In this study, alternative methods to improve sewage sludge management were investigated, with a view to the circular economic model. Sewage sludge is the waste (EWC code: 190805) generated by biological wastewater treatment plants; at least 50 million tonnes are produced annually by the European Union and approximately 40 million tonnes individually by the United States and China (Xi Zhang; 2020) and its production is expected to increase with urbanisation and industrial development. The management of this type of waste is very complex due to the high moisture content, the presence of pathogens and pollutants. The high water content of sludge, which is largely responsible for the high volumes and difficult handling, is due to the presence of extracellular polymeric substances (EPS), which are secreted by bacteria to organise themselves into biofilms. In order to reduce sludge moisture, it is therefore necessary to alter these structures. The experimentation was divided into two phases: 1 - sewage sludge from the Cassano d'Adda wastewater treatment plant (with a moisture content of 77 wt.%) was subjected to thermochemical treatment (treatment temperature of 220°C, pressure of 30 bar and treatment time of 30 minutes) in the plant designed and developed within the Life Freedom project. 2- the cake was then used as a secondary raw material in the production of experimental bricks and geopolymers. Four bricks were prepared: a control sample made with only water and clay, and three bricks in which part of the clay was replaced with cake in different percentages (10wt.%; 30wt.% and 50wt.%); the mixtures were fired in the industrial kiln of the Robecco Pavese kiln, the firing cycle lasted 44 hours and the maximum temperature (940°C) was maintained for 3 hours. For the production of the experimental geopolymers, metakaolin was used as a precursor, waterglass as an alkaline activator, an alkaline aqueous solution to regulate the solid-liquid ratio (solid/liquid=0.4), Silica Fume to regulate the silica-aluminium ratio (Si/Al=2), a control geopolymer and three geopolymers were also prepared in which cake was added as an additive in quantities of 5wt. %, 10wt.% and 15wt.% of the precursor; the geopolymers were placed in the muffle furnace at 80°C for 3 hours. In the first phase of the study, it was found that the thermochemical treatment of sewage sludge can be an optimal method to decrease the moisture content of the sludge, thus favouring a reduction in its volume and thus facilitating its transport and subsequent handling. The characterisation of the sewage sludge and cake (XRPD analysis; SEM-EDS analysis; thermogravimetric analysis) revealed the heterogeneity of the materials and the need to combine several types of analysis in order to correctly characterise them. In the second part of the study, it was observed that cake has suitable characteristics to be reused as a secondary raw material in the production of both bricks and geopolymers; in particular, interesting aspects observed were: the increase in porosity induced by the addition of cake, both in bricks and geopolymers; in bricks, it was also found that cake increased the degree of sintering; it is therefore appropriate to continue investigating such alternative and sustainable management routes.
Nel presente studio sono stati indagati metodi alternativi volti a migliorare la gestione dei fanghi di depurazione, nell’ottica del modello economico circolare. I fanghi di depurazione sono il rifiuto (codice CER: 190805) generato dagli impianti biologici di depurazione delle acque reflue; almeno 50 milioni di tonnellate sono prodotti annualmente dall'Unione Europea e circa 40 milioni di tonnellate individualmente da Stati Uniti e Cina (Xi Zhang; 2020) e si prevede che la loro produzione aumenterà con l'urbanizzazione e lo sviluppo industriale. La gestione di tale tipo di rifiuto è molto complessa a causa dell’elevato contenuto di umidità, la presenza di patogeni e sostanze inquinanti. L’elevato contenuto d’acqua dei fanghi, responsabile in larga parte dei volumi elevati e della difficoltà di gestione, è dovuto alla presenza di sostanze polimeriche extracellulari (EPS), che vengono secrete dai batteri per organizzarsi in biofilm. Al fine di ridurre l’umidità dei fanghi è dunque necessario alterare tali strutture. La sperimentazione è stata articolata in due fasi: 1- i fanghi di depurazione provenienti dall’impianto di depurazione delle acque reflue di Cassano d’Adda (caratterizzati da un contenuto di umidità del 77wt.%) sono stati sottoposti ad un trattamento termochimico (temperatura di trattamento di 220°C, pressione di 30 bar e tempo di trattamento di trenta minuti), nell’impianto progettato e sviluppato all’interno del progetto ‘’Life Freedom’’. Tale trattamento ha così consentito poi di filtrare il prodotto in uscita con filtro-pressa meccanica e separare la fase solida (cake) dalla fase liquida (liquor). 2- il cake è stato poi utilizzato come materia prima seconda nella produzione di laterizi e geopolimeri. Sono stati preparati quattro campioni sperimentali di laterizi: un campione di controllo fatto con solo acqua ed argilla e tre campioni in cui parte dell’argilla è stata sostituita con il cake in differenti percentuali (10wt.%; 30wt.% e 50wt.%); gli impasti sono stati essiccati a 100°C e poi cotti nel forno industriale della Fornace Robecco Pavese, il ciclo di cottura è stato di 44 ore e la massima temperatura (940°C) è stata mantenuta per 3 ore. Per la realizzazione dei geopolimeri è stato utilizzato il metacaolino come precursore , il waterglass come attivatore alcalino, soluzione acquosa alcalina per regolare il rapporto solido-liquido (solido/acqua=0.4), Silica Fume per regolare il rapporto silice-alluminio (Si/Al=2), è stato preparato anche il tal caso un geopolimero di controllo e tre geopolimeri in cui è stato aggiunto il cake come additivo in quantità del 5wt.%, 10wt.% e 15wt.% rispetto al precursore; i geopolimeri sono stati messi in stufa a 80°C per 3 ore. Nella prima fase dello studio è stato verificato come un trattamento termochimico dei fanghi di depurazione possa essere una metodica ottimale per diminuire il contenuto di umidità dei fanghi, favorendo dunque una riduzione del volume degli stessi e quindi agevolando il trasporto e la successiva gestione. La caratterizzazione del fango di depurazione e del cake (analisi XRPD; analisi al SEM-EDS; analisi termogravimetrica) ha messo in luce l’eterogeneità dei materiali studiati e la necessità di combinare molteplici tipi di analisi per poterli correttamente caratterizzare. Nella seconda parte dello studio è stato osservato che il cake presenta caratteristiche idonee per essere riutilizzato come materia prima seconda sia nella produzione di laterizi che di geopolimeri; in particolare aspetti interessanti osservati sono: l’incremento della porosità indotta dall’aggiunta del cake, sia nei laterizi che nei geopolimeri; nei laterizi inoltre è stato riscontrato che il cake ha aumentato il grado di sinterizzazione; è dunque opportuno continuare ad indagare tali vie di gestione alternative e sostenibili.
Disidratazione idrotermale dei fanghi di depurazione e trasformazione in materia prima seconda
DI FILIPPO, CHIARA
2021/2022
Abstract
In this study, alternative methods to improve sewage sludge management were investigated, with a view to the circular economic model. Sewage sludge is the waste (EWC code: 190805) generated by biological wastewater treatment plants; at least 50 million tonnes are produced annually by the European Union and approximately 40 million tonnes individually by the United States and China (Xi Zhang; 2020) and its production is expected to increase with urbanisation and industrial development. The management of this type of waste is very complex due to the high moisture content, the presence of pathogens and pollutants. The high water content of sludge, which is largely responsible for the high volumes and difficult handling, is due to the presence of extracellular polymeric substances (EPS), which are secreted by bacteria to organise themselves into biofilms. In order to reduce sludge moisture, it is therefore necessary to alter these structures. The experimentation was divided into two phases: 1 - sewage sludge from the Cassano d'Adda wastewater treatment plant (with a moisture content of 77 wt.%) was subjected to thermochemical treatment (treatment temperature of 220°C, pressure of 30 bar and treatment time of 30 minutes) in the plant designed and developed within the Life Freedom project. 2- the cake was then used as a secondary raw material in the production of experimental bricks and geopolymers. Four bricks were prepared: a control sample made with only water and clay, and three bricks in which part of the clay was replaced with cake in different percentages (10wt.%; 30wt.% and 50wt.%); the mixtures were fired in the industrial kiln of the Robecco Pavese kiln, the firing cycle lasted 44 hours and the maximum temperature (940°C) was maintained for 3 hours. For the production of the experimental geopolymers, metakaolin was used as a precursor, waterglass as an alkaline activator, an alkaline aqueous solution to regulate the solid-liquid ratio (solid/liquid=0.4), Silica Fume to regulate the silica-aluminium ratio (Si/Al=2), a control geopolymer and three geopolymers were also prepared in which cake was added as an additive in quantities of 5wt. %, 10wt.% and 15wt.% of the precursor; the geopolymers were placed in the muffle furnace at 80°C for 3 hours. In the first phase of the study, it was found that the thermochemical treatment of sewage sludge can be an optimal method to decrease the moisture content of the sludge, thus favouring a reduction in its volume and thus facilitating its transport and subsequent handling. The characterisation of the sewage sludge and cake (XRPD analysis; SEM-EDS analysis; thermogravimetric analysis) revealed the heterogeneity of the materials and the need to combine several types of analysis in order to correctly characterise them. In the second part of the study, it was observed that cake has suitable characteristics to be reused as a secondary raw material in the production of both bricks and geopolymers; in particular, interesting aspects observed were: the increase in porosity induced by the addition of cake, both in bricks and geopolymers; in bricks, it was also found that cake increased the degree of sintering; it is therefore appropriate to continue investigating such alternative and sustainable management routes.È consentito all'utente scaricare e condividere i documenti disponibili a testo pieno in UNITESI UNIPV nel rispetto della licenza Creative Commons del tipo CC BY NC ND.
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https://hdl.handle.net/20.500.14239/15922