Technology categories

Technology main category: Biological nutrient recovery: composting, anaerobic digestion, microalgae technology
Sub category Definition FP ID link for products are in matured phase
Anaerobic digestion

Anaerobic digestion is a sequence of processes by which microorganisms break down biodegradable material in the absence of oxygen.

The production of biogas from biomass as a renewable alternative to natural gas achieved via anaerobic digestion (AD). The AD process not only produces renewable energy, the processed biomass can also help soil fertility e.g. conversion of the organic carbon to a stable form which will persist in the soil. It also has other advantages, such as increasing soil organic carbon in the soil, being a carbon sink, as mentioned in the abatement of climate change objectives by the COP-21 Paris Agreement. Manures are often anaerobically digested, mixed with other organic waste with a higher biomethane potential and more optimal C/N ratio: green waste, food industry by-products, household separated organic waste. Co-digestion of manure with other substrates enables better biogas production and more stable  reactor  operations  as  well  as  more  optimal  economics. AD can now be considered  as a key technology in the nutrient recovery value chain. This is due to its ability to mineralise organic substrates so that the nutrients (N, P and others) contained within them can be more easily taken up by plants. This  implies  that  digestate and derived products can  be  more  suitable  as  fertilisers  than the raw resources from which they originated (such as sludges, slurries, biowastes, etc.). Nonetheless, varying input sources can lead to large differences in digestate composition between biogas installations as well as digestate outputs with different and unpredictable composition at each single biogas plant. However, farmers require homogeneous and “predictable” mineral nutrients, as is commonly the case in synthetic mineral fertilisers, so biogas plant owners may need to address this constraint. [1]

During the anaerobic digestion the organic nitrogen is mineralised and at the end of the process the ratio of ammonia and total nitrogen may reach 70-80%.  The increase of the ammonia nitrogen concentration also leads to an increase in pH usually between 8 and 9. However, the digestion process does not modify the total amount of nitrogen in the products.  The anaerobic digestion represents a potentially very effective treatment in reducing greenhouse gas emissions: in fact, during the process the majority of fermentable organic matter is degraded to methane and carbon dioxide, obtaining an effluent (digestate) having a lower GHGs emission potential than the incoming products. However, some studies have reported an increase  in methane emissions during storage of digestate. This effect may be related to the hydraulic retention time in the digester that might be too short to complete the degradation of the material in the biogas plant and thus continues in the storage. In addition, significant biogas losses can occur even from the pipes and the covers of digesters that can easily reach 10% of the biogas produced. This treatment may increase the ammonia emissions in the subsequent storage phase. The digestate is usually characterized by higher ammonia nitrogen content and pH than to the starting slurry, both factors that facilitate the volatilization of the ammonia process. Another factor that facilitates these emissions is the content and type of solids present in the digestate; these in fact are reduced in quantity and size because of degradation processes, features that do not facilitate the formation of a crust, which may represent an effective barrier to emissions. [1]

Benefits:

  • Producing renewable energy
  • Abatement of odours
  • Stabilization of manure and co-substrates: the demolition of the carbonaceous organic load resulting from the anaerobic digestion gives the manure a sufficient stability in subsequent periods of storage; it causes a slowdown of the degradation and fermentation processes.
  • Reduction of the pathogens: the anaerobic digestion in mesophilic environment (40 °C) can partially reduce the pathogenic charge in the manure. By operating in thermophilic conditions (55 °C) it is possible,  instead,  to  get  the  full  hygienization  of  the  sewage  with  the  total  destruction  of  the pathogens. [1]

[1] https://ec.europa.eu/eip/agriculture/sites/agri-eip/files/eip-agri_fg_nutrients_recycling_final_report_2017_en.pdf

 

Composting

Composting is the decomposition process of organic waste by the action of aerobic bacteria, fungi, and other organisms.

Composting is one of the oldest techniques to create a more stable and hygienic product. Stability and hygienisation are essential for recycling minerals from complex and variable organic products such as biowastes. These organic products tend to function more as soil enhancers, containing higher organic carbon load as well as P which can be considered more slow-release. Product quality when working from biowastes is of major concern for the end-user, which implies that sound quality assessment protocols need to be in place, preferably audited and controlled by independent auditing and certifying entities. The natural heat released during composting also results in “biothermal drying” reducing the water content and so making the product more transportable. [1]

Composting (self-heating) of the product at temperatures exceeding 70°C is only possible if a maximum of 30 wt% of solid fraction of pig manure is used. This can then be combined with the solid fraction of cattle slurry, cattle manure with straw, horse manure or poultry manure to obtain enough structure and an optimal C/N ratio. Some sites also add vegetal biomass or vegetable, fruit and garden (VFG) waste or green waste compost. This mostly occurs in a closed shed consisting of several tunnels which can be closed off and aerated separately (large capacity). It can also be done by use of an aerated drum (feasible at farm scale level). The material can also be placed in rows on the floor and is turned over manually (extensive composting). Farm-level composting could be used for optimisation of the quality of the solid fraction of manure as fertilizer/soil improver, and reduce nutrient losses during storage. [2]

If a farmer has invested in a separation system (screw press), he can use the liquid fraction as an NK-fertiliser on his land, and can do a hygienisation of the solid fraction. In this way he obtains an organic fertiliser which can be exported outside the country, or can be sold to the private market (gardening, etc.). In the Netherlands, the pasteurised solid fraction is also used as bedding material for cows. This gives an extra guaranty that no infection will occur. In the Netherlands there are two companies who offer a decentralized (farm-scale  level)  aerated  drum in which solid  fraction of manure can be pasteurised. This is an aerated rotating drum in which manure/digestate is pasteurised without any external heat. Because of the rotation, and the air that is blown with a ventilator into the drum, a natural composting process starts. Farm-level composting is an extensive process, in open air, where no external aeration is used. To obtain a good composting process, it is necessary to have a good ratio of carbon-rich input materials and N-rich input materials. Also the temperature, CO2- and moisture content are important parameters. To aerate and homogenise the pile it is necessary to turn it over from time to time. At farm level this can be done with a windrow turner. The follow-up and turning of the pile requires extra time and labour for the farmer.The objective of extensive farm composting is to produce a homogeneous and stable product to apply on Flemish agricultural land; this for maintenance of the soil (application of OM). If the product can be pasteurised by the composting process, the end product can be exported. [2]

[1] https://ec.europa.eu/eip/agriculture/sites/agri-eip/files/eip-agri_fg_nutrients_recycling_final_report_2017_en.pdf

[2] https://ec.europa.eu/eip/agriculture/sites/agri-eip/files/fg19_minipaper_1_state_of_the_art_en.pdf

Anaerobic digestion + composting

Combined use of anaerobic digestion and composting to process organic waste.

Microalgae/duckweed/insect/enzyme technology

Microalgae/duckweed/insect/enzyme technology refers to the use of microalgae/duckweed/insect/enzyme in waste streams to recover nutrients and produce biomass as animal feed or crop fertilizers.

Algae cultivation: The process is to biologically accumulate and retrieve nutrients from complex liquid waste  water-streams. Algal biomass can subsequently serve different purposes – both in bulk as in fine chemical applications. This can be for example for animal feed or renewable energy but also for example for the recovery of colorants (e.g. fycocyanin via Spirulina). [1]

A potential method of nutrient extraction  from  organic  wastes is the  production of proteinaceous biomass by cultivating microalgae. This increases the value and manageability of the nutrients. Recycling the nutrients from manure and assimilating them into algal biomass can result in high quality fertilizers without incurring the environmental and monetary costs of using chemical fertilizers while simultaneously remediating the waste effluent from this process. Manure digestate is an especially attractive feedstock to grow microalgae for biofertilizers production, as it is less contaminated than untreated effluents and rich in nitrogen and phosphorus. Microalgae could be used to recover nutrients from the liquid fraction of digestate and as microalgae incorporate these nutrients into their biomass, a fertilizer is created that is less prone to nutrient losses towards the environment. By reducing the volume of the liquid digestate, the nutrients become more manageable and  some  reclaimed water may be produced. Living microalgae can also be used as a nitrogen fixator to bring atmospheric nitrogen into the soil and as soil conditioner. Microalgae can be also further processed (e.g. hydrolyzed) in order to obtain more elaborated biofertilizers and biostimulants. [2]

[1] https://ec.europa.eu/eip/agriculture/sites/agri-eip/files/eip-agri_fg_nutrients_recycling_final_report_2017_en.pdf

[2] https://ec.europa.eu/eip/agriculture/sites/agri-eip/files/fg19_minipaper_1_state_of_the_art_en.pdf

Technology main category: Phosphorus precipitation from liquid manure, waste water and drain water
Sub category Definition FP ID link for products are in matured phase
Phosphorus precipitation from manure/digestate

This technology refers to the recovery of soluble phosphate  from manure or digestate by adding chemical solutions containing multivalent metal ions like calcium, magnesium and iron, etc.

Phosphorus precipitation from multi organic wastes

This technology refers to the recovery of soluble phosphate  from mixture of multi organic wastes by adding chemical solutions containing multivalent metal ions like calcium, magnesium and iron, etc.

Phosphorus precipitation from wastewater/sludge

This technology refers to the recovery of soluble phosphate from wastewater or sludge by adding chemical solutions containing multivalent metal ions like calcium, magnesium and iron, etc.

Phosphorus in liquid streams (such as wastewaters or liquid fractions) can be retrieved in purified form by selective precipitation processes. Most known target precipitates are struvite (MgNH4PO4 ) and calcium-phosphate (CaPO4 ). In the potato processing industry, struvite reactors have found progressive market uptake whereas in other sectors (such as manure processing) development of recovery techniques for calcium phosphate have seemed to gain momentum in recent years. [1]

[1] https://ec.europa.eu/eip/agriculture/sites/agri-eip/files/eip-agri_fg_nutrients_recycling_final_report_2017_en.pdf

Technology main category: Thermochemical nutrient recovery
Sub category Definition FP ID link for products are in matured phase
Reductive thermochemical Phosphorus recovery

The mono feed, animal bone grist, reductive thermochemical P recovery is a specific pyrolysis process in absence of air with zero emission performance for production of Bio-Phosphate. This is explicitly designed for 850°C high material core reductive thermal processing of animal bone grist by-products to recover high concentrated Phosphorus products in economical large industrial scale for controlled release organic fertiliser and adsorbent applications.

Benefits: high processing efficiency, zero emission environmental performace and full recycling-reuse of all mono feed input material streams. Specifically developed/designed high tech for bone processig, autothermal and producing large amount of surplus green electricity. Providing sustainable economics by converting unexploted biomass into high output product value. Continously operated in economical industrial scale with „just in time” supply system and low operational costs. Challenges: capital investment intensive high tech technology.

Multi feed reductive thermochemical process

The multi feed reductive thermochemical pyrolysis is a traditional 450°C low temperature reductive thermal processing in absence of air to carbonize cellulose based and other by-product/waste materials in smaller-decentralized scales to produce biochar.

Benefits: multi feed approached, different types of pyroylsis systems may be applied in small-medium scale. Challenges: feed material supply and logistics, environmental impacts, economical scale ups, high operational costs in small-medium capaity sizes, management of the sustainable economy of the productive operation under market competitive commercial conditions.

Oxidative thermochemical Phosphorus recovery

Oxidative thermochemical processing is incineration for burning of materials in full oxidative environment or gasification (partial burning of materials in semi oxidative environment) with burned-off solid ash product outputs are followed by P recovery chemical post processing.

Benefits: traditional and well known technologies.Challenges: feed material supply and logistics, significant environmental and climate impacts, complex and costly chemical post processing of the water insoluble ash solid products, low output product value.

Technology main category: Physic-chemical nitrogen recovery from manure, digestate and wastewaters: separation, stripping and membrane processes
Sub category Definition FP ID link for products are in matured phase
Nitrogen recovery from air

This technology capture nitrogen from air. In the absorption system, the nitrogen is absorbed to form a stable N fertilizer, increasing the mineral-N content from the input product.

Mobile manure acidification

Acid solutions are added to manure to reduce the N emission during storage and application and increase the N fertilizer value.

Acidification of slurries and digestate is not a nutrient recovery technique as such but is promoted in some EU member states as a mitigating measure to reduce ammonia emissions related to manure/digestate management. Those in favour of acidification argue that it improves manure storage and stabilisation and enables a better return of nitrogen to crops when manure is spread, so improving nutrient use efficiency of the manure. A point of attention when using acidification is the type of acid used: when using sulphuric acid, it is important to avoid anaerobic microbial formation of hydrogen sulfide (H2S) in storage afterwards. Hydrogen sulphide is not only odorous and can cause nuisance as a result, but this product is also very toxic and even lethal at low concentrations when inhaled.  Therefore,  strict  operational  guidelines must be  followed. The disadvantage of the acidification of manure is the loss of buffering capacity which is present in the raw manure and digestate in the form of free carbonates.  Acidification will decrease the liming capacity of such products and convert the carbonates to CO2, releasing them to the atmosphere.[1]

Animal manure is  a rich source of nitrogen, namely ammonium (NH4+) that  is  directly  available for plants. However, part of this ammonium nitrogen can be lost during storage or field application due to ammonia (NH3) volatilization. NH3 emissions are a severe environmental problem, 80% of the total NH3 emissions from agricultural activities are from barns and slurry stores represent up and more than 50% of the applied N can be lost by NH3 emissions during and after slurry application to soil. Such losses led to two main problems in term of nutrients use efficiency: a decrease of the fertilizer value of slurry in terms of nitrogen and a significant variability of N concentrations in slurry during field application. [2]

Slurry acidification is a simple solution to avoid NH3 emissions but such technique is today used at farm scale exclusively in Denmark and in some countries of North and Eastern Europe. The main reason for this low implementation at farm scale in other European countries is probably the fear of farmers relative to the handling of concentrated acids (mainly sulfuric acid). Indeed, such operation has to be performed by trained staff and implies, in most cases, to rely on contractors. Slurry acidification can also lead to significant CO2 emissions during the process as well as H2S emissions during storage. The consequences of long term application of acidified slurry to soil are still unclear and the decrease of soil pH and increase of S content in soil are often presented as the principal threat of acidified (H2SO4) slurry application to soil. Finally, tools for fast and accurate measurement of slurry pH are still missing what difficult oversight operation by authorities in farms using such technique. Slurry acidification is promoted to minimize N losses but it might also increase P plant availability since the amount of soluble P increase significantly when slurry is acidified with sulfuric acid. [2]

[1] https://ec.europa.eu/eip/agriculture/sites/agri-eip/files/eip-agri_fg_nutrients_recycling_final_report_2017_en.pdf

[2] https://ec.europa.eu/eip/agriculture/sites/agri-eip/files/fg19_minipaper_1_state_of_the_art_en.pdf

 

Membrane filtration

Membrane filtration covers all engineering approaches for the transport of substances between two fractions with the help of permeable membranes. According to the operational conditions, this technology can be divided into: microfiltration, ultrafiltration, nanofiltration, reverse osmosis, forward osmosis and electrodialysis.

Membrane (MF) and ultrafiltration (UF) techniques consist in physical separation by forcing stream input (i.e. liquid fraction of manure or digestate after decanter digestate centrifugation, through membrane by  means of pressure. Membranes used to process digestate can be classified as in the following according to pore size: MF- (pores > 0.1 μm, 0.1-3 bar), UF- (pores > nm, 2-10 bar) and RO-membranes (no pores, 10-100 bar). Membranes used can be either organo-polymeric or ceramic. The first ones are less expensive but they are difficult to be cleaned and they do not support high pressure. Ceramic membrane, used above all for ultrafiltration, are easier to be cleaned (they have resistance to chemicals) and they allow higher performance because of high pressure used. Nevertheless, the higher the separation performance, the higher the energy consumption what might be the main limitation for the implementation of such technique. [1]

Membrane cascades, ending in ultrafiltration or reversed osmosis allow for suspended particles to be filtered out and for mineral nutrients in the form of dissolved salts (mostly N and/or K) to be further concentrated. Reversed osmosis as a final step in the membrane cascade also results in ultra-pure water which could be recycled and re-used. This type of technology allows the production of mineral concentrates, although the concentrations are not as high as in synthetic mineral fertilisers. Also, the use of membranes in the farm environment has encountered operational issues related to (bio-) fouling and clogging of membrane pores leading to loss of performance and (in such cases) excessive operational costs. New developments are underway to tackle these negative effects, nonetheless it has proven to be challenging to introduce membrane technology in farming environments. [2]

[1] https://ec.europa.eu/eip/agriculture/sites/agri-eip/files/fg19_minipaper_1_state_of_the_art_en.pdf

[2] https://ec.europa.eu/eip/agriculture/sites/agri-eip/files/eip-agri_fg_nutrients_recycling_final_report_2017_en.pdf

Stripping + Scrubbing

The stripping is performed by blowing air through N-rich waste streams while increasing temperature or pH (e.g. with CaOH) which will gasify the mineral nitrogen (NH3). This is considered a pre-treatment needed before the scrubbing N recovery process where the NH3-filled air will be washed with acidified (HNO3 or H2SO4) water (scrubbing) to capture the  ammonium  in liquid form (ammonia sulphate from H2SO4 or ammonia nitrate from HNO3).

Ammonia can be stripped by air, steam or vacuum through the liquid fraction in a packed tower. Ammonia stripping can be obtained directly from the manure or digestate or even from their respective liquid fraction by heating at 80 °C. Nevertheless, a pH lifting (with NaOH) up to 10.5 and temperature of 70 °C, allow 85-90% of ammonia to be stripped.
Stripped gas rich in ammonia is then recovered by washing air flux with strong acid solution (H2SO4), which produces ammonium sulfate (N = 3-8 % w/w). Besides H2SO4  as sorbent, also nitric acid (HNO3) can be applied to obtain ammonium nitrate. Another solution is represented by “cold ammonia stripping” to be operated on mineral concentrate. In this case N stripping can be operated at ambient temperature by adjusting pH with CaO (or similar). Performance reported for full scale plant are of 80-90 % of ammonia stripped. [1]

The chemical equilibrium between water soluble ammonium (NH4+) and its volatile counterpart ammonia (NH3) is determined  almost entirely by temperature and pH. Concretely, by increasing pH and temperature, ammonium can be driven out in the form of gaseous ammonia. Passing the air saturated with ammonia through an acidic scrubber system then recaptures the nitrogen as soluble ammonium. Depending on which counter acid is used  in the scrubber (e.g. sulphuric acid, nitric acid) a pure ammonium sulphate (NH4SO4 ) or ammonium nitrate (NH4NO3) mineral fertiliser product can be obtained. The resulting products consist entirely out of mineral nitrogen and therefore have 100% nitrogen use efficiency (NUE), similar to synthetic fertilisers. Both nitrogen and sulphur (S) are plant essential nutrients, so the scrubber water in the form of NH4SO4 also provides the farmer with a good source of mineral S. Nonetheless, the ratio N/S found in the scrubber fluid can differ from the actual N/S ratio required for the crop which could therefore lead to S over-fertilisation. This constraint is not encountered when working with NH4NO3 scrubber waters. [2]

[1] https://ec.europa.eu/eip/agriculture/sites/agri-eip/files/fg19_minipaper_1_state_of_the_art_en.pdf

[2] https://ec.europa.eu/eip/agriculture/sites/agri-eip/files/eip-agri_fg_nutrients_recycling_final_report_2017_en.pdf

 

Physical separation

Physical sepration of manure/digestate not including addition of chemicals or membranes (stables, centrifuges, drying, etc.)

Mechanical separation of raw manure or digestate results in the concentration of nitrogen (and potassium) in the liquid fraction, and the concentration of phosphorus and organic material in the solid fraction. This technique is mostly applied as pre-treatment for nutrient recovery techniques. However, separation can already be an interesting manure management technique. The liquid N-rich fraction, can be used on arable land/grassland on the farm to reduce the use of mineral fertiliser. The solid fraction contains a high concentration of phosphorus and is mainly used in regions with low P-soils and/or with a high demand of carbon. By concentration of P in the solid fraction, a high amount of P can be transported in a small volume (15-20% solid fraction). Separation of manure can be achieved by different techniques as screw press, centrifuge or belt press. The main objective of pig farmers to separate manure is to dispose P from the farm (pig manure has a low N:P ratio) while, for cattle/dairy farmers, also the use of the liquid fraction on grassland/agricultural land (easy to spread, high N:P ratio) is a major motivation. A centrifuge is more expensive (investment and operational cost) than a screw press and, as individual investment, a centrifuge is in most cases not feasible while a screw press, more adapted to moderate volumes of sludge or manure is more affordable. [1]

This consists in the separation of manure or digestate in a solid fraction with a higher organic matter and P content and a liquid fraction with a higher mineral N and K content. The liquid fraction usually has a higher ratio of mineral N over total N which implies a higher direct plant availability compared to raw unseparated digestate or animal slurries. On the other hand, as P is less soluble, it tends to end up predominantly in the solid fraction. This results in different N/P ratios in the two fractions compared to the unseparated digestate or slurry. Considering crop requirements and fertilisation management, the liquid fractions that have a high mineral N over total N as well as high N over P are considered better fertilising products than manure or digestate in their raw unprocessed form. However, the separation efficiency and subsequent division of P and N, depends on the ingoing raw manure or digestate which are subject to variability. This means that here as well, product homogeneity may become a key issue to address. [2]

[1] https://ec.europa.eu/eip/agriculture/sites/agri-eip/files/fg19_minipaper_1_state_of_the_art_en.pdf

[2] https://ec.europa.eu/eip/agriculture/sites/agri-eip/files/eip-agri_fg_nutrients_recycling_final_report_2017_en.pdf