
Related: landfill gas for leachate stripping. See Environment Agency guidance on permits.
FAQ
What is ammonium hydroxide?
Ammonia dissolved in water, here 15% strength.
Can ammonia be recovered as gas?
Yes, with an extra separation loop.
Why not always destroy it?
Destruction uses fuel.
Which route uses least heat?
Acid scrubbing.
Choosing an ammonia route?
Talk to us about your wastewater and heat.