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Hydrothermal carbonization of digested sewage sludge and microalgae biomass: phosphorus and energy recovery
Umeå University, Faculty of Science and Technology, Department of Chemistry.ORCID iD: 0000-0003-3607-3888
2024 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Hydrotermal karbonisering av rötat avloppsslam och mikroalgerbiomassa : fosfor och energiåtervinning (Swedish)
Abstract [en]

Sewage sludge and microalgae biomass are by-products of wastewater treatment, requiring careful management to avoid environmental and health risks. Both sewage sludge and microalgae have high moisture content, making thermochemical conversion challenging and energy intensive. Hydrothermal carbonization (HTC) presents a promising solution for converting these wet feedstocks into valuable resources. The thesis aimed to study HTC of sewage sludge and microalgae biomass, individually and combined (i.e., co-HTC). It focused on process parameters, mixing ratios, product characteristics, primary and secondary char formation, and resource recovery, with especial emphasis was on phosphorus and energy recovery as potential applications of the resulting hydrochars. Both the HTC and co-HTC experiments were conducted at 180, 215 and 250°C for 2 h (Papers I–IV).

Paper I investigated co-HTC by combining microalgae and sewage sludge in various ratios, from 0 to 100% of sewage sludge. Results showed that higher sewage sludge proportions and carbonization temperatures led to lower degradation and carbonization rates. The addition of sewage sludge influenced secondary char formation and composition, reducing carboxylic acid and ketones while increasing higher molecular weight cholesterols. Moreover, sewage sludge hydrochars contained larger phosphorus quantities.

In acid-leaching experiments (Papers II and III) using sewage sludge, phosphorus-extraction efficiencies surpassed 75%. Complete phosphorus recovery (100%) was achieved only with oxalate extraction at pH=1. Organic acids, utilized at a lower concentration (0.25 M) compared to mineral acids (2.5 M), acted as both acids and chelating agents, facilitating phosphorus recovery. Regardless of acid type, leaching from hydrochar transferred not only phosphorus but metals and heavy metals into the P-rich leachate, requiring post-treatment purification. 

Combustion studies of microalgae and sewage sludge co-hydrochar, and phosphorus extracted hydrochar from sewage sludge as solid fuels showed notable improvements in physicochemical and energy-related properties. Acid treatment improved carbon content, heating values, and fuel ratio, while significantly reducing ash content compared to untreated hydrochars (Paper IV). These properties decreased in the co-hydrochars with higher sewage sludge proportions due to differing carbon, volatile matter, and ash content between microalgae and sewage sludge. Thermodynamic equilibrium calculations predicted liquid slag and solid phase formation at combustion temperatures up to 1200°C. Experimental comparison with combustion ashes, analyzed through DRIFTS, SEM-EDS, and XRD, validated simulated compounds including Fe2O3, SiO2, feldspar, whitlockite, CaCO3, and CaSO4 in Paper IV. Notably, CaCO3 presence in ashes was confirmed by XRD but not reflected in calculation results. Microalgae hydrochar ashes were primarily composed of calcium phosphates and Fe2O3, visually confirmed by EDS mapping due to XRD limitations.

The result of this thesis suggests that the HTC process offers a pre-treatment means of improving hydrophobicity and significantly reducing feedstock volumes. Additionally, the resource-recovery approach studied in this thesis, which uses sewage sludge and microalgae-derived hydrochars generated in wastewater treatment plants, is a step towards being an efficient management strategy for by-products generated by these plants.

Abstract [sv]

Avloppsslam och biomassa från mikroalger är biprodukter från rening av avloppsvatten, vilket kräver noggrann hantering för att undvika miljö- och hälsorisker. Både avloppsslam och mikroalger har hög fukthalt, vilket gör den termokemiska omvandlingen utmanande och energikrävande. Hydrotermisk karbonisering (HTC) presenterar en lovande lösning för att omvandla dessa våta råvaror till värdefulla resurser. Avhandlingen syftade till att studera HTC av avloppsslam och biomassa från mikroalger, individuellt och kombinerat (d.v.s. co-HTC). Den fokuserade på processparametrar, blandningsförhållanden, produktegenskaper, primär och sekundär kolbildning och resursåtervinning, med särskild tonvikt på fosfor och energiåtervinning som potentiella tillämpningar av de resulterande kolvätena. Både HTC- och co-HTC-experimenten utfördes vid 180, 215 och 250 °C under 2 timmar (Paper I–IV).

Paper I undersökte co-HTC genom att kombinera mikroalger och avloppsslam i olika förhållanden, från 0 till 100 % av avloppsslam. Resultaten visade att högre andelar av avloppsslam och förkolningstemperaturer ledde till lägre nedbrytnings- och förkolningshastigheter. Tillsatsen av avloppsslam påverkade sekundär kolbildning och sammansättning, vilket minskade karboxylsyra och ketoner samtidigt som kolesteroler med högre molekylvikt ökade. Dessutom innehöll kolväten i avloppsslam större mängder fosfor.

I syralakningsexperiment (Paper II och III) med avloppsslam översteg fosforutvinningseffektiviteten 75%. Fullständig fosforåtervinning (100%) uppnåddes endast med oxalatextraktion vid pH=1. Organiska syror, som användes i en lägre koncentration (0.25 M) jämfört med mineralsyror (2.5 M), fungerade som både syror och kelatbildare, vilket underlättade fosforåtervinningen. Oavsett syratyp överförde utlakning från kolväte inte bara fosfor utan metaller och tungmetaller till det P-rika lakvattnet, vilket krävde efterbehandlingsrening.

Förbränningsstudier av mikroalger och avloppsslam co-hydrochar, och fosfor extraherad hydrochar från avloppsslam som fasta bränslen visade påtagliga förbättringar i fysikalisk-kemiska och energirelaterade egenskaper. Syrabehandling förbättrade kolhalten, värmevärdena och bränsleförhållandet, samtidigt som askhalten minskade avsevärt jämfört med obehandlade kolväten (Paper IV). Dessa egenskaper minskade i samkolväten med högre andel avloppsslam på grund av olika innehåll av kol, flyktiga ämnen och aska mellan mikroalger och avloppsslam. Termodynamiska jämviktsberäkningar förutspådde bildning av flytande slagg och fast fas vid förbränningstemperaturer upp till 1200°C. Experimentell jämförelse med förbränningsaska, analyserad genom DRIFTS, SEM-EDS och XRD, validerade simulerade föreningar inklusive Fe2O3, SiO2, fältspat, whitlockit, CaCO3 och CaSO4 i Paper IV. Noterbart bekräftades CaCO3-närvaro i aska genom XRD men återspeglades inte i beräkningsresultat. Mikroalger hydrochar aska bestod främst av kalciumfosfater och Fe2O3, visuellt bekräftat av EDS-kartläggning på grund av XRD-begränsningar.

Resultatet av denna avhandling tyder på att HTC-processen erbjuder ett förbehandlingsmedel för att förbättra hydrofobiciteten och avsevärt minska råvaruvolymerna. Dessutom är den resursåtervinningsmetod som studeras i denna avhandling, som använder avloppsslam och mikroalger som genereras i avloppsreningsverk, ett steg mot att bli en effektiv hanteringsstrategi för biprodukter som genereras av dessa anläggningar.

Place, publisher, year, edition, pages
Umeå: Umeå University, 2024. , p. 84
Keywords [en]
HTC, hydrochar, phosphorus release, solid fuel, acid treatment, thermodynamic calculations, primary char, secondary char, hydrochar formation
National Category
Chemical Process Engineering Inorganic Chemistry Energy Systems
Identifiers
URN: urn:nbn:se:umu:diva-223968ISBN: 9789180704052 (print)ISBN: 9789180704069 (electronic)OAI: oai:DiVA.org:umu-223968DiVA, id: diva2:1855813
Public defence
2024-05-31, Stora hörsalen (KB.E3.03), KBC-huset, Umeå, 09:00 (English)
Opponent
Supervisors
Available from: 2024-05-08 Created: 2024-05-03 Last updated: 2024-05-06Bibliographically approved
List of papers
1. Co-hydrothermal carbonization of microalgae and digested sewage sludge: Assessing the impact of mixing ratios on the composition of primary and secondary char
Open this publication in new window or tab >>Co-hydrothermal carbonization of microalgae and digested sewage sludge: Assessing the impact of mixing ratios on the composition of primary and secondary char
2024 (English)In: Waste Management, ISSN 0956-053X, E-ISSN 1879-2456, Vol. 174, p. 429-438Article in journal (Refereed) Published
Abstract [en]

The role of microalgae cultivation in wastewater treatment and reclamation has been studied extensively, as has the potential utility of the resulting algal biomass. Most methods for processing such biomass generate solid residues that must be properly managed to comply with current sustainable resource utilization requirements. Hydrothermal carbonization (HTC) can be used to process both individual wet feedstocks and mixed feedstocks (i.e., co-HTC). Here, we investigate co-HTC using microalgae and digested sewage sludge as feedstocks. The objectives were to (i) study the material's partitioning into solid and liquid products, and (ii) characterize the products’ physicochemical properties. Co-HTC experiments were conducted at 180–250°C using mixed microalgae/sewage sludge feedstocks with the proportion of sewage sludge ranging from 0 to 100 %. Analyses of the hydrochar composition and the formation and composition of secondary char revealed that the content of carbonized material in the product decreased as the proportion of sewage sludge in the feedstock increased under fixed carbonization conditions. The properties of the hydrochars and the partitioning of material between the liquid phase and the hydrochar correlated linearly with the proportion of microalgae in mixed feedstocks, indicating that adding sewage sludge to microalgae had weak or non-existent synergistic effects on co-HTC outcomes. However, the proportion of sewage sludge in the feedstock did affect the secondary char. For example, adding sewage sludge reduced the abundance of carboxylic acids and ketones as well as the concentrations of higher molecular weight cholesterols. Such changes may alter the viable applications of the hydrochar.

Place, publisher, year, edition, pages
Elsevier, 2024
Keywords
Chemical composition, Co-HTC, Hydrochar, Mixed feedstocks, Thermogravimetric analysis, Wastewater treatment by-products
National Category
Other Chemistry Topics
Identifiers
urn:nbn:se:umu:diva-218868 (URN)10.1016/j.wasman.2023.11.039 (DOI)38104415 (PubMedID)2-s2.0-85180417079 (Scopus ID)
Funder
Bio4Energy
Available from: 2024-01-05 Created: 2024-01-05 Last updated: 2024-05-03Bibliographically approved
2. Acid-Induced Phosphorus Release from Hydrothermally Carbonized Sewage Sludge
Open this publication in new window or tab >>Acid-Induced Phosphorus Release from Hydrothermally Carbonized Sewage Sludge
Show others...
2021 (English)In: Waste and Biomass Valorization, ISSN 1877-2641, E-ISSN 1877-265X, Vol. 12, no 12, p. 6555-6568Article in journal (Refereed) Published
Abstract [en]

Phosphorus (P) recovery from P-rich residues is crucial to sustain food and industrial demands globally, as phosphate rock reserves are being depleted. The aim of this study is to investigate the speciation and recovery of P from hydrochars (HC) of a metal-bearing sewage sludge (SS) produced by hydrothermal carbonization (HTC). We here focus on extractions by acid leaching as P cannot be directly recovered by HTC due to insoluble metal-P compounds. Acid leaching of SS and HCs was investigated using H2SO4 and HCl over a range of leaching times, and explained in terms of how composition affects P and metal release efficiency. HTC at 180, 215 and 250 °C showed that P remained immobilized (> 75% of total P) in the HCs. More than 95% was present as inorganic P, and was the direct consequence of the double addition of iron salts in the wastewater treatment plant. Leaching experiments in 2.5 M acid solutions showed that a near complete release of P could be achieved in HCs, while it was only incomplete in SS (up to 85%). Lower acid concentrations were ineffective for total P recovery. Treatment temperature exceeding 180 °C however decreased P release rates, such that total removal took at least 2 h of reaction time instead of a few minutes. On the other hand, acid leaching transferred more than 70% of iron, manganese, copper and zinc into the leachate, necessitating a post-treatment purification process. This work therefore reveals that HC produced at low HTC temperatures could offer promising avenues for time- and energy-efficient P recovery from SS. Graphic Abstract: [Figure not available: see fulltext.]

Place, publisher, year, edition, pages
Springer Netherlands, 2021
Keywords
Acid leaching, Digested sewage sludge, Hydrothermal carbonization, Metal release, Phosphorus release
National Category
Other Chemistry Topics
Identifiers
urn:nbn:se:umu:diva-183709 (URN)10.1007/s12649-021-01463-5 (DOI)000651092200001 ()2-s2.0-85106005763 (Scopus ID)
Available from: 2021-05-31 Created: 2021-05-31 Last updated: 2024-05-03Bibliographically approved
3. Phosphorus release from hydrothermally carbonized digested sewage sludge using organic acids
Open this publication in new window or tab >>Phosphorus release from hydrothermally carbonized digested sewage sludge using organic acids
Show others...
2022 (English)In: Waste Management, ISSN 0956-053X, E-ISSN 1879-2456, Vol. 151, p. 60-69Article in journal (Refereed) Published
Abstract [en]

Hydrothermal carbonization (HTC) is a treatment technique with great potential for sanitizing digested sewage sludge (SS) and converting it into valuable products. In particular, phosphorus (P) recovery from hydrothermally carbonized SS has attracted special attention in recent years. This work aims to examine the leaching efficiency of P and the consequent release of metals and heavy metals from SS hydrochars (at 180, 215 and 250 °C) using organic acids (oxalate and citrate) over a range of pH values (0–4) and extraction times (5 min-24 h). Both organic acids triggered P extraction efficiencies exceeding 75 % at the lowest pH, but only oxalate reached a nearly complete P release from hydrochars at pH > 0 and for all carbonization temperatures. Low HTC temperature (180 °C) and short extraction time (5 min) were the optimal conditions treatment for P recovery when reacted in oxalate solutions of maximal pH buffering capacity (pH = 1.4). However, oxalate leaching also transferred metals/heavy metals into the P-leachate, with the exception of Ca being retained in the solid residue from HTC as Ca-oxalate precipitate. Different characterization methods confirmed the presence of this precipitate, and provided information about the surface and morphological changes of the SS hydrochars following acid treatment. The results suggest that HTC not only a promising technique to sanitize and reduce the volume of SS, but also an efficient means for P recovery using oxalic acid, thus contributing to the circular economy of P.

Place, publisher, year, edition, pages
Elsevier, 2022
Keywords
Characterization, Chemical treatment, Digested sewage sludge, Hydrochar, Hydrothermal carbonization, Metal release, Phosphorus
National Category
Other Chemistry Topics
Identifiers
urn:nbn:se:umu:diva-198565 (URN)10.1016/j.wasman.2022.07.023 (DOI)000965058600006 ()35926282 (PubMedID)2-s2.0-85135307735 (Scopus ID)
Funder
Swedish Research Council FormasUmeå University
Available from: 2022-08-24 Created: 2022-08-24 Last updated: 2024-05-03Bibliographically approved
4. Combustion and ash transformation of hydrothermally carbonized digested sewage sludge after phosphorus recovery
Open this publication in new window or tab >>Combustion and ash transformation of hydrothermally carbonized digested sewage sludge after phosphorus recovery
(English)Manuscript (preprint) (Other academic)
National Category
Energy Engineering
Identifiers
urn:nbn:se:umu:diva-223967 (URN)
Available from: 2024-05-03 Created: 2024-05-03 Last updated: 2024-05-03

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89101112131411 of 18
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