Unravelling key factors controlling vivianite formation during anaerobic digestion of waste activated sludge

Resource recovery of high value-added products from wastewater: Current status and prospects

Wastewater resource recovery not only allows the extraction of value-added products and offsets the operational costs of wastewater treatment, but it is also conducive to alleviating adverse environmental issues due to energy and chemical inputs and associated emissions. A number of attractive compounds such as alginate-like polymers, struvite, polyhydroxyalkanoates, and sulfated polysaccharides, were found and successfully obtained from wastewater and have a wide range of application prospects. The aim of this work is to provide a comprehensive review of recent advances in recovery of these popular products from wastewater, and their physicochemical properties, main sources, and current recovery status are summarized. Various factors influencing the recovery performance of these materials are thoroughly discussed. Moreover, the research needs and future directions towards wastewater resource recovery are highlighted. This study can provide valuable insights for future research endeavors aiming to improve wastewater resource recovery through the retrieval of high value-added products.

Effect of iron sources on methane production and phosphorous transformation in an anaerobic digestion system of waste activated sludge

The iron materials are commonly employed to enhance resource recovery from waste activated sludge through anaerobic digestion (AD). The influence of different iron sources, such as Fe2O3, Fe3O4, and FeCl3 on methane production and phosphorus transformation in AD systems with thermal hydrolyzed sludge as the substrate was assessed in this study. The results indicated that iron oxides effectively promote methane yield and methane production rate in AD systems, resulting in a maximum increase in methane production by 1.6 times. Soluble FeCl3 facilitated the removal of 92.3% of phosphorus from the supernatant through the formation of recoverable precipitates in the sludge. The introduction of iron led to an increase in the abundance of bacteria responsible for hydrolysis and hydrogenotrophic methanogenesis. However, the enrichment of microbial communities varied depending on the specific irons used. This study provides support for AD systems that recover phosphorus and produce methane efficiently from waste sludge.

Tracking the formation potential of vivianite within the treatment train of full-scale wastewater treatment plants

Phosphorus recovery is a vital element for the circular economy. Wastewater, especially sewage sludge, shows great potential for recovering phosphate in the form of vivianite. This work focuses on studying the iron, phosphorus, and sulfur interactions at full-scale wastewater treatment plants (Viikinmäki, Finland and Seine Aval, France) with the goal of identifying unit processes with a potential for vivianite formation. Concentrations of iron(III) and iron(II), phosphorus, and sulfur were used to evaluate the reduction of iron and the formation potential of vivianite. Mössbauer spectroscopy and X-ray diffraction (XRD) analysis were used to confirm the presence of vivianite in various locations on sludge lines. The results show that the vivianite formation potential increases as the molar Fe:P ratio increases, the anaerobic sludge retention time increases, and the sulfate concentration decreases. The digester is a prominent location for vivianite recovery, but not the only one. This work gives valuable insights into the dynamic interrelations of iron, phosphorus, and sulfur in full-scale conditions. These results will support the understanding of vivianite formation and pave the way for an alternative solution for vivianite recovery for example in plants that do not have an anaerobic digester.

Strategies for optimizing biovivianite production using dissimilatory Fe(III)-reducing bacteria

Vivianite (Fe3(PO4)2·8H2O), a sink for phosphorus, is a key mineralization product formed during the microbial reduction of phosphate-containing Fe(III) minerals in natural systems, and also in wastewater treatment where Fe(III)-minerals are used to remove phosphate. As biovivianite is a potentially useful Fe and P fertiliser, there is much interest in harnessing microbial biovivianite synthesis for circular economy applications. In this study, we investigated the factors that influence the formation of microbially-synthesized vivianite (biovivianite) under laboratory batch systems including the presence and absence of phosphate and electron shuttle, the buffer system, pH, and the type of Fe(III)-reducing bacteria (comparing Geobacter sulfurreducens and Shewanella putrefaciens). The rate of Fe(II) production, and its interactions with the residual Fe(III) and other oxyanions (e.g., phosphate and carbonate) were the main factors that controlled the rate and extent of biovivianite formation. Higher concentrations of phosphate (e.g., P/Fe = 1) in the presence of an electron shuttle, at an initial pH between 6 and 7, were needed for optimal biovivianite formation. Green rust, a key intermediate in biovivianite production, could be detected as an endpoint alongside vivianite and metavivianite (Fe2+Fe3+2(PO4)2.(OH)2.6H2O), in treatments with G. sulfurreducens and S. putrefaciens. However, XRD indicated that vivianite abundance was higher in experiments containing G. sulfurreducens, where it dominated. This study, therefore, shows that vivianite formation can be controlled to optimize yield during microbial processing of phosphate-loaded Fe(III) materials generated from water treatment processes.

Deciphering the interaction of heavy metals with Geobacter-induced vivianite recovery from wastewater

Vivianite recovery from wastewater driven by Geobacter is one of the promising approaches to address the challenges of phosphorus (P) resource shortage and eutrophication. However, the interfere of heavy metals which are prevalent in many actual wastewater with this process is rarely reported. In this study, we investigated the impact of heavy metals (i.e., Cu and Zn ions) on microbial activity, Fe reduction, P recovery efficiency, and their fate during Geobacter-induced vivianite recovery process. The experimental results showed that low and medium concentrations of Cu and Zn prolonged the Fe reduction and P recovery time but had little effect on the final P recovery efficiency. However, high concentrations of Cu and Zn ultimately inhibit vivianite formation. In addition, the different concentrations of Cu and Zn showed different effects on the morphology of the recovered vivianite. The migration of Cu and Zn was analysed by stepwise extraction of heavy metals in the vivianite. Medium concentrations of Cu and Zn were more likely to co-precipitate with vivianite, while adsorption was the primary mechanism at low concentrations. Furthermore, there were differences in the fate of Cu and Zn, and a competition mechanism was observed. Finally, we found that increasing the Fe/P ratio can significantly reduce the residues of heavy metals in vivianite. It also increased the adsorbed Cu and Zn proportion and reduced co-precipitation. These results provide insights into improving the efficiency of vivianite recovery and managing the environmental risks of heavy metal in the recovered product.

Toward Energy Neutrality: Novel Wastewater Treatment Incorporating Acidophilic Ammonia Oxidation

Chemically enhanced primary treatment (CEPT) followed by partial nitritation and anammox (PN/A) and anaerobic digestion (AD) is a promising roadmap to achieve energy-neutral wastewater treatment. However, the acidification of wastewater caused by ferric hydrolysis in CEPT and how to achieve stable suppression of nitrite-oxidizing bacteria (NOB) in PN/A challenge this paradigm in practice. This study proposes a novel wastewater treatment scheme to overcome these challenges. Results showed that, by dosing FeCl3 at 50 mg Fe/L, the CEPT process removed 61.8% of COD and 90.1% of phosphate and reduced the alkalinity as well. Feeding by low alkalinity wastewater, stable nitrite accumulation was achieved in an aerobic reactor operated at pH 4.35 aided by a novel acid-tolerant ammonium-oxidizing bacteria (AOB), namely, Candidatus Nitrosoglobus. After polishing in a following anoxic reactor (anammox), a satisfactory effluent, containing COD at 41.9 ± 11.2 mg/L, total nitrogen at 5.1 ± 1.8 mg N/L, and phosphate at 0.3 ± 0.2 mg P/L, was achieved. Moreover, the stable performances of this integration were well maintained at an operating temperature of 12 °C, and 10 investigated micropollutants were removed from the wastewater. An energy balance assessment indicated that the integrated system could achieve energy self-sufficiency in domestic wastewater treatment.

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.

Vivianite recovery from high concentration phosphorus wastewater with mine drainage as iron sources

High concentration phosphorus wastewater has attracted much attention due to the safety of water ecology and the potential crisis of phosphorus resource, which is caused by large amounts of phosphorus discharging into natural water bodies. Vivianite (Fe₃(PO₄)₂·8H₂O) crystallization has been considered as an effective technology for phosphorus recovery. In this study, we develop a potentially low-cost, sustainable approach to recover phosphorus from high concentration phosphorus wastewater using mine drainage as iron source. Inoculated with both sewage and Geobacter, mine drainage was suitable for vivianite recovery from high concentration phosphorus wastewater with PO₄^3- concentration between 6 and 18 mM. When the PO₄^3- concentration increased gradually, both phosphorus removal efficiency (R_P) and vivianite recovery efficiency (R_V) decreased significantly. The highest R_V of 48 % was obtained with 9 mM PO₄^3- in Geobacter batches (CJ2 batches), which was 15 % higher than that in the paralleled sewage batches (33 % in HJ2). Simultaneously, vivianite accounted for 91 % of the solid phosphate compounds in CJ2 batches due to the enhancement of Geobacter.