The production of sewage sludge from wastewater plants is increasing, also due to higher performances of wastewater treatment technologies in removing organic matter. At the same time, the direct use of sludge for agricultural purposes is facing stricter limits and therefore the need arises to develop different sustainable and reliable disposal methods. A recent alternative is represented by hydrothermal carbonization (HTC). In the perspective of circular economy, HTC technology allows the conversion of sludge into useful products such as hydrochar and phosphorus (P), one of the critical raw materials. Process water, rich in volatile fatty acids, can also be recovered as an additional substrate for anaerobic digestion. In the present work leaching tests were performed to extract phosphorus both from the slurry after HTC treatment and from the hydrochar after mechanical dewatering. The leaching agents used on both matrices were H2SO4, HNO3 and oxalic acid (C2H2O4), while an additional one, citric acid (C6H8O7), was tested on the hydrochar samples only. Phosphorus in the acidic solutions obtained from both matrices was then precipitated with calcium-silicate-hydrate (CSH), while additional precipitating agents (strong bases such as KOH and NaOH) were investigated in the case of P recovery from the hydrochar samples. Results showed higher overall P recovery efficiency from the hydrochar than from the slurry, with values of 82-92% when HNO3 was used. P recovery from the slurry was higher by acid leaching with H2SO4, with values in the range of 55 to 75%. Oxalic acid presented high efficiencies for both matrices (up to 100%), but the high variability among the results suggest the needs for further testing. If compared to the regulatory limits for use as fertilizer, the precipitated solids from the slurry and hydrochar were too rich in Fe and Al and thus further research is required to find methods efficient in achieving a higher purity product.
La produzione di fanghi dagli impianti di depurazione è in aumento anche grazie maggiori prestazioni delle tecnologie di trattamento delle acque reflue nella rimozione della sostanza organica. Allo stesso tempo, l'uso diretto dei fanghi in agricoltura si trova ad affrontare limiti più severi e pertanto sorge la necessità di sviluppare metodi diversi di smaltimento sostenibili e affidabili. Una recente alternativa è rappresentata dalla carbonizzazione idrotermica (HTC). Nella prospettiva dell'economia circolare, la tecnologia HTC consente la conversione dei fanghi in prodotti utili come l'idrochar e il fosforo (P), una delle materie prime critiche. L'acqua di processo, ricca di acidi grassi volatili, può anche essere recuperata come ulteriore substrato per la digestione anaerobica. Nel presente lavoro sono state eseguite prove di lisciviazione per estrarre fosforo sia dalla miscela liquida (slurry) dopo trattamento HTC, sia dall'idrochar ottenuto dopo disidratazione meccanica. I liscivianti utilizzati su entrambe le matrici erano H2SO4, HNO3 e acido ossalico (C2H2O4), mentre sui campioni di idrochar è stato testato un acido aggiuntivo, ossia acido citrico (C6H8O7). Il fosforo nella soluzione acida ottenuta da entrambe le matrici è stato quindi precipitato con calcio-silicato-idrato (CSH), mentre ulteriori agenti precipitanti (basi forti come KOH e NaOH) sono stati studiati nel caso di recupero di P dai campioni di idrochar. I risultati hanno mostrato una maggiore efficienza complessiva di recupero del P dall'hydrochar rispetto all'impasto liquido, con valori dell'82-92% quando è stato utilizzato HNO3. Il recupero di P dall'impasto liquido è stato maggiore mediante lisciviazione acida con H2SO4, con valori compresi tra il 55 e il 75%. L'acido ossalico ha presentato efficienze elevate per entrambe le matrici (fino al 100%), ma l'elevata variabilità tra i risultati suggerisce la necessità di ulteriori prove. Rispetto ai limiti normativi per l'uso come fertilizzante, i solidi precipitati dal liquame erano troppo ricchi di Fe e Al, ed è quindi necessaria un'ulteriore ricerca per definire i metodi utili per ottenere un prodotto di purezza superiore.
Recovery of phosphorus from HTC slurry and hydrochar derived from sewage sludge
SESSOLO, LUCA
2020/2021
Abstract
The production of sewage sludge from wastewater plants is increasing, also due to higher performances of wastewater treatment technologies in removing organic matter. At the same time, the direct use of sludge for agricultural purposes is facing stricter limits and therefore the need arises to develop different sustainable and reliable disposal methods. A recent alternative is represented by hydrothermal carbonization (HTC). In the perspective of circular economy, HTC technology allows the conversion of sludge into useful products such as hydrochar and phosphorus (P), one of the critical raw materials. Process water, rich in volatile fatty acids, can also be recovered as an additional substrate for anaerobic digestion. In the present work leaching tests were performed to extract phosphorus both from the slurry after HTC treatment and from the hydrochar after mechanical dewatering. The leaching agents used on both matrices were H2SO4, HNO3 and oxalic acid (C2H2O4), while an additional one, citric acid (C6H8O7), was tested on the hydrochar samples only. Phosphorus in the acidic solutions obtained from both matrices was then precipitated with calcium-silicate-hydrate (CSH), while additional precipitating agents (strong bases such as KOH and NaOH) were investigated in the case of P recovery from the hydrochar samples. Results showed higher overall P recovery efficiency from the hydrochar than from the slurry, with values of 82-92% when HNO3 was used. P recovery from the slurry was higher by acid leaching with H2SO4, with values in the range of 55 to 75%. Oxalic acid presented high efficiencies for both matrices (up to 100%), but the high variability among the results suggest the needs for further testing. If compared to the regulatory limits for use as fertilizer, the precipitated solids from the slurry and hydrochar were too rich in Fe and Al and thus further research is required to find methods efficient in achieving a higher purity product.| File | Dimensione | Formato | |
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2022_04_Sessolo.pdf
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https://hdl.handle.net/10589/187133