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

Mobility and bio-accessibility of available phosphorus in sewage sludge: Influencing mechanism of hydrothermal pretreatment and incineration

Accurate assessment and enhancement of phosphorus (P) availability are critical for land application of sewage sludge and its thermal-treated products. By simulating different functioning pathways of P in soil, a novel multivariable scheme was developed to evaluate P availability from the perspective of mobility and bio-accessibility, then was applied to investigate the effects of hydrothermal pretreatment (HT), carbonaceous skeleton-assisted HT (CSkel-HT), and incineration on this topic. Sludge contained predominantly slow-release and microbial-available P (>50.0 % of total P). HT and incineration reduced available P through filtrate discharge, organic-P decomposition, and Fe/Al-P volatilization. Surprisingly, CSkel-HT addition promoted soluble Ca/MgHPO4 and thermal-stable Fe/AlPO4 formation under acidic conditions, which not only retained the slow-release and microbial-available P in hydrochar and ash, but also increased the rapid-available and plant-available P contents by 25.0 % and 300.0 %. Our scheme provided more informative insights than traditional single-index methods, and revealed the enhancing mechanism of CSkel-HT on P availability.

Intensified Enrichment and Leaching Strategy of Phosphorus in Sewage Sludge via Species-Targeted Conversion

Wet-chemical method for extracting phosphorus (P) from sludge incineration ash suffers from great P loss in the incineration and P-leaching stages due to the multidirectional migration of diversified P species. Existing single-process optimizations are difficult to meet the diverse and often conflicting needs of the two upstream and downstream processes to recover different P species. Herein, we developed a system-oriented strategy, HyperPhosⓅ, to synergistically address multiprocess requirements. It employs a specially designed one-pot pretreatment to create a unique reaction window for the most recoverable Fe/Al-P, achieving the targeted transformation of unstable P species into homogeneous Fe/Al-orthophosphate of 85.6% in proportion. These compounds exhibited remarkable thermal stability during incineration, with volatilization loss of only 60% of conventional technical route (Conv-Tech) even at 1000 °C. Subsequently, these parts of P were completely leached out using H2C2O4 or NaOH through redox reaction and chelating displacement, with the solvent concentration and the codissolution amounts of heavy metals less than half of the mean levels in the literature. Techno-economic analysis showed that HyperPhosⓅ obtained 1.1-2.4 times more P than Conv-Tech, while the unit cost was reduced by up to 49.5%. These findings provide new opportunities to close the P-cycle in a sustainable, economical, and environmentally friendly approach.

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.

Effects of sewage sludge ash as a recycled phosphorus source on the soil microbiome

Ash byproducts have been used as soil amendments to recycle nutrients and modify soil properties such as pH or density. Interest in these practices has continued with increasing emphasis on sustainability, particularly regarding phosphorus reuse from incinerated sewage sludge. Given recent advancements in microbial analyses, the impacts of these practices can now be studied from the soil microbiome perspective. Next-generation DNA sequencing technologies provide information about the taxonomic composition of bacterial, archaeal, and fungal communities in a complex environment like soil. In this review, we discuss the results of microbial analyses of soils amended with recycled ash products, including a pilot study of sewage sludge incinerator ash as a phosphorus source. These results indicated that changes in soil microbial community composition require high amounts of amendment for detectible effects. Future research efforts could include more focused investigations into phosphorus-related microorganisms, such as phosphorus-solubilizing bacteria or polyphosphate-accumulating organisms.

Selective leaching and recovery of phosphorus from incinerated sewage sludge ash with CaO addition

Incinerated sewage sludge ash (ISSA) with high P2O5 content is a potential phosphorus resource that can replace the non-renewable phosphorus rocks. However, extracting phosphorus from ISSA using hydrometallurgical methods also dissolves a large amount of impurity metals into the leachate. Therefore, this study proposes a new method combining high-temperature reaction with CaO addition, selective leaching, and chemical precipitation for efficient and low-cost phosphorus recovery from ISSA. During thermal treatment at 1050 °C, the addition of CaO significantly influenced the types and amounts of phosphate mineral phases in ISSA. When 20 % CaO was added, the Al/Fe-phosphate phases were completely converted into acid-soluble Ca-phosphate phases, while Al and Fe were retained in acid-insoluble phases (e.g., Al- and Fe-containing oxides and silicates). Subsequently, by controlling the pH of acidic leachate (2.0 to 1.5), Ca-phosphate phases were selectively dissolved. At a pH of 1.5, better selective leaching was achieved, with a leaching efficiency of 94.71 % for P and less than 10 % for Al, Fe and Si. The XRF results showed that the majority of SiO2 (38.39 %), Al2O3 (33.26 %) remained in the post-leaching solid residue, which was expected to be further resource utilization. Without purification treatment to remove Al, Fe and Si, almost all phosphate ions were precipitated at pH = 8.5, with P recovery efficiency of 94.07 %. XRD results showed that the precipitate was mainly composed of HAP, with a P2O5 content of 37.51 % and low level of contaminants, thereby realizing the effective recovery of phosphorus from ISSA.

Efficient removal of lipophilic compounds from sewage sludge: Comparative evaluation of solvent extraction techniques

Municipal sewage sludge, a by-product of wastewater treatment plants, presents environmental challenges due to its complex composition. Particular concern is the lipophilic and aliphatic compounds that pose risks to the environment and human health. This study focuses on the efficient removal of those compounds from sewage sludge using several organic solvents (hexane, toluene, chloroform, dichloromethane, acetone, hexane-methanol mixture, ethanol, and methanol) and ionic liquids (ILs) like tetrakis(hydroxymethyl)phosphonium chloride and 1-ethyl-3-methylimidazolium acetate by solvent extraction techniques. To determine optimal conditions, various factors such as solvent types, contact time, and temperature were examined. The results reveal that solvent polarity significantly impacts extract composition, with non-polar solvents like hexane and toluene yielding profiles characteristic of lipid-type compounds. An in-depth analysis of contaminants present in the sewage sludge was studied by Fourier-transform infrared spectroscopy (FTIR). Additionally, nuclear magnetic resonance (NMR) was used to identify the extracted compounds, including triglycerides, aliphatic esters, aliphatic alcohols, and free carboxylic acids. NMR provides data on the composition of the sewage sludge and indicates that among all the solvents used, tetrakis(hydroxymethyl) phosphonium chloride was the most suitable solvent for removing lipophilic and aliphatic compounds. Regeneration potential and reusability of the IL were conducted and verified by NMR. The results showed that tetrakis(hydroxymethyl) phosphonium chloride ionic liquid could be used for several extraction cycles. Identifying these compounds in the extracted mixture demonstrates that it adds value and potential for various applications. Towards environmental sustainability and circular economy, this effort develops strategies for the safe management, disposal, and recyclability of sewage sludge and, the reduction in environmental and health hazards associated with organic compounds.

Microplastic accumulation and transport in agricultural soils with long-term sewage sludge amendments

Land application of sewage sludge brings microplastic contamination to soil. However, studies regarding the occurrence and mobility of sludge-borne microplastics in soil are insufficient. In the present study, based on an experimental field, the effects of sludge application amount on the accumulation and migration of microplastics in 0-20 (upper) and 20-40 cm (lower) soil layers were evaluated. After 16 years of continuous sludge application (36 t/ha per year), the microplastic content and migration ratio in upper soil reached 6811 particles/kg and 148 %, which was about 5 and 20 times, respectively, higher than that of the control soil without sludge. The microplastics in upper and lower soil layers, were mainly 0.2-0.5 mm in size, mostly fibrous in shape, primarily transparent in color, and predominantly rayon in composition. Microplastic surfaces may persistently adsorb clay minerals and iron/titanium oxides from soil, posing potential environmental risks. Sludge application had a significant positive correlation with soil microplastic abundance, resulting in a good fit of predictive model constructed for microplastic accumulation in sludge-amended soils. These findings help to improve the knowledge on environmental behavior of microplastics in sludge-amended soil, and can provide a scientific basis for the regulation of microplastic pollution during sludge land application.

Optimizing iron removal from sewage sludge ash for enhanced phosphate fertilizer bioavailability

Sewage sludge derived fertilizer is a promising solution for phosphorus (P) recovery from biowaste, however, the inherent iron content in the sludge ash (SSA) impedes the P availability of the fused calcium magnesium phosphate fertilizer (FCMP). To achieve the goal of iron removal during the process, carbothermal reduction was adopted for the first time and the performance of carbon addition was systematically evaluated. Results showed that carbon addition at 4.50 % significantly increased the P availability from 9.50 % to 11.00 % and decreased the required amounts of calcium/magnesium. Moreover, ferrophosphate with 20.20 % P can be produced and the melting point of the system can be reduced by manipulating carbon addition. Finally, a process design was provided for the co-production of FCMP and ferrophosphate. This study highlights the addition of carbon to facilitate iron removal in SSA for the production of FCMP with enhanced bioavailability at a reduced energy consumption scenario.

Valorization of sewage sludge incineration ash as a novel soilless growing medium for urban agriculture and greenery

Limited open areas for urban agriculture and greenery have led to the search for innovative, sustainable growing media to strengthen the food supply and improve atmospheric quality for a resilient city. Rampant land developments have caused soil to become increasingly scarce. Sewage sludge incineration ash (SSIA), the by-product of waste-to-energy (WtE) incineration of sewage sludge, is a major municipal waste containing phosphorus-fertilizing nutrients. For the first time, we investigated the novel application of SSIA as a soilless plant-growing medium with built-in fertilizer. SSIA outperformed topsoil in bulk density, water-holding capacity, porosity, and nutrient content. However, it was found that SSIA has a high salinity and should be treated first. Wheatgrass (Triticum aestivum L.), a fast-growing glycophyte, thrived in the desalinated SSIA, showing growth and nutrient content comparable to the topsoil case. Simultaneously, it demonstrated phytoremediation. The SSIA residue was then recycled into cementitious materials, using desalinating water for mixing. SSIA upcycle into a growing medium facilitates urban resource management by utilizing nutrients in sewage waste for eco-friendly plant cultivation, benefiting urban agriculture and greenery. It is also a prudent valorization step before further recycling SSIA to reduce landfill requirements.

Co-composting of sewage sludge as an effective technology for the production of substrates with reduced content of pharmaceutical residues

Sewage sludge is a valuable source of elements such as phosphorus and nitrogen. At the same time, heavy metals, emerging organic compounds, micropollutants (pharmaceuticals, pesticides, PCPs, microplastics), or some potentially dangerous bacteria can be present. In this study, the sewage sludge was aerobically treated by composting with other materials (co-composted), and the resulting substrate was tested for suitability of its use in agriculture. Closer attention was focused on the pharmaceuticals (non-steroidal antiphlogistics, sartanes, antiepileptics, caffeine, and nicotine metabolites) content and ecotoxicity of the resulting substrates in the individual phases of sludge co-composting. It has been verified that during co-composting there is a potential for reduction of the content of pharmaceutical in the substrates up to 90 %. The course of the temperature in the thermophilic phase is decisive. Growth and ecotoxicity experiments demonstrated that with a suitable co-composting procedure, the resulting stabilized matter is suitable as a substrate for use in plant production, and the risk of using sewage sludge on agricultural land is substantially reduced.

Recycling of waste glass and incinerated sewage sludge ash in glass-ceramics

Disposal of waste glass and incinerated sewage sludge ash (ISSA) in landfills is a waste of resources and poses significant environmental risks. This work aims to recycle waste glass and ISSA together to form value-added glass-ceramics. The physical and mechanical properties, leaching behaviour, and microstructure of the glass-ceramics produced with different proportions of waste glass powder (WGP) and ISSA were investigated. Thermodynamic calculations were performed to predict the formation of crystalline phases and the phase transformation involved. The results showed the potential of WGP and ISSA as raw materials in glass-ceramics production. WGP effectively densified the microstructure of the glass-ceramics by forming a viscous phase. As WGP content increased, the total porosity of glass-ceramics decreased whereas the density increased, accompanied by the formed anorthite transforming into wollastonite. The incorporation of WGP densified and refined the pore structure of the glass-ceramics, thereby improving the mechanical properties and reducing the water absorption. The glass-ceramics produced with a 50:50 blend of WGP and ISSA exhibited the highest compressive strength of 43.7 MPa and the lowest water absorption of 0.3 %. All fabricated glass-ceramics exhibited innocuous heavy metal leaching. The co-sintering of ISSA and WGP can produce additive-free glass-ceramics, characterized by reduced energy consumption and notable heavy metal immobilization capacity. These materials hold promise for utilization in construction as building materials.

Phosphate recovery from sludge-incinerated ash by adsorption with hydrotalcite synthesized by metals in the ash

Selective adsorption of phosphorus (P) from the acidic leachate of sludge-incinerated ash (SIA) becomes more attractive due to avoiding removing heavy metals. Especially, layered double hydroxides (LDHs) as an anion adsorbent could be applied into this area owing to their good capacity on P-adsorption and low cost on preparation. Interestingly, SIA contains more aluminum (Al) and iron (Fe) needed to be removed prior to P-recovery, and removed Al and Fe could be utilized to synthesize LDHs, like Mg/Al-LDH and Mg/Fe-LDH. With this study, Mg/Al-LDH-r and Mg/Fe-LDH-r were economically synthesized with Al and Fe removed from SIA, which were similar in their chemical structures to commercial LDHs. The synthesized LDHs had a high P-adsorption capacity, up to 95.0%. The maximal phosphate capacity of the recovered LDHs (Mg/Al-LDH-r and Mg/Fe-LDH-r) was 239.0 and 199.8 mg P/g LDHs, respectively. "NaOH + desalinated brine" as a new desorption solution could achieve a desorption ratio at about 80%, which could reduce the liquid-solid ratio by at least 60%, greatly decreasing the desorption cost. Pot trials demonstrated that the desorbed and precipitated CaP could promote the growth of maize as well as a commercial P-fertilizer. Furthermore, the adsorbed phosphate by LDHs could be directly used as a slow-released P-fertilizer and also improve the pH value of acidic soil, completely deleting the desorption process.

Steps to circularity: Impact of resource recovery and urban agriculture in Seattle and Tacoma, Washington

Capturing the value in urban residuals (food scraps and wastewater) is a critical component of urban sustainability and a circular nutrient economy. Food production in urban areas has also been recognized as an important component of urban health. Data from two cities (Seattle and Tacoma, WA) with active resource recovery and community garden programs were used to quantify nutrient recovery and food production potential. Yield data from growth trials conducted using soil amendments produced from locally generated organic residuals were used to model yields in existing urban agriculture programs. Our survey showed much lower than expected volume of food scraps from both residential and multifamily housing for both cities. Nutrient generation rates from food scraps were estimated as 0.55-0.67 kg N and 0.09-0.11 kg P capita-1 yr-1. Recovery rates for Seattle with an established food scrap collection program were 0.21 kg N and 0.006 kg P capita-1 yr-1. Nutrient recovery from wastewater biosolids was higher; 1-1.67 kg N and 0.23-0.76 kg P capita-1 yr-1. Data on effluent quantity and nutrient concentrations from these programs suggests that effluent has a high potential for nutrient recovery (4.03-5 kg N and 0.3-0.5 kg P capita yr-1). Yield was modeled for kale (brassica oleracea) considering the number of people that could be fed per hectare for one year using a 67 g portion by comparing yields from synthetic fertilizer and residuals-based amendments in both high and low quality urban soils. The Tacoma biosolids potting soil yielded enough for 310 and 736 people ha-1 yr-1 for the high and low quality soils, respectively. The modeled food/yard compost produced from the food scraps yielded sufficient kale for 148 to 353 people ha-1 yr-1. Relative yield from fertilizer for the low and high quality soils was 15 and 263 people ha-1yr-1, respectively. Considering yield, enough biosolids are produced to meet 6.7-29.2% of the vegetable needs of each city. These results suggest that significant nutrients can be recovered using existing infrastructure. With enhanced nutrient capture from wastewater effluent, sufficient nutrients could be recovered to meet the N and P needs for food crops for the residents of each city.

Utilizing CaCl2 to promote the enrichment and bioavailability of phosphorus in incinerated sludge ash

Recovering phosphorus from incineration sludge sewage ash (ISSA) is a well-established technology, with a greater recovery potential than that of supernatant or sludge. ISSA can be utilized as a secondary raw material in the fertilizer industry, or as a fertilizer if heavy metal concentrations do not exceed permissible limits, thus reducing the cost of phosphorus recovery. Increasing the temperature to produce ISSA with higher solubility and plant availability of phosphorus is advantageous for both pathways. But a decrease in the extraction of phosphorus is also observed at high temperatures, thereby diminishing the overall economic benefits. In this study, CaCl₂ was utilized to mitigate the decrease in the extraction rate and also to promote the bioavailability of phosphorus. The addition of CaCl₂ (80 g/kg of dry sludge) effectively promoted the conversion of non-apatite inorganic phosphorus to apatite inorganic phosphorus at a rate of 87.73% at 750 °C. Furthermore, the decrease in the extraction rate of phosphorus at 1,050 °C was comparatively smaller in the presence of CaCl₂. If iron flocculants are used to capture P in wastewater management, it may be necessary to pay special attention to the amount of addition and incineration temperature to maximize the economic potential of recycling.

Land application of industrial wastes: impacts on soil quality, biota, and human health

Globally, waste disposal options such as landfill, incineration, and discharge to water, are not preferred long-term solutions due to their social, environmental, political, and economic implications. However, there is potential for increasing the sustainability of industrial processes by considering land application of industrial wastes. Applying waste to land can have beneficial outcomes including reducing waste sent to landfill and providing alternative nutrient sources for agriculture and other primary production. However, there are also potential hazards, including environmental contamination. This article reviewed the literature on industrial waste applications to soils and assessed the associated hazards and benefits. The review investigated wastes in relation to soil characteristics, dynamics between soils and waste constituents, and possible impacts on plants, animals, and humans. The current body of literature demonstrates the potential for the application of industrial waste into agricultural soils. The main challenge for applying industrial wastes to land is the presence of contaminants in some wastes and managing these to enhance positive effects and reduce negative outcomes to within acceptable limits. Examination of the literature also revealed several gaps in the research and opportunities for further investigation: specifically, a lack of long-term experiments and mass balance assessments, variable waste composition, and negative public opinion.

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.

Synergy of phosphate recovery from sludge-incinerated ash and coagulant production by desalinated brine

Wet-chemical approach is widely applied for phosphate recovery from incinerated ash of waste activated sludge (WAS), along with metals removed/recovered. The high contents of both aluminum (Al) and iron (Fe) in WAS-incinerated ash should be suitable for producing coagulants with some waste anions like Cl^- and SO₄^2- With acid (HCl) leaching and metals' removing, approximately 88 wt% of phosphorus (P) in the ash could be recovered as hydroxylapatite (HAP: Ca₅(PO₄)₃OH); Fe³⁺ in the acidic leachate could be selectively removed/recovered by extraction with an organic solvent of tributyl phosphate (TBP), and thus a FeCl₃-based coagulant could be synthesized by stripping the raffinate with the original brine (containing abundant Cl^- and SO₄^2-). Furthermore, a liquid poly-aluminum chloride (PAC)-based coagulant could also be synthesized with Al³⁺ removed from the ash and the brine, which behaved almost the same in the coagulation performance as a commercial coagulant on both phosphate and turbidity removals. Both P-recovery from the ash and coagulant production associated with the brine would enlarge the markets of both 'blue' phosphate and 'green' coagulants.

Mining phosphorus from waste streams at wastewater treatment plants: a review of enrichment, extraction, and crystallization methods

Two interrelated problems exist: the non-renewability of phosphate rock as a resource and the excess phosphate in the water system lead to eutrophication. Removal and recovery of phosphorus (P) from waste streams at wastewater treatment plants (WWTPs) is one of the promising solutions. This paper reviews strategies for P recovery from waste streams in WWTPs are reviewed, and the main P recovery processes were broken down into three parts: enrichment, extraction, and crystallization. On this basis, the present P recovery technology was summarized and compared. The choice of P recovery technology depends on the process of sewage treatment and sludge treatment. Most P recovery processes can meet the financial requirements since the recent surge in phosphate rock prices. The safety requirements of P recovery products add a high cost to toxic substance removal, so it is necessary to control the discharge of toxic substances such as heavy metals and persistent organic pollutants from the source.

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.

Recovery of phosphorus from wastewater: A review based on current phosphorous removal technologies

Phosphorus (P) as an essential nutrient for life sustains the productivity of food systems; yet misdirected P often accumulates in wastewater and triggers water eutrophication if not properly treated. Although technologies have been developed to remove P, little attention has been paid to the recovery of P from wastewater. This work provides a comprehensive review of the state-of-the-art P removal technologies in the science of wastewater treatment. Our analyses focus on the mechanisms, removal efficiencies, and recovery potential of four typical water and wastewater treatment processes including precipitation, biological treatment, membrane separation, and adsorption. The design principles, feasibility, operation parameters, and pros & cons of these technologies are analyzed and compared. Perspectives and future research of P removal and recovery are also proposed in the context of paradigm shift to sustainable water treatment technology.

Innovative Technological Approach for the Cyclic Nutrients Adsorption by Post-Digestion Sewage Sludge-Based Ash Co-Formed with Some Nanostructural Additives under a Circular Economy Framework

This paper presents a new, innovative technological approach, in line with Circular Economy principles, to the effective management of sludge generated during municipal wastewater treatment processes and subsequently used for biogas production. This approach allows for optimal, functional, and controlled cascade-type biotechnological thermal conversion of carbon compounds present in sewage sludge, later in solid digestate residues (after biogas production), and finally in the ash structure (after incineration, purposefully dosed nanostructural additives make the production of a useful solid product possible, especially for cyclic adsorption and slow release of nutrients (N, P, K) in the soil). The idea is generally targeted at achieving an innovative conversion cycle under a Circular Economy framework. In particular, it is based on an energy carrier (methane biogas) and direct energy production. The functionalized combustion by-products can be advantageous in agriculture. The use of ashes with nanostructural additives (halloysite, kaolinite) from combustion of sewage sludge after the anaerobic fermentation as an adsorbent of selected nutrients important in agriculture (Na+, K+, NO3-, SO42-, PO43-, Cl-) was verified at laboratory scale. The tests were carried out both for pure ash and for the ash derived from combustion with the purposeful addition of kaolinite or halloysite. The equilibrium conditions for nitrate, potassium, sodium, phosphate(V), sulphate(VI), and chloride ions from aqueous solutions with the use of the three adsorbent structures were determined. The obtained innovative results were interpreted theoretically with adsorption isotherm models (Langmuir, Freundlich, Temkin, Jovanović). The most spectacular and clearly favorable results related to the influence of nanostructural additives in the process of sludge combustion, and formation of sorption surfaces under high temperature conditions were identified in the case of sorption-based separation of phosphate(V) ions (an increase from 1.13% to 61.24% with the addition of kaolinite, and even up to 76.19% with addition of halloysite).

Phosphorus speciation in sewage sludge from three municipal wastewater treatment plants in Sweden and their ashes after incineration

Given the high efficiency in phosphorus removal at municipal wastewater treatment plants (MMWWTP), sewage sludge constitutes a promising resource for phosphorus (P) recovery. Sewage sludge is, however, a complex matrix and its direct use as fertiliser is limited by its content of metals/metalloids and organic pollutants. In order to increase its usability as a potential resource of P, there is a need for increased knowledge on phosphorus speciation in these matrices. The sludge composition is highly influenced by local conditions (i.e. wastewater composition and treatment method), and it is therefore important to study sludge from several MMWWTPs. In this study, three different protocols for sequential extraction were utilised to investigate the chemical speciation of phosphorus in sludge from three different MMWWTP sludges in Sweden, as well as in corresponding ashes following incineration. The results showed that the total amounts of phosphorus ranged from 26 to 32 mg g^-1 sludge (dry weight), of which 79-94% was inorganically bound (IP). In the sludge, 21-30% of the IP was associated with calcium (Ca-P), which is the preferred species for fertiliser production. Following incineration, this fraction increased to 54-56%, mainly due to transformation of iron-associated phosphorus (Fe-P), while aluminium-associated species of phosphorus (Al-P) remained unaltered. The results from this study confirm that incineration is a suitable treatment for sewage sludge in terms of potential phosphorus recovery.

Sewage sludge treatment methods and P-recovery possibilities: Current state-of-the-art

With the growing emphasis on environmental protection, the ways of sewage sludge treatment are changing. In this review, we analyse different methods of sewage sludge treatment in terms of potential environmental risk and raw materials recovery. The review begins with a comparison and assessment of existing reviews on this topic. Then, it focuses on the properties and current utilisation of sewage sludge in agriculture and a brief description of sludge thermal treatment methods (mono- and co-incineration, pyrolysis, and gasification). The final part of the review is devoted to technologies for treating sludge ash from mono-incinerators to recover phosphorus, a substance listed as a critical raw material by the EU. Our results show that direct use of sewage sludge likewise composts containing sewage sludge should no longer be considered as a direct source of nutrients and organic matter in agriculture, because of its pollutant content. Co-incineration and landfilling represent a dead-end in sludge treatment due to the loss of raw materials, whereas pyrolysis is sustainable for remote locations with low heavy metal content sludge. Heavy metals also pose a problem for the direct use of sludge ash and must be therefore removed. There are already sludge ash processing technologies that are capable of processing ash to form a variety of raw materials such as phosphorus. These regeneration approaches are currently in their infancy, but are gradually being introduced. The sewage sludge treatment industry is rapidly evolving, and we have attempted to summarise and discuss the current state of knowledge in this review, which will provide a baseline towards the future of sewage sludge suitable treatment.

Making Waves: A sea change in treating wastewater - Why thermodynamics supports resource recovery and recycling

Entropy is a concept defined by the second law of thermodynamics. Applying this concept to the world we live in, entropy production must be minimized and negentropy (negative entropy production) should be accelerated, in order to produce a healthy and stable ecological system. The present wastewater treatment, however, contributes to entropy production. This means that conventional wastewater treatment, without recovery of resource and energy, will gradually but inevitably contribute to a deteriorating ecological balance. When the self-cleaning ability of the natural ecological system is limited, the need to develop sustainable wastewater treatment in order to delay entropy production and accelerate negentropy becomes urgent. Resource and energy recovery from wastewater should be the first priority, as they can contribute significantly towards minimizing entropy production and accelerating negentropy. Sustainable wastewater treatment must focus on recovering recyclable high value-added organic chemicals from wastewater and/or excess sludge to minimize entropy production caused by methane (CH₄, once combusted, is converted into CO₂ - an even higher substance in entropy) via anaerobic digestion. Instead of CH₄, thermal energy present in the effluent can be utilized for heating/cooling buildings and also for drying excess sludge towards incineration to recover more energy. Overall, this can lead to a carbon-neutral operation and even creating a "carbon sink" could be possible for wastewater treatment.

Phosphorus recovery from cattle manure bottom ash by extraction and precipitation methods

Phosphorus, a limiting element, is essential for living organisms, but the total amount available is decreasing with its increasing use. This problem can be solved by studying the methods of phosphorus recovery from waste. Phosphorus (P₂O₅, 13.75%) is abundantly present in cattle manure bottom ash (CMBA), indicating its potential as a source for phosphorus recovery. Herein, phosphorus recovery from CMBA was investigated by acid extraction and precipitation methods. The optimum concentration of sulfuric acid for extraction was 1.4 M, which eluted approximately 90% of the phosphorus contained in CMBA. In the precipitation method, sodium hydroxide and calcium silicate hydrate (CSH, CaSiO₃∙nH₂O) were used to adjust the solution pH to 4 and 8, where more than 99% of the eluted phosphorus was recovered when the pH was adjusted to 8 using CSH alone. The chemical composition and crystal forms of the recovered precipitates were analyzed using X-ray fluorescence and an X-ray powder diffractometer. The results indicated monetite and brushite were the main crystal forms of precipitates at pH 4, and struvite, hydroxyapatite, and tricalcium phosphate were the main crystal forms at pH 8. The availability of phosphorus in the precipitates was also evaluated by quinoline gravimetric analysis using water and 2% citric acid, and the water-soluble precipitate was less than 35%, whereas it ranged from 65 to 97% in 2% citric acid. This study suggests that CMBA can be used as a promising source to recover phosphorus via acid extraction and precipitation processes.

Closing the Carbon Loop in the Circular Plastics Economy

Today, plastics are ubiquitous in everyday life, problem solvers of modern technologies, and crucial for sustainable development. Yet the surge in global demand for plastics of the growing world population has triggered a tidal wave of plastic debris in the environment. Moving from a linear to a zero-waste and carbon-neutral circular plastic economy is vital for the future of the planet. Taming the plastic waste flood requires closing the carbon loop through plastic reuse, mechanical and molecular recycling, carbon capture, and use of the greenhouse gas carbon dioxide. In the quest for eco-friendly products, plastics do not need to be reinvented but tuned for reuse and recycling. Their full potential must be exploited regarding energy, resource, and eco efficiency, waste prevention, circular economy, climate change mitigation, and lowering environmental pollution. Biodegradation holds promise for composting and bio-feedstock recovery, but it is neither the Holy Grail of circular plastics economy nor a panacea for plastic littering. As an alternative to mechanical downcycling, molecular recycling enables both closed-loop recovery of virgin plastics and open-loop valorization, producing hydrogen, fuels, refinery feeds, lubricants, chemicals, and carbonaceous materials. Closing the carbon loop does not create a Perpetuum Mobile and requires renewable energy to achieve sustainability. This article is protected by copyright. All rights reserved.

Phosphorus recovery from a pilot-scale grate furnace: influencing factors beyond wet chemical leaching conditions

Phosphorus is a non-renewable resource going to be exhausted in the future. Sewage sludge ash is a promising secondary raw material due to its high phosphorus content. In this work, the distribution of 19 elements in bottom and cyclone ashes from pilot-scale grate furnace have been monitored to determine the suitability for the phosphorus acid extraction. Moreover, the influence of some parameters beyond wet chemical leaching conditions were investigated. Experimental results showed that bottom ash presented lower contamination in comparison to cyclone ash and low co-dissolution of heavy metals (especially Cr, Pb and Ni), while high phosphorus extraction efficiencies (76-86%) were achieved. High Al content in the bottom ash (9.4%) negatively affected the phosphorus extraction efficiency as well as loss on ignition, while the particle size reduction was necessary for ensuring a suitable contact surface. The typology of precipitating agents did not strongly affect the phosphorus precipitation, while pH was the key parameter. At pH 3.5-5, phosphorus precipitation efficiencies higher than 90% were achieved, with a mean phosphorus content in the recovered material equal to 16-17%, comparable to commercial fertilizers. Instead, the co-precipitation of Fe and Al had a detrimental effect on the recovered material, indicating the need for additional treatments.

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.

A review on the integration of mainstream P-recovery strategies with enhanced biological phosphorus removal

Phosphorus (P), an essential nutrient for all organisms, urgently needs to be recovered due to the increasing demand and scarcity of this natural resource. Recovering P from wastewater is a feasible and promising way widely studied nowadays due to the need to remove P in wastewater treatment plants (WWTPs). When enhanced biological P removal (EBPR) is implemented, an innovative option is to recover P from the supernatant streams obtained in the mainstream water line, and then combine it with liquor-crystallisation recovery processes, being the final recovered product struvite, vivianite or hydroxyapatite. The basic idea of these mainstream P-recovery strategies is to take advantage of the ability of polyphosphate accumulating organisms (PAO) to increase P concentration under anaerobic conditions when some carbon source is available. This work shows the mainstream P-recovery technologies reported so far, both in continuous and sequenced batch reactors (SBR) based configurations. The amount of extraction, as a key parameter to balance the recovery efficiency and the maintenance of the EBPR of the system, should be the first design criterion. The maximum value of P-recovery efficiency for long-term operation with an adequate extraction ratio would be around 60%. Other relevant factors (e.g. COD/P ratio of the influent, need for an additional carbon source) and operational parameters (e.g. aeration, SRT, HRT) are also reported and discussed.

High-temperature technology survey and comparison among incineration, pyrolysis, and gasification systems for water resource recovery facilities

Solids from wastewater treatment undergo processing to reduce mass, minimize pathogens, and condition the products for specific end uses. However, costs and contaminant concerns (e.g., per- and polyfluoroalkyl substances [PFAS]) challenge traditional landfill and land application practices. Incineration can overcome these issues but has become complicated due to evolving emissions regulations, and it suffers from poor public perception. These circumstances are driving the re-emergence of pyrolysis and gasification technologies. A survey of suppliers was conducted to document differences with technologies. Both offer advantages over incineration with tailored production of a carbon-rich solid, currently less stringent air emission requirements, and lower flue gas flows requiring treatment. However, incineration more simply combines drying and thermal processing into one reactor. Equipment costs provided favor pyrolysis and gasification at lower capacities but converge with incineration at higher capacities. Long-term operational experience will confirm technology competitiveness and elucidate whether pyrolysis and gasification warrant widespread adoption. PRACTITIONER POINTS: Pyrolysis and gasification systems are gaining traction in the wastewater industry with several full-scale installations operating, in construction, or design Several advantages, but some disadvantages, are considered in comparison with incineration Organic contaminants, including PFAS, will undergo transformation and potentially complete mineralization through each process.

Assessment of a simple and replicable procedure for selective phosphorus recovery from sewage sludge ashes by wet chemical extraction and precipitation

The selective phosphorus recovery by wet chemical extraction and precipitation was assessed at the laboratory scale aiming at identifying a simple and replicable procedure that could be effectively applied to different types of sewage sludge ashes. The experimental work was performed on five samples of sewage sludge ashes, of which three were obtained from muffle-furnace incineration and two from full-scale mono-incineration plants. A single-step extraction procedure has been investigated by applying different operating conditions (type of leaching acid, liquid-to-solid ratio, contact time). Experimental results indicated that phosphorus recovery efficiency varied between 54 and 92% with limited co-dissolution of metals and metalloids, except for arsenic. Operating conditions, sewage sludge ashes characteristics and phosphorus removal processes in the wastewater treatment plant were the main factors affecting phosphorus recovery efficiency. The application of optimal operating conditions (0.2 M sulfuric acid, liquid-to-solid ratio of 20 and contact time of 2 h) resulted in phosphorus recovery from 76 to 92% on four samples. Subsequently, precipitation of phosphorus from acidic leachate was carried out by lime dosing. After filtering and drying, the recovered products presented a P₂O₅ content between 11.5 and 36.7% dry weight, with a fraction of soluble phosphorus between 75 and 91%, a good percentage for application as fertilizer or animal feed. Since few undesired elements (i.e., As, Cu and Zn) exceeded the limits for fertilizer application (exception was represented by Ni and Pb, which were present at low concentration), an additional purification step may be required. Overall, experimental results highlighted the influence of process parameters on phosphorus recovery.

Towards Precision Fertilization: Multi-Strategy Grey Wolf Optimizer Based Model Evaluation and Yield Estimation

Precision fertilization is a major constraint in consistently balancing the contradiction between land resources, ecological environment, and population increase. Even more, it is a popular technology used to maintain sustainable development. Nitrogen (N), phosphorus (P), and potassium (K) are the main sources of nutrient income on farmland. The traditional fertilizer effect function cannot meet the conditional agrochemical theory’s conditional extremes because the soil is influenced by various factors and statistical errors in harvest and yield. In order to find more accurate scientific ratios, it has been proposed a multi-strategy-based grey wolf optimization algorithm (SLEGWO) to solve the fertilizer effect function in this paper, using the “3414” experimental field design scheme, taking the experimental field in Nongan County, Jilin Province as the experimental site to obtain experimental data, and using the residuals of the ternary fertilizer effect function of Nitrogen, phosphorus, and potassium as the target function. The experimental results showed that the SLEGWO algorithm could improve the fitting degree of the fertilizer effect equation and then reasonably predict the accurate fertilizer application ratio and improve the yield. It is a more accurate precision fertilization modeling method. It provides a new means to solve the problem of precision fertilizer and soil testing and fertilization.

Electro-oxidation to convert dissolved organic nitrogen and soluble non-reactive phosphorus to more readily removable and recoverable forms

Conventional wastewater treatment processes cannot effectively remove dissolved organic nitrogen (DON) and soluble non-reactive phosphorus (sNRP), which can pose regulatory compliance challenges for total nitrogen and total phosphorus discharges. Moreover, DON and sNRP are not easily recoverable for beneficial reuse as part of the waste to resource paradigm. Conversion of DON and sNRP to more readily removable dissolved inorganic nitrogen (DIN) and soluble reactive phosphorus (sRP), respectively, will help meet stringent nutrient limits and facilitate nutrient recovery. In this study, electro-oxidation (EO) was evaluated for conversion of four DON compounds to DIN and five sNRP compounds to sRP. EO was more efficient and provided higher extents of conversion of the recalcitrant nutrient fractions compared to a more traditional advanced oxidation process, UV/H₂O₂. Direct electron transfer was likely the dominant oxidation mechanism for EO-based DON and sNRP conversion, with DON being more recalcitrant. Among the DON compounds tested, greater availability of primary amine (C-N bonds) yielded greater conversion compared to compounds with fewer primary amine or those with secondary amine (C-N-C bond). Among the sNRP compounds tested, those with P-O-C bonds (organic sNRP) converted more readily than those with P-O-P bonds (inorganic sNRP), presumably because cleavage of the latter bond requires greater energy. Using 30 min of EO treatment, the highest DON and sNRP compound conversion was 11.7 ± 0.09% for urea and 31.1 ± 0.75% for beta-glycerol phosphate. A similar extent of EO-based conversion of DON (6.41 ± 1.5%) and sNRP (32.7 ± 3.3%) was observed in real wastewater.

Can the Application of Municipal Sewage Sludge Compost in the Aided Phytostabilization Technique Provide an Effective Waste Management Method?

(1) Background: sewage sludge is a by-product of wastewater treatment, which needs to be managed appropriately, e.g., in composting processes. The application of municipal sewage sludge composts (MSSCs) as a soil amendment is a potential way to effectively manage sewage sludge. (2) Methods: this paper presents the results of a vegetation pot experiment undertaken to assess the suitability of Dactylis glomerata L. and MSSC in the aided phytostabilization technique when applied on soils from an area effected by industrial pressure; this is characterized by high levels of heavy metal (HM). The contents of HMs in the test plant (the roots and above-ground parts), as well as in the soil and MSSC, were determined via an atomic spectrometry method. (3) Results: the application of MSSC positively contributed to an increased production of plant biomass and an increase in the pH in the soil. Concentrations of Cu, Cd, Pb, Zn, and Cr were higher in the roots than in the above-ground parts of Dactylis glomerata L. The addition of MSSC contributed most significantly to the considerable reduction in Ni, Pb, and Zn contents in the soil after the experiment. (4) Conclusions: MSSC can support the phytostabilization of soils contaminated with high levels of HMs.