Formation of vivianite in digested sludge and its controlling factors in municipal wastewater treatment

The challenges and limitations of vivianite quantification with 2,2'-bipyridine extraction

Vivianite presents a significant phosphorus pool in iron-rich, reducing environments, necessitating the development of an affordable and routine quantification method. A novel extraction protocol using 2,2'-bipyridine (Bipy) was proposed as a promising approach. However, the efficacy of this protocol in achieving complete vivianite extraction remains uncertain and lacks robust analytical validation. This study systematically assessed the Bipy extraction protocol on known amount of synthetic and two magnetically recovered environmental vivianite samples, with varying oxidation levels, impurity content, and particle size. Extraction efficiencies of iron and phosphorus were 13-44 % and 13-55 %, respectively, indicating incomplete extraction under the original protocol conditions (0.2 % Bipy, 24 h). An initially rapid release of iron and phosphorus slowed down across all samples, indicating non-constant reaction rates, and suggesting that the extraction is governed by mechanisms beyond kinetic control. Extending the extraction time to 48 h and increasing the Bipy concentration to 1 % yielded marginal improvements, with efficiency gains of 14 % or less. Grinding, which reduced particle size, nearly doubled extraction efficiency. Conversely, the sample with the highest Fe2+ content showed the overall lowest overall extraction efficiencies. Similarly, recovered vivianite samples containing impurities, namely magnesium and calcium, were extracted less efficiently than synthetic vivianite. Additionally, the extracted iron-to‑phosphorus ratio exceeded the theoretical value of 0.67, indicating non-stoichiometric extraction. To establish Bipy extraction as a reliable analytical method, it is crucial to address incomplete extraction and determine whether vivianite can be fully extracted or only to a certain extent. A potential strategy involves reducing extraction time and repeating the extraction step.

Rapid Eutrophication of a Clearwater Lake: Trends and Potential Causes Inferred From Phosphorus Mass Balance Analyses

Many clearwater lakes increasingly show symptoms of eutrophication, but the underlying causes are largely unknown. We combined long-term water chemistry data, multi-year sediment trap measurements, sediment analyses and simple mass balance models to elucidate potential causes of eutrophication of a deep temperate clearwater lake, where total phosphorus (TP) concentrations quadrupled within a decade, accompanied by expanding hypolimnetic anoxia. Discrepancies between modeled and empirically determined P inputs suggest that the observed sharp rise in TP was driven by internal processes. The magnitude of seasonal variation in TP greatly increased at the same time, both in surface and deep water, partly decoupled from deep water oxygen conditions. A positive correlation between annual P loss from the upper water column and hypolimnetic P accumulation could hint at a short-circuited P cycle involving lateral TP transport from shallow-water zones and deposition and release from sediments in deep water. This hypothesis is also supported by P budgets for the upper 20 m during stable summer stratification, suggesting that sediments in shallow lake areas acted as a P net source until 2018. These changes are potentially related to shifts in submerged macrophytes from wintergreen charophyte meadows (Nitellopsis obtusa) to annual free-floating hornwort (Ceratophyllum demersum) and to increased sulfide formation, promoting iron fixation in the sediments. Iron bound to sulfur is unavailable for binding P, resulting in a positive feedback between P release in shallow lake areas, primary productivity, macrophyte community structure and redox-dependent sediment biogeochemistry. Overall, our results suggest that relationships more complex than the commonly invoked increase in internal P release under increasingly anoxic conditions can drive rapid lake eutrophication. Since the proportion of littoral areas is typically large even in deep stratified lakes, littoral processes may contribute more frequently to the rapid lake eutrophication trends observed around the world than is currently recognized.

Improving nutrients ratio in class A biosolids through vivianite recovery: Insights from a wastewater resource recovery facility

Class A biosolids from water resource recovery facilities (WRRFs) are increasingly used as sustainable alternatives to synthetic fertilizers. However, the high phosphorus to nitrogen ratio in biosolids leads to a potential accumulation of phosphorus after repeated land applications. Extracting vivianite, an FeP mineral, prior to the final dewatering step in the biosolids treatment can reduce the P content in the resulting class A biosolids and achieve a P:N ratio closer to the 1:2 of synthetic fertilizers. Using ICP-MS, IC, UV-Vis colorimetric methods, Mössbauer spectroscopy, and SEM-EDX, a full-scale characterization of vivianite at the Blue Plains Advanced Wastewater Treatment Plant (AWTTP) was surveyed throughout the biosolids treatment train. Results showed that the vivianite-bound phosphorus in primary sludge thickening, before pre-dewatering, after thermal hydrolysis, and after anaerobic digestion corresponded to 8 %, 52 %, 40 %, and 49 % of the total phosphorus in the treatment influent. Similarly, the vivianite-bound iron concentration also corresponded to 8 %, 52 %, 40 %, and 49 % of the total iron present (from FeCl3 dosing), because the molar ratio between total iron and total incoming phosphorus was 1.5:1, which is the same stoichiometry of vivianite. Based on current P:N levels in the Class A biosolids at Blue Plains, a vivianite recovery target of 40 % to ideally 70 % is required in locations with high vivianite content to reach a P:N ratio in the resulting class A biosolid that matches synthetic fertilizers of 1:1.3 to 1:2, respectively. A financial analysis on recycling iron from the recovered vivianite had estimated that 14-25 % of Blue Plain's annual FeCl3 demand can potentially be met. Additionally, model simulations with Visual Minteq were used to evaluate the pre-treatment options that maximize vivianite recovery at different solids treatment train locations.

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.