From plate to field: how a food-waste AD plant turns digestate into water and fertiliser
Short answer: On a food-waste AD plant in eastern Switzerland, a 4-stage thermal vacuum evaporator closes the loop from restaurant leftovers to the field. It takes around 40 m³ of water a day out of the liquid digestate, recovers the nitrogen as ammonium sulphate solution (ASL) for a liquid fertiliser, keeps the nutrient concentrate usable for organic farming and returns all distillate as process water to the plant's own de-packaging line. Reduce. Recover. Reuse. On one site, in operation since early 2020.
In November 2025, around 70 people from the Swiss biomass sector saw this chain on site at a Biomasse Suisse field day titled "Vom Teller aufs Feld" (from plate to field). The organiser's review and all slides are public: field day review on the Biomasse Suisse website.
The chain
Plate: food waste from restaurants and supermarkets, packaged food and by-products from the food industry.
Digester: biogas for power and heat in a CHP unit.
Evaporator: the liquid digestate is evaporated with CHP waste heat. Ammonia from the vapour is bound with sulphuric acid in a scrubber.
Three products: distillate, ASL and a nutrient concentrate.
Field: ASL and concentrate go to farms as fertiliser products; the distillate stays on site.
Reduce: less volume to store, haul and spread
Liquid digestate is mostly water. Every cubic metre the evaporator takes out is a cubic metre that needs no storage capacity, no tanker and no spreading slot. On this plant that is around 40 m³ of distillate a day.
Heat is often the limit on an AD plant. The evaporator has four stages and uses the same heat several times, so each kWh of heat evaporates around 4.5 litres of water. In operation, around 350 kW of CHP waste heat is enough for those 40 m³ a day. Digestate volume goes down even where there is little heat to spare.
What the smaller volume is worth depends on the site. Here, transport including storage costs around €20 per m³ of digestate.
The fertiliser service company that markets the products named the practical pressures at the field day: seasonal demand, storage capacity that is often tight, and a lot of effort to get customers to take product in autumn. A smaller volume eases all three.
Recover: nitrogen and nutrients back to the field
When digestate is evaporated, ammonia moves into the vapour. The evaporator's scrubber binds it with sulphuric acid as ammonium sulphate solution (ASL).
This is a key point for the field: the acid only goes into the scrubber, not into the digestate. The nutrient concentrate is therefore not acidified and stays usable for organic farming. Which products organic farms may use is set by the organic input lists in each country.
A Swiss agricultural service company markets both nutrient products:
ASL is the basis of "brinamon", a liquid nitrogen-sulphur fertiliser (N 8 %, SO3 8.5 %, pH-neutral), used among other things to upgrade slurry. It also goes into custom liquid fertiliser blends.
The nutrient concentrate goes into the organic product line "brinogia", or directly to low-emission spreading.
The service company converted one of its slurry stores into an ammonium sulphate store in 2020, the year the evaporator started.
Reuse: all distillate stays on site
The distillate is used as process water in the plant's de-packaging line. The evaporator runs around the clock, the de-packaging line does not run at weekends. A distillate buffer tank now collects the water produced over the weekend, and the line uses it during the week. Before the tank was built, the weekend share went to the municipal sewage plant. Today none of it does.
Whether distillate can be discharged on another site depends on local limits and on the finishing steps after the evaporator.
Without and with the evaporator
Without evaporator | With evaporator (this plant) | |
|---|---|---|
Digestate volume | Full volume stored, hauled and spread | Around 40 m³ of water a day taken out |
Transport incl. storage (around €20 per m³) | Paid on the full volume | Paid only on the reduced volume |
Heat | Not needed for digestate | Around 350 kW of CHP waste heat, used four times over |
Nitrogen | Stays in the liquid digestate | Recovered as ASL, basis of brinamon |
Nutrient concentrate | No separate concentrate | Not acidified, organic product line brinogia |
Process water | Fresh water and well water | All distillate reused in the de-packaging line |
See it for yourself
The slides of the field day, including the talks of the service company and of MKR Cleanwater, are on the Biomasse Suisse website.
Reference visits and a talk with the operator are possible on request. Measured operating data from the same type of evaporator are in our article "How does an evaporator perform in daily operation?".
FAQ
What happens to the ammonia in the digestate?
It moves into the vapour during evaporation and is bound with sulphuric acid in a scrubber. The result is ammonium sulphate solution (ASL), which is sold as the basis of a liquid fertiliser.
Is the digestate acidified?
No. The acid is only used in the scrubber. The nutrient concentrate stays unacidified and is used in an organic fertiliser line.
What is the distillate used for?
On this plant, as process water in the de-packaging line. A buffer tank stores the weekend production, so no distillate goes to the sewage plant any more.
Does the evaporator need much heat?
It runs on waste heat from the plant's CHP unit. With four stages, each kWh of heat evaporates around 4.5 litres of water, so the volume goes down even when heat is limited.
Can I visit the plant?
Yes, reference visits and a talk with the operator are possible on request.
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