Recovery of phosphate as struvite from low-temperature combustion sewage sludge ash (LTCA) by cation exchange

Comparative evaluation of phosphorus recovery from sewage sludge thermal products via magnesium ammonium phosphate and hydroxyapatite methods

Shortage of phosphorus resource has become a global concern. Due to the high phosphorus content in sewage sludge, phosphorus recovery can be realized from thermal products of sewage sludge. Phosphorus recovery performance of smoldering ash (SA), incineration ash (IA) and pyrolysis char (PC) was investigated. The precipitate rate of phosphorus in acid and alkali leaching solutions is over 94 % by magnesium ammonium phosphate (MAP) and hydroxyapatite (HAP) methods. For MAP method, the recovered P contents in the precipitations of SA, IA and PC are 23.25 ± 0.35, 31.71 ± 0.79 and 23.76 ± 0.24 mg/g, respectively. For HAP recovery, the phosphorus contents per unit mass of precipitated products are lower than that by MAP, ranging from 13.67 ± 0.10 to 22.89 ± 0.34 mg/g. The purity of the recovered products was evaluated based on the contents of major elements and heavy metals in recovered products. Most of major elements and heavy metals can coprecipitate with phosphorus in the recovery products by acid leaching-MAP method. Due to the low impurity content in the alkali leaching solution and insolubility of most heavy metals in it, the products recovered by alkali leaching-HAP shows higher purity than that by acid leaching-MAP method. The phosphorus recovery performance, reagent consumption and purity of recovered products of the two methods were compared. Acid leaching-MAP recovery is optimal for IA due to its highest P recovery and purity, with lower reagent consumption compared to alkali leaching-HAP. For SA and PC, alkali leaching-HAP recovery is preferable due to its higher P recovery purity and market price of hydroxyapatite products.

Optimizing phosphorus precipitation from acidic sewage sludge ash leachate: Use of Mg-rich mining by-products for enhanced nutrient recovery

Phosphorus recovery from Sewage Sludge Ashes (SSA) by wet chemical extraction followed by selective precipitation has gained great attention in recent years, attempting to reduce the anthropic pressure on natural reserves. This study investigates the selective precipitation process at lab- and small pilot-scales by means of two conventional and one innovative precipitating agents, the latter derived from a low-grade magnesium oxide mining by-product (LG-MgO named PC8), assessing the role of the most relevant operating parameters. Lab-scale experiments were performed on leachates obtained from bottom and fly ashes, in which several operating conditions were tested, differing in the type of precipitating agent, target pH and nutrient molar ratio. Based on experimental results, small pilot-scale experiments were conducted with Ca(OH)2 and PC8 at pH 7. Effective phosphorus precipitation was obtained at lab-scale at pH equal to 4 for high Al/P molar ratio, while SSA leachate with low Al/P molar ratio promoted improved phosphorus precipitation (>90%) only at pH higher than 8 with PC8. Small pilot-scale findings confirmed the effectiveness of PC8 in increasing simultaneously the pH and the nutrient content of the solid precipitate. The comprehensive assessment of the samples denoted compliance with the European Regulation (EU 2019/1009), which allows the formulation of different fertilizers with agronomic relevance. This is the first time that experiments from small pilot-scale tests in the field of phosphorus recovery from SSA were investigated using an innovative precipitant providing key information for the process scale-up.

Optimization of struvite from sewage sludge ash as phosphorus source

Phosphorus is an essential macronutrient for crop production. Struvite precipitation and reuse from phosphorus-rich sewage sludge are cost-effective measures to improve phosphorus utilization efficiency and decrease its negative environmental impact. In this study, the objectives were to optimize the phosphorus extraction (using sulfuric acid) and recovery (as struvite) processes and determine the most appropriate process conditions using RSM. This was done by evaluating the effect of different parameters such as acid concentration (mol/l) (0.02 to 0.8), H2SO4/ISSA ratio (liquid/solid) (20 to 150 ml/g), and time (0.5 to 4 h) for leaching tests and N: P ratio (1 to 2), Mg: P ratio (1 to 2), and pH (8 to 11) for struvite precipitation. The optimization of the factors affecting PO43--P extraction from SSA by acidic leaching showed that applying 0.5 mol/l H2SO4 and 57 ml/g L/S ratio at 2 h achieved the highest PO43--P extraction (99.8%). The extraction of phosphorus also leached heavy metals; however, using a cation exchange resin, it was possible to effectively remove heavy metals from P-rich solutions. The optimal phosphorus recovery as struvite (98.5%) was achieved at the lowest pH and N/P ratio, while an increase in Mg/P ratio from 1 to 2 positively affected phosphorus recovery. The obtained struvite had a high content 94.6%, and the heavy metal content in struvite was lower than the value of standard, so that the obtained struvite sample can be used as fertilizer.

Co-precipitation of Cd with struvite during phosphorus recovery

During the struvite recovery process, Cd, a hazardous metal commonly found in waste streams, can be sequestered by struvite. This study investigated the influence of Cd2+ on the precipitation of struvite. Quantitative X-ray diffraction (QXRD) results showed that the purity of struvite decreased from 99.1% to 73.6% as Cd concentration increased from 1 to 500 μM. Scanning electron microscopy (SEM) revealed a roughened surface of struvite, and X-ray photoelectron spectroscopy (XPS) analysis indicated that the peak area ratio of Cd-OH increased from 19.4% to 51.3%, while the area ratio of Cd-PO4 decreased from 86.6% to 48.7% as Cd concentrations increased from 10 to 500 μM. The findings suggested that Cd2+ disrupted the crystal growth of struvite, and mainly combined with -OH and -PO4 to form amorphous Cd-bearing compounds co-precipitated with struvite. Additionally, Mg-containing amorphous phases were formed by incorporating Mg2+ with -OH and -PO4 during struvite formation.

Evaluation of combined thermo-chemical processes for the treatment of landfill leachate using virgin and recovered FeCl3 coagulants

Sludge resulting from treatment of municipal waste landfill leachate contains suitable cationic substances such as Fe-based recovered coagulants which, if not recovered, can cause environmental problems. The present study aimed to maximise coagulant recoverability and investigate its potential reuse for the treatment of municipal waste landfill leachate. The study focused on establishing (i) the effect of mineral acids on leaching of Fe, (ii) the % of maximum recovery of Fe coagulant, (iii) the impact of ultrasound on recovery, and (iv) effectiveness of recovered coagulant when reused in coagulation-flocculation treatment of landfill leachate. Sulfuric acid outran hydrochloric acid in performance, with the acid leaching process leading to the recovery of 70.12% of Fe (acid concentration = 3.80 M, solid-to-liquid ratio = 8%, and heating time = 5 h). Subsequently, a developed acid leaching process was tested, which results showed that the highest rate of Fe recovery occurred without ultrasound treatment, meaning the use of it could reduce the recovery rate due to the increase in the iron (III) oxide-hydroxide [Fe(OH)3] sedimentation. Comparative experiments were undertaken with the recovered and virgin coagulants. These revealed that Fe-based recovered coagulant led to the 60.21% and 91.40% removal of COD and total suspended solid respectively, while the values of the COD and total suspended solid removal with the virgin FeCl3 were 7.66% and 6.42% lower than that of Fe under optimal conditions (dosage = 9.38 g/L, pH = 8.94, settling time = 52.9 min). The present study established that Fe recovered could be exploited as an eco-friendly coagulant to replace FeCl3 in the landfill leachate treatment.

Phosphorus recovery from sewage sludge ash (SSA): An integrated technical, environmental and economic assessment of wet-chemical and thermochemical methods

Incineration is a promising disposal method for sewage sludge (SS), enriching more than 90% of phosphorus (P) in the influent into the powdered product, sewage sludge ash (SSA), which is convenient for further P recovery. Due to insufficient bioavailable P and enriched heavy metals (HMs) in SSA, it is limited to be used directly as fertilizer. Hence, this paper provides an overview of P transformation in SS incineration, characterization of SSA components, and wet-chemical and thermochemical processes for P recovery with a comprehensive technical, economic, and environmental assessment. P extraction and purification is an important technical step to achieve P recovery from SSA, where the key to all technologies is how to achieve efficient separation of P and HMs at a low economic and environmental cost. It can be clear seen from the review that the economics of P recovery from SSA are often weak due to many factors. For example, the cost of wet-chemical methods is approximately 5∼6 €/kg P, while the cost of recovering P by thermochemical methods is about 2∼3 €/kg P, which is slightly higher than the current P fertilizer (1 €/kg P). So, for now, legislation is significant for promoting P recovery from SSA. In this regard, the relevant experience in Europe is worth learning from countries that have not yet carried out P recovery from SSA, and to develop appropriate policies and legislation according to their own national conditions.

Efficient and selective recovery of iron phosphate from the leachate of incinerated sewage sludge ash by thermally induced precipitation

Phosphorus recovery from incinerated sewage sludge ash (ISSA) is important but hindered by low selectivity. Here, a novel strategy of acid leaching followed by thermally induced precipitation was proposed for the efficient and selective recovery of FePO₄ from ISSA samples. A high phosphorus leaching efficiency of ∼ 99.6% was achieved with 0.2 mol/L H₂SO₄ and liquid to solid (L/S) ratio of 50 mL/g. Without removing various co-existing ions (Al³⁺, Ca²⁺, SO₄^2-, etc.), high-purity FePO₄ of ∼ 92.9% could be facilely produced from this highly acidic H₂SO₄ leachate (pH = 1.2) by simple addition of Fe(III) at a molar ratio of 1:1 to the phosphorus and reacted at 80 °C for thermally induced precipitation. The remained acid leachate could be further reused for five times to continue leaching phosphorus from the ISSA samples and produce the FePO₄ precipitates with a high phosphorus recovery efficiency of 81.1 ± 1.8%. The selective recovery of FePO₄ from the acid leachate was demonstrated more thermodynamically favorable compared to other precipitates at this acidic pH of 1.2, and elevated temperature of 80 °C towards thermally induced precipitation. The estimated cost of this strategy was ∼$26.9/kg-P and lower than that of other existing technologies. The recovered FePO₄ precipitates could be used as a phosphate fertilizer to promote the growth of ryegrass, and also as a precursor to synthesize high-value LiFePO₄ battery material, demonstrating the high-value application potential of the phosphorus from the ISSA.

CaO-assisted hydrothermal treatment combined with incineration of sewage sludge: Focusing on phosphorus (P) fractions, P-bioavailability, and heavy metals behaviors

Dewatering of sewage sludge (SS) was the prerequisite for saving its drying energy and sustaining its stable combustion. Hydrothermal treatment (HT) has been a promising technology for improving SS dewaterability with high energy efficiency. However, the knowledge of phosphorus (P) transformation and heavy metals (HMs) behaviors in the combined HT and incineration process was still lack. P fractions, P-bioavailability, HMs speciation, and their environmental risk in the ash samples from this combination process were evaluated and compared with those from the co-incineration of SS and CaO. The combination process was superior to the latter one in the light of P and HMs. CaO preferred to enhance the transformation of non-apatite inorganic phosphorus (NAIP) to apatite phosphorus (AP) initially with enriched P and increased P-bioavailability in the resultant ash samples. The combination process further reduced the values of risk assessment code and individual contamination factor with the increment of the stable F4 fraction in HMs. Significant reduction of potential ecological risk was observed with the lowest global risk index of 43.76 in the combination process. Optimum CaO addition of 6% was proposed in terms of P and HMs. The work here can provide theoretical references for the potential utilization of P from SS to mitigate the foreseeable shortage of P rocks.

Land application of sewage sludge incinerator ash for phosphorus recovery: A review

Phosphorus (P) is essential for all living things and an integral part of food production. However, significant amounts of P are functionally lost when wastewater byproducts, such as biosolids or sewage sludge incinerator ash (SSA), are not beneficially reused. Around 20% of sewage sludge produced in the US is incinerated and nearly 25% of sewage sludge is incinerated in European Union member countries. SSA contains significant amounts of P (up to 14% total P) and other beneficial elements but is typically sent to landfills for disposal. However, SSA has also been explored as one method of capturing and redirecting P back into the food system. Research investigating SSA characterization, P availability, and contaminant concentrations and behavior in soil is required to understand the effects of SSA land application on soil chemical properties and crop production. Several approaches for recovering P from SSA have been investigated that consider these factors. Ultimately, the opportunity for land application of SSA depends on the individual characteristics of a given SSA, ex. total P and contaminant concentrations, and the requirements and regulations of the region where it is produced and applied. In this review, we address the history of P recovery from SSA and discuss research regarding characterization, contaminants, P availability, and land application of SSA.

Simultaneous amorphous silica and phosphorus recovery from rice husk poultry litter ash

The livestock sector is one of the most important sectors of the agricultural economy due to an increase in the demand for animal protein. This increase generates serious waste disposal concerns and has negative environmental consequences. Furthermore, the food production chain needs phosphorus (P), which is listed as a critical raw material due to its high demand and limited availability in Europe. Manure contains large amounts of P and other elements that may be recycled, in the frame of circular economy and "zero waste" principles, and reused as a by-product for fertilizer production and other applications. This paper focuses on the extraction and recovery of amorphous silica from rice husk poultry litter ash. Two different extraction procedures are proposed and compared, and the obtained silica is characterized. This work shows that amorphous silica can be recovered as an almost pure material rendering the residual ash free of P. It also addresses the possibility of more specific phosphorous extraction procedures via acid leaching.

Phosphate substances transformation and vivianite formation in P-Fe containing sludge during the transition process of aerobic and anaerobic conditions

Excess sludge was considered as a promising raw material for phosphorus recovery. In this study, the P-Fe containing sludge came from the aerobic membrane bioreactor with electrocoagulation (EC), which was refluxed to the anaerobic unit for iron reduction. Under anaerobic condition, the ORP and pH maintained at -350 mV and 7.5, which exactly met the conditions for vivianite formation. According to the analysis of X-ray polycrystalline diffraction (XRD) and field emission scanning electron microscopy (FE-SEM), the final product of the sludge after anaerobic condition was mainly vivianite. Microbial analysis showed that there were iron reducing bacteria (IRB) in sludge before and after anaerobic process, including Dechloromonas, Desulfovibrio. Aeromonas and Methanobacterium. During the transition process of aerobic and anaerobic conditions, amorphous phosphate substances in P-Fe containing sludge could be transformed vivianite just with long term standing, which could promote the recovery of phosphate resource from wastewater.