Glossary

TERMS AND DEFINITIONS

Terms used in digestate and slurry evaporation, explained with measured data from our plants where we have it. Below the terms you will find common misconceptions and what is actually true.

Our terms

Evaporation ratio

The evaporation ratio states how much feed becomes how much concentrate. At 1:3, around 3 m³ of liquid fraction becomes around 1 m³ of concentrate, which is a volume reduction of around two thirds.

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. In extended tests on food-waste digestate, 1:3 to 1:4 was reached.

Synonyms: volume reduction, concentration factor.

Chloride in the concentrate

What determines the type of evaporator and the material is the chloride content of the concentrate. It equals the chloride in the feed times the evaporation ratio: 1,000 mg/l in the feed becomes a calculated 10,000 mg/l in the concentrate at 1:10.

We concentrate digestate in the DV at a lower temperature up to 15,000 mg/l chloride in the concentrate, in the neutral to alkaline range. For more than 15,000 and up to below 30,000 mg/l, we use MVR evaporators in highly corrosion-resistant materials and keep the pH of the concentrate at 8 or higher. Which evaporator and which material fit a specific case is decided per project, based on the laboratory test.

Laboratory evaporation test

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 laboratory evaporation tests. In several of them we advised against evaporation or limited the evaporation ratio.

Technical availability (VDI 3423)

Put simply, technical availability is the share of planned operating time in which the plant was not stopped by a technical fault. The exact definition and calculation are set out in the guideline VDI 3423, and its wording is what counts.

Reference Plant Western Germany has two DV lines, both multi-stage vacuum evaporators. Its remote monitoring data show an availability of at least 95 % in the 2023/24 evaporation season (October to February) and at least 88 % in the 2024/25 season. The calculation is conservative: every minute with an active common fault signal counts as downtime, even where the cause lay outside the plant, such as a lack of heat.

Design value and measured value

The design value is what a plant is designed for. The measured value is what it achieves in operation. We publish both.

The four-stage evaporator at Reference Plant Switzerland is designed for 4.3 litres of distillate per kWh of heat. At full load of 350 to 370 kW heat it produced 4.5 to 4.7 l/kWh in 2026.

Distillate finishing

For distillate to be discharged to a watercourse, the post-treatment after the evaporator is matched to the discharge permit. Two operating plants with a permit (COD 200 mg/l, BOD5 40 mg/l, NH4-N 10 mg/l) use biological post-treatment.

At the first, COD in laboratory samples from 2015 to 2019 had a median of 32 mg/l and was below the limit in 67 of 68 samples; NH4-N was below 10 mg/l in 59 of 69 samples. At the second, NH4-N was below 10 mg/l in all 19 laboratory samples from 2014 to 2016, with a median of 0.95 mg/l. Nitrogen is the value for which the ongoing analyses most often show that readjustment is needed.

Ammonia recovery in the vapour scrubber

Ammonia evaporates with the vapour. In the vapour scrubber, sulphuric acid binds it as ammonium sulphate solution (ASL) before the vapour condenses. The acid only comes into contact with the vapour; the digestate itself stays free of acid.

ASL is a separate liquid fertiliser. Phosphorus, potassium and organic nitrogen stay in the concentrate. DV and MVR use the same vapour scrubber principle.

Idle mode on heat loss

If heat is briefly unavailable, a DV line waits in idle mode for an adjustable time, typically 60 to 90 minutes, and carries on evaporating as soon as the heat returns. If no heat comes, the plant shuts down automatically.

After pauses of up to eight hours, a line at Reference Plant Western Germany is evaporating again after around 70 minutes on average and reaches 90 % of its output after around 80 minutes. This allows the evaporator to follow a flexible CHP operating schedule.

Operation without staff on site

The evaporators keep running when nobody is at the plant, for example at night and at weekends. Staff look after them during normal working hours, alongside their other duties. The plant is monitored remotely.

At Reference Plant Western Germany, which treats food-waste digestate, staff are on site Monday to Friday in two shifts. In the 2023/24 evaporation season, more than half of the distillate was produced at night and at weekends, with nobody on site. During these hours the lines were evaporating 83 % of the time.

Specific energy demand

Specific energy demand states how much energy an evaporator needs for a given quantity of distillate. We state it as litres of distillate per kWh: per kWh of heat for the DV, per kWh of electricity for the MVR.

The four-stage DV at Reference Plant Switzerland is designed for 4.3 l/kWh of heat. At full load in 2026 it produced 4.5 to 4.7 l/kWh. The MVR produces 22.5 litres of distillate per kWh of electricity, measured for the complete plant. That is around 44 kWh of electricity per m³ of distillate.

Other suppliers state kWh per tonne of evaporated water or kWh per m³ of feed. When comparing figures, always check what they refer to and whether heat or electricity is meant.

Synonyms: specific energy consumption, energy ratio; kWh per tonne evaporated.

Operating cost per m³ of feed (OPEX)

We state an evaporator's 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 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.

Number of stages (effects)

A multi-stage vacuum evaporator uses its heat several times: the vapour from one stage heats the next. The more stages, the less heat it needs for the same quantity of distillate.

A three-stage DV line at Reference Plant Western Germany and the four-stage DV4000 at Reference Plant Switzerland each produce around 40 m³ of distillate on a full operating day. The four-stage plant needs around 350 kW of heat for this, the three-stage line around 490 kW. The right number of stages depends on how much heat is available on site.

Synonyms: effects, multiple-effect evaporation.

Thin liquid fraction

A thin liquid fraction is a liquid fraction with little dry solids, up to around 2.2 % DS. Examples are filtrate after flocculation and the liquid fraction of pig slurry. In laboratory tests, such media reached evaporation ratios of 1:10 to 1:15.

The electrically driven MVR is suited to thin liquid fractions. It reduces the volume by up to 90 %. The DV is designed for thicker liquid fractions of up to 4.5 % DS.

Second separation stage

A second separation stage removes further solids from the liquid fraction after the first solid-liquid separation. It is needed when the liquid fraction leaving the first stage contains too much dry solids or particles that are too coarse for the evaporator. The laboratory test shows whether it is needed. This is then clear before the investment, not during operation.

Choice of material

Which material an evaporator needs depends mainly on the chloride content of the concentrate. Up to 15,000 mg/l chloride in the concentrate, we concentrate digestate in the DV at a low temperature. For more than 15,000 and up to below 30,000 mg/l, we use MVR evaporators in highly corrosion-resistant materials and keep the pH of the concentrate at 8 or higher.

This is decided per project, based on the laboratory test. See also: Chloride in the concentrate.

Direct and indirect discharge

Direct discharge means the treated distillate goes to a watercourse. Indirect discharge means it goes to the public sewer and from there to a wastewater treatment plant. The applicable limits are set by the relevant permit.

For direct discharge, the post-treatment after the evaporator is matched to the discharge permit. Two operating plants with a permit (COD 200 mg/l, BOD5 40 mg/l, NH4-N 10 mg/l) use biological post-treatment for this. See also: Distillate finishing.

Storage and spreading

Evaporation reduces the volume that has to be stored, transported and spread. At Reference Plant Western Germany, which treats food-waste digestate, the DV lines reduce this volume by around two thirds: around 3 m³ of digestate becomes around 1 m³ of concentrate. The rest leaves the plant as distillate and as ammonium sulphate solution (ASL).

Synonyms: storage capacity, tanker movements.

Proof before the investment

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

Biomethane plant without CHP

A biomethane plant without CHP lacks the waste heat a DV runs on. The electrically driven MVR is then an option. It handles thin liquid fractions up to 2.2 % DS. If the liquid fraction contains more dry solids, separation must first bring it down to this level, for example with a decanter. The laboratory test shows whether this works.

Common misconceptions

"Evaporation only works with waste heat."

In fact, the DV uses waste heat, while the MVR runs on electricity. It produces 22.5 litres of distillate per kWh of electricity and suits thin liquid fractions up to 2.2 % DS.

"Distillate is clean water."

In fact, distillate still carries a residual load. If it is to be discharged to a watercourse, the post-treatment after the evaporator is matched to the discharge permit. Two operating plants with a permit (COD 200 mg/l, BOD5 40 mg/l, NH4-N 10 mg/l) use biological post-treatment for this.

"Concentrate can be thickened as far as you like."

In fact, chloride, viscosity and fouling set the limit, and that limit depends on the medium. We test it on your sample in our laboratory before we commit to an evaporation ratio. In several tests this led us to advise against evaporation or to limit the evaporation ratio.

"The ammonia ends up in the distillate."

In fact, ammonia evaporates with the vapour. The vapour scrubber binds it as ammonium sulphate solution (ASL) before the vapour condenses.

"An evaporator needs staff round the clock."

In fact, at Reference Plant Western Germany 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.

"Evaporators foul all the time."

In fact, the DV lines at Reference Plant Western Germany are cleaned chemically (CIP) every 240 operating hours, about every two weeks. That is around 2.5 % of the time in the evaporation season. How often cleaning is needed depends on the medium.

"If the heat fails, the evaporator has to be restarted from scratch."

In fact, 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.

"Laboratory results say nothing about operation."

In fact, before Reference Plant Western Germany was built, we evaporated its digestate in our laboratory to 1:2.5. In operation it now runs at around 1:3. The laboratory test was on the safe side.

"Acidifying digestate is the same as an ASL scrubber."

In fact, acidification adds acid to the entire digestate. In the vapour scrubber, acid only comes into contact with the vapour, and the digestate stays free of acid. Acidified digestate places demands on storage tanks and their approval and can release more hydrogen sulphide. ASL, by contrast, is a separate product, while phosphorus, potassium and organic nitrogen stay in the concentrate.

"Availability is the same as running time."

In fact, technical availability states how much of the planned operating time the plant is not stopped by faults. Whether it actually evaporates during that time also depends on heat, feed, cleaning and maintenance. The two figures therefore answer different questions. The exact definition is set out in VDI 3423.

"An MVR can handle thick digestate too."

In fact, the MVR is designed for thin liquid fractions up to 2.2 % DS. The DV evaporates at a lower temperature and handles liquid fractions up to 4.5 % DS.

"More stages are always better."

In fact, more stages save heat. A four-stage line needs around 350 kW for around 40 m³ of distillate a day, a three-stage line around 490 kW. The right number of stages depends on how much heat is available on site.

"Reverse osmosis is always cheaper."

In fact, it depends on volume, medium and available heat. Roughly above 200,000 m³ of digestate a year, reverse osmosis can be the right choice. Even then, an RO concentrate remains that can be evaporated.

"Raw digestate can be evaporated directly."

In fact, in most cases this does not make sense. What goes into the evaporator is 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 a medium is suitable.

Standard terms

Digestate

The residue from anaerobic digestion in a biogas plant. It contains water, nutrients and organic residues.

Slurry

Liquid manure from livestock farming, mainly from cattle and pigs. As with digestate, it is the liquid fraction after separation that is evaporated. The liquid fraction of pig slurry reached evaporation ratios of 1:10 to 1:15 in laboratory tests. When slurry is digested in a biogas plant, the result is digestate. Synonyms: liquid manure.

Liquid fraction

The liquid part of digestate after solid-liquid separation. Only this fraction goes into the evaporator, not the raw digestate. Synonyms: liquid digestate, centrate, reject water.

Separation

The mechanical separation of digestate into a solid and a liquid fraction, for example with screw presses or centrifuges.

Concentrate

What remains after evaporation. Phosphorus, potassium and organic nitrogen stay in it, at a much smaller volume.

Distillate

The condensed vapour after the vapour scrubber. Whether it can be discharged or reused depends on the post-treatment. Synonym: condensate.

Vapour

The vapour produced when the liquid fraction boils. It contains water and volatile substances such as ammonia.

Ammonium sulphate solution (ASL)

The nitrogen fertiliser produced in the vapour scrubber, where sulphuric acid binds the ammonia from the vapour. ASL is a separate liquid product.

RENURE

Short for "REcovered Nitrogen from manURE": nitrogen from manure that may be used like mineral fertiliser. ASL from the vapour scrubber falls under the RENURE product type "ammonium salt (scrubbing salt)" under Directive (EU) 2026/288. This applies only where the nitrogen comes from manure and the quality criteria are met, and only in countries that permit RENURE.

CIP (cleaning in place)

Chemical cleaning of the evaporator without dismantling it. At Reference Plant Western Germany it takes place every 240 operating hours.

Dry solids (DS)

The share of solids in a liquid, in per cent. Feed limits: DV up to 4.5 % DS, MVR up to 2.2 % DS.

Thermal vacuum evaporator (DV)

An evaporator that boils under vacuum at a low temperature and is driven by heat, for example waste heat from a CHP unit. At Reference Plant Western Germany the stages evaporate at around 50 to 63 °C.

MVR (mechanical vapour recompression)

A compressor raises the pressure and temperature of the vapour so it can heat the evaporator again. The MVR runs on electricity, at atmospheric pressure, and produces 22.5 litres of distillate per kWh of electricity.

MVR and TVR

Both compress vapour so it can be reused as heating steam. MVR uses an electrically driven compressor, TVR (thermal vapour recompression) a steam-jet compressor driven by motive steam.

Forced-circulation falling-film heat exchanger

A pump circulates the liquid fraction, which runs down the heating surfaces as a thin film. MKR's DV and MVR both use this principle.

Flow temperature

The temperature of the heating water entering the evaporator. The DV works with a flow temperature of 65 to 85 °C.

Fouling

Deposits on heating surfaces that reduce heat transfer. At low evaporation temperatures, far less fouling builds up.

Boiling point elevation

Dissolved salts raise the boiling point of a liquid, increasingly so as it is concentrated. This reduces the usable temperature difference in the evaporator.

Foam and non-condensable gases

When heated, digestate can foam and release gases such as CO2. Gases that do not condense must be removed, otherwise they impair condensation and vacuum.

NH4-N (ammonium nitrogen)

Nitrogen dissolved as ammonium. It is available to plants and can be driven off as ammonia.

COD and BOD5

Parameters for the organic load in water. COD (chemical oxygen demand) covers everything that can be chemically oxidised, BOD5 (biochemical oxygen demand over 5 days) the biodegradable part.

Reverse osmosis (RO)

A membrane separates water (permeate) from dissolved substances (RO concentrate). For digestate it needs extensive pre-treatment, and a concentrate remains that can be evaporated.

Ammonia stripping

Air or steam drives ammonia out of the liquid, usually at raised pH or temperature. Unlike evaporation, this removes only ammonia, not water, so the volume stays almost the same.

Zero liquid discharge (ZLD)

Treatment in which no liquid effluent leaves the site. Together with a dryer for the concentrate, we offer ZLD.

Waste heat and CHP

A CHP unit produces electricity and heat. Heat that is not needed on site can drive a DV.