HRS Digestate Concentration System (DCS) Q&A | HRS Heat Exchangers

HRS Digestate Concentration System (DCS) Q&A

September 29, 2016 | Categories: Environmental - Opinion Piece | by
HRS Concentration of Environmental Waste for Renewable Energy (DCS)

Using patented technology designed and manufactured by HRS Heat Exchangers, the DCS is a superior and energy-efficient way of concentrating digestate.


Q: What is your product/technology?

The HRS Digestate Concentration System (DCS) reduces the volume of digestate while increasing its nutrient content, reusing water from the CHP engine and surplus heat from other parts of the AD plant to do so.

Q: How does it work? (why do you need it?)

The DCS uses concentration under vacuum to significantly reduce digestate volumes while, at the same time, maximizing the nutrient content.

The first part of the DCS process involves heating the liquid digestate in heat exchangers; minimal additional water and energy is required, as the surplus water from the plant’s CHP engine (which is usually available at 185°F) is used as the heating media. The digestate is then pumped into a cyclone separator; the high speed rotating airflow causes the steam to flash off under vacuum and the solid particles to fall to the bottom of the cyclone, where they can be removed.

The steam produced from this first cycle (usually available at 158°F) is then used as the heating media for the second effect, whereby the process is repeated. The subsequent steam (usually available at 140°F) is used as the heating media for the third cycle – the number of effects is determined by the level of dry solids required, and the amount of surplus heat available, up to a maximum of four cycles. After the final effect, the steam is condensed back into water, which can then displace mains water used in the front end of the digester; a completely closed loop system. The DCS is virtually self-sufficient – minimal energy or water is bought in, nothing is wasted, and the surplus energy from the CHP is re-used up to four times.

Q: Where would it ideally be used? (and are there places where it might not work so well?)

Any AD plant which produces a liquid digestate containing solids. It is particularly suitable for sites with limited digestate storage or on-farm plants looking to maximize the nutrients provided by digestate fertilizer.

Q: How is it different? (e.g. to products on the market? / is it unique in any way).

Unlike conventional digestate dryers, which require a lot of heat, the DCS is extremely efficient in term of digestate concentration. The heat from the biogas CHP is recovered and re-used up to a maximum of four cycles giving a high level of water removal per kW. The DCS can be retrofitted to any plant. Features include:

  • The use of surplus heat from the CHP engine – and reuse of this initial heat throughout the process – which is up to four times more energy efficient than conventional digestate dryers. This also allows the DCS to treat a large quantity of digestate, reducing the effect on the process flow and helping to maintain overall plant throughput;
  • The combined use of HRS corrugated tubular heat exchangers and the company’s patented Unicus heat exchangers. Water carrying the recycled heat energy from the CHP engine travels in the space between the inner and outer tubes. This not only results in more efficient heat transfer, but also helps to reduce fouling and maintenance requirements, and reduces the system’s overall footprint by an average of 30% compared to using smooth tube heat exchangers;
  • An integrated cleaning system for Cleaning in Place (CIP), to maintain concentrator efficiency;
  • The use of non-welded heat exchangers, which make it easier to inspect, clean and replace them when required;
  • Acid-dosing the digestate with sulphuric acid to overcome the release of ammonia caused by high temperatures associated with concentration. This decreases the pH levels and turns ammonia into ammonium sulphate, decreasing odors and maximizing nutrient content (ammonium sulphate is an ideal biofertiliser).

Q: In what ways does the DCS optimize the efficiency of the AD process?

  • Removing up to 80% of the liquid volume of digestate to produce a material containing up to 20% dry solids, with the use of the patented Unicus heat exchanger. This makes digestate easier and cheaper to handle, store, transport and apply to land (storage and transport requirements can be halved);
  • Increasing the nutrient concentration of digestate;
  • Making use of surplus heat from the CHP engine, up to a maximum of four times;
  • Reusing water removed from the digestate to displace mains water used in the front end of the AD plant, thereby creating a completely closed loop system;
  • Reducing odors and maximizing nutrient content by turning ammonia into ammonium sulphate;
  • Creating an energy-efficient alternative to conventional digestate dryers, which use a lot of power and do not concentrate digestate liquid to the same levels as the DCS;
  • Enabling plant operators to increase their profit margins by claiming RHI for using their heat within the DCS;
  • Being completely bespoke – as a plug-and-play, fully automated system, the DCS is completely flexible and can be tailored to suit each individual AD operation.

Q: Where has it been used? (e.g. give an example of a place/company that is using it)

Willen Biogas in Middlesex and Kernal Export in Spain.

Q: Cost? (e.g. give a rough costing of a piece or pieces of plant to give an idea)

Cost is dependent on various factors including the number of concentration effects installed and other options, but as a guide £200,000 to £1 million depending on capacity.

Q: Any savings? (e.g. how much either in pounds or saved time or production levels).

Some examples from a 1.5 MW plant include 60% reduction in tanker volumes, £20,000/yr saving in water costs and displacement of ~1,100 t/yr of artificial fertilizer.


Q: What about maintenance? (e.g. can this be done in-house or externally and how often and how long does this take and does it involve any down time).

Using corrugated tubular heat exchangers reduces maintenance requirements. Cleaning is by integrated Cleaning in Place (CIP) system and non-welded heat exchangers make it possible to inspect and clean the unit easily.


Q: What is its life span? (e.g. how long does it last either in years or production cycles)

The designed working life is 20 years.


Q: How is it future-proofed? (e.g. waste legislation is changing constantly how have you made sure it can keep up and not become obsolete).

The equipment is easy to upgrade and service, while the efficiency benefits provided by the unit will justify its use throughout the lifetime of the plant.


Q: Any future applications? (e.g. do you plan to apply the technology to other processes? How does the equipment fit in with the company’s future plans)?

We are investigating other applications where the solid content of solutions needs to be increased and surplus process heat is available.


Q: How many have you sold and where? (or where do you see it being sold)

Three units are currently operational and we are in discussions with other potential customers.