Ammonia can be recovered from anaerobic digestate by thermal stripping, followed by absorption in cold water to make a concentrated aqueous ammonia product. A design for a plant on a digestate stream of roughly 400 m³/day aimed to recover about 1,000 kg of ammonia nitrogen per day as a product of more than 8% nitrogen by weight, leaving a treated effluent below 200 mg/l.
The challenge: nitrogen-rich digestate
Food waste and manure digesters produce a liquid digestate with very high ammonia. In this project the feed was the filtrate from an ultrafiltration (UF) stage, which removed most solids before the liquid reached the stripper. The design data included:
- A flow of roughly 400 m³/day (about 105,000 US gallons).
- Ammonia nitrogen of about 3,300 mg/l.
- Suspended solids below 100 mg/l after UF, and total solids below 1%.
- A feed temperature of about 35 to 40°C and a pH of 7.5 to 8.5.
- Excess biogas available as a heat source, but no natural gas, steam or engine cooling heat.
The site was in a cold climate, so freezing temperatures had to be considered for the outdoor equipment.
The approach
Why thermal stripping
The proposal compared five options. Biological nitrification and denitrification needs large tanks, may need an added carbon source, and cannot recover the ammonia. It also forms nitrous oxide, a greenhouse gas that the proposal put at roughly 270 times the warming effect of carbon dioxide per tonne. pH-driven stripping needs large amounts of chemicals. Membrane removal needs chemicals and power that may not be justified. Breakpoint chlorination needs about 8 kg of chlorine for each kg of ammonia and is only suitable for clean water.
Thermal stripping depends on physical chemistry, not biology, and it does not produce sludge or nitrous oxide. The proposal reported 98.5% ammonia removal at an existing plant with a feed of 6,700 mg/l, against a discharge requirement of 100 mg/l.
The process
- Filtration. A final filter prevents solids entering the stripper.
- Heating. A heat-recovery exchanger passes heat from the treated effluent to the incoming flow. A shell-and-tube exchanger supplies the final heat from biogas.
- Stripping. Hot digestate trickles down a packed column against air.
- Scrubbing. The ammoniated air passes to a cold-water scrubber, where the ammonia dissolves to form ammonium hydroxide. The vent air from the scrubber is reused as stripping air, so ammonia is not released.
- Rectification. A final column adjusts the concentration of the product. It is managed by the rate at which the product is drawn off.
Why scaling dominates the design
The packing in a stripper can foul or scale, and managing this decides how reliable the plant is. The proposal listed the dissolved metals that cause trouble, with thresholds above which scaling is likely:
| Component | Scaling risk above |
|---|---|
| Calcium | 40 mg/l |
| Magnesium | 10 mg/l |
| Iron | 0.3 mg/l |
| Manganese | 0.05 mg/l |
Iron can start microbial growth as it precipitates, which makes the build-up worse. When a feed exceeds these limits, a duty and standby pair of columns is recommended, so one can be taken off line for cleaning. In this design, duty and standby columns were provided with a slow acid cleaning cycle. The UF stage upstream is the first line of defence against solids.
Proving the performance
The performance requirements were specific and measurable. They set the feed conditions the plant must receive (feed rate, solids, pH, temperature and ammonia up to 5,000 mg/l) and the outputs it must deliver:
- A concentrated ammonia product above 8% nitrogen by weight, adjustable between 8 and 15%.
- Treated effluent below 200 mg/l ammonia nitrogen and below 200 mg/l suspended solids.
- At least 98% of the feed flow returned as treated effluent.
The test ran over 30 consecutive days, with 24-hour composite samples of the feed, product and effluent. Each composite combined at least six grab samples taken no more than once every two hours. Flows were measured by installed magnetic flow meters and totalised by the PLC. Standard methods were used for solids and ammonia analysis. If the inlet fell outside the agreed ranges, the test was paused.
What to consider for your own project
- Treat solids upstream. A UF stage or equivalent protects the stripper and reduces the cleaning frequency.
- Test your digestate. The offer depended on laboratory analysis of a sample, including the scaling metals.
- Have a heat source. Surplus biogas is the obvious one on a digester site.
- Check the market for the product. Concentration and certification requirements vary by customer.
- Plan for cold weather. Freezing can affect outdoor columns and pipework.
Information on UK anaerobic digestion can be found on the NNFCC Biogas Information Portal.
FAQ
What is aqueous ammonia?
A solution of ammonia in water, also called ammonium hydroxide. It is used as a fertiliser and in industrial processes.
Why not nitrify and denitrify instead?
Biological removal destroys the nitrogen rather than recovering it, needs space and carbon, and can emit nitrous oxide.
Is a standby column necessary?
If the feed contains scaling metals above the thresholds, yes. It allows continuous running during cleaning.
Can the plant be tested before purchase?
Yes. A pilot plant on hire can test the real digestate first.
Recovering nitrogen from digestate?
Contact us with your digestate analysis and flow and we will discuss a design.