Struvite—An Innovative Fertilizer from Anaerobic Digestate Produced in a Bio-Refinery

Advancing sustainable wastewater management: A comprehensive review of nutrient recovery products and their applications

Wastewater serves as a vital resource for sustainable fertilizer production, particularly in the recovery of nitrogen (N) and phosphorus (P). This comprehensive study explores the recovery chain, from technology to final product reuse. Biomass growth is the most cost-effective method, valorizing up to 95 % of nutrients, although facing safety concerns. Various techniques enable the recovery of 100 % P and up to 99 % N, but challenges arise during the final product crystallization due to the high solubility of ammonium salts. Among these techniques, chemical precipitation and ammonia stripping/ absorption have achieved full commercialization, with estimated recovery costs of 6.0-10.0 EUR kgP-1 and 4.4-4.8 £ kgN-1, respectively. Multiple technologies integrating biomass thermo-chemical processing and P and/or N have also reached technology readiness level TRL = 9. However, due to maturing regulatory of waste-derived products, not all of their products are commercially available. The non-homogenous nature of wastewater introduces impurities into nutrient recovery products. While calcium and iron impurities may impact product bioavailability, some full-scale P recovery technologies deliver products containing this admixture. Recovered mineral nutrient forms have shown up to 60 % higher yield biomass growth compared to synthetic fertilizers. Life cycle assessment studies confirm the positive environmental outcomes of nutrient recycling from wastewater to agricultural applications. Integration of novel technologies may increase wastewater treatment costs by a few percent, but this can be offset through renewable energy utilization and the sale of recovered products. Moreover, simultaneous nutrient recovery and energy production via bio-electrochemical processes contributes to carbon neutrality achieving. Interdisciplinary cooperation is essential to offset both energy and chemicals inputs, increase their cos-efficiency and optimize technologies and understand the nutrient release patterns of wastewater-derived products on various crops. Addressing non-technological factors, such as legal and financial support, infrastructure redesign, and market-readiness, is crucial for successfully implementation and securing the global food production.

Environmental impact and optimization suggestions of pig manure and wastewater treatment systems from a life cycle perspective

With the rapid development of the pig farming industry in China, a large amount of pig manure is inevitably generated. The management and utilization of pig manure in a sustainable approach require a systematic analysis of the environmental impacts generated from the existing pig manure treatment and disposal routes. In this study, three typical pig manure treatment and disposal routes: pig manure solid-liquid separation, i) wastewater biological treatment, direct land application of manure/sludge; ii) black-film anaerobic digestion of wastewater, mono-composting of manure/sludge and land use; iii) wastewater biological treatment, co-composting of manure/sludge and land use, were comparatively assessed in terms of their environmental impacts using the life cycle assessment. The results show that the added chemicals in wastewater treatment and the consumed electricity in composting the manure/sludge are the two main contributors to all environmental impacts. Thus, using green chemicals, controlling the dosage of added chemicals accurately, and selecting composting types with low energy consumption will significantly reduce the environmental burden of pig manure treatment and disposal routes. For the global warming potential of the evaluated three pig manure treatment and disposal routes, direct emissions of greenhouse gases during the composting process contribute the most, accounting for 77 %, 95 %, and 79 %, respectively. Furthermore, the struvite recovery with anaerobic digestion from pig manure will bring excellent environmental benefits, which will markedly offset the toxicity impacts and carbon emissions derived from pig manure treatment and disposal process. Overall, this work quantitatively evaluates the potential environmental impacts of the existing pig manure treatment and disposal routes, providing insights on optimization for future technical improvement and development.

Effect of solar and artificial lighting on microalgae cultivation and treatment of liquid digestate

A comparative study was carried out to assess the effect of two light sources on microalgae cultivation and the treatment of liquid digestate. The R1 photobioreactor operated with LED lightning allowed to achieve moderate nutrient removal rates whereas soluble COD (Chemical Oxygen Demand) was reduced in 90%. After switching this reactor into sunlight, the removal rate of phosphates increased to 66%. However, the greatest removal rate of both nutrients and sCOD of up to 93% was observed in the R2 photobioreactor operated only under sunlight. Microglena sp. was the dominant algae growing in the R1 reactor, and the main bacteria families detected were Chitinophagaceae, Sphingomonadaceae and Xanthobacteraceae. In contrast, Tetradesmus obliquus dominated in the R2 reactor and Rhodanobacteraceae, Chitinophagaceae and A4b were predominant bacteria in this run. Furthermore, much greater biomass productivity as well as overall biomass density was observed in the R2 photobioreactor cultivated exclusively with solar lightning.

Effect of struvite (Crystal Green) fertilization on soil element content determined by different methods under soybean cultivation

Struvite is regarded as a promising phosphorus fertilizer alternative to mineral fertilizers; however before fertilizing, soil tests should be undertaken to determine fertilizer recommendations. In May 2022, soil was sampled from a pot experiment with the application of phosphorus set up at the Wroclaw University and Environmental and Life Sciences. Chemical analysis of the soil included total and available phosphorus, potassium, magnesium determined by the Egner-Riehm, Mehlich 3 and Yanai methods. The purpose of the article is to compare soil element extraction by three different methods under struvite fertilization and its use in soybean cultivation. The application of these methods indicated an unambiguous increase in soil Mg content after struvite application. Broadcast soybean fertilization affected the phosphorus content of the soil. The results of the study indicated that different extraction methods presented different contents of P from soil. The content of available phosphorus was circa 122-156 mg kg-1 dm, 35.4-67.5 mg kg-1 dm and 100-159 mg kg-1 dm according to the Mehlich, Yanai and Egner-Riehm methods, respectively. A positive correlation was found between the content of Mg and K in soil determined by the Mehlich 3 and Yanai methods, which may suggest that the Yanai method could be introduced into standard soil chemical analysis in Poland. Such a correlation was not found for phosphorus, which is a difficult element to determine due to the multitude of factors affecting its availability.

Recycling of nutrients from landfill leachate: A case study

The continuous increase in the consumption of natural resources requires different solutions directed to the recovery and recycling of different materials and products, including the nutrients used as fertilizers for food production. In this context, this research assessed the feasibility of using landfill leachate as a source of nutrients for the growth of maize. Leachate was treated to precipitate struvite, a rich magnesium, phosphate, and ammonium mineral that can be applied directly as fertilizer. It was used for the growth of maize, which was sowed in three different parcels. A commercial DAP + urea mixture was used to compare, and non-fertilized parcels were used as controls. Struvite was successfully obtained and applied in the fields. A marginal higher maize yield was achieved in two sites when using struvite (6.36% and 2.16%) compared to the commercial fertilizer, even if it was applied in a lower dose to weather conditions. An increase in N and Mg in soil could be observed, which allowed for the assimilation of nutrients in the plants. Concerning safety, the use of struvite did not produce the transfer of heavy metals or pathogens to the soil or plants. This research shows a promising way of dealing with leachate, which could be attractive in countries where organic waste is buried in landfills.

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Landfill leachate can be used as a source of nutrients for the grow of maize by precipitation of struvite.

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A field trial in real scale was performed.

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A marginal higher maize yield was achieved in two of the sites (6.36% and 2.16%) when compared to the commercial fertilizer.

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Struvite did not cause presence of pathogens or heavy metals in the crops.

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It offers an alternative to conventional leachate treatment options, aligned with the principles of the circular economy.

Correlations between the Composition of Liquid Fraction of Full-Scale Digestates and Process Conditions

Fast development of centralized agricultural biogas plants leads to high amounts of digestate production. The treatment and disposal of liquid fractions after on-site digestate solid–liquid separation remains problematic due to their high organic, nutrient and aromatic contents. This work aims to study the variability of the remaining compounds in the digestate liquid fractions in relation to substrate origin, process parameters and solid–liquid separation techniques. Twenty-nine digestates from full-scale codigestion biogas plants and one waste activated sludge (WAS) digestate were collected and characterized. This study highlighted the combined effect of the solid–liquid separation process and the anaerobic digestion feedstock on the characteristics of liquid fractions of digestates. Two major clusters were found: (1) liquid fractions from high efficiency separation process equipment (e.g., centrifuge and others with addition of coagulant, flocculent or polymer) and (2) liquid fractions from low efficiency separation processes (e.g., screw press, vibrating screen and rotary drum), in this latter case, the concentration of chemical oxygen demand (COD) was associated with the proportion of cow manure and energy crops at biogas plant input. Finally, SUVA254, an indicator for aromatic molecule content and the stabilization of organic matter, was associated with the hydraulic retention time (HRT).

Evaluating the Struvite Recovered from Anaerobic Digestate in a Farm Bio-Refinery as a Slow-Release Fertiliser

Biogas production in agricultural biogas plants generates digestate—liquid waste containing organic matter and mineral nutrients. Utilisation of the digestate on farm fields adjacent to the biogas plants is limited. Therefore, bio-refineries implement advanced forms of digestate processing, including precipitation of struvite (MgNH4PO4.6H2O). Struvite can be transported over long distances and dosed precisely to meet the nutritional needs of the plants. Divergent opinions on the fertilising value of struvite and its function over time call for further research on its effects on crop yields in the first and subsequent years after application. This study investigates the effects of struvite (STR), struvite with ammonium sulphate (STR + N), and commercial ammonium phosphate (AP) on the yields, nutrient concentration in the crops, nutrient uptake by the crops, and soil N, P, and Mg content in the second growing period after the application of fertilisers to silty loam (SL) and loamy sand (LS) soils under grass cultivation. Struvite was recovered from the liquid fraction of digestate obtained from a bio-refinery on the De Marke farm (Netherlands). The soils investigated in the pot experiment originated from Obory (SL) and Skierniewice (LS) (Central Poland). The results obtained over the first growing period following fertilisation were published earlier. In our prior work, we showed that the majority of the struvite phosphorus remains in the soil. We hypothesised that, in the second year, the yield potential of the struvite might be higher than that of commercial P fertiliser. Currently, we have demonstrated that, in the second growing period following the application, struvite causes an increase in grass yield, nutrient uptake by the crops, and P and Mg content in the soil. On SL and LS soils, the yields of the four grass harvests from the STR and STR + N treatments were higher than those from AP by approximately 8% and 16.5%, respectively. Our results confirm that struvite is more effective as a fertiliser compared to commercial ammonium phosphate. Struvite can be, therefore, recommended for fertilising grasslands at higher doses once every two years.

Duckweed from a Biorefinery System: Nutrient Recovery Efficiency and Forage Value

This paper presents the results of an interdisciplinary study aimed at assessing the possibility of using duckweed to purify and recover nutrients from the effluent remaining after struvite precipitation and ammonia stripping from a liquid fraction of anaerobic digestate in a biorefinery located at a Dutch dairy cattle production farm. The nutritional value of duckweed obtained in a biorefinery was assessed as well. Duckweed (Lemna minuta) was cultured on a growth medium with various concentrations of effluent from a biorefinery (EFL) and digested slurry (DS) not subjected to the nutrient recovery process. The study’s results showed that duckweed culture on the media with high contents of DS or EFL was impossible because they both inhibited its growth. After 15 days of culture, the highest duckweed yield was obtained from the ponds with DS or EFL contents in the medium reaching 0.39% (37.8 g fresh matter (FM) and 16.8 g FM per 8500 mL of the growth medium, respectively). The recovery of N by duckweed was approximately 75% and 81%, whereas that of P was approximately 45% and 55% of the growth media with EFL0.39% and DS0.39%, respectively. Duckweed obtained from the biorefinery proved to be a valuable high-protein feedstuff with high contents of α-tocopherol and carotenoids. With a protein content in duckweed approximating 35.4–36.1%, it is possible to obtain 2–4 t of protein per 1 ha from EFL0.39% and DS0.39% ponds, respectively.

Struvite formation associated with the microalgae biofilm matrix of a rotating algal biofilm reactor (RABR) during nutrient removal from municipal wastewater

Struvite was observed within the microalgae biofilm matrix of an outdoor, pilot-scale rotating algal biofilm reactor (RABR) designed to remove nitrogen and phosphorus from municipal anaerobic digester filtrate. The bottom layer of cells (2.5-month growth) and two top layers of cells (1-week and 2.5-month growth) were evaluated on east- and west-facing sides of the RABR. Sun orientation and shading effects of upper biofilm layers impacted the species composition and microalgae content of the bottom biofilm layers. Struvite formed within the microalgae biofilm matrix, and a higher struvite content appeared to be correlated with a higher microalgae content. The highest struvite content (expressed as %wt. of total solids) was observed in the east- and west-facing bottom layers of growth and west-facing 1-week growth (5.0%, 4.3%, and 4.1%, respectively). The lowest struvite content was observed in east- and west-facing 2.5-month growth and east-facing 1-week growth (1.1%, 1.5%, and 1.1%, respectively). Despite RABR influent component ion molar ratios with potential for various magnesium and calcium precipitates, microalgae biofilm provided pH and nucleation sites favorable to struvite precipitation. This evaluation is the first in the refereed literature the authors are aware of that reports on the association of struvite formation in the presence of a microalgae biofilm.

Ammonium Sulphate from a Bio-Refinery System as a Fertilizer—Agronomic and Economic Effectiveness on the Farm Scale

This paper presents the results of a pot experiment aimed at the assessment of the agronomic and economic effectiveness of ammonium sulphate from an agro bio-refinery (Bio-AS). The Bio-AS was obtained by means of the ammonia stripping process from effluent after struvite precipitation from a liquid fraction of digestate. The agronomic effectiveness of Bio-AS in a pot experiment with maize and grass in two different soils, silty loam (SL) and loamy sand (LS), was investigated. The fertilising effect of Bio-AS was compared to commercial ammonium sulphate fertilizer (Com-AS) and control treatment (without fertilisation). The crop yields were found to depend on both soil type and nitrogen treatment. Crop yields produced under Bio-AS and Com-AS exceeded those under control treatments, respectively for SL and LS soils, by 88% and 125% for maize and 73% and 94% for grass. Crop yields under Bio-AS were similar to those under the Com-AS treatment. The fertilizer use of Bio-AS affected the chemical composition of plants and soil properties similarly as Com-AS. This suggests that Bio-AS from a bio-refinery can replace industrial ammonium sulphate, resulting in both economic and environmental benefits.

Organic wastes as alternative sources of phosphorus for plant nutrition in a calcareous soil

Recycled organic wastes (OW) can be a valuable P source; however, their P-fertilising capacity is still poorly known. In this study, we selected three anaerobic digestates [wastewater sludge (D₁), winery sludge (D₂), and bovine-slurry/energy crops (BD)] and two animal effluents [bovine slurry (BS) and swine slurry (SS)] to test their P-release and P-fertilising capacities via sequential chemical extraction (SCE), X-ray diffraction (XRD), and ³¹P-nuclear magnetic resonance (³¹P NMR). Subsequently, the three digestates (30 mg P kg^-1 of soil) were compared for the release of Olsen-P during a soil incubation and for plant-P apparent recovery (ARF) in a pot experiment using ryegrass (112 days) in a soil with poorly available-P (Olsen-P < 5 mg kg^-1), under a non-limiting N environment. The amount of labile-P (H₂O + NaHCO₃), as determined from SCE, related well to the Olsen-P following OW addition to the soil. It was shown via ³¹P NMR spectroscopy that orthophosphate was the leading P-form in highly P-releasing OW. The amount of labile-P, however, was affected by soil adsorption, thereby reducing plant-P uptake. The plant-P ARF (%) showed that the recycled P-sources were clustered in highly (BD and SS: ≈20%), intermediately (D₁ and BS: ≈15%), and poorly performing OWs (D₂: ≈10%) vs. chemical P-source (P-chem: 20%). Therefore, only BD and SS were effective alternatives to P-chem; however, the other OW can be efficient P-sources in soils with higher Olsen-P. Thus, crop fertilisation can be tailored on a P-basis by SCE as a function of soil adsorption capacity and on an N-basis according to the demand.