Ammonia recovery from digestate works by stripping ammonia out of the liquid fraction with low-pressure steam, then condensing it into a saleable ammonia water. In a bid for a chicken manure biogas plant in the US Midwest, the design basis was to recover more than 90% of the ammonia in the feed and concentrate it to 19% ammonia water. That protects the digester from ammonia inhibition and turns a problem nitrogen stream into a fertiliser product.
The challenge: chicken manure is rich in nitrogen
Poultry manure is one of the more difficult feedstocks for anaerobic digestion. It is high in nitrogen, and as the proteins break down the nitrogen is released as ammonia. At high concentrations, free ammonia slows or stops the microorganisms that produce methane. The NNFCC Biogas Info portal covers this inhibition effect in its general guidance on anaerobic digestion.
The plant in this enquiry was a renewable natural gas (RNG) facility. Its flow sheet was:
- Chicken manure is pre-fermented.
- A decanter separates the slurry into a solid fraction and a liquid digestate.
- The liquid digestate goes to an ammonia stripper.
- The ammonia-reduced liquid returns to the process, which keeps the digester’s ammonia level down.
- The recovered ammonia becomes an “organic” liquid nitrogen fertiliser.
The stripper therefore had two jobs. It protects the biogas process, and it makes a product.
The approach: what the design basis asked for
The owner’s enquiry gave a heat and mass balance, and the bidder’s response accepted it without deviation. The key figures, rounded:
| Item | Design basis | What it means |
|---|---|---|
| Liquid feed | About 28 tonnes per hour | Roughly 125 US gallons per minute to the stripper feed pump |
| Ammonia in the feed | About 210 kg/h, or roughly 0.75% of the feed by mass | A dilute stream, so most of the work is moving water |
| Ammonia recovery | More than 90% of the feed ammonia | At least about 190 kg/h recovered |
| Product | About 1 tonne per hour of ammonia water at 19% w/w | Roughly 8,000 tonnes a year at the stated duty |
| Steam | Low pressure, up to about 1 bar g | The recovery unit is not a pressure-vessel-heavy design |
The ratio is the useful number. Starting from 0.75% ammonia in the feed, the process concentrates it to 19% in the product, a concentration of about 25 times. Stripping efficiency sets how much ammonia is left in the liquid that returns to the digester, so the guarantee of more than 90% is also a guarantee about digester protection.
Scope and battery limits
The stripper package covered the feed pump, the ammonia recovery unit, and the pumps for ammonia water and process water. The terminal points were set at the outlet of the owner’s stripper feed tank and the inlet of the owner’s ammonia storage tank. In the bid, two items the enquiry listed were treated as not required: a separate cooling system for the stripper, and a tank and pump for dosing a pH-adjustment agent.
That second point is worth noticing. Many ammonia stripping designs raise the pH of the feed with caustic so that ammonia leaves as a gas. Dropping a pH-adjustment tank and pump removes a chemical supply, a hazardous store and a running cost. The enquiry document does not explain the process route in detail, so the reasoning here is limited to what the source states.
Why it works: the practical constraints
Cold-climate operation
The site’s recorded air temperatures ran from about -31°C to nearly +38°C. The enquiry required continuous operation across that range and a winterisation philosophy covering insulation, heat tracing and freeze protection. Process water, steam and condensate lines all need protecting at the cold end of that range, so winterisation shapes the layout as much as the process does.
Ammonia safety
The requirements included hazardous-area classification for ammonia handling and storage, ammonia detection, alarms and emergency shutdown. Off-gas from the recovery unit was routed to an odour treatment system. The ammonia water tank sat in a containment basin and was sized for seven days of production plus 10%, filled to no more than 90%. In the UK, the equivalent starting point for hazardous-area duties is the HSE’s DSEAR guidance.
Storage and load-out
A week of storage lets the plant keep running if road collections are missed. Ammonia water was pumped to the tank and then to a truck loading station. Preventing vapour losses during filling and emptying matters, because lost ammonia is both lost product and an odour source.
Heat
Natural-gas-fired boilers supplied the steam and hot water for the recovery unit and for tank heating. Energy efficiency and low-temperature recovery, including partial vacuum, were named as secondary selection drivers, behind equipment performance and availability.
What to consider for your own project
- Know the ammonia load, not just the flow. A 0.75% stream at 28 tonnes an hour is a very different job from the same stream at a tenth of the flow.
- Define the product. A 19% ammonia water is a regulated product with its own storage, loading and sales issues.
- Decide where the stripped liquid goes. Here it returned to a process water tank before re-mixing, which is what makes the digester protection work.
- Plan for the weather. Heat tracing and freeze protection add scope, not just cost.
- Agree battery limits early. Who supplies the storage tank, the boilers and the odour treatment should be written down.
For thermal stripping in a landfill setting, see our worked example of a thermal ammonia stripping system.
FAQ
Why does ammonia need removing from digestate?
High free ammonia inhibits methane-producing microorganisms. Removing it from the recirculated liquid keeps the digester stable on nitrogen-rich feedstocks such as poultry manure.
What is ammonia water?
It is ammonia dissolved in water. In this design the target was 19% by weight, produced by stripping, condensing and concentrating the recovered ammonia.
Does ammonia stripping need high-pressure steam?
Not in this design. The recovery unit was specified for low-pressure steam, with an operating pressure of no more than about 1 bar g.
Can the recovered ammonia be sold?
The project was built around that aim, as an organic liquid nitrogen fertiliser. Whether a given product can be sold depends on local rules for fertilisers and waste-derived materials.
Planning ammonia recovery for a biogas plant?
Send us your digestate analysis and flow and we will advise on stripper sizing, product concentration and scope.