When waste heat is limited, a thermal ammonia stripper with an acid scrubber can treat sludge centrate and turn the ammonia into ammonium sulphate, a saleable fertiliser salt, instead of destroying it. In a design for a wastewater treatment works handling about 2,000 m³/day of centrate, this route needed around 1.5 MW of waste heat and produced roughly 4.7 tonnes of ammonium sulphate per day.
The challenge: ammonia returning to the works
Centrate is the liquid pressed out of digested sewage sludge. It is rich in ammonia, and when it goes back to the head of the works it adds to the nitrogen load that the biological plant must treat. Taking ammonia out of the centrate stream can relieve the main treatment process.
The design basis in this case was:
- Flow of about 2,000 m³/day
- Ammonium of about 700 mg/l in the feed, with a target below 100 mg/l (an 85% reduction was the stated objective)
- pH of around 8 to 8.5
- Ambient temperatures from 0 to 35°C
The difference between feed and target is about 600 mg/l. Over 2,000 m³ per day, that is around 1,200 kg of ammonia removed per day.
The approach: two decisions
Decision 1: how to remove the ammonia from the water
The proposal looked at biological removal, pH-driven stripping, steam stripping and thermal stripping. Biological removal may need a large land area, an expensive carbon source and specialised bacteria that can upset reliable operation. pH-driven stripping needs large quantities of chemicals to raise and then lower the pH. Steam stripping works, but its process train is more complex and likely to cost more. Thermal stripping, which uses heat rather than chemicals to drive the ammonia into the air, was chosen.
Decision 2: what to do with the ammonia in the air
There are three ways to deal with the ammoniated air:
| Option | Advantages | Considerations |
|---|---|---|
| Thermal oxidiser | Ammonia destroyed, nothing to handle. Heat and destruction in one unit if there is surplus gas. | Needs a fuel supply |
| Water scrubber | Recovers aqueous ammonia, which can be purified further | Needs further processing |
| Acid scrubber | Makes an ammonium salt. The heat load can fall by up to about 40% compared with thermal destruction. | Needs acid delivered to site |
Where biogas is in short supply, cutting the heat requirement matters. The acid scrubber was selected to reduce the need for primary fuel, waste heat and cooling, and because ammonium sulphate has a market value.
How the plant works
- Centrate is heated using an on-site waste heat source and sent to the top of the ammonia stripper columns.
- Hot treated centrate leaves the base of the column and passes through a cooling tower before release.
- Stripping air is drawn through the cooling tower, then topped up with steam to reach the right temperature and humidity.
- The ammoniated air leaves the stripper and enters the acid scrubber, where sulphuric acid captures the ammonia as ammonium sulphate. The salt concentrates in the base of the scrubber and is drawn off as a slurry.
- The cleaned air returns to the stripper for reuse. Because the scrubber reaction gives off heat, the air does not need reheating.
A crystalliser can be added to produce uniform crystals and improve the quality of the product.
Quantities worth knowing
| Item | Approximate daily figure |
|---|---|
| Ammonia removed | 1,200 kg |
| Sulphuric acid used | 3,500 kg |
| Ammonium sulphate produced | 4,700 kg |
| Waste heat needed | 1.5 MW thermal |
| Electricity | 150 to 200 kW |
The acid demand is the main running cost to be weighed against the value of the salt. For this reason the route works best where there is an outlet for the ammonium sulphate, for example to agricultural users.
Reliability and standby
The proposal did not recommend a complete duplicate plant. Instead it suggested carrying enough spares to bring the plant back within about 24 hours of a component failure. Uptime can be as high as 95% if required. It also recommended not sizing the plant too close to the maximum expected flow, and leaving a margin of at least 20%.
What to consider for your own project
- Find your heat source first. Thermal stripping depends on waste heat from engines, boilers or biogas. Without it, running costs can be too high.
- Decide where the salt goes. Ammonium sulphate needs a buyer, or a safe disposal route.
- Plan acid storage and handling. Several tonnes of sulphuric acid a day requires a safe delivery and storage arrangement.
- Add a design margin. A margin of at least 20% on flow avoids sizing the plant right at its limit.
- Compare with destruction. Our post on fuel choices for thermal stripping covers the other route.
Government guidance on anaerobic digestion in the UK is gathered in the Anaerobic Digestion Strategy and Action Plan.
FAQ
What is centrate?
The liquid separated from digested sludge in a centrifuge or press. It usually has a high ammonia concentration.
Does thermal ammonia stripping need chemicals?
The stripper itself needs only a small amount of anti-foam agent. An acid scrubber adds acid, which is the main chemical cost.
How big is the plant?
The first plant built using the technology removed 12 tonnes of ammonia per day on a footprint of about 20 m by 20 m.
Is ammonium sulphate useful?
It is used as a fertiliser. Ammonium nitrate or phosphate can also be made by using a different acid.
Considering ammonia removal from centrate?
Tell us your flow, ammonia concentration and available heat and we will assess the options.