Struvite Precipitation for Sustainable Recovery of Nitrogen and Phosphorus from Anaerobic Digestion Effluents of Swine Manure

Use of Phosphorus-Solubilizing Microorganisms as a Biotechnological Alternative: A Review

Microorganisms with the ability to dissolve phosphorus have the potential to release this essential nutrient into the soil through natural solubilization processes, which allows for boosting plant growth and development. While literature reviews acknowledge their potential, unexplored territories concerning accessibility, application, and effective integration into sustainable agriculture necessitate further research. This manuscript employed distinct methodologies to execute a bibliometric analysis and a literature review. The combined application of both methodologies enables a holistic understanding of the domain landscape and its innovative facets. For the bibliometric analysis, the propositions of Donthu and Jia were utilized, supplemented by tools, such as Bibliometrix. The literature review adhered to a systematic methodology predicated on Petersen's guidelines to represent the domain accurately, pinpointing trends and gaps that could steer future, more detailed research. This investigation uncovers an escalating interest in studying these microorganisms since the 2000s, emphasizing their significance in sustainable agriculture and the context of phosphorus scarcity. It was also discerned that India and China, nations with notable agricultural sectors and a high demand for phosphorus fertilizers, spearheaded research output on this subject. This signifies their substantial contribution to the progression of this scientific field. Furthermore, according to the research consulted, phosphorus-solubilizing microorganisms play a pivotal role in the symbiotic interaction of soil with plant roots and represent an efficacious strategy to counteract the low availability of phosphorus in the soil and sustainably enhance agricultural systems. Finally, this review contributes to the relevant domain by examining existing empirical evidence with special emphasis on sustainable agriculture, improved understanding of phosphorus solubilization mechanisms, and recognition of various microbial entities.

Heavy metal sorption on struvite recovered from livestock wastewaters and release properties of granular forms

Phosphorus recovery from wastewater is receiving more attention due to its non-renewable property. As copper (Cu) and zinc (Zn) usually occur in livestock wastewater, this study focused on metal sorption in struvite from swine wastewater and the release properties of granular struvite in solution with varying pH conditions (2, 4, 7). The results demonstrated pH values presented a slightly decreasing trend with increasing Cu/Zn ratio, and Zn exhibited higher sorption performance on struvite crystals than that of Cu. Under the high content of metals in the wastewater, Cu/Zn ratios in the wastewater contributed to varying metal binding forms and mechanisms, resulting in the difference in the leaching properties of nutrients and metal. For the granular struvite manufactured with the adhesion of alginate, the P release percentage achieved 30.3-40.5% after 96 h in the wastewater of pH 2, whereas they were only 5.63-8.92% and 1.05-1.50% in the wastewater of pH 4 and 7, respectively. Acid wastewater contributed to the release of two metals, and the release amount of Zn was higher than that of Cu, which is associated with their sorption capacity in crystals. During the latter soil leaching test of adding granular struvite, the NH4+-N and PO43--P concentration in the effluent ranged from 0.34 to 1.26 and 0.62 to 2.56 mg/L after 96 h, respectively. However, the Cu and Zn could not be measured due to lower than the detection limit under varying treatments. Struvite might be accompanied by quicker metal leaching and slower nutrient leaching when surface sorption dominates in wastewater with lower metal concentrations.

Nutrient released characteristics of struvite-biochar fertilizer produced from concentrated sludge supernatant by fluidized bed reactor

With the exacerbating water eutrophication globally, it is important to recover nitrogen (N) and phosphorus (P) from sewage for recycle. In this study, coconut shell biochar and ethylene diamine tetraacetic acid (EDTA) were added into the designed fluidized bed reactor (FBR) to create struvite-biochar. N and P released from struvite-biochar and the recovery efficiency of N and P from concentrated sludge supernatant were analyzed. Results showed that the optimal operation condition for hydraulic retention time (HRT), pH, Mg/P molar ration, and addition amount EDTA were 90 min, 9.5, 1.2, and 0.2 g/L, respectively. The recovery efficiency of NH₄⁺-N and PO₄^3--P, and purity struvite for FBR were 34.41%-38.05%, 64.95-68.40%, and 84.15%, respectively. The recovery efficiency of NH₄⁺-N and PO₄^3--P were respectively increased by 7.23% and 5.36% when FBR with addition of 0.33 g/L coconut shell biochar, but purity struvite from struvite-biochar decreased by 45.70%. Contents of As, Cd, Pb, and Cr in struvite and struvite-biochar were all lower than Chinese Standard Limits of Fertilizer. Compared to commercial chemical fertilizer, such as superphosphate and urea, struvite-biochar and struvite have slowly released N and P. The amounts of released P, NO₃^--N and NH₄⁺-N from struvite-biochar were higher than struvite during the five leaching times. Compared with struvite, the total amounts of released P, NO₃^--N and NH₄⁺-N from struvite-biochar increased by 4.9%, 3.5% and 8.3%, respectively. Therefore, it is valuable to add biochar into FBR to recovery N and P from concentrated sludge supernatant and make struvite-biochar as a slow-release fertilizer.

Real-Time Control Technology for a Bio-Liquor Circulation System in a Swine Barn with Slurry Pit: Pilot Scale Study

The livestock industry, especially swine production, has been pressurized by vicinity complaints about odor in Korea. Therefore, a lot of effort has been undertaken regarding reducing the odor emissions from pigsties, widely carried out and the washing out manure in slurry pit by liquid-phase compost has particularly been spotlighted with outstanding performance of odor reduction. However, such a washing out manure called bio-liquor circulation system (BCS) has been controlled by a timer with designated reaction time, which cannot guarantee the system performance. This research proposes an effective real-time control technology for BCS, which circulates bio-liquor to the slurry pit of swine barns. The real-time control system was operated through accurate detection of the designated control points on the oxidation reduction potential (ORP) and pH time profiles for the nitrate knee point (NKP) and nitrogen break point (NBP) in anoxic and aerobic conditions with 100 and 99.6% performances, respectively. The duration of the anoxic and aerobic phases was also automated and noticeably lowered the concentration of nutrients in the manure in the slurry-pit, which served as a source of malodor. The real-time control strategy may be an innovative way to reduce odor and simultaneously produce liquid fertilizer, and provides a reference for the optimization of the industrial scale.

Sustainable and efficient technologies for removal and recovery of toxic and valuable metals from wastewater: Recent progress, challenges, and future perspectives

Due to their numerous effects on human health and the natural environment, water contamination with heavy metals and metalloids, caused by their extensive use in various technologies and industrial applications, continues to be a huge ecological issue that needs to be urgently tackled. Additionally, within the circular economy management framework, the recovery and recycling of metals-based waste as high value-added products (VAPs) is of great interest, owing to their high cost and the continuous depletion of their reserves and natural sources. This paper reviews the state-of-the-art technologies developed for the removal and recovery of metal pollutants from wastewater by providing an in-depth understanding of their remediation mechanisms, while analyzing and critically discussing the recent key advances regarding these treatment methods, their practical implementation and integration, as well as evaluating their advantages and remaining limitations. Herein, various treatment techniques are covered, including adsorption, reduction/oxidation, ion exchange, membrane separation technologies, solvents extraction, chemical precipitation/co-precipitation, coagulation-flocculation, flotation, and bioremediation. A particular emphasis is placed on full recovery of the captured metal pollutants in various reusable forms as metal-based VAPs, mainly as solid precipitates, which is a powerful tool that offers substantial enhancement of the remediation processes' sustainability and cost-effectiveness. At the end, we have identified some prospective research directions for future work on this topic, while presenting some recommendations that can promote sustainability and economic feasibility of the existing treatment technologies.

A comprehensive techno-economic analysis of income-generating sources on the conversion of real sheep slaughterhouse waste stream into valorized by-products

The present analysis was conducted as the first research to assess the techno-economic viability of the value-added by-products (struvite, blood meal, bone meal, and raw sheepskin) from a medium-scale sheep slaughterhouse facility with a slaughtering capacity of 300 sheep per day. For this aim, a comparative technical and economic feasibility analysis was performed to assess the synergistic use of slaughterhouse-oriented rendering wastes and struvite recovery from real sheep abattoir effluent within the framework of detailed cost breakdown, break-even point, and payback period analyses. The experimental findings clearly showed that under the optimal conditions (chemical combination of MgCl₂.6H₂O + NaH₂PO₄.2H₂O, a molar ratio of Mg²⁺:NH₄⁺-N:PO₄^3--P = 1.2:1:1, a reaction pH of 9.0, an initial ammonium concentration of 240 mg NH₄⁺-N/L, and a reaction time of 15 min), struvite precipitation could effectively remove about 73%, 64%, 59%, and 82% of NH₄⁺-N, TCOD, SCOD, and color, respectively, from the real sheep slaughterhouse waste stream. Based on various up-to-date techno-economic items considered within the break-even point analysis, the sheep slaughterhouse facility was estimated to achieve the targeted net income (€100/day) for any selling prices of €1041.30/ton, €640.05/ton, €263.72/ton, and €1.012/hide, respectively, for struvite, blood meal, bone meal, and raw sheepskin. Steel construction and chemicals were determined as the most costly components for CAPEX (capital expenditures) and OPEX (operating expenditures), respectively, and selling prices of bone meal and raw sheepskin were found to be the most critical income items on the profitability of the slaughterhouse facility. Co-monetary assessment of the struvite process and valorized compounds corroborated the economic viability of the proposed project with the payback periods of about 6.3 and 5.5 years, respectively, for the current market and the profit-oriented conditions without subsidy. The findings of this feasibility analysis, as the first of its own, could be used as guideline for simplifying the decision-making with regards to the feasibility of similar facilities and commercialization of profitable by-products.

A review on the incorporation and potential mechanism of heavy metals on the recovered struvite from wastewater

Phosphorus, as a non-renewable element, is flowing out too fast in the past decades. To sustain the development of this globally scarce resource, efficient measures were taken to recover more phosphorus in the struvite form from wastewater. However, heavy metals in the wastewater might produce an inhibitory effect on phosphorus recovery, and even worse, pollutants might be incorporated in/onto the crystals precipitated. Impurities on struvite will reduce the quality of struvite as a potential slow-release fertilizer and affect the safe application of struvite in agriculture. This review aims to identify the trends in the literature to present the residues of heavy metals in struvite. It summarizes the current status in the residues of main metal elements on crystals and its response to wastewater properties, composition, and oxidation state of metals. The adsorption process and potential adsorption mechanism of heavy metals during the struvite crystallization are deeply explored, which might determine the latter release rate of metals when applying into the soil. Possible solutions are further provided to minimize the amounts of heavy metals mainly through adjusting operational conditions or employing pretreatment methods. Finally, this review critically analyzes the limitation gap between theory and actual generalization and potential application of struvite products in the market, and corresponding perspectives in the future are given to safely utilize the phosphorus resource from wastewater in the form of struvite.

High-Solid Anaerobic Digestion: Reviewing Strategies for Increasing Reactor Performance

High-solid and solid-state anaerobic digestion are technologies capable of achieving high reactor productivity. The high organic load admissible for this type of configuration makes these technologies an ideal ally in the conversion of waste into bioenergy. However, there are still several factors associated with these technologies that result in low performance. The economic model based on a linear approach is unsustainable, and changes leading to the development of a low-carbon model with a high degree of circularity are necessary. Digestion technology may represent a key driver leading these changes but it is undeniable that the profitability of these plants needs to be increased. In the present review, the digestion process under high-solid-content configurations is analyzed and the different strategies for increasing reactor productivity that have been studied in recent years are described. Percolating reactor configurations and the use of low-cost adsorbents, nanoparticles and micro-aeration seem the most suitable approaches to increase volumetric production and reduce initial capital investment costs.