Evaporation can reduce the volume of digestate and sludge, making these difficult residues easier to transport, treat or use. Mechanical vapour recompression, or MVR, attracts attention because it recycles evaporated steam and runs mainly on electricity, often lowering operating costs compared with boiler-fed thermal evaporation.
The economics change when the feed is thick, viscous or prone to fouling. An MVR compressor usually raises steam temperature by only 8–15°C, so the process needs a large heat-transfer area. Digestate may therefore require oversized heat exchangers and pumps, raising capital cost and eroding some of the expected energy savings.
Suspended solids make the material more viscous as water is removed and often have to be filtered before evaporation. Digestate also commonly contains 2,000–3,000 ppm of ammonia. Acid dosing can lower pH and keep ammonia from entering and damaging the compressor, but it adds chemicals, equipment and operational complexity.
Traditional evaporation uses hotter steam and can achieve the same heat transfer with smaller exchangers, which may make it the better investment for challenging feeds despite higher thermal-energy use. The practical choice depends on testing the actual sludge or digestate and comparing the complete system—including filtration, chemical treatment, pumps and cleaning—rather than selecting a technology from energy prices alone.
