Fertilizers and artificial fertilizer replacements from organic waste products: Experiences from a large-scale demonstration project

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.

GZV has therefore invested in separation technologies where phosphorus is separated in a solid fraction. A part of the nitrogen is reclaimed in a mineral concentrate, and water is separated as clean dischargeable water. Initially, the goal was to discharge 45% of the digestate as clean water, but this was not feasible. However, there have been successful steps to reduce the use of sulfuric acid, polymers, and other additives. Around 30% of the mineral nitrogen is reclaimed in the concentrate via reverse osmosis. This product is sold as mineral concentrate, although the mineral levels are relatively low compared to liquid artificial fertilizers (0.8% N). Phosphorus is reclaimed in a solid fraction, which is then sold in the Eiffel region (France), where there is a market for phosphorus fertilizers.

The mineral concentrate works well as an artificial fertilizer replacement because more than 90% of the nitrogen present is in the mineral form (NH₄). The law still needs to recognize this as an artificial fertilizer replacement, instead of a manure product, to use it this way. The European Commission is still coming to a decision about a definitive exemption from the laws surrounding the use of animal manures for artificial fertilizer replacements from animal manure, also known as RENURE (Recovered Nitrogen from manURE) products. For now, GZV works with a temporary exception under the pilot project ‘Achterhoek free from artificial fertilizers’, where the mineral concentrate can be used in addition to the amount of animal manure allowed.

Extra nitrogen, in the form of urea (ammonium nitrate), is added to the product to improve its marketability, after which it is sold to local farmers under the brand name ‘Groene weidemestof’. The addition of nitrogen is needed to create a product where nitrogen, potassium, and sulfur, are present in the ratios needed by the crop. The product is sold at the price of artificial fertilizers. A part of the income is used for storage outside of the growing season, transportation, and application, because these costs are greater for use of liquid artificial fertilizer replacements than for use of artificial fertilizer granules.

An innovative nitrogen stripper

The demonstration location at Benas (Bremen, Duitsland) ferments energy crops and chicken manure. Fermentation of the nitrogen-rich chicken manure is only possible by reducing the ammonia levels in the fermenter, because high ammonia concentrations are toxic to biogas forming bacteria. To reduce the ammonia levels, an innovative nitrogen stripper and scrubber is installed at the site of the fermenter. This is an innovative technology because it uses gypsum (CaSO₄, an industrial byproduct) instead of the expensive sulfuric acid used in regular scrubbers. The process at the BENAS location produces an ammonium sulfate, a calcium fertilizer, and a digestate. All the fertilizers are used by BENAS on their own land to produce energy crops. Although this is a nice example of circularity, this also generates a risk of high sulfur dosage to the soil through use of ammonium sulfate.

The fermentation facility at BENAS has a solid business case by selling energy. The biogas is converted to electricity or upgraded to green gas depending on the demand on the electrical grid. The 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 a higher stability of the electricity grid and provides a higher income for the producer.

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.

Using residual heat to dry and evaporate digestate

Two demonstration companies are in Flanders, a region where the sale of digestate is difficult because of a surplus of animal manure in the market. AmPower (Ieper, Belgium) digests waste streams from the agri-food industry and household organic waste. Waterleau New Energy (Ieper, Belgium) ferments pig manure in addition to waste streams from the agri-food industry and household organic waste. The companies have a similar process for producing digestate. After the first separation step, the solid fraction is dried to produce a solid manure suited to export.

The remaining fluid fraction is evaporated by use of residual heat. This reduces the volume of the product to be exported. Ammonia and carbon dioxide also escapes during the 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.

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 the animal manure limitations (170 kg N/ha). However, they still have difficulty selling the fertilizers in the region because of phosphorus application limitations and because injecting a liquid concentrate is more expensive than spreading artificial fertilizer granules.

Both companies use high amounts of thermal energy to dry their products. They use the residual heat from the production of electricity from gas. About 40% of the energy worth of biogas is converted to electricity, 40% is residual heat, and 20% is lost. Although the energy demand of digestate evaporation is provided by sustainable energy, there may be a question if the residual heat can be more sustainably used in another way, such as heating homes. Another disadvantage is that these companies are dependent on residual heat and cannot transition to production of green gas because of the quality requirements to make it comparable to natural gas.

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 to use phosphate equilibrium fertilization to prevent the buildup of excess phosphorus in the soil, and to reduce heavy metal application to the soil . Reclamation and reuse of phosphorus out of sewage is crucial to closed loop phosphorus utilization. Despite this, there is concern for contamination by use of sewage sludge in Italy.

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.

The 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. The 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.

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 the 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.

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.

Large differences in plant available phosphorus

In most of the demonstration plants, a solid fraction is separated from digestate that is not further dried. These solid fractions have been analyzed for phosphorus availability, and the analysis shows large differences in the availability of phosphorus between different fertilizers from digestate. Phosphorus availability is high in digestate from animal manure, but this reduces as soon as an iron-containing feedstock is added to the mixture. The solid phosphorus-rich fertilizers are largely sold in France, where their use is not limited by phosphorus regulations. Low phosphorus availability does not limit marketability in the experience of the producers. This is also influenced by the fact that it is normal to present only the total phosphorus level on fertilizing products, so the buyer is not aware of the availability level.

No organic contaminates in dischargeable water

The researchers also examined the levels of organic micropollutants in the different products. Digestate contains residues of herbicides, pesticides, and occasionally residues of veterinary medicines. Three of the demonstration companies have a separation process where clean water is reclaimed and discharged on the surface water. None of the water discharge streams contained residues that would create a pollution risk for the surrounding region. There were also no harmful residues found in ammonium sulfate. The digestate from sewage sludge, that was digested at a relatively high temperature (thermophilic fermentation, 55 °C), contained no residues of veterinary medicines. The mesophilic fermentation (40 °C) at the other locations did result in digestate containing residues of veterinary medicines.

Negative CO₂ footprint via production of biogas and replacement of artificial fertilizer

The researchers also calculated the CO₂ footprint of the demonstration plants. All the companies have a negative CO₂ footprint when it is assumed that biogas is used as a replacement for fossil fuels. Using animal manure and sewage sludge as feedstock results in a relatively low biogas production because these waste streams contain a high amount of liquid. High biogas production is achieved through fermenting co-products or, in Germany, energy crops.

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 the 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.

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 fertilizer. 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.

The participating companies were all located in areas of intensive livestock production, therefore there is no demand for unprocessed digestate. Therefore, they have an economic advantage when they produce reclaimed fertilizers which biogas companies with a different regional situation do not, because in other regions the unprocessed digestate can be sold. The local fertilizer market is the driving factor for investing in processing facilities for digestate.

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.