Vacuum evaporators for slurry and digestate
A vacuum evaporator extracts water from digestate or slurry. Under vacuum conditions, water boils at significantly lower temperatures than 100 °C – makes waste heat useable. The process yields a highly reduced concentrate and clean distillate; the nitrogen content is captured as a fertiliser-grade ammonium sulphate solution. MKR offers two processes: the thermal DV series, which utilises existing waste heat, and the electric MVR series with mechanical vapour recompression for sites without waste heat. The ideal solution depends on your heat availability, the input material, and the overall volume.
DV Series - thermal, Multistage Vacuum Evaporators
The DV series uses waste heat from your biogas plant to evaporate digestate – developed for food waste biogas plants and on site since 2013. It operates in multiple stages (1–4 stages) and is modularly expandable. Reduction: up to 70 % of digestate; Heat requirement: 180–600 kW thermal; flow 65–85 °C; Heat efficiency: up to 4.3 litres of distillate per kWh thermal; Operating costs: from 5.50 €/m³; Design: 5,000 to over 40,000 m³ of digestate/year; Models: DV1000 (approx. 30 m3/day, 190 kW) to DV4000 (approx. 100 m³/day, max. 600 kW);
New in 2026: Now also available with a heat pump – for gas injection plants without their own waste heat.
MVR Series – electric, Mechanical Vapour Recompression
The MVR series requires no heat source – it operates entirely electrically using the principle of mechanical vapour recompression. Ideal for locations without usable waste heat. Reduction: up to 90 % of slurry; Energy: approx. 20 l distillate per kWh el. (50 Wh/l); no heat demand; Operating costs: from 7.50 €/m³; Clean recycled water, virtually nitrogen-free and odourless; Zero-liquid operation possible;
How does the technology work?
Both MKR series remove water from digestate or slurry, but in different ways. The DV series evaporates under vacuum, so water boils well below 100 °C, and reuses waste heat across several stages. The MVR series works at atmospheric pressure with no external heat, reusing the energy of its own vapour by recompressing it.
How does a multi-stage vacuum evaporator work? (DV series)
A multi-stage evaporator uses the same heat several times over, so it produces far more distillate per kilowatt-hour than a single-stage unit.
Waste heat from the biogas plant (65–85 °C flow) heats the first stage, where the digestate boils under vacuum. The vapour produced in that stage isn't wasted — it becomes the heating medium for the next stage, which runs at a lower pressure and therefore a lower boiling point.
Across one to four stages this "multiple-effect" principle reuses the same energy step by step, reaching up to 4.3 litres of distillate per kWh of thermal energy. The result is a strongly reduced concentrate, clean distillate, and nitrogen captured as ammonium sulphate solution (ASL).
How does mechanical vapour recompression (MVR) work?
MVR series needs no external heat source — it runs on electricity alone by recompressing its own vapour and reusing that energy.
Unlike the DV series, the MVR series works at atmospheric pressure, not under vacuum.
The digestate is boiled and the vapour it releases is drawn into a mechanical compressor. Compressing the vapour raises its pressure and condensation temperature, so it becomes a usable heat source again that evaporates more digestate and condenses into clean distillate.
Atmospheric operation is the efficiency reason: the vapour stays dense, so the compressor moves far less volume, and it never has to work against a vacuum — it only adds a small pressure boost above atmospheric instead of lifting vapour out of a vacuum. Both effects cut the compressor's energy use. Only the compressor draws power, about 20 litres of distillate per kWh of electricity, which makes the MVR series ideal for sites without waste heat and capable of zero-liquid operation.
What does an evaporator produce?
A vacuum or MVR evaporator turns digestate or slurry into three usable outputs: a nitrogen fertiliser (ASL), clean distillate water and a nutrient-rich concentrate. There is no waste stream - every fraction has a defined use.
Ammonium Sulphate Solution (ASL)
Recovered nitrogen in mineral fertiliser quality (pH 5.5–6.5). RENURE-compliant and immediately usable.
Distillate (clean water)
The separated water is odourless and virtually nitrogen-free. Usable as process or recycled water.
Concentrate
Highly reduced, nutrient-rich concentrate. Suitable as farm fertiliser for soil fertility.
What are the technical specifications of MKR's evaporators?
MKR offers three core models. The DV1000 (3-stage, thermal) takes in 1,320 l/h and produces up to 660 l/h of distillate at 190 kW. The DV4000 (4-stage, thermal) handles 4,300 l/h with up to 2,300 l/h distillate at max. 600 kW, reaching up to 4.3 l distillate per kWh thermal. The MVR15 (electric, mechanical vapour recompression) processes 1,500 l/h with up to 1,350 l/h distillate at about 20 l per kWh of electricity and no heat demand. Full specifications below.
DV1000 (3-stage)
Intake volume flow: 1,320 l/h
Distillate output: 3.5 l/kWh th.
Distillate flow max.: 660 l/h
Heat demand: 190 kW
Flow temperature: 82 °C
Dim.(L×W×H): 10.5 × 2.5 × 4.8 m
Tare weight: 11.2 t
- Ideal for small to medium biogas plants
- Low heat requirement with stable distillate quality
- Compact design, easy to retrofit in existing sites
DV4000 (4-stage)
Intake volume flow 4,300 l/h
Distillate output 4.3 l/kWh th.
Distillate flow max. 2,300 l/h
Heat demand max. 600 kW
Flow temperature 82 °C
Dim.(L×W×H) 24.0 × 3.0 × 5.5 m
Tare weight approx. 38 t
- For large plants with high throughput
- High energy efficiency through four-stage design
- Designed for continuous operation in industrial settings
MVR15
Intake volume flow 1,500 l/h
Distillate output 20 l/kWh el.
Distillate flow max. 1,350 l/h
Heat demand 0 kW
TS-content approx. 2 % TS
Dim.(L×W×H) 4.5 × 2.8 × 3.1 m
Tare weight approx. 8 t
- Electric operation without external heat source
- Very low operating costs per m³ of distillate
- Particularly suitable for limited waste heat availability
Which evaporator is right for me?
With usable waste heat, the thermal DV series; without it, the electric MVR series — or the DV series with a heat pump for thicker media. Try our evaporator calculator for a first recommendation in seconds.
How does vacuum evaporation compare to other digestate treatment methods?
Vacuum evaporation is the only method that does all three tasks at once: it strongly reduces volume, recovers clean water (distillate) and captures nitrogen as a RENURE-compliant fertiliser (ASL) — in a closed, odour-free process. Separation, reverse osmosis, ammonia stripping and drying each solve one part of the problem and are often used alongside evaporation rather than instead of it (e.g. separation upstream, drying downstream for zero-liquid).
Criteria
Volume reduction
Nutrient recovery
Clean water recovered
Energy source
Odour / emissions
Zero liquid possible
Typical role
Vacuum evaporation (MKR)
High — up to 70% (DV) / 90% (MVR)
Yes — ammonium sulphate solution (ASL), RENURE-compliant
Yes — clean distillate, reusable
Heat or Elec.
Closed, odour-free
Yes (especially combined with drying)
Complete treatment: volume + water fertiliser
Solid–liquid separation
Low — only splits solid/liquid
Partial — nutrients split between phases
No
Elec. (Low)
Open — odour remains
No
Pre-treatment step (often upstream of evaporation)
Reverse osmosis
Moderate — limited by fouling/osmotic pressure
Concentrate retains nutrients (no defined product)
Permeate (quality varies; ammonia can pass)
Elec. (High)
Closed
No - concentrate remains
Polishing of low-solids streams
Ammonia stripping
None — targets nitrogen, not volume
Yes — ammonium salt (nitrogen only)
No
Heat + Elec.
Emission control needed
No
Targeted nitrogen recovery
Drying
Very high
Solid fertiliser (possible N losses as ammonia)
No — water lost as vapour unless condensed
Heat (High)
Ammonia/odour unless scrubbed
Yes - solid output
Final concentrate/solids step (often after evaporation)
What each method does
Vacuum evaporation (MKR): The only method that delivers all three at once — strong volume reduction, clean reusable distillate and RENURE-compliant ASL fertiliser, closed and odour-free.
Solid–liquid separation: A useful first step that splits solid and liquid — but it neither reduces the overall nutrient load nor produces clean water (MKR often uses it upstream).
Reverse osmosis: Works on low-solids streams, but high-solids digestate fouls the membranes, needs heavy pre-treatment and leaves a large concentrate.
Ammonia stripping: Recovers nitrogen only — it doesn't reduce water volume and usually needs pH adjustment and energy.
Drying: Produces a transportable solid, but is energy-intensive, can release ammonia/odour, and is often used after evaporation to reach zero-liquid.