Phosphorus recovery from wastewater via calcium phosphate precipitation: A critical review of methods, progress, and insights

Sustainable media feedstocks for cellular agriculture

The global food system is shifting towards cellular agriculture, a second domestication marked by cultivating microorganisms and tissues for sustainable food production. This involves tissue engineering, precision fermentation, and microbial biomass fermentation to establish food value chains independent of traditional agriculture. However, these techniques rely on growth media sourced from agricultural, chemical (fossil fuels), and mining supply chains, raising concerns about land use competition, emissions, and resource depletion. Fermentable sugars, nitrogen, and phosphates are key ingredients derived from starch crops, energy-intensive fossil fuel based processes, and finite phosphorus resources, respectively. This review explores sustainable alternatives to reduce land use and emissions associated with cellular agriculture media ingredients. Sustainable alternatives to first generation sugars (lignocellulosic substrates, sidestreams, and gaseous feedstocks), sustainable nitrogen sources (sidestreams, green ammonia, biological nitrogen fixation), and efficient use of phosphates are reviewed. Especially cellulosic sugars, gaseous chemoautotrophic feedstocks, green ammonia, and phosphate recycling are the most promising technologies but economic constraints hinder large-scale adoption, necessitating more efficient processes and cost reduction. Collaborative efforts are vital for a biotechnological future grounded in sustainable feedstocks, mitigating competition with agricultural land and emissions.

Assessment of three different approaches for integrating phosphorus recovery from sewage sludge and derived products in existing wastewater treatment plants

Three different technological solutions, namely acidogenic fermentation and chemical extraction (alkaline or acidic), followed by precipitation with 1% Ca(OH)2, were investigated in the view of integrating phosphorus recovery into existing wastewater treatment plants. Experiments were conducted at the lab-scale using (i) sludge taken from biologically and chemically promoted phosphorus removal activated sludge processes and (ii) ashes obtained from sludge muffle incineration. Results highlighted the benefits of enhanced biological phosphorus removal (EBPR) systems rather than chemically promoted phosphorus removal in not only phosphorus extraction (up to 40% with EBPR) and recovery directly from secondary sludge (P precipitation between 66 and 92%), but after sludge incineration as well (P extraction up to 96% and precipitation above 96%). Acidogenic fermentation ensured the highest phosphorus release from EBPR sludge (equal to a concentration in solution of 122 mg/L P-PO43-), while the derived ashes had a lower level of metal contamination (particularly Fe and Al content < 2%). The phosphorus-rich product obtained by means of the recovery process showed relevant metal contamination (Cu, Zn, and Ni) under some operating conditions, suggesting the need for further treatments.

Role of phosphate in microalgal-bacterial symbiosis system treating wastewater containing heavy metals

Phosphorus is one of the important factors to successfully establish the microalgal-bacterial symbiosis (MABS) system. The migration and transformation of phosphorus can occur in various ways, and the effects of phosphate on the MABS system facing environmental impacts like heavy metal stress are often ignored. This study investigated the roles of phosphate on the response of the MABS system to zinc ion (Zn2+). The results showed that the pollutant removal effect in the MABS system was significantly reduced, and microbial growth and activity were inhibited with the presence of Zn2+. When phosphate and Zn2+ coexisted, the inhibition effects of pollutants removal and microbial growth rate were mitigated compared to that of only with the presence of Zn2+, with the increasing rates of 28.3% for total nitrogen removal, 48.9% for chemical oxygen demand removal, 78.3% for chlorophyll-a concentration, and 13.3% for volatile suspended solids concentration. When phosphate was subsequently supplemented in the MABS system after adding Zn2+, both pollutants removal efficiency and microbial growth and activity were not recovered. Thus, the inhibition effect of Zn2+ on the MABS system was irreversible. Further analysis showed that Zn2+ preferentially combined with phosphate could form chemical precipitate, which reduced the fixation of MABS system for Zn2+ through extracellular adsorption and intracellular uptake. Under Zn2+ stress, the succession of microbial communities occurred, and Parachlorella was more tolerant to Zn2+. This study revealed the comprehensive response mechanism of the co-effects of phosphate and Zn2+ on the MABS system, and provided some insights for the MABS system treating wastewater containing heavy metals, as well as migration and transformation of heavy metals in aquatic ecosystems.

Internal phosphorus loading in a chain of eutrophic hardwater lakes in Saskatchewan, Canada

Sediments can act as a source or sink of phosphorus (P) for the water column of lakes. In iron (Fe)-rich softwater lakes, redox processes are important contributors to sediment P flux. However, the contribution of redox processes to P flux in hardwater lakes, with high pH and high concentrations of redox-insensitive calcium (Ca) is unknown. Intact sediment cores, collected in different seasons (summer or fall) from a chain of eutrophic hardwater lakes in southeastern Saskatchewan, Canada, were used to quantify sediment P fluxes in laboratory incubations under hypoxic or oxic conditions at temperatures consistent with the season of sample collection. Geochemical analyses determined concentrations of sediment total (TP) and organic P (TPo), organic matter (OM), total Ca and magnesium, and total and extractable manganese, Fe and aluminum. Sediment P pools were determined using sequential fractionation and solution 31P nuclear magnetic resonance spectroscopy. Sediment P fluxes were significantly higher in sediment cores incubated under hypoxic conditions (-24.4 to 28.5 mg P m-2 d-1) than oxic conditions (-60.3 to 14.2 mg P m-2 d-1). There were significant seasonal and lake differences for TP, TPo and cation concentrations, with Ca the dominant cation in all but one lake. Phosphate bound in the redox-sensitive pool was the only sediment P fraction that significantly differed among the lakes (0.10 to 0.18 mg P g-1 d.w.; 9 to 16 % of TP), with an inverse relationship to sediment P flux. Principal component analysis suggests that high concentrations of internally-generated TPo forms and OM in surface sediments play a key role in internal P loading in these lakes. However, sediment Ca appears to have an overriding effect on sediment P, partially masking the impact of redox control on internal P loading in these hardwater prairie lakes.

Utilizing waste eggshells as a calcium precursor for contact precipitation of phosphorus from digested sludge centrate

Phosphorus (P) recovery from wastewater is an essential component of the global P cycle. A contact precipitation process using chicken eggshells as a calcium (Ca) precursor was used to recover P from synthetic wastewater and real digested sludge centrate. Up to 96.4 % of P could be recovered from the digested sludge centrate after three repeated cycles of the contact precipitation process. In addition, 36.1 % of total chemical oxygen demand and 37.6 % of total ammonia nitrogen were removed from the centrate. Finally, most of the precipitates could be collected by a simple washing step. Scanning electron microscopy-energy dispersive spectroscopy and x-ray diffraction results indicated that the eggshells played three roles in this process: Ca source, precipitation substrate, and filter medium. Precipitates were mainly brushite. This research provides a new perspective on P recovery from wastewater using waste eggshells, and if further optimized, has a potential for practical future applications.

Enhancing microalgae biomass production: Exploring improved scraping frequency in a hybrid cultivation system

Recently, hybrid systems, such as those incorporating high-rate algal ponds (HRAPs) and biofilm reactors (BRs), have shown promise in treating domestic wastewater while cultivating microalgae. In this context, the objective of the present study was to determine an improved scraping frequency to maximize microalgae biomass productivity in a mix of industrial (fruit-based juice production) and domestic wastewater. The mix was set to balance the carbon/nitrogen ratio. The scraping strategy involved maintaining 1 cm wide stripes to retain an inoculum in the reactor. Three scraping frequencies (2, 4, and 6 days) were evaluated. The findings indicate that a scraping frequency of each 2 days provided the highest biomass productivity (18.75 g total volatile solids m-2 d-1). The species' behavior varied with frequency: Chlorella vulgaris was abundant at 6-day intervals, whereas Tetradesmus obliquus favored shorter intervals. Biomass from more frequent scraping demonstrated a higher lipid content (15.45%). Extrapolymeric substance production was also highest at the 2-day frequency. Concerning wastewater treatment, the system removed 93% of dissolved organic carbon and ∼100% of ammoniacal nitrogen. Combining industrial and domestic wastewater sources to balance the carbon/nitrogen ratio enhanced treatment efficiency and biomass yield. This study highlights the potential of adjusting scraping frequencies in hybrid systems for improved wastewater treatment and microalgae production.

Competition between homogeneous and heterogeneous crystallization of CaCO3 during water softening

Homogeneous and heterogeneous crystallization of CaCO3 simultaneously occur in seed-induced crystallization during water softening, while suppressing homogeneous crystallization is necessary due to the production of fine particulates that poorly precipitate. However, homogeneous crystallization is difficult to distinguish from heterogeneous crystallization. Consequently, a central focus in improving water softening is understanding their competing activities. In this study, a novel method for distinguishing homogeneous and heterogeneous calcium carbonate crystallization is described that utilizes magnetite as seed particles. Results showed that saturation index (SI) was the primary driver of both homogeneous and heterogeneous crystallizations. Heterogeneous crystallization was preferentially promoted at low SI, while homogeneous crystallization was promoted at high SI. The highest suppression effect to homogeneous crystallization occurred at SI of about 1.01. Seed dosage and mean particle size were the primary parameters related to the competition of the crystallization types. Higher seed dosage and smaller seed particle sizes promoted heterogeneous crystallization and suppressed homogeneous crystallization. Due to the good adaptability of heterogeneous crystallization at low SI, the absorption of CO2 from the air into the solutions also improved the efficiency of hardness removal. The introduction of seed particles did not change crystalline product phases, with calcite being the only observed phase and possessing rhombohedral forms with highly regular and smooth edges. Water softening pilot test results showed that SI of 1.5 was more favorite for CaCO3 layer formation on seed surface and hardness removal in comparison with SI of 1.0 and 2.0. Overall, the results from this study demonstrate that the introduction of seed particles is a promising approach to suppress the homogeneous crystallization of CaCO3. Moreover, these results can serve as a framework for improved seed-induced crystallization during water softening.

Enhanced removing of phosphate ions from agricultural drainage wastewater by using microwave-assisted synthesized attapulgite (Fullers earth) @carboxymethylcellulose nanocomposite

Contamination of various water resources with phosphate pollutant owing to the excessive use of phosphate fertilizers was labeled by dangerous consequences. Most of the water remediation methods are not efficient for phosphate recovery and always generate secondary wastes. Therefore, the current study is aimed to prepare a novel ecofriendly and sustainable APT500@CMC nanocomposite via simple covalent binding of thermally treated attapulgite clay at 500 °C (APT500) with carboxymethyl cellulose (CMC) using microwave irradiation process. The assembled nanocomposite was confirmed by diverse techniques. The optimum conditions for efficient 10, 25 and 50 mg/L PO43- removal were detected at pH 3, time 30 min, temperature 25 °C and mass 200 mg. The kinetic and isotherms were fitted both to a combination of pseudo 1st - 2nd orders and Langmuir model, while thermodynamic parameters verified PO43- removal via spontaneous and exothermic reaction behavior. The mode of interaction and binding of PO43- ions onto the surface of APT500@CMC were suggested via ion-pair interaction process. Excellent PO43- recovery (98.8 %) from real agricultural drainage wastewater was established. The explored APT500@CMC afforded good stability for five regeneration cycles. Therefore, the collected results confirm the validity of APT500@CMC for excellent removal of PO43- from real agricultural drainage wastewater.

Wastewater Treatment for Carbon Dioxide Removal

Wastewater treatment is notorious for its hefty carbon footprint, accounting for 1-2% of global greenhouse gas (GHG) emissions. Nonetheless, the treatment process itself could also present an innovative carbon dioxide removal (CDR) approach. Here, the calcium (Ca)-rich effluent of a phosphorus (P) recovery system from municipal wastewater (P recovered as calcium phosphate) was used for CDR. The effluent was bubbled with concentrated CO2, leading to its mineralization, i.e., CO2 stored as stable carbonate minerals. The chemical and microstructural properties of the newly formed minerals were ascertained by using state-of-the-art analytical techniques. FTIR identified CO3 bonds and carbonate stretching, XRF and SEM-EDX measured a high Ca concentration, and SEM imaging showed that Ca is well distributed, suggesting homogeneous formation. Furthermore, FIB-SEM revealed rhombohedral and needle-like structures and TEM revealed rod-like structures, indicating that calcium carbonate (CaCO3) was formed, while XRD suggested that this material mainly comprises aragonite and calcite. Results imply that high-quality CaCO3 was synthesized, which could be stored or valorized, while if atmospheric air is used for bubbling, a partial direct air capture (DAC) system could be achieved. The quality of the bubbled effluent was also improved, thus creating water reclamation and circular economy opportunities. Results are indicative of other alkaline Ca-rich wastewaters such as effluents or leachates from legacy iron and steel wastes (steel slags) that can possibly be used for CDR. Overall, it was identified that wastewater can be used for carbon mineralization and can greatly reduce the carbon footprint of the treatment process, thus establishing sustainable paradigms for the introduction of CDR in this sector.

Development of Hydroxyapatite Coatings for Orthopaedic Implants from Colloidal Solutions: Part 1-Effect of Solution Concentration and Deposition Kinetics

This study introduces and explores the use of supersaturated solutions of calcium and phosphate ions to generate well-defined hydroxyapatite coatings for orthopaedic implants. The deposition of hydroxyapatite is conducted via several solutions of metastable precursors that precipitate insoluble hydroxyapatite minerals at a substrate-solution interface. Solutions of this nature are intrinsically unstable, but this paper outlines process windows in terms of time, temperature, concentration and pH in which coating deposition is controlled via the stop/go reaction. To understand the kinetics of the deposition process, comparisons based on ionic strength, particle size, electron imaging, elemental analyses and mass of the formed coating for various deposition solutions are carried out. This comprehensive dataset enables the measurement of deposition kinetics and identification of an optimum solution and its reaction mechanism. This study has established stable and reproducible process windows, which are precisely controlled, leading to the successful formation of desired hydroxyapatite films. The data demonstrate that this process is a promising and highly repeatable method for forming hydroxyapatites with desirable thickness, morphology and chemical composition at low temperatures and low capital cost compared to the existing techniques.

Chitosan Hydrogels for Water Purification Applications

Chitosan-based hydrogels have gained significant attention for their potential applications in water treatment and purification due to their remarkable properties such as bioavailability, biocompatibility, biodegradability, environmental friendliness, high pollutants adsorption capacity, and water adsorption capacity. This article comprehensively reviews recent advances in chitosan-based hydrogel materials for water purification applications. The synthesis methods, structural properties, and water purification performance of chitosan-based hydrogels are critically analyzed. The incorporation of various nanomaterials into chitosan-based hydrogels, such as nanoparticles, graphene, and metal-organic frameworks, has been explored to enhance their performance. The mechanisms of water purification, including adsorption, filtration, and antimicrobial activity, are also discussed in detail. The potential of chitosan-based hydrogels for the removal of pollutants, such as heavy metals, organic contaminants, and microorganisms, from water sources is highlighted. Moreover, the challenges and future perspectives of chitosan-based hydrogels in water treatment and water purification applications are also illustrated. Overall, this article provides valuable insights into the current state of the art regarding chitosan-based hydrogels for water purification applications and highlights their potential for addressing global water pollution challenges.