Maximising the value of digestate from food waste

A novel system that reduces the requirement to store and apply digestate to land by around 60 per cent is helping an anaerobic digestion (AD) plant in the UK convert 25,000 tonnes a year of London’s food waste into a nutrient-rich biofertiliser.

D. Williams & Co. is a UK farming company that grows and processes speciality herbs alongside arable crops at the 810 ha Cattlegate Farm near Enfield, north of London. The owners formed Willen Biogas in 2008 to investigate the potential of a biogas plant on the farm. From the start they were determined to maximise the value of all the outputs from their AD plant, including the gas, heat, and digestate.

Early in the development process for the 1.5MW facility, Willen Biogas approached HRS Heat Exchangers to see how the HRS Digestate Concentration System (DCS) could reduce digestate volumes and improve the quality of this valuable organic fertiliser. This was essential if the new business was to integrate with the existing herb production and farming operations.

With the farm located just 21km north of the centre of the capital on heavy London clay soil, it became clear that digestate from the AD process could help the farm reduce the cost of cultivations, minimise its use of manmade fertilisers, reduce its environmental impact, and deliver financial savings. Digestate production was one of the main drivers for the plant but with 41,000 tonnes of liquid digestate being generated each year, Willen Biogas needed to find a way to reduce the liquid fraction without losing any of the valuable nutrients.

Digestate concentration
The HRS DCS removes up to 80 per cent of the water contained in the liquid fraction and concentrates it to 20 per cent dry solids while, at the same time, maximising the nutrient content. Heating the digestate with surplus water from the plant’s CHP engine, the DCS has reduced the liquid fraction of Willen’s digestate by 60 per cent (from 50,000m3 to 20,000m3 per year), making digestate handling, storage, transport and application not only easier for Willen Biogas, but less costly, too.

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 is used as the heating media. The steam produced from this first cycle is then used as the heating media for the second effect, whereby the process is repeated with further cycles. The number of effects is determined by the level of dry solids required, and the amount of spare heat available, up to a maximum of four cycles. After the final effect, the steam is condensed back into water and is used to displace mains water employed in the front end of the digester; a closed loop system. The DCS is virtually self-sufficient – minimal energy or water is brought in, nothing is wasted, and the surplus energy from the CHP is re-used up to four times.

Using patented technology designed and manufactured by HRS Heat Exchangers, the DCS is a superior and energy-efficient way of concentrating digestate. It is four times more efficient than conventional digestate drying technology and is enabling D. Williams & Co to displace 1,093 tonnes of artificial fertiliser each year, including 688 tonnes of ammonium nitrate.

Another benefit of the DCS is odour control, which helps increase the nutrient content of the digestate. The high temperatures needed to concentrate digestate can cause the release of ammonia, largely responsible for the odours associated with digestate. However, the DCS overcomes this by acid-dosing the digestate with sulphuric acid, thereby decreasing the pH levels. This turns the ammonia into ammonium sulphate, which is not only less odorous, but is also an ideal nutrient.

As well as creating a more concentrated, richer fertiliser, the DCS delivers the following additional benefits for Willen Biogas:

• By reducing the liquid fraction by 60 per cent, transport, water and storage costs have been cut, reducing operational overheads significantly.
• The amount of traffic going across the farmland has been significantly reduced (down from 2,174 tanker movements per year to 870).
• Unlike conventional digestate dryers – which use a lot of power to create a small amount of product – the DCS is energy efficient; surplus heat from the CHP engine is reused up to four times over, and the wastewater is then used to displace mains water in the front end of the digester.
• Minimal additional energy and water is required, and none is wasted.
• Willen Biogas can claim payments under the UK’s Renewable Heat Incentive, which supports renewable energy projects.
• As a plug-and-play, fully automated system, the DCS is completely flexible and has been uniquely tailored to suit the needs of Willen Biogas.

Digestate pasteurisation
To prevent the spread of weed seeds, crop diseases and other potential pathogens, either the feedstock or the digestate should be treated appropriately, with pasteurisation being a tried and tested technique across the world. In many areas including much of Europe, certification schemes exist to back-up such pasteurisation so that digestate that has been so treated can be used, sold, traded and applied as an agricultural and horticulture fertiliser, rather than a waste product.

One of the most energy-, and therefore cost-efficient methods to pasteurise digestate is the HRS Digestate Pasteurisation System (DPS), which is based on heat exchangers rather than tanks with heating jackets. Using heat exchangers means that effective digestate pasteurisation is possible using surplus heat while allowing additional thermal regeneration levels of up to 60 per cent. This saved heat can then be used for other processes, such as evaporation of the digestate to remove water.

The standard three-tank DPS provides continuous pasteurisation, with one tank being pasteurised while one is filling, and another being emptied. The HRS pasteuriser uses a double tube heat exchanger to heat the digestate to 75˚C above the required pasteurisation temperature. This allows for variation in the sludge consistency and its incoming temperature, making sure that the digestate is always properly pasteurised. The tanks can also be used individually, for example to allow for routine maintenance.

The DPS and DCS are examples of several systems that HRS produce to improve the efficiency and sustainability of biogas plants and the anaerobic digestion process, with other examples including systems to dry biogas and recover waste heat from exhaust systems.

Local support
In Australia, HRS has a team that offers comprehensive support of our clients to make sure their needs are met during a project’s lifecycle – from cradle to grave, and for the operational life of the project.

“We implement a variety of evaporation processes, whether it be falling film, forced recirculation, mechanical vapour recompression, or thermal vapour recompression,” said HRS ANZ director Chris Little. “All come with a high degree of energy efficiency and we can configure the system to meet the specific needs of the client and the client application. It is a bespoke solution for each client. There is nothing off the shelf about what we do.”

With this type of technology available, what HRS is offering is a purely onsite means to be able to deal with the effluent that these projects produce. Whether it is digestate from an anaerobic digestion process, or other effluent from a food factory, the cost to dispose of it or treat it can be prohibitive, according to Little.

“If we are able to deploy an onsite means to be able to treat that product, ideally utilising waste heat from elsewhere in the process, then the client will be avoiding the cost of transport and disposal, which in itself can go a long way to repaying a project while reducing their carbon footprint,” he said.

“In addition, what we are seeing in the market is an ever-increasing focus on nutrient recovery. Very often there is value in the concentrate we produce that can be reused, monetised or otherwise utilised. We not only see this in the food waste AD sector but also in dairy and other areas. Extracting value at  every end of the process without harming the environment, while saving energy and reducing waste are crucial considerations in today’s world. Concentration through evaporation can go a long way to achieve those goals.”