For centrate with ammonium above 20,000 mg/l, a thermally driven ammonia stripper is a strong candidate when a source of waste heat is available. It uses very little chemical, needs no bacteria and fits on a small site. This article follows a proposal for a European waste facility treating 450 m³ a day of centrate, and shows how five treatment methods were ranked.
The challenge: very high ammonium, little space
The design conditions were demanding:
- Flow: about 450 m³ a day of centrate.
- Influent ammonium: about 21,400 mg/l.
- Effluent target: below 600 mg/l, a reduction of more than 97%.
- Climate: ambient temperatures between about -6°C and 17°C.
- Site: land space was at a premium.
Centrate is the liquid pressed or spun out of digested sludge. It carries most of the nitrogen released during digestion, so concentrations far exceed those in normal municipal wastewater.
The approach: five methods compared
1. Biological removal
Aerated and anoxic reactors work well at lower concentrations. Here they would need a large land area, probably an external carbon source and a stable population of specialised bacteria. Any upset in the bacteria affects performance.
2. pH-driven air stripping
Raising the pH converts ammonium to ammonia gas, which can then be stripped. The drawback is chemical use: large quantities to raise the pH, then more to bring it back down before discharge.
3. Breakpoint chlorination
This is only practical at low ammonia concentrations. At 21,400 mg/l the chlorine demand would be very high.
4. Steam stripping
Steam stripping is feasible, but the process train is more complex and so likely to cost more than the thermal alternative.
5. Thermally driven stripping
Wastewater is heated using waste heat and passed down a packed column against a counter-current air stream. Ammonia moves into the air. Because the process is physical rather than biological, it needs only an anti-foam agent in normal operation and produces no carbon dioxide from the removal itself. The first plant built on this principle removed around 12 tonnes of ammonia a day on a footprint of about 20 m by 20 m.
Thermal stripping needs a heat source. Waste heat from landfill gas, biogas, syngas or a power generation plant is ideal. This design assumed about 1.5 MW of thermal energy would be supplied by the client.
Treating the ammoniated air
Stripping moves ammonia from water into air, so the air stream needs its own treatment. There are three main options:
| Option | Advantage | Disadvantage |
|---|---|---|
| Thermal destruction | Ammonia is destroyed and no by-product is handled | Needs fuel; only viable where surplus gas is available |
| Water scrubbing | Recovers ammonium hydroxide, with minimal emissions because the stripping air is recycled | By-product needs storage and removal; higher heat demand |
| Acid scrubbing | Produces ammonium salts; heat load can be cut by up to about 40% | Needs acid supply and handling |
Water scrubbing was chosen. Without surplus fuel gas, thermal destruction was not viable. Acid scrubbing would have reduced heat demand but added chemical logistics. In this design the scrubber produces about 62 tonnes a day of 15% ammonium hydroxide solution. If ammonia gas is wanted, the solution can be heated and pressurised to drive off the gas and condense it to liquid.
How the plant works
- Centrate is heated using the waste heat source.
- It flows down the stripper column while recycled air flows up.
- The hot treated liquid passes through a heat recovery exchanger, which pre-heats returning air and cools the discharge.
- Ammonia-laden air passes through the water scrubber, where ammonia dissolves until the target concentration is reached.
- Air returns to the stripper. A steam injection step brings it to the correct conditions.
Electrical demand is about 150 to 200 kW.
What to consider for your own project
- Confirm the heat source first. The economics depend on waste heat being available.
- Plan the by-product. Ammonium hydroxide needs storage and a route off site.
- Leave a margin. Do not size the plant too close to the maximum flow. A margin of at least 20% on theoretical maximum flow is recommended.
- Decide on standby. Two full duty-standby plants are not essential. Holding enough spares to recover within about 24 hours may be adequate. Uptime of up to 95% is achievable.
- Allow time for approvals. The proposal allowed up to six months for design approvals on top of roughly 12 weeks of design, 27 weeks of fabrication, 20 weeks of installation and 4 weeks of commissioning.
For UK projects, check discharge consents and permitting requirements with the Environment Agency. See the government guidance on environmental permits.
FAQ
What is centrate?
Centrate is the liquid removed from digested sludge by a centrifuge. It is rich in ammonium, which is why it is often treated separately.
Does thermal ammonia stripping need chemicals?
Only a small amount of anti-foam agent in normal operation, according to the proposal. This is a major difference from pH-driven stripping.
Can the ammonia be reused?
Yes. It can be recovered as ammonium hydroxide, ammonium salts or compressed liquid ammonia, depending on the scrubbing option chosen.
Is waste heat essential?
A heat source is needed. Waste heat from gas engines, boilers or other processes makes the plant economic. Using primary fuel quickly becomes expensive.
Treating ammonia-rich liquor?
Send us your flow, concentration and available heat and we will outline a suitable process.