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CHP WASTE HEAT · TECHNICAL ARTICLE

Digestate drying: from separated solids to a dry product

After digestion, digestate can be mechanically separated into liquid and solid fractions. The liquid can go to storage and agricultural use; the solids can be dried with CHP waste heat to simplify storage and transport or prepare them for further fertiliser production.

7 min readSources are listed at the end of the article
Isometric illustration of a digester, separator and dryer for the solid digestate fraction connected to a CHP unit
One possible process flow: digestate → separation → drying the solids with CHP waste heat.

The separated solid fraction still contains considerable water. It takes up space, is relatively heavy to transport and is harder to turn into a standardised product. Drying can be the next processing step if unused CHP heat is available on site.

Basic process flow

After digestion, digestate enters a separator. The liquid fraction goes to final storage or a lagoon for later use in accordance with an agronomic plan. The solid fraction passes through a receiving hopper and dryer feed system to the dryer, then to dry-material storage. Heat is usually supplied as hot water through a water-to-air heat exchanger.

What does drying achieve?

  • lower mass and volume to store and transport
  • material that is easier to meter, bag or store in bulk
  • potential for subsequent pelletising or granulation
  • a new use for heat that would otherwise be rejected

Drying alone does not create a premium product. Value depends on digestate composition, nutrient content, hygiene requirements, contaminants and the market for the intended use.

Exhaust-air treatment is a crucial part of the process

Heating digestate can release odours, ammonia, dust and moisture. Ventilation and air treatment must therefore be included from the start. Depending on the material and permit, biofilters, chemical air scrubbers, condensation or combined systems may be needed.

Examples of available technology

  • RHS RHINO – a German agitated dryer developed for agriculture as well. RHS states that it can handle separated digestate and the liquid fraction, drying material to a high dry matter content depending on requirements.
  • Dorset Green Machines – belt dryers and back-mixing systems for digestate, with additional air-cleaning equipment.
  • ANDRITZ – offers several technologies for digestate, including separation and belt, paddle and drum dryers that can use low-temperature waste heat.
  • STELA Laxhuber – belt systems and long-standing references for drying digested and other wet streams with biogas-plant heat.

What comes after the dryer?

The simplest route is to use the dry material in bulk where permitted and commercially justified. A further step might be pelletising and granulating digestate, mixing with other components or making a specific organic or organo-mineral product. That is a separate project requiring laboratory analysis, an agronomic concept and regulatory review.

Key pointDrying reduces water and helps with logistics, but market value depends on what is done with the material afterwards. The intended final product should therefore be defined before selecting a separator and dryer.

Sources and further reading

This article is for information. An actual installation requires a heat balance, equipment manufacturer data and checks of local conditions, permits and applicable regulations. The illustration shows a functional concept, not a detailed engineering design.