To improve the efficiency of phosphorus (P) release from marine sediments and contribute to P loop closure, this study proposed a novel strategy combining bio-inoculation with polyphosphate-accumulating organisms (PAOs) and chemical enhancement via chelating agents. Based on prior findings, two-stage experiments were conducted. In Stage 1, anaerobic batch tests assessed the effect of different chelating agents for P release. While citrate showed no promoting effect, the addition of ethylenediaminetetraacetic acid (EDTA) significantly enhanced total P release, reaching 48.5 % within 15 days. In Stage 2, PAO-acclimated sediments were introduced into the system, followed by alternating anaerobic-aerobic fed-batch operation for 7 days, and subsequent EDTA addition with anaerobic incubation for another 6 days. This combined approach achieved a total P release efficiency 83.4 %, with final soluble P concentrations reaching 145.9 mg/L. During this process, PAOs were rapidly enriched, with their relative abundance increasing from 12.9 % to 65.0 %. Metagenomic analysis revealed that EDTA promoted environmental filtering, selectively enriching PAOs (Candidatus Accumulibacter) and thereby reinforcing their specific contributions to P functional genes. The resulting P-rich supernatant was then subjected to precipitation. PHREEQC simulations guided the prediction of optimal precipitation conditions, and laboratory experiments confirmed that most soluble P, especially Fe-bound forms, could be efficiently recovered, with maximum precipitation efficiencies of 98.8 %.

Phosphorus (P) is an essential nutrient for all living organisms, yet its availability is limited within the European Union (EU), leading to a complete reliance on imports to meet the region’s P demand. This dependency highlights the urgent need to explore alternative P sources, with recovery from secondary resources emerging as a viable solution. Concurrently, marine eutrophication, particularly in the Baltic Sea, has become a global environmental crisis due to decades of nutrient over-enrichment. According to Baltic Marine Environment Protection Commission’s (HELCOM) eutrophication assessment, over 97% of the Baltic Sea region fails to meet good environmental status, with 12% classified as being in the worst condition. Addressing these dual challenges, P resource scarcity and P-driven eutrophication, requires innovative approaches. Recovering P from eutrophic Baltic Sea sediments presents a promising strategy, offering the dual benefits of P recycling and eutrophication mitigation. This Ph.D. research aims to develop a chemical bioengineering-method for P release and recovery from Baltic Sea sediments, contributing to both resource sustainability and environmental restoration. 

Anaerobic batch experiments demonstrated that among various carbon sources, propionic acid and glucose exhibited significantly higher stimulation of P release, indicating their stronger potential for promoting P mobilization under anaerobic conditions. Subsequent long-term sequencing batch reactor (SBR) operations further verified that supplementation with propionic acid at 200 mg/L chemical oxygen demand (COD) effectively facilitated both P anaerobic release and aerobic uptake from marine sediments, while selectively enriching polyphosphate-accumulating organisms (PAO)-related family (Rhodocyclaceae), whose abundance increased from 0% to 16% within 42 days.

Building on these findings, optimized operational conditions were applied to achieve enhanced PAO enrichment and P concentration for subsequent struvite precipitation. Cyclic anaerobic-aerobic cultivation promoted PAO abundance at the genus level from 0.06% to 7.1%, while achieving satisfactory P release (8.61 mg P/g VSS·h-1) and uptake (8.43 mg P/g VSS·h-1) within the sediment-inoculated SBR. Additionally, extended anaerobic operation enabled the concentration of low-P solutions into high-P supernatants (up to 99.5 mg/L), facilitating efficient P recovery with a rate exceeding 95%. Notably, this P extraction process further accelerated PAO enrichment at the genus level, increasing their abundance from less than 15% to 52.1% by the end of the operation. 

Furthermore, a combined strategy involving PAO inoculation and ethylenediaminetetraacetic acid (EDTA) addition was proposed to further enhance P release from marine sediments. This approach achieved a final P release efficiency exceeding 80% within 13 days, with the P concentration reaching approximately 150 mg/L and PAO abundance increasing rapidly from <13% to over 65%. PHREEQC simulations and precipitation experiments confirmed the feasibility of recovering the released P via precipitation. 

Overall, this study provides new insights into the microbial mechanisms and engineering strategies for P release and recovery from marine sediments, offering a promising approach for sustainable P resource management and eutrophication mitigation.

Fosfor (P) är ett livsnödvändigt näringsämne för alla levande organismer, men tillgången inom Europeiska unionen (EU) är begränsad, vilket har lett till ett fullständigt beroende av import för att möta regionens P-behov. Detta beroende understryker det akuta behovet av att undersöka alternativa P-källor, där återvinning från sekundära resurser håller på att utvecklas till en hållbar lösning. Samtidigt har årtionden av övergödning utvecklats till en global miljökris och särskilt Östersjön är hårt drabbad. Enligt data från HELCOMs eutrofieringsbedömning har över 97 % av Östersjöregionen inte god miljöstatus, där 12 % klassificeras som värsta möjliga tillstånd. För att hantera dessa dubbla utmaningar, brist på P-resurser och P-relaterad övergödning, krävs innovativa lösningar. Återvinning av P från eutrofa sediment i Östersjön är en lovande strategi som kombinerar resurshantering med miljöförbättring. Syftet med detta doktorandprojekt är att utveckla en kemisk bioteknisk metod för frisättning och återvinning av P från Östersjösediment, vilket bidrar till både resurshållbarhet och ekologisk återställning.

Anaeroba satsförsök visade att propionsyra och glukos, bland ett flertal kolkällor, kraftigt stimulerade frisättning av P, vilket indikerar deras stora potential för att mobilisera P under dessa förhållanden. Efterföljande långtidsförsök i sekventiell batchreaktor (SBR) bekräftade att tillsats av propionsyra vid 200 mg/L kemisk syreförbrukning (COD) effektivt underlättade både anaerob frisättning och aerobt upptag av P från marina sediment, samtidigt som den selektivt berikade PAO-relaterade bakterier (Rhodocyclaceae), vars förekomst ökade från 0 % till 16 % inom 42 dagar.

Med utgångspunkt av dessa resultat tillämpades optimerade driftsförhållanden för att ytterligare förstärka PAO-berikning och P-koncentrering inför efterföljande utfällning av struvit. Cyklisk anaerob–aerob odling ökade förekomsten av PAO på genusnivå från 0,06 % till 7,1 %, samtidigt som en hög P-frisättning (8,61 mg P/g VSS·h-1) och ett högt P-upptag (8,43 mg P/g VSS·h1) uppnåddes i den sedimentinokulerade SBRen. Vidare möjliggjorde förlängd anaerob drift en koncentrationsökning från låg-P-lösningar till hög-P-supernatanter (upp till 99,5 mg/L), vilket möjliggjorde effektiv återvinning med en effektivitet över 95 %. Noterbart påskyndade denna P-utvinningsprocess ytterligare PAO-berikningen, med en ökning från mindre än 15 % till 52,1 % vid driftens slut.

Dessutom utvärderades en kombinerad strategi med PAO-inokulation och tillsats av etylendiamintetraättiksyra för att ytterligare öka P-frisättningen från marina sediment. Denna metod uppnådde över 80 % P-frisättning inom 13 dagar, med en P-koncentration som uppnådde cirka 150 mg/L och en snabb ökning av PAO från <13 % till över 65 %. PHREEQC-simuleringar och utfällningsförsök bekräftade möjligheten att återvinna den frigjorda P genom utfällning.

Sammanfattningsvis ger denna studie nya insikter i mikrobiella mekanismer och tekniska strategier för frisättning och återvinning av P från marina sediment. Studien erbjuder ett lovande tillvägagångssätt för hållbar resurshantering och bekämpning av övergödning.

To evaluate the feasibility of enriching polyphosphate-accumulating organisms (PAOs) within marine sediment for achieving phosphorus (P) recovery, two sediment-inoculated sequencing batch reactors (SBRs), fed with propionic acid (R1) and glucose (R2), were operated for 119 days. For comparison, two sewage sludge-inoculated reactors (R3 and R4) were also set up. The sediments/sludge fed with 200 mg/L chemical oxygen demand (COD) equivalent propionic acid exhibited satisfactory P release/uptake performance after 56 days of culture. The maximum P release and uptake rates for R1 were 3 mg P/g VSS•h−1 and 2.5 mg P/g VSS•h−1, respectively, while for R3 they were 2.6 mg P/g VSS•h−1 and 5.8 mg P/g VSS•h−1, respectively. Meanwhile, the PAO family (Rhodocyclaceae) in R1 increased from almost 0 % initially to 16.0 % after 42 days. However, the glucose-fed SBRs did not exhibit enhanced biological phosphorus removal (EBPR) performance throughout the operation. As the COD feeding concentration increased to 400 mg/L, the reactors showed EBPR deterioration. Total P in R1 and R3 significantly decreased from 423.7 mg to 307.2 mg and from 368.0 mg to 94.9 mg, respectively. Key intracellular polymer responses indicated that introduction of excessively high COD significantly reduced poly-P content and the anaerobic synthesis of polyhydroxyalkanoate. Microbial analysis suggested that the breakdown of EBPR performance could be attributed to glycogen-accumulating organisms outcompeting PAOs under high carbon feeding conditions. Additionally, PHREEQC simulations confirmed that P-rich supernatant from the anaerobic phase could theoretically be recovered as struvite, with a recovery efficiency of up to 94 %.

To investigate the possibility of phosphorus (P) recovery from marine sediment and explore the role of the carbon: nitrogen ratio in affecting the internal P release under anaerobic conditions, we experimented with the external addition of carbon (acetic acid and glucose) and ammonia nitrogen (NH4-N) to expose P release mechanisms. The 24-day anaerobic incubations were conducted with four different carbon: nitrogen dosing groups including no NH4-N addition and COD/N ratios of 100, 50, and 10. The P release showed that extra NH4-N loading significantly suppressed the decomposition of P (p < 0.05) from the marine sediment, the maximum P release was 4.07 mg/L and 7.14 mg/L in acetic acid- and glucose-fed systems, respectively, without extra NH4-N addition. Additionally, the results exhibited that the imbalance of carbon: nitrogen not only failed to induce the production of organic P mineralization enzyme (alkaline phosphatase) in the sediment but also suppressed its activity under anaerobic conditions. The highest enzyme activity was observed in the group without additional NH4-N dosage, with rates of 1046.4 mg/(kg∙h) in the acetic acid- and 967.8 mg/(kg∙h) in the glucose-fed system, respectively. Microbial data analysis indicated that a decrease in the abundance of P release-regulating bacteria, including polyphosphate-accumulating organisms (Rhodobacteraceae) and sulfate-reducing bacteria (Desulfosarcinaceae), was observed in the high NH4-N addition groups. The observed reduction in enzyme activity and suppression of microbial activity mentioned above could potentially account for the inhibited P decomposition in the presence of high NH4-N addition under anaerobic conditions. The produced P-enriched solution from the bioreactors may offer a promising source for future recovery endeavors.

The aim of this study was recovery of phosphorus (P) from marine sediment, and our results revealed the influence of P release from the sediment stimulated with different types and concentrations of carbon sources. During the 15-day anaerobic operation, the sediments stimulated with 1 g/L propionic acid and glucose exhibited more prominent effects compared to other trials, with 5.98 mg/L and 6.44 mg/L of P released, respectively, with a total solid content of 4 %. Notably, the excessive addition of carbon sources was shown to can partially inhibit P release. As microbial activity intensified, P was utilized for microbial synthesis, resulting in a decreased P in the supernatant. For example, in glucose-fed systems with concentrations of 5 g/L and 10 g/L, the P concentration decreased from 5 mg/L on Day 3 to approximately 3 mg/L on Day 15. The sequencing results indicated distinct evolutions within different carbon source-fed systems over the 15-day operations. Feeding high concentrations of glucose resulted in rapid enrichment of fermentative bacteria under anaerobic conditions, while sulfate-reducing bacteria promoted P release in volatile fatty acids-fed systems. Metabolic analysis revealed that carbon sources not only influence gene expression in different systems, but also impact the metabolic pathways involved in nutrient cycling, which can be interrelated. For example, a significant positive correlation was observed between the abundance of P and sulfur cycling functional genes (phoD, cysD).

Developing novel strategies to enhance volatile fatty acid (VFA) yield from abundant waste resources is imperative to improve the competitiveness of biobased VFAs over petrochemical-based VFAs. This study hypothesized to improve the VFA yield from food waste via three strategies, viz., pH adjustment (5 and 10), supplementation of selenium (Se) oxyanions, and heat treatment of the inoculum (at 85 °C for 1 h). The highest VFA yield of 0.516 g COD/g VS was achieved at alkaline pH, which was 45% higher than the maximum VFA production at acidic pH. Heat treatment resulted in VFA accumulation after day 10 upon alkaline pretreatment. Se oxyanions acted as chemical inhibitors to improve the VFA yield at pH 10 with non-heat-treated inoculum (NHT). Acetic and propionic acid production was dominant at alkaline pH (NHT); however, the VFA composition diversified under the other tested conditions. More than 95% Se removal was achieved on day 1 under all the conditions tested. However, the heat treatment was detrimental for selenate reduction, with less than 15% Se removal after 20 days. Biosynthesized Se nanoparticles were confirmed by transmission and scanning electron microscopy and and energy dispersive X-ray analyses. The heat treatment inhibited the presence of nonsporulating bacteria and methanogenic archaea (Methanobacteriaceae). High-throughput sequencing also revealed higher relative abundances of the bacterial families (such as Clostridiaceae, Bacteroidaceae, and Prevotellaceae) that are capable of VFA production and/or selenium reduction.

Phosphorus (P), one of the essential and indispensable elements for the growth of living organisms, is finite and nonrenewable for a multitude of applications. Economic P recycling can contribute to sustainable P management and lessen the pressure on P imports of the EU. Several European countries now have paved the way for establishing P recovery legislation and successively implemented compulsory recycling from P-rich wastes. This paper provided a comprehensive review for analyzing the potential of existing P recycling resources and engineering technologies in terms of their feasibility and applicability in Europe. Potential P-rich flows from the waste streams and eutrophic environments were deeply investigated. Regarding their technical principles, application, and availability, different engineering approaches for P release enhancement and P recovery were also reviewed in a critical manner. Moreover, assessments on engineering applications for P recycling considering economic feasibility and environmental benefits were also presented. Wastewater treatment plants show promising recovery efficiency, whereas industrial processes with sludge ashes as input allow up to 95% recovery of P. Collectively, in Europe, efficient and sustainable management of P has been recognized as a necessary strategy to meet the growing demand for mineral P.