Efficiency of phosphate removal during anaerobic digestion of activated sludge by dosing iron(III) (2023)

Anaerobic digestion (AD) and composting are common methods of sewage sludge management, but the influence of different types of coagulants on these biological processes and their stabilized biomass properties has not been fully elucidated. The paper examines the effect of the coagulant used in municipal sewage treatment plants on the biological stabilization of sewage sludge. Fully controlled and monitored AD and small-scale composting bioreactors were used. The coagulants tested included inorganic coagulant (IC), polyaluminium chloride (PAC) and organic coagulants, (OC) polyamine (pAmine) and chitosan (Chit). In general, it was found that the coagulant used had a measurable effect on the biological stabilization of the sludge. It was found that the presence of complex organic compounds OC in sludge biomass reduces biomass biodegradability while increasing gas production. During AD, Chit sludge achieved higher biogas production than pAmine and PAC sludge (13% and 16% respectively in Nm³CH₄T⁻¹USA). In composting, the pamin sludge reached the highest raw material temperature (34-35 °C) and CO₂gaseous emissions followed by Chit (33 °C) and PAC sludge (32 °C). In general, the concentration of tot-P in PAC sediments was higher than in pAmine and Chit sediments both before (20, 17 and 15 g/kg DM respectively) and after AD (23, 21 and 20.5 g/kg DM respectively) and in the composting process (31, 29.5 and 26 g/kg DM respectively). Tot-N concentrations (g/kg DM) showed a significant increase after AD (pAmine and PAC approx. 50% and Chit 81%), while a decrease was observed after composting, especially in PAC sludge (PAC 28%, pAmine and Chit ⁓5 %). The choice of the most appropriate coagulant by wastewater treatment plants depends on the purpose of the biological stabilization process. When AD was used and biogas yield was selected as the target yield, semi-natural OC Chit turned out to be the best option among the tested coagulants. Similarly, when the nutrient content of the resulting biosolids (AD or composting) is more relevant, the sludge produced by OC was found to contain higher concentrations of N, while the sludge produced by IC contained slightly higher concentrations of P.

This study was aimed at extracting orthophosphate (ortho-P) from high-fat waste AF liquor (AFL) by adsorption of Mg/Al layered double hydroxides (Mg/Al LDH), assessing the effect of carbonates and investigating the adsorption mechanisms. An experiment on the effect of carbonates on P-rich synthetic wastewaters showed that a low carbonate content favors the extraction of P by LDH. Then, true AFL rich in volatile fatty acids (VFA), carbonates and ortho-P was used as the adsorbate to test the adsorption efficiency of Mg/Al LDH. Experimental results showed that 4g/L Mg/Al LDH could extract 88.3% ortho-P from low carbonate AFL (4829.83 mg CaCO₃/L) and the adsorption amount was 62.99 mgP/g LDH, but negligible VFA was extracted. Kinetic and mechanism analysis showed that the adsorption of P on Mg/Al LDH was a fast physicochemical process involving ion exchange and surface adsorption. Ultimately, the nutrient release test confirmed the slow release properties of intercalated P.

Several approaches to P recovery after metal precipitation have been proposed in the literature recently, but a solid techno-economic comparison of these approaches is still lacking. Five methods of phosphorus recovery using sewage sludge or sludge ash as feed material were compared based on their specific recovery operating costs by scaling the processes to uniform sizes and operating conditions. The selected technologies are (1) wet leaching + struvite precipitation, (2) magnetic separation of vivianites, (3) gasification of molten sludge, (4) thermochemical sodium sulphate process and (5) recovery of white phosphorus. The analyzes were based on literature values ​​together with a plant-wide model for estimating chemical consumption and sludge properties associated with phosphorus and metals. Technologies were assessed based on operating costs, final product quality, effectiveness of corrective actions and technology maturity. The choice of recovery process depends on the precipitating agent used in the wastewater treatment processes. Sewage sludge ash technologies (4) and (5) were characterized by the highest recovery efficiency, technical maturity and product quality, but require mono-incineration. Technology (3) had a better recovery efficiency than (1), but the final product had a much lower P content. Technology (2) had the lowest recovery efficiency of all technologies compared, but produced a final product with the second highest P content. Operating costs are calculated for energy and chemical costs for the same scale and operating conditions. Specific repair costs range from €6 to €38/kgP_restored.

The recovery of phosphorus (P) from activated sludge (WAS) from wastewater treatment plants is significant in a P-deficient world. Recently, vivianite crystallization has been recognized as the primary method for recovering P from anaerobic digestion (AF) of WAS. In this study, acid/base pretreatment (pH 3/pH 10) was performed on AF WAS to enhance iron reduction and viviaite formation. The results showed that the maximum iron reduction rate (R_maks) in the pH 3 and pH 10 groups increased 1.9 and 1.7 times compared to the Fe control group, and the iron reduction efficiency (mi_Fe) increased by 17.5% and 12.0%, respectively. The share of P bound to Fe (Fe-P) in sediments with pH 3 and pH 10 increased by 50.0% and 33.7%, respectively. In addition, the relative abundance of iron-reducing bacteriaClostridium_sensusensuin the pH 3 group it was higher; and the content of Fe-P in the sediment and the size of wiviaite crystals after AF were higher. With these results, a pH 3 pretreatment was preferred for Fe promotion²⁺vivianite release and formation during AF.

As a product of phosphorus recovery from the anaerobic digestion (AD) of activated sludge (WAS), vivianite is of increasing interest. However, the main factors controlling the formation of vivianites have not been fully addressed. Therefore, this study was initiated to determine the key factors controlling the formation of vivianites. The chemical equilibrium simulation indicates that interfering ions such as metal ions and inorganic compounds can affect vivianite formation, especially in PO₄^3-concentration less than 3 mM. Experiments showed that the rate of iron bioreduction by metal-reducing dissimilar bacteria (DMRBs) and the competition of methane-producing bacteria (MPBs) with DMRBs for VFAs (acetates) were not the main factors controlling the formation of viviaite, and iron-DMRB bioreduction can take place with sufficient amounts of Fe³⁺exists in WAS. Determined affinity constants (K_S) both DMRB and MPB on acetate revealed that K_HAcconstant (4.2 mmol/g VSS) of DMRB was almost 4 times lower than MPB (15.67 mmol/g VSS) and therefore MPB could not seriously compete for VFA (acetate) with DMRB. As a result, the formation of vivianites was mainly controlled by the amount of Fe³⁺in WAS. In practice, a Fe/P molar ratio of 2:1 should be sufficient to form viviaite in AD or WAS. Otherwise, the exogenous dose of Fe³⁺z Fe²⁺in AD must be used in AD.

Membrane bioreactors (MBRs) are well known as an advanced wastewater treatment technology that combines activated sludge with a pressurized membrane separation process. They are widely used in municipal and industrial wastewater treatment plants due to their advantages over conventional activated sludge systems. However, MBRs still face major problems that hinder their widespread implementation, such as poor effluent quality due to low retention of trace organic pollutants, instability due to changing operating conditions, and contamination of organic membranes. Membrane distillation bioreactor (MDBR) is a new wastewater treatment technology that combines MBR with an MD unit, either separated in a side stream or submerged in a bioreactor. The concept of a hybrid MDBR setup was first introduced in 2007, followed by numerous journal articles. These studies demonstrated the potential of MDBR for water, nutrients, bioenergy and other added-value products for the recovery of domestic and industrial wastewater; however, a comprehensive study demonstrating the full potential of MDBR is still lacking. Therefore, there is a need for a dedicated review that summarizes and critically analyzes these research findings and highlights research gaps to provide a clear path for future research. This review outlines the availability of the hybrid MDBR system and application. In particular, the review includes a brief introduction to MBR and MD technologies, followed by the development of MDBR as a new technology for municipal and industrial wastewater treatment. Various applications of MDBR systems are reviewed with an emphasis on system performance and removal efficiency. The potential of MDBR for wastewater reuse and recovery of nutrients and biogas was also demonstrated. Finally, the main challenges hindering the development of MDBR were discussed and recommendations were made regarding future prospects for MDBR commercialization.

Water Research, tom 104, 2016, s. 449-460

Iron is an important element for modern wastewater treatment, including the removal of phosphorus from wastewater. However, recovering phosphorus from sewage sludge containing iron and phosphorus, without incineration, is not yet economically viable. We believe that advancing our understanding of iron and phosphorus speciation in sewage sludge can help identify new pathways for phosphorus recovery. Surpluses and digestate from two sewage treatment plants were studied. Plants relied solely on iron-based phosphorus removal or biological phosphorus removal assisted by iron dosing. Mössbauer spectroscopy showed that vivianite and pyrite were the dominant iron compounds in excess sediments and anaerobic digested sediments at both plants. Mössbauer and XRD spectroscopy suggest that vivianite-bound phosphorus comprised between 10 and 30% (in plants that rely primarily on biological removal) and between 40 and 50% of the total phosphorus (in plants that rely on iron-based phosphorus removal) . Furthermore, Mössbauer spectroscopy showed that none of the samples contained a significant amount of Fe(III), although there were aeration steps and although Fe(III) was dosed in addition to Fe(II). We hypothesize that the chemical/microbial reduction of Fe(III) in treatment lines is relatively rapid and causes the formation of vivianites. Once formed, vivianite can withstand oxygenated treatment zones due to slow oxidation kinetics and oxygen diffusion limitations in the silt flocs. These results indicate that vivianite is the most important iron phosphorus compound in sewage treatment plants with a moderate iron dosage. We hypothesize that vivianite predominates in most plants where iron is dosed to remove phosphorus, which may provide new pathways for phosphorus recovery.

Journal of Environmental Sciences, tom 92, 2020, s. 224-234

Recently, more and more attention is paid to the strong oxidizing properties of the newly prepared potassium ferrate (NAPF) in the sludge reduction process, while less attention is paid to the conversion of phosphorus in this process. The feasibility of phosphorus migration and transformation during excess sludge reduction pre-treatment by NAPF pre-oxidation in combination with anaerobic digestion was investigated. After pre-treatment with NAPF 70 mg/g slurry and 16 days of anaerobic digestion, the volatile solids in the solid phase were reduced by 44.2% and a lot of organic matter was released in the liquid phase and then degraded during digestion by native microorganisms. During the process of pre-oxidation of sludge, in the form of PO₄^3-, with a peak of 100 mg/l. During anaerobic fermentation, Fe³⁺in the liquid phase it was gradually reduced to Fe²⁺, and then formed Fe²⁺-NO₄^3-complex crystals and migrated back to the solid phase. PO concentration₄^3-decreased to 17.08 ± 1.1 mg/L in the liquid phase after anaerobic digestion. Finally, phosphorus in Fe²⁺-NO₄^3-the compound accounts for 80% of the total phosphorus in the solid phase. A large number of viviaite crystals have been observed in the sediment. Therefore, this technology not only effectively reduces sludge, but also increases the PO content₄^3-in the precipitate in the form of vivianite.

Bioresources Technology, Volume 293, 2019, Article 122088

The paper presents an innovative method of recovering phosphorus (P) in the form of wiviaite from activated sludge (WAS) waste by optimizing the iron dose and pH value during anaerobic digestion (AF). The optimal conditions for the formation of viviaite were in the pH range of 6.0–9.0 with the initial PO₄^3->5mg/L and Fe/P molar ratio of 1.5. Specifically, FeCl₃showed superiority over ZVI in terms of joint issue of Fe²⁺not after₄^3-during the fermentation of WAS, especially under acidic conditions. FeCl₃dosing at pH 3.0 can contribute to 78.81% Fe²⁺release and 85.69% of total PO₄^3-liberation from YOU. They were finally recovered as high purity viviaite (93.67%).Clostridiaceae(40.25%) was the dominant bacterium in FeCl₃-pH3 reactors that played a key role in triggering the reduction of non-Fe-like iron²⁺creation. Therefore, recovering P as wiwiaite from WAS fermentation may be a promising and highly valuable approach to mitigate the P crisis.

Science of the Total Environment, tom 762, 2021, artykuł 143076

Vivianet (Fot₃(NA₄)₂eight o'clock₂O) is a common hydrated ferrous phosphate that is often found under reducing conditions, especially in an anaerobic non-sulfide environment containing high concentrations of iron (Fe²⁺) nie ortofosforan (PO₄^3-). Vivianite is an important product of dissimilar iron reduction and a promising way to recover phosphorus from wastewater. Its formation is closely related to extracellular electron transfer (EET), a key mechanism of microbial respiration and a key explanation for the reduction of metal oxides in soils and sediments. Despite its natural ubiquity, easy availability and attractive economic value, vivianite's utility value has not received much attention. This review presents the characterization, occurrence, and biosynthesis of vivianite from microbial EET and systematically analyzes the environmental value of vivianite, including heavy metal (HM) immobilization, carbon tetrachloride (CT) dechlorination, sedimentary phosphorus sequestration, and eutrophication mitigation. In addition, the possible functions of a slow-release fertilizer are also discussed. Vivian is generally not expected to make a major contribution to future scientific research, especially addressing environmental issues. Overcoming the misunderstanding and certain technical limitations will be beneficial for the further environmental application of vivianite.

Science of the Total Environment, tom 791, 2021, artykuł 148213

Recovering phosphorus (P) from industrial wastewater solves both P deficiency and P contamination. Sequential iron-air fuel cell launched to recover P from synthetic wastewater containing 0.6 g-P/L Na₂HPO₄. In the cell, ferric iron passes from the iron anode into the fluid, precipitating soluble P and forming vivianite. Electrons travel from the iron anode to the air cathode through an external circuit to generate energy. Within 3 months of continuous use, the P removal efficiency steadily reached about 97.6%, and the average output voltage of the cell was 404 mV. After prolonged use, a deterioration in the performance of the iron fuel cell was observed due to the passivation of the electrode caused by the accumulation of P deposit on the surface of the iron anode. The deposit layer on the iron anode hindered, but did not block, the transfer of iron(II) mass to the anode fluid. The cell continued to operate with a 25% drop in output voltage, an 86% drop in current density, an 87% drop in power density, and a 9-fold increase in internal resistance. Further XRD, FITR and Mössbauer analyzes showed that vivianite was the main component of both deposits on the surface of the iron anode and at the bottom of the anode chamber with 66% and 30% content, respectively. Vivianite on the iron anode surface was preferred due to its higher content for P recovery. thus to the sustainable development of wastewater treatment.

Chemosfera, tom 226, 2019, s. 246-258

Due to the scarcity of phosphorus resources and the limitations of existing phosphorus recovery methods, the recovery of phosphorus in the form of vivianite is of great interest due to its natural ubiquity, ready availability and predictable economic value. This review systematically summarizes the chemistry of vivianite, including material characteristics, formation process, and influencing factors. In addition, the potential for the recovery of phosphorus as wiwiaite from wastewater was also extensively investigated from the point of view of economic value and technical feasibility. In general, this method is theoretically and practically feasible and provides some additional benefits in wastewater treatment plants. However, insufficient understanding of vivianite recovery in wastewater/sludge has been delaying the development and exploration of such a sophisticated approach. Further research and support in various fields would facilitate the refinement of this technique in the future.