Digestate and slurry evaporator: how vacuum and MVR evaporation reduce the volume

Michael Köhnlechner

Oct 18

6 min read

A digestate evaporator boils water out of the liquid fraction of digestate or slurry (liquid manure) and leaves a concentrate with the nutrients. A vacuum evaporator (DV) driven by waste heat reduces the volume by around two thirds; an electric MVR reduces it by up to 90 % on thin liquid fractions up to 2.2 % DS. The ammonia is recovered separately as ammonium sulphate solution (ASL).

This article explains which digestate and slurry can be evaporated, how to choose between DV and MVR, what comes out of the plant and what we have measured in daily operation.

Which digestate and slurry can be evaporated?

What goes into the evaporator is not raw digestate or raw slurry but the liquid fraction after solid-liquid separation. In most cases, evaporating the raw material directly does not make sense.

Two limits matter:

Dry solids (DS) in the feed. The DV handles liquid fractions up to 4.5 % DS, the MVR thin liquid fractions up to 2.2 % DS. If the liquid fraction is thicker, a second separation stage brings it down first.
Chloride in the concentrate. Chloride is concentrated together with the nutrients. At an evaporation ratio of 1:10, 1,000 mg/l in the feed becomes a calculated 10,000 mg/l in the concentrate. Up to 15,000 mg/l we evaporate digestate in the DV at a low temperature; above that and below 30,000 mg/l we use MVR evaporators in highly corrosion-resistant materials.

How far a medium can be concentrated depends on chloride, viscosity and fouling. Thin liquid fractions such as filtrate after flocculation or the liquid fraction of pig slurry reached 1:10 to 1:15 in laboratory tests. At Reference Plant Western Germany, food-waste digestate runs at around 1:3, which means around 3 m³ of liquid fraction becomes around 1 m³ of concentrate.

Slurry (liquid manure)

Slurry from cattle and pigs is treated in the same way as digestate: it is separated first, and the liquid fraction goes into the evaporator. When slurry is digested in a biogas plant, the result is digestate, so the same rules apply to both.

The liquid fraction of pig slurry can be thin. In laboratory tests it reached evaporation ratios of 1:10 to 1:15, and on thin liquid fractions up to 2.2 % DS the electric MVR reduces the volume by up to 90 %. That makes the MVR a good fit for slurry on sites without waste heat. Thicker liquid fractions up to 4.5 % DS go to the DV.

For slurry, the ammonia matters twice: it escapes as emissions during storage and spreading, and it is a valuable nutrient. In the evaporator it is recovered as ammonium sulphate solution (ASL). Because the nitrogen comes from manure, this ASL can fall under the EU RENURE rules, where the quality criteria are met and the country permits RENURE.

Vacuum (DV) or MVR: the heat on site decides

Thermal vacuum evaporator (DV). The DV boils the liquid fraction under vacuum at a low temperature and is driven by heat, for example waste heat from a CHP unit, with a flow temperature of 65 to 85 °C. It is multi-stage: the vapour from one stage heats the next. The four-stage DV4000 is designed for 4.3 litres of distillate per kWh of heat. At Reference Plant Switzerland it produced 4.5 to 4.7 l/kWh at full load in 2026.

Electric MVR (mechanical vapour recompression). A compressor raises the pressure and temperature of the vapour so that it heats the evaporator again. The MVR runs on electricity at atmospheric pressure and produces 22.5 litres of distillate per kWh of electricity, measured for the complete plant. That is around 44 kWh per m³ of distillate.

The rule of thumb is simple:

Waste heat available and liquid fraction up to 4.5 % DS: DV.
No waste heat, for example on a biomethane plant without CHP, and a thin liquid fraction up to 2.2 % DS: MVR.

When you compare suppliers, check what the energy figure refers to. Some state kWh per tonne of evaporated water, others kWh per m³ of feed, and some mean heat while others mean electricity.

What comes out of a digestate evaporator?

Concentrate. Phosphorus, potassium and organic nitrogen stay in it, at a much smaller volume. This is what reduces storage, tanker movements and spreading.
Ammonium sulphate solution (ASL). Ammonia evaporates with the vapour. In the vapour scrubber, sulphuric acid binds it as ASL before the vapour condenses. The acid only comes into contact with the vapour; the digestate itself stays free of acid. This is different from acidifying digestate, where acid is added to the entire digestate. Where the nitrogen comes from manure and the quality criteria are met, ASL can fall under the EU RENURE rules.
Distillate. The condensed vapour. It is not simply clean water. Whether it can be reused or discharged depends on the post-treatment, which is matched to the permit. Two operating plants that discharge to a watercourse use biological post-treatment for this.

Measured in daily operation

We publish measured values next to design values. From our reference plants:

Around 40 m³ of distillate on a full operating day. The four-stage DV4000 at Reference Plant Switzerland needs around 350 kW of heat for this. A three-stage line at Reference Plant Western Germany produces about the same quantity from around 490 kW.
Operation without staff on site. At Reference Plant Western Germany, which treats food-waste digestate, more than half of the distillate in the 2023/24 season was produced at night and at weekends, with nobody on site. The plant is monitored remotely.
Idle mode when heat is lost. If heat is briefly unavailable, a DV line waits in idle mode, typically for 60 to 90 minutes, and carries on evaporating as soon as the heat returns. After pauses of up to eight hours it is evaporating again after around 70 minutes on average. The evaporator can therefore follow a flexible CHP schedule.
Cleaning. The DV lines at Reference Plant Western Germany are cleaned chemically (CIP) every 240 operating hours, about every two weeks.

The full data set, with how we evaluated it, is on our page Operating data and evidence.

Operating cost per m³ of feed

We state operating cost per m³ of feed, meaning per m³ of liquid fraction entering the plant. Related to distillate, the same plant would look more expensive, because the distillate is only part of the feed.

Guide values: DV from €5.50 per m³ of feed, MVR from €8 per m³ of feed. The figure for a specific plant comes from the concept.

Proof before the investment

Before we issue a binding offer, we evaporate a sample of your medium in our laboratory. A test on 2 litres shows which evaporation ratio your medium allows, which material the evaporator needs and whether the upstream separation is sufficient. We have carried out dozens of such tests, and in several of them we advised against evaporation or limited the evaporation ratio.

For thin liquid fractions up to 2.2 % DS, our MVR test unit adds continuous operation on your site.

Frequently asked questions

Do I need waste heat for a digestate evaporator?

No. The DV uses waste heat, the MVR runs on electricity. The MVR suits thin liquid fractions up to 2.2 % DS.

Can slurry be evaporated as well as digestate?

Yes. The liquid fraction of slurry after separation is evaporated in the same way. Thin liquid fractions of pig slurry reached 1:10 to 1:15 in laboratory tests, and the electric MVR suits them up to 2.2 % DS.

How thick can the digestate or slurry be?

The evaporator takes the liquid fraction after separation: up to 4.5 % DS for the DV and up to 2.2 % DS for the MVR. The laboratory test shows whether your liquid fraction is suitable.

Is the distillate clean water?

Not automatically. Distillate still carries a residual load. If it is to be discharged, the post-treatment after the evaporator is matched to the permit.

When is reverse osmosis the better choice?

It depends on volume, medium and available heat. Roughly above 200,000 m³ of digestate a year, reverse osmosis can be the right choice, and its concentrate can be evaporated. We compare both in Evaporation or reverse osmosis for liquid digestate.

How large does my plant need to be?

There is no lower limit. One DV4000 line handles up to approx. 100 m³ of feed a day (approx. 34,000 m³ a year). One MVR module handles up to 4 m³/h of feed (approx. 32,000 m³ a year). Several lines or modules in parallel scale to over 150,000 m³ of feed a year.

Terms such as evaporation ratio, chloride in the concentrate or specific energy demand are explained in our glossary. For the technical data of the DV and MVR series, see Vacuum (DV) and MVR evaporators.