For ammonia-rich sludge dewatering liquor on a space-constrained site, thermal stripping with ammonia recovery avoids the land, carbon source and nitrous oxide of biological treatment. A proposal for a new UK wastewater works compared six removal routes and chose a thermally driven stripper that returns the ammonia as a 20% ammonium hydroxide product.
The challenge: liquor from advanced digestion
A sludge treatment centre with advanced anaerobic digestion produces dewatering liquor that is high in ammonia. It had to be treated so the discharge met the Environment Agency permit before further polishing at the main works. The plant could be either a liquor treatment plant that removes ammonia through the nitrogen cycle, or a nutrient harvesting plant that captures it. The choice was left open until after planning consent, and each had a strict envelope of limits of deviation: 75 m by 40 m by 9 m high for the first and 50 m by 50 m by 18 m for the second.
| Liquor parameter | Start-up | Design duty | Range |
|---|---|---|---|
| Flow (m³/day) | 335 | 627 | 335 to 790 |
| Ammonia (mg/l) | 1,920 | 1,760 | 780 to 2,850 |
| Ammonia load (kg/day) | 642 | 1,105 | 385 to 1,550 |
The brief required at least 85% ammonia removal or 200 mg/l, whichever was tighter. The proposal offered 95% and 150 mg/l.
Six ways to remove ammonia
- Biological (anoxic, e.g. sequencing batch reactor): large land area, may need a bought-in carbon source, and can form nitrous oxide, which is about 270 times as potent as CO₂ per tonne.
- Anammox: still developing, with slow-growing, sensitive bacteria.
- pH-driven stripping: large chemical use to raise then lower pH.
- Membrane removal: chemicals and power that may not justify the cost.
- Breakpoint chlorination: needs a chlorine ratio of 8:1 by mass and suits only clean water.
- Thermal stripping: chosen.
How the chosen process works
Liquor is filtered, then heated by recovering heat from the outgoing treated liquor and by a final heat exchanger. It trickles down a packed column against a counter-flow of air. The ammoniated air goes to a cold-water scrubber where the ammonia dissolves into ammonium hydroxide, which is further concentrated to 20% (up to 25% is possible). The stripping air is recycled, and a small bleed (about 5%) passes through an acid wash.
The only routine chemical is anti-foam, at about 0.5 litres per hour. Treated effluent leaves below 40°C, with a pH below 9.
Managing scale
Columns scale up with calcium above about 100 mg/l, magnesium above 50 mg/l, and iron or manganese above 0.5 mg/l. Packing is cleaned about every three to six months, or flushed in place with citric acid. A duty and standby column is recommended where limits are exceeded.
Energy and footprint
The heat requirement was about 1.5 MW thermal, equivalent to about 250 Nm³/h of biogas at 60% methane, with 165 to 175 kW of electricity. On the numbers given, that is about 2.2 kWh of electricity per kilogram of nitrogen removed. The proposal also lists equipment weights and shipping origins for embodied carbon reporting.
What the process does not do
It does not remove BOD or suspended solids. Metals and other contaminants pass through to the effluent, because the process only concentrates ammonia into water. Uptime is about 90% without redundancy and close to 100% with it.
What to consider for your own project
- Do you have heat? Thermal stripping suits waste heat or biogas.
- Can you use the product? Aqueous ammonia needs an off-taker and storage.
- Check ammonia strength. Below roughly 1,000 mg/l, percentage removal starts to fall.
- Plan for scale cleaning.
See Environment Agency guidance for discharge permits. Related: thermal ammonia stripping at a wastewater works.
FAQ
Why not biological treatment?
It needs land, may need extra carbon and can emit nitrous oxide.
What is recovered?
Ammonium hydroxide at 15 to 25%, depending on the off-taker.
How often must columns be cleaned?
Typically every three to six months.
Does it remove BOD?
No, impact is minimal.
Treating high-ammonia liquor?
Send us your liquor data for a process comparison.