AmPower also evaporates their watery digestate, but without separating the nitrogen. The digestate is made acidic with sulfuric acid to prevent the release of ammonia during the heating process. This process produces an organic fertilizer rich in nitrogen and sulfur. AmPower does not process animal manure, so this product can be used without restriction by animal manure limitations (170 kg N/ha). However, they still have difficulty selling fertilizers in the region because of phosphorus application limitations and because injecting a liquid concentrate is more expensive than spreading artificial fertilizer granules.
Circular use of phosphorus through sewage sludge
Sewage sludge is also suited to the production of fertilizers, as shown by the Italian company Aqua&Sole located near Milan. Thermophilic fermentation at 55 °C produces a stable, hygienic digestate. Thermophilic fermentation requires control of the ammonia concentrations in the fermentation facility because high concentrations can halt the production of biogas. To address this, Aqua&Sole has invested in an advanced nitrogen stripper which extracts up to 40% of the mineral nitrogen from the digestate in the form of an ammonium sulfate solution. The digestate is used locally by arable farmers, while the ammonium sulfate can be fully utilized as an artificial fertilizer replacement thanks to its waste end-product status. No phosphorus regulations apply to the region, so the use of the digestate results in a surplus application of phosphorus. Researchers advise using phosphate equilibrium fertilization to prevent the buildup of excess phosphorus in the soil and to reduce heavy metal application to the soil. Reclamation and reusing phosphorus out of sewage is crucial to closed-loop phosphorus utilization. Despite this, there is a concern about contamination by the use of sewage sludge in Italy.
Furthermore, there is also an advantage to replacing artificial nitrogen fertilizers with reclaimed fertilizers. The production of artificial nitrogen fertilizers requires a high amount of energy, and replacing this with digestate or an artificial fertilizer replacement results in a saving of CO2 emissions.
Reclaimed fertilizers do not always result in profit currently
The largest income source for these companies is energy production. Producing reclaimed fertilizers from digestate had a limited effect on the total business case for these companies because the market opportunities of these fertilizers are often limited. This is partly because they have a low perceived value on the market compared to artificial fertilizers and partly because reclaimed fertilizers are less concentrated than artificial fertilizers, making the logistical cost associated with their application higher than by use of artificial fertilizers. The processing of digestate into fertilizers increases the flexibility for marketing of reclaimed fertilizers compared to unprocessed digestate. It should be noted that the economic situation was analyzed before the current energy and natural resource crisis. Artificial fertilizer replacements from digestate can now profit more greatly because of the high prices for artificial fertilizer.
High-quality application for organic fibers from digestate
Groot Zevert Vergisting and BENAS also create extra value by extracting organic fibers from digestate for use in valuable products. BENAS produces their organic fibers from energy crops, and they process them into cardboard products, such as plant pots, at their own location. The fibers are also suited to use in potting soil and may be used in organic agriculture. GZV has invested in a system where organic fibers are washed to remove salts and phosphorus. This makes the product suited as a peat replacement in potting soils. Potting soil companies aim to reduce the use of peat in potting soil, so there is much interest in quality alternatives. Both companies have had a long development process, and despite good perspectives, there is still no long-term sale of the products.
The remaining fluid fraction is evaporated by the use of residual heat. This reduces the volume of the product to be exported. Ammonia and carbon dioxide also escapes during evaporation. This can be reclaimed via condensation to produce a solution of ammonium-bicarbonate. This is called ammonium water in practice. This solution is not suitable as fertilizer because the ammonia can quickly escape due to the high pH. Waterleu sells the ammonia-rich water to a nearby company as a replacement for conventional ammonium water in air gas cleaning installations. Although this is a sustainable solution, the company owner has plans to reclaim the nitrogen with an acid wash to produce ammonium sulfate, which can be used as a fertilizer. The company is waiting until the use of ammonium sulfate from animal manure is approved as a fertilizer replacement.
Transforming digestate into new, more concentrated fertilizers has a small positive effect on the CO2 footprint of the demonstration plants. Though producing more concentrated fertilizers results in a reduction of the CO2 emissions associated with transport, the energy used to separate digestate results in a net positive CO2 footprint for this part of the process. A small amount of biogas energy is used in digestate processing, and all the companies deliver energy to the electrical/energy grid in the form of gas or electricity, resulting in a total negative footprint.
Depending on the technique, up to 35% of the total nitrogen from digestate can be reclaimed as a mineral artificial fertilizer replacement. The other 70% of nitrogen is present in organic fertilizers, including digestate and solid fertilizers. It is important to see examine not only the artificial fertilizer replacements but also the organic fertilizers when examining the effects of nitrogen emissions because these can also contribute to nitrogen emissions.
Ammonium sulfate is a suitable artificial fertilizer replacement, but the high sulfur levels require attention. Only a small part of the nitrogen requirement of the plant can be provided by ammonium sulfate. Otherwise, a surplus of sulfur will be delivered and will leave the system through runoff. This process will also lead to losses of microelements because the excess sulfur binds to the microelements when it exits the system. Ammonium sulfate cannot be mixed with digestate because this can create harmful manure gasses. High sulfur levels also require attention in concentrates from reverse osmosis or evaporation because sulfuric acid is used to prevent ammonia emissions. It is important to consider the sulfur application levels when creating a fertilizing plan with reclaimed fertilizers.
Nitrogen is present as ammonium in artificial fertilizer replacements, which creates a risk of ammonia emissions. This is especially the case for mineral concentrate, which is characterized by a relatively high pH. Ammonia emissions can be reduced to levels lower than animal slurry by injecting the concentrate into the soil because it can infiltrate the soil more quickly. GZV commissioned the development of a new type of fertilizer injector to facilitate injection. Field experiments show that good application practices result in nitrogen uptake and yields, like the use of artificial fertilizer. Mixing the mineral concentrate with animal manure is not recommended because this reduces the nitrogen effectivity because of increases in ammonia emissions. Mixing also reduces yields because of the lower nitrogen effectivity.
The fermentation facility at BENAS has a solid business case in selling energy. The biogas is converted to electricity or upgraded to green gas depending on the demand on the electrical grid. Green gas has a methane content equal to natural gas and can be delivered via the natural gas network. The option to produce electricity from biogas on demand contributes to the higher stability of the electricity grid and provides a higher income for the producer.
A plentiful knowledgebase for other biogas facilities
The SYSTEMIC project has delivered a high amount of data and new insights about digestate processing techniques, energy and chemical use, quality of reclaimed fertilizers, costs for processing, and environmental advantages. This data is now openly available via the project website and WUR library.
From manure to artificial fertilizer replacement
The Dutch company Groot Zevert Vergisting (GZV, Beltrum) was one of the demonstration locations in the project. This company focuses on the digestion of manure and co-products. The co-products come from the agri-food industry and produce high amounts of biogas. The biogas is transported via pipes to a nearby milk production factory. Digestate of manure cannot be easily applied in the region because of a surplus of manure. However, the region’s farmers are interested in replacements for artificial nitrogen fertilizer reclaimed from digestate of manure. These artificial fertilizer replacements would provide nitrogen in addition to what is allowed from animal manure. There is no regional market for phosphorus fertilizer replacements because of high phosphorus levels in the region’s soils and regulations which prevents the addition of more phosphorus to the soil.
Artificial fertilizer replacements
Diverse organic and mineral fertilizers were generated in this project, which also included artificial fertilizer replacements such as ammonium sulfate and mineral concentrate. Artificial fertilizer replacements are fertilizers where at least 90% of the nitrogen is present in mineral form.