A thermal ammonia stripper with a thermal oxidiser can remove at least 90% of ammonia from sludge centrate and burn it using biogas as fuel. A technical submission for a large UK wastewater works sized one for an average of about 900 m³/day of centrate, with a heat demand of about 3.5 MW and a design effluent ammonia below 300 mg/l.
The challenge: a short deadline and a long wish list
The water company wanted to know whether the technology could be deployed at a works near a large city. In the first instance, ammonia would be destroyed in a thermal oxidiser. In the longer term, the owner wanted the option to recover it as a commercial product. The submission was assembled at short notice to support a decision.
Design basis
| Centrate | Minimum | Average | Peak |
|---|---|---|---|
| Flow (m³/day) | 479 | 909 | 1,338 |
| Ammonia (mg/l) | 1,200 | 2,200 | 3,300 |
| Ammonia (kg/day) | 1,029 | 2,063 | 3,094 |
The design effluent was about 1,000 m³/day at below 300 mg/l ammonia and 20°C. COD was treated as unchanged, since the volatile organics the stripper removes are not reliably measurable as COD.
Configuration
- Two stripping columns, each rated 50% of throughput, built in a high-alloy stainless steel (256SMO or equivalent).
- A feedwater heater, thermal oxidiser with analysers and gas train, and an economiser.
- Automatic standby on all equipment except the columns and the oxidiser.
- Space reserved (25 m by 25 m) for a future ammonia recovery plant, with the oxidiser designed to be mothballed later.
- Compliance with the Industrial Emissions Directive requirements and the site’s explosion-risk standards.
Performance proof
The guarantees covered effluent ammonia, a thermal energy ceiling of 4 MW, availability of at least 95% and a discharge temperature limit. Proof was through a 28-day performance test for oxidiser emissions and a 7-day reliability test.
Energy and carbon
The heat can be met by about 563 Nm³/h of biogas at 55% methane. Biogas is biogenic and carries no greenhouse gas factor in the assessment. If natural gas were used instead, the submission estimated about 5,300 tonnes of CO₂ a year. Electrical load of about 218 kW gives about 1.9 GWh a year and roughly 444 tonnes of CO₂e at the UK grid factor stated.
Top three risks and opportunities
Risks:
- Price of heat. The plant works best on waste heat.
- Low ammonia. Below about 1,000 mg/l, removal efficiency declines.
- Solids, calcium and magnesium can block packing and raise cleaning costs.
Opportunities:
- Recovering ammonia as an aqueous product.
- Converting recovered ammonia to hydrogen.
- Concentrating dilute ammonia streams to improve recovery.
What to consider for your own project
- Define the dewatering pattern. Centrifuges running 16 hours or 24/7 change buffering needs.
- Keep a recovery route open. Reserving space is cheap.
- Agree test durations for emissions and reliability early.
- Insulate and mix the centrate tank to stabilise the feed.
For emissions rules see the Environment Agency. Compare process selection for sludge liquor.
FAQ
What does the oxidiser do?
It burns the stripped ammonia, with biogas as fuel.
Why two columns?
For redundancy and to match throughput.
Does it remove COD?
The effect on COD is not reliably measurable.
When does it struggle?
With low ammonia, high solids or high calcium and magnesium.
Considering ammonia removal at your works?
Contact us with your centrate data.