Recycled Products from Municipal Wastewater: Composition and Effects on Phosphorus Mobility in a Sandy Soil

Phosphorus Fertilizers from Sewage Sludge Ash and Animal Blood as an Example of Biobased Environment-Friendly Agrochemicals: Findings from Field Experiments

Wastes of biological origin from wastewater treatment systems and slaughterhouses contain substantial amounts of phosphorus (P) with high recovery potential and can contribute to alleviating the global P supply problem. This paper presents the performance of fertilizer (AF) and biofertilizer (BF) from sewage sludge ash and animal blood under field conditions. BF is AF incorporated with lyophilized cells of P-solubilizing bacteria, Bacillus megaterium. In the experiments with spring or winter wheat, the biobased fertilizers were compared to commercial P fertilizer, superphosphate (SP). No P fertilization provided an additional reference. Fertilizer effects on wheat productivity and on selected properties of soil were studied. BF showed the same yield-forming efficiency as SP, and under poorer habitat conditions, performed slightly better than AF in increasing yield and soil available P. Biobased fertilizers applied at the P rate up to 35.2 kg ha^-1 did not affect the soil pH, did not increase As, Cd, Cr, Ni, and Pb content, and did not alter the abundance of heterotrophic bacteria and fungi in the soil. The findings indicate that biobased fertilizers could at least partially replace conventional P fertilizers. Research into strain selection and the proportion of P-solubilizing microorganisms introduced into fertilizers should be continued.

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.

Options for Improved Phosphorus Cycling and Use in Agriculture at the Field and Regional Scales

Soil phosphorus (P) cycling in agroecosystems is highly complex, with many chemical, physical, and biological processes affecting the availability of P to plants. Traditionally, P fertilizer recommendations have been made using an insurance-based approach, which has resulted in the accumulation of P in many intensively managed agricultural soils worldwide and contributed to the widespread water quality issue of eutrophication. To mitigate further environmental degradation and because future P fertilizer supplies are threatened due to finite phosphate rock resources and associated geopolitical and quality issues, there is an immediate need to increase P use efficiency (PUE) in agroecosystems. Through cultivar selection and improved cropping system design, contemporary research suggests that sufficient crop yields could be maintained at reduced soil test P (STP) concentrations. In addition, more efficient P cycling at the field scale can be achieved through agroecosystem management that increases soil organic matter and organic P mineralization and optimizes arbuscular mycorrhizal fungi (AMF) symbioses. This review paper provides a perspective on how agriculture has the potential to utilize plant and microbial traits to improve PUE at the field scale and accordingly, maintain crop yields at lower STP concentrations. It also links with the need to tighten the P cycle at the regional scale, including a discussion of P recovery and recycling technologies, with a particular focus on the use of struvite as a recycled P fertilizer. Guidance on directions for future research is provided.

X-Ray Spectroscopic Quantification of Struvite and Dittmarite Recovered from Wastewater

Phosphorus recovery from wastewater as struvite (MgNHPO⋅6HO) or dittmarite (MgNHPO⋅HO) can decrease water pollution risk, as well as produce a P-rich material suitable as fertilizer. However, most studies to date have focused on the removal of P from wastewater, rather than on characterization of the recovered P materials. The objective of this work was to apply microfocused X-ray fluorescence (XRF) spectroscopy, and both bulk and microfused X-ray absorption near edge structure (XANES) spectroscopy, to provide insight into the speciation of recovered P in various struvite-containing and struvite-like materials. Three materials were investigated: homogeneous crystalline struvite on apatite seed, homogeneous dittmarite, and heterogeneous struvite with sand contamination (referred to as the "sandy" material). The struvite materials were recovered from dairy wastewater, whereas the dittmarite was from a cheese processing plant. Phosphorus speciation in the crystalline struvite on apatite seed material was ∼17% apatite and 83% struvite; in the "sandy" material, P was ∼24% apatite and ∼76% struvite, with an uncertainty of approximately ±15%. The P -edge XANES spectra of recovered dittmarite appeared pure. These findings highlight the heterogeneity of recovered P materials and underscore the importance of P speciation to understand P release behavior and bioavailability from recovered phosphates.

Solubility, Diffusion and Crop Uptake of Phosphorus in Three Different Struvites

Phosphate (P) fertilisers produced from waste recycling (e.g., struvite) are considered to be more sustainable than those conventionally produced from the processing of rock P (e.g., highly soluble triple superphosphate, TSP). In this study, we used 33P to monitor struvite dissolution and P diffusion into the soil in comparison to TSP. We evaluated three distinct chemical formulations of struvite, namely: (1) Crystal Green® (CG) produced in an industrial process from sewage sludge; (2) natural struvite (NS) precipitated in swine manure pipelines; and (3) laboratory precipitated struvite (PS) from chicken manure by a new process of P recovery. P diffusion was evaluated in soil columns over a 21-day period. This was complimented with a pot experiment in which wheat and soybean were cultivated in a Eutric Cambisol for 38 days in the presence of either struvite or TSP. P fertilisers were applied at a dose equivalent to 17.5 kg P ha−1 and fertiliser solubility determined by recovering soil solution. All three types of struvite tested showed reduced P solubility and mobility relative to TSP, but a comparison of the three struvites has shown that their P solubilities differed by a factor of two, with the greatest P release (up to 85% of total P) obtained from a struvite recovered from poultry manure and containing other useful nutrients (K, S and Ca). All struvites enhanced crop growth and P uptake of wheat and soybean relative to a nil P control, with up to 80% P recovery compared to TSP. These results further support the more widespread use of struvite as a sustainable source of P to plants despite its low water solubility.

Phosphorus Sorption to Aluminum-based Water Treatment Residuals Reacted with Dairy Wastewater: 2. X-Ray Absorption Spectroscopy

Phosphorus capture from wastewater can decrease water pollution and provide a P-rich fertilizer alternative for use in agricultural production. This study was conducted to elucidate P retention mechanisms in Al-based water treatment residuals (Al-WTR) to gain insight regarding P sorption and the potential for P release from Al-WTR after reaction with dairy wastewater. Synchrotron-based microfocused X-ray fluorescence (micro-XRF) spectrometry, bulk P -edge X-ray absorption near edge structure spectroscopy (XANES), and P -edge micro-XANES spectroscopy were used to determine P distribution and speciation within the Al-WTR materials. Bulk XANES analyses indicated a shift from ∼56 P atom % Ca-associated P in the initial Al-WTR to ∼32% P atom % Ca-associated P after reaction with wastewater; Al-associated P made up the remainder of the P species. According to XANES analyses, adsorption appeared to be the primary P retention mechanism in the Al-WTR materials. However, micro-XANES analyses depicted a more complicated picture of P retention mechanisms, with regions of primarily Al-associated P, regions of primarily Ca-associated P, regions of mixed Al- and Ca-associated P, and distinct apatite- or octocalcium phosphate-like P grains. Synchrotron micro-XRF mapping further suggested that exposure of the aggregate exteriors to wastewater caused P to diffuse into the porous Al-WTR aggregates. Organic P species were not explicitly identified via P -edge XANES despite high organic matter content, suggesting that organic P may have been predominantly associated with mineral surfaces. Although diffusion and sorption to Al may decrease P bioavailability, Ca-associated P may increase P bioavailability from Al-WTR that is reused as a soil amendment.

Handling the phosphorus paradox in agriculture and natural ecosystems: Scarcity, necessity, and burden of P

This special issue of Ambio compiles a series of contributions made at the 8th International Phosphorus Workshop (IPW8), held in September 2016 in Rostock, Germany. The introducing overview article summarizes major published scientific findings in the time period from IPW7 (2015) until recently, including presentations from IPW8. The P issue was subdivided into four themes along the logical sequence of P utilization in production, environmental, and societal systems: (1) Sufficiency and efficiency of P utilization, especially in animal husbandry and crop production; (2) P recycling: technologies and product applications; (3) P fluxes and cycling in the environment; and (4) P governance. The latter two themes had separate sessions for the first time in the International Phosphorus Workshops series; thus, this overview presents a scene-setting rather than an overview of the latest research for these themes. In summary, this paper details new findings in agricultural and environmental P research, which indicate reduced P inputs, improved management options, and provide translations into governance options for a more sustainable P use.

Effect of iron sulphate on the phosphorus speciation from agro-industrial sludge based and sewage sludge based compost

Composting is considered a suitable process for organic waste management, providing stable products that can be safely utilized as fertilizers, but little is still known about the variation of phosphorous (P) extractability during the stabilization process. In this work, sequential chemical extraction (SCE) with increasing strength extractants (H₂O; 0.5M NaHCO₃ pH 8.5; 0.1M NaOH, 1M HCl) was applied for P speciation over 56days of composting of either agro-industrial or urban wastewater sludge with green waste treated (AIC_Fe+; SSC_Fe+) or not (AIC_Fe-; SSC_Fe-) with FeSO₄ (2%v/v). Composting strongly reduced the H₂O-P, promoting the organic-P (P_o) mineralization from the labile fraction (H₂O+NaHCO₃ 40%), in addition to the increases of NaHCO₃- and HCl-extractable inorganic-P (P_i) in both AIC_Fe- and SSC_Fe- (+20% on average). The FeSO₄ treatment did not negatively affect the process, reducing the P_o mineralization during composting by increasing the NaOH-P, also protecting this fraction from fixation in the sparingly soluble fraction. The final P fractionation (%) was in AIC_Fe-: NaOH (41)=NaHCO₃ (38)>HCl (18)>H₂O (3); in AIC_Fe+: NaOH (53)>NaHCO₃ (24)=HCl (22)>H₂O (2); in SSC_Fe-: NaOH (46)>NaHCO₃ (29)>HCl (21)>H₂O (4) and in SSC_Fe+: NaOH (66)>NaHCO₃ (13)>HCl (20)>H₂O (1). Composting reduced the more easily leachable fraction (labile-P_o), reducing the risk of P loss by increasing the long-term available P fraction (NaOH-P). This was enhanced by the FeSO₄ addition. Further investigation into soil behaviour and plant availability of P from this source is needed.