Insight into chemical phosphate recovery from municipal wastewater

A critical review on ammonium recovery from wastewater for sustainable wastewater management

The growing global population's demand for ammonium has triggered an increase in its supply, given that ammonium plays a crucial role in fertilizer production for the purpose of food security. Currently, ammonia used in fertilizer production is put through what is known as the industrial Haber Bosch process, but this approach is substantially expensive and requires much energy. For this reason, looking for effective methods to recover ammonium is important for environmental sustainability. One of the greatest opportunities for ammonium recovery occurs in wastewater treatment plants due to wastewater containing a large quantity of ammonium ions. The comprehensively and critically review studies on ammonium recovery conducted, have the potential to be applied in current wastewater treatment operations. Technologies and their ammonium recovery mechanisms are included in this review. Furthermore the economic feasibility of such processes is analysed. Possible future directions for ammonium recovery from wastewater are suggested.

Interaction of calcium, phosphorus and natural organic matter in electrochemical recovery of phosphate

To address the issues of eutrophication and the potential risk of phosphorus (P) shortage, it is essential to remove and recover P from P-containing streams to close this nutrient cycle. Electrochemical induced calcium phosphate (CaP) precipitation was shown to be an efficient method for P recovery. However, the influence of natural organic matter (NOM) is not known for this treatment. In this paper, the behavior of NOM and its effect on CaP precipitation was studied. In contrast to studies where NOM hindered CaP precipitation, results show that the interaction of NOM with CaP improves the removal of P, independent of the types of NOM. The P removal at the average increased from 43.8 ± 4.9% to 58.5 ± 1.2% in the presence of 1.0 mg L^-1 NOM. Based on the yellow color of the CaP product, NOM is co-precipitated. The bulk solution pH with and without buffers has totally different effects on the precipitation process. Without buffer, CaP precipitates on the cathode surface in a wide pH range (pH 4.0-10.0). However, the precipitation process is completely inhibited when the bulk solution is buffered at pH 4.0 and 6.0. This is probably due to neutralization of OH^- by the buffers. Regardless of the presence or absence of NOM and solution pH, the recovered products are mainly amorphous CaP unless the electrolysis time was increased to seven days with 4.0 A m^-2, in which crystalline CaP formed. These findings advance our understanding on the interaction of Ca, P and NOM species for the application of electrochemical method for P recovery from real wastewater.

Is There a Precipitation Sequence in Municipal Wastewater Induced by Electrolysis?

Electrochemical wastewater treatment can induce calcium phosphate precipitation on the cathode surface. This provides a simple yet efficient way for extracting phosphorus from municipal wastewater without dosing chemicals. However, the precipitation of amorphous calcium phosphate (ACP) is accompanied by the precipitation of calcite (CaCO₃) and brucite (Mg(OH)₂). To increase the content of ACP in the products, it is essential to understand the precipitation sequence of ACP, calcite, and brucite in electrochemical wastewater treatment. Given the fact that calcium phosphate (i.e., hydroxyapatite) has the lowest thermodynamic solubility product and highest saturation index in the wastewater, it has the potential to precipitate first. However, this is not observed in electrochemical phosphate recovery from raw wastewater, which is probably because of the very high Ca/P molar ratio (7.5) and high bicarbonate concentration in the wastewater resulting in formation of calcite. In the case of decreased Ca/P molar ratio (1.77) by spiking external phosphate, most of the removed Ca in the wastewater was used for ACP formation instead of calcite. The formation of of brucite, however, was only affected when the current density was decreased or the size of cathode was changed. Overall, the removal of Ca and Mg is much more affected by current density than the surface area of cathode, whereas for P removal, the reverse is true. Because of these dependencies, though there is no definite precipitation sequence among ACP, calcite, and brucite, it is still possible to influence the precipitation degree of these species by relatively low current density and high surface area or by targeting phosphorus-rich wastewaters.

Elucidating the impacts of initial supersaturation and seed crystal loading on struvite precipitation kinetics, fines production, and crystal growth

To reduce intra-plant nutrient cycling, and recover phosphorus (P) fertilizers from nutrient-rich sidestreams, wastewater utilities increasingly elect to employ struvite precipitation processes without a clear understanding of the inherent tradeoffs associated with specific design and operating decisions. Specifically, the impact of reactor conditions on struvite crystallization rate, and distribution between formation of fines particles and secondary growth onto large diameter seed crystals represent critical knowledge gaps limiting the predictive capabilities of existing process models. In this work, the relative impacts of initial supersaturation (S_i), and seed loading, on P removal kinetics, and struvite solids distribution were investigated. In experiments conducted at different levels of initial supersaturation (1.7-2.4) and seed loading (0-25 g L^-1), struvite fines represented the majority of phosphate solids formed in 10 of 12 conditions. While total P removal was dependent on S_i, and primarily attributed to formation of fines, the concentration of struvite seed granules had a significant impact on the rate of P removal. Struvite seed granules increased the rate of precipitation by reducing induction time of primary nucleation of struvite fines. Secondary crystal growth represented the majority of struvite solids formed at high seed loading and low S_i, but presented the tradeoff of low total removal and low rate of removal. To convey the significance of these findings on process modeling, we show how a prominent kinetic model with a first-order dependency on solid struvite concentration over-predicts P removal rate when total mass is dominated by large diameter seeds (0.9 mm). This works reveals the critical role of struvite fines in P removal, and highlights the need to account for their production and kinetic importance in struvite process design and operation.

Removal of phosphate from water by amine-functionalized copper ferrite chelated with La(III)

Eutrophication has become a worldwide environmental problem and removing phosphorus from water/wastewater before discharge is essential. The purpose of our present study was to develop an efficient material in terms of both phosphate adsorption capacity and magnetic separability. To this end, we first compared the performances of four spinel ferrites, including magnesium, zinc, nickel and copper ferrites. Then we developed a copper ferrite-based novel magnetic adsorbent, by synthesizing 1,6-hexamethylenediamine-functionalized copper ferrite(CuFe₂O₄) via a single solvothermal synthesis process followed by LaCl₃ treatment. The materials were characterized with X-ray diffraction, transmission electron microscope, vibrating sample magnetometer, Fourier transform infrared spectra and N₂ adsorption-desorption. The maximum adsorption capacity of our material, calculated from the Langmuir adsorption isotherm model, attained 32.59mg/g with a saturation magnetization of 31.32emu/g. Data of adsorption kinetics were fitted well to the psuedo-second-order model. Effects of solution pH and coexisting anions (Cl^-, NO₃^-, SO₄^2-) on phosphate adsorption were also investigated, showing that our material had good selectivity for phosphate. But OH^- competed efficiently with phosphate for adsorption sites. Furthermore, increasing both NaOH concentration and temperature resulted in an enhancement of desorption efficiency. Thus NaOH solution could be used to desorb phosphate adsorbed on the material for reuse, by adopting a high NaOH concentration and/or a high temperature.

Growth and development of skeletal anomalies in diploid and triploid Atlantic salmon (Salmo salar) fed phosphorus-rich diets with fish meal and hydrolyzed fish protein

Diploid and triploid Atlantic salmon, Salmo salar were fed high-protein, phosphorus-rich diets (56-60% protein; ca 18g phosphorus kg-1 diet) whilst being reared at low temperature from start-feeding until parr-smolt transformation. Performances of salmon fed diets based on fish meal (STD) or a mix of fishmeal and hydrolysed fish proteins (HFM) as the major protein sources were compared in terms of mortality, diet digestibility, growth and skeletal deformities. Separate groups of diploids and triploids were reared in triplicate tanks (initially 3000 fish per tank; tank biomass ca. 620 g) from 0-2745 degree-days post-start feeding (ddPSF). Growth metrics (weight, length, condition factor) were recorded at ca. 4 week intervals, external signs of deformities to the operculum, jaws and spinal column were examined in parr sampled at 1390 ddPSF, and external signs of deformity and vertebral anomalies (by radiography) were examined in fish sampled at the end of the trial (2745 ddPSF). The triploid salmon generally had a lower mass per unit length, i.e. lower condition factor, throughout the trial, but this did not seem to reflect any consistent dietary or ploidy effects on either dietary digestibility or the growth of the fish. By the end of the trial fish in all treatment groups had achieved a weight of 50+ g, and had completed the parr-smolt transformation. The triploids had slightly, but significantly, fewer vertebrae (Triploids STD 58.74 ± 0.10; HFM 58.68 ± 0.05) than the diploids (Diploids STD 58.97 ± 0.14; HFM 58.89 ± 0.01), and the incidence of skeletal (vertebral) abnormalities was higher in triploids (Triploids STD 31 ± 0.90%; HFM 15 ± 1.44%) than in diploids (Diploids STD 4 ± 0.80%; HFM 4 ± 0.83%). The HFM diet gave a significant reduction in the numbers of triploid salmon with vertebral anomalies in comparison with the triploids fed the STD diet possibly as a result of differences in phosphorus bioavailability between the two diets. Overall, the incidence of skeletal deformities was lower than reported in previous studies (Diploids 20+%, Triploids 40+%), possibly as a result of the combination of rearing at low-temperature and phosphorus-rich diets being used in the present study.

Magnetite/Lanthanum hydroxide for phosphate sequestration and recovery from lake and the attenuation effects of sediment particles

An effective approach for eutrophication control and phosphate recovery remains a longstanding challenge. Herein, we present a new technique for phosphate sequestration in lake and phosphate recovery using novel magnetically recoverable magnetite/lanthanum hydroxide [M-La(OH)₃] hybrids that can be prepared using a simple one-pot synthesis method. Batch studies show that M-La(OH)₃ exhibits a strong sorption towards phosphate with sorption capacities of up to 52.7 mg-P/g at pH 7.0 in water. A simple model indicates that the efficiency of M-La(OH)₃ for phosphate sequestration in lake is significantly attenuated by 34-45% compared to that in water, due to interference from sediment particles. However, our results demonstrate that sediments suspensions mixed with a M-La(OH)₃ content of 1-3% exhibit a capability of up to 1.2 mg-P/g for sequestering external phosphate compared with that of 0.2 mg-P/g for pristine sediment at pH 7.3. M-La(OH)₃-mixed sediment suspensions appear to effectively sequester phosphate over an environmentally relevant pH range from 4 to 8.5. Phosphorus (P) fractionation experiments indicate that the enhanced phosphate sorption by M-La(OH)₃-mixed sediment suspensions is mainly due to the increased fractions of NaOH-P and inorganic P. This work indicates that the M-La(OH)₃ has the potential for phosphate sequestration and recovery from lake.

Ti4O7/g-C3N4 Visible Light Photocatalytic Performance on Hypophosphite Oxidation: Effect of Annealing Temperature

The oxidation of hypophosphite to phosphate is the key to recover the phosphorus resource from the hypophosphite wastewater. In the present work, Ti₄O₇/g-C₃N₄ composites were synthesized at two different temperatures (100 and 160°C) and their performance on photocatalytic oxidation of hypophosphite under visible light irradiation and the corresponding mechanism were evaluated. A hydrolysis method using g-C₃N₄ and Ti₄O₇ was applied to synthesize the Ti₄O₇/g-C₃N₄ composites with their hybrid structure and morphology confirmed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectra (XPS). The annealing temperature significantly affected the photocatalytic performance of Ti₄O₇/g-C₃N₄ that the 160-Ti₄O₇/g-C₃N₄ composite (fabricated at 160°C) showed the highest oxidation efficiency of hypophosphite of 81% and the highest photocatalytic oxidation rate of 0.467 h^-1 comparing with the 100-Ti₄O₇/g-C₃N₄ composite (fabricated at 100°C) and pure g-C₃N₄. The enhanced photocatalytic performance of 160-Ti₄O₇/g-C₃N₄ could be ascribed to the effective charge separation and enhanced photoabsorption efficiency. Additionally, electron spin resonance (ESR) results showed that hydroxyl radicals and superoxide anion radicals were mainly responsible to the oxidation of hypophosphite with superoxide anion radicals accounting for a more significant contribution. Moreover, Ti₄O₇/g-C₃N₄ photocatalysts showed the remarkable stability in the repetitive experiments.

Phosphorous recovery from sewage sludge using calcium silicate hydrates

Phosphorous is an essential limiting nutrient for which there is no substitute. Its efficient recovery from sewage treatment plants is important to mitigate both dependence on limited reserves of exploitable phosphate rock and eutrophication of surface waters. Here, we evaluate the use of calcium silicate hydrates (CSH) to recover phosphorous eluted from sewage sludge. Phosphorous elution experiments were conducted with acid and base leaching solutions. The phosphorous recovery efficiency with CSH was compared to that with other calcium compounds, and the final product was analyzed to assess its potential value as fertilizer. Dried sewage sludge from the West Lake Ecological Water Resource Center, South Korea, having 123 g-P kg^-1, was used for these tests. About 55% of the phosphorus in the sludge was released with an elution solution of 0.1 M H₂SO₄. A dose of 15 g L^-1 of CSH recovered 89.6% of the eluted phosphorous without the need for additional pre-treatment, and the resulting calcium phosphate product (in brushite form, based on XRD analysis) exhibited superior settleability than that resulting from Ca(OH)₂- and CaCl₂-induced precipitation. XRD peaks of the calcium sulfate hydrate (in gypsum form) and residual CSH were also observed. The final product contained a relatively high content of the total P₂O₅ eluted in a 2% citric acid solution (43.1%), which suggests that it might be readily used to fertilize crops.

Lanthanum oxide nanorods for enhanced phosphate removal from sewage: A response surface methodology study

Lanthanum-based adsorbents are ideal candidates for phosphate removal because of their excellent affinity to phosphate. However, their application in the removal of trace-levels of phosphate from sewage is still unsatisfactory due to the limited adsorption capacity and inadequate optimization of the operational parameters. To overcome these drawbacks, we have developed a novel lanthanum hydroxide (LH), using a facile precipitation and hydrothermal process that involves a nanorod-like structure with the lengths ranging from 124 to 1700 nm, depending on the La/OH molar ratio. The phosphate adsorption capacity of the developed LH is up to 170.1 mg-P g^-1 in synthetic water, while a slightly lower adsorption capacity of 111.1 mg-P g^-1 is observed in a sewage sample. A polynominal model consisting of three variables (i.e. dosage, reaction time and initial phosphate concentration) for predicting efficiency of phosphate removal has been successfully developed using a face-centred central composite design (CCD)-based methodology. The results also suggest a strong interactive effect of the dosage with the phosphate concentration, and reaction time, which can significantly affect the optimization of the phosphate removal by LH. Both X-ray photoelectron spectroscopy and X-ray diffraction studies indicate that the inner sphere complexation of phosphate with LH is probably the major mechanism governing phosphate removal.

Recovery of nutrients from digested sludge as struvite with a combination process of acid hydrolysis and Donnan dialysis

In this study, the effects of the combination of acid hydrolysis and Donnan dialysis on the high-quality struvite production from digested sewage sludge were examined. The Box-Behnken design was applied in the hydrolysis using oxalic acid for the optimization of the conditions that affect the nutrients and metal release from digested sludge. An optimal condition was obtained at oxalic acid 0.5 M, acid/sludge ratio (mL/g) 10/1, and reaction time 60 min. The separation of the metals from hydrolyzed sludge liquid was carried out with the Donnan dialysis using a Nafion 117 cation exchange membrane. At the end of the 4 hours of operating time, the recovery values for Zn, K, Na, Mg, Fe, and Al were obtained as 67.9%, 62.1%, 57.6%, 39.4%, 5.3%, and 2.5%, respectively. The produced struvite meets the legal limits for fertilizer use in terms of Cd, Cu, Ni, Pb, Zn, Hg, and Cr content specified by Turkish regulations. A high Ca/Mg molar ratio and the presence of K, Na, and Al affected the efficiency of struvite crystallization. The use of oxalic acid resulted in low Ca release, and the metal ions that affect struvite purity were eliminated using Donnan dialysis.

Sludge treatment: Current research trends

Sludge is produced during wastewater treatment as a residue containing most insoluble and adsorbed soluble impurities in wastewaters. This paper summarized the currently available review papers on sludge treatments and proposed the research trends based on the points raised therein. On partition aspect, sludge production rate and the reduction of production rate and the fate and transformation of involved emergent contaminants including endocrine disrupting chemicals and pharmaceuticals and personal care products are widely studied. On release aspect, development of thermal processes on sludge with migration and transformation of heavy metals in sludge during treatment is a research focus. The use of detailed fluid and biological reaction models and advanced instrumentation and control systems is studied to optimize treatment performances. On recovery part, co-digestion of sludge with co-substrates at mesophilic and hyperthermophilic conditions and the recovery of phosphorus at low costs are research highlights.

Electrochemical Induced Calcium Phosphate Precipitation: Importance of Local pH

Phosphorus (P) is an essential nutrient for living organisms and cannot be replaced or substituted. In this paper, we present a simple yet efficient membrane free electrochemical system for P removal and recovery as calcium phosphate (CaP). This method relies on in situ formation of hydroxide ions by electro mediated water reduction at a titanium cathode surface. The in situ raised pH at the cathode provides a local environment where CaP will become highly supersaturated. Therefore, homogeneous and heterogeneous nucleation of CaP occurs near and at the cathode surface. Because of the local high pH, the P removal behavior is not sensitive to bulk solution pH and therefore, efficient P removal was observed in three studied bulk solutions with pH of 4.0 (56.1%), 8.2 (57.4%), and 10.0 (48.4%) after 24 h of reaction time. While P removal efficiencies are not generally affected by bulk solution pH, the chemical-physical properties of CaP solids collected on the cathode are still related to bulk solution pH, as confirmed by structure characterizations. High initial solution pH promotes the formation of more crystalline products with relatively high Ca/P molar ratio. The Ca/P molar ratio increases from 1.30 (pH 4.0) to 1.38 (pH 8.2) and further increases to 1.55 (pH 10.0). The formation of CaP precipitates was a typical crystallization process, with an amorphous phase formed at the initial stage which then transforms to the most stable crystal phase, hydroxyapatite, which is inferred from the increased Ca/P molar ratio from 1.38 (day 1) to the theoretical 1.76 (day 11) and by the formation of needle-like crystals. Finally, we demonstrated the efficiency of this system for real wastewater. This, together with the fact that the electrochemical method can work at low bulk pH, without dosing chemicals and a need for a separation process, highlights the potential application of the electrochemical method for P removal and recovery.

Phosphate recovery from liquid fraction of anaerobic digestate using four slow pyrolyzed biochars: Dynamics of adsorption, desorption and regeneration

Four slow pyrolyzed biochars produced from wood (WDB), corncobs (CCB), rice husks (RHB) and sawdust (SDB) were evaluated for adsorption, desorption and regeneration of phosphate (PO₄^3--P) from anaerobically digested liquid swine manure. The PO₄^3--P adsorption capacity increased followed by initial concentrations increasing. Maximum PO₄^3--P adsorptions at initial 150 mg/L of PO₄^3--P (highest load) were average of 7.67, 6.43, 5.73 and 5.41 mg/g for WDB, CCB, RHB and SDB, respectively. Pseudo second order kinetics model could best fit PO₄^3--P adsorption, which indicated the chemisorption via precipitation was the main mechanism for PO₄^3--P removal. The sorption process was reversible and the adsorbed PO₄^3--P could be desorbed in both neutral (57-78%) and acidic solution environments (75-88%) for all biochars. Meanwhile, regenerated biochar could re-adsorb up to 5.62 mg/g at the highest initial PO₄^3--P of 150 mg/L. The present finding implied biochar could be effectively used to recover PO₄^3--P from anaerobic digestate.

Effectiveness of phosphate removal during anaerobic digestion of waste activated sludge by dosing iron(III)

Phosphate-Fe(II) precipitation induced by Fe(III) reduction during the anaerobic digestion of excess activated sludge was investigated for the removal of phosphorus and its possible recovery. The experiments were conducted with three Fe(III) sources at 35 °C and 55 °C. The results show that ferrihydrite-Fe(III) was effectively reduced during the anaerobic sludge digestion by 63% and 96% under mesophilic and thermophilic conditions, respectively. Whereas FeCl₃-Fe(III) was only mesophilically reducible and the reduction of hematite-Fe(III) was unnoticeable at either temperature. Efficient precipitation of vivianite was not observed although high saturation index values, e.g., >14 (activity reduction not considered), had been reached. This reveals the complexity of vivianite precipitation in anaerobic digestion systems; for example, Fe(II) complexation and organic interference could not be ignored. With ferrihydrite amendments at a Fe/TP of 1.5, methane production from sludge digestion was reduced by 35.1% at 35 °C, and was unaffected when the digestion temperature went up to 55 °C. But, acidic FeCl₃ severely inhibited the methane production and consequently the sludge biomass degradation.