Natural Organic Matter (NOM) is an element to which particular attention must be paid in the water treatment sector, particularly in disinfection processes. The presence of NOM, that gives negative organoleptic characteristics to water, leads to the formation of disinfection by-products (DBPs) in processes that use chlorine-derived disinfectants, of which the most widespread is sodium hypochlorite (NaClO), and chlorine dioxide (ClO2). Trihalomethanes (THMs) and haloacetic acids (HAAs) are formed in the case of chlorine-derived disinfectants and chlorite (ClO2-) and chlorate (ClO3-) in the case of chlorine dioxide, not resulting from the direct reaction of the disinfectant with NOM but rather from the decomposition of ClO2, which can be favored by the presence of NOM. The presence of DBPs represents a source of danger for humans, being THMs and HAAs carcinogenic and ClO2- and ClO3-, although not carcinogenic, impacting human health. The use of ClO2 is of interest, given the lower amount of THMs formed. The removal of NOM by adsorption on activated carbon represents a valid strategy for reducing the formation of DBPs. In this thesis work, the formation of DBPs produced by ClO2 in different aqueous matrices was studied as a function of the concentration and type of NOM. 3 activated carbons, with a different porosity, were also tested to evaluate the efficiency of NOM removal by adsorption: Rapid Small-Scale Column Test (RSSCT) tests were carried out, varying the Empty Bed Contact Time (EBCT) and the diffusivity model (proportional and constant). NOM was evaluated by absorbance and fluorescence spectroscopic methods. From the disinfection tests, results related to the quantity of DBPs formed using ClO2, and further information related to the residual disinfectant in water after different contact times, were obtained. A greater generation of THMs, in particular chloroform (CHCl3), and chlorite was highlighted for waters with a higher NOM content and longer contact times. From the RSSCT tests, mesoporous activated carbon proved to be the most efficient in NOM removal with longer operating times of the activated carbon bed compared to microporous and mesoporous activated carbons. Furthermore, the constant diffusivity model, compared to the proportional diffusivity model, leads to breakthrough curves that are widespread and more gradual over time leading, for all the activated carbons tested, to breakthrough reached in longer times.
La sostanza organica di origine naturale (NOM, Natural Organic Matter) costituisce un elemento a cui prestare particolare attenzione nel processo di potabilizzazione, in particolare in disinfezione. La presenza di NOM, oltre a conferire caratteristiche organolettiche sgradevoli, porta alla formazione di sottoprodotti di disinfezione (DBPs) nei processi che utilizzano cloro-derivati, di cui il più diffuso è l’ipoclorito di sodio (NaClO), e biossido di cloro (ClO2). In particolare, si formano trialometani (THMs) e acidi aloacetici (HAAs) nel caso di cloro-derivati e clorito (ClO2-) e clorato (ClO3-) nel caso di biossido di cloro, non derivanti dalla reazione diretta del disinfettante con la NOM bensì dalla decomposizione del ClO2, che può essere favorita dalla presenza di NOM. La presenza di DBPs rappresenta una fonte di pericolo per l’uomo, essendo THMs e HAAs cancerogeni e ClO2- e ClO3-, seppur non cancerogeni, impattanti sulla salute umana. L’utilizzo di ClO2 suscita interesse data la ridotta formazione di THMs. La rimozione della NOM per adsorbimento su carbone attivo rappresenta una valida strategia di riduzione della formazione dei DBPs. Nel presente lavoro di tesi si è studiata la formazione di DBPs prodotti da ClO2 in differenti matrici acquose in funzione della concentrazione e tipologia di NOM. Sono anche stati testati 3 carboni attivi, a diversa porosità, per valutare l’efficienza di rimozione della NOM per adsorbimento: sono state svolte prove in colonna di tipo Rapid Small-Scale Column Test (RSSCT), variando il tempo di contatto (EBCT, Empty Bed Contact Time) e il modello di diffusività (proporzionale e costante). La NOM è stata valutate tramite metodi spettroscopici di assorbanza e fluorescenza. Dalle prove di disinfezione sono stati ottenuti risultati legati al quantitativo di DBPs formati dall’utilizzo di ClO2 e ulteriori informazioni legate al disinfettante residuo in acqua dopo differenti tempi di contatto. Si è evidenziata una maggiore generazione di THMs, in particolare cloroformio (CHCl3), e clorito per le acque a più alto contenuto di NOM e maggiori tempi di contatto. Dalle prove RSSCT il carbone attivo mesoporoso si è dimostrato il più efficiente nella rimozione della NOM con tempi di operatività del letto di carbone attivo maggiori rispetto ai carboni attivi microporoso e mesoporoso. Inoltre, il modello di diffusività costante, rispetto al modello di diffusività proporzionale, conduce a curve di perforazione diffuse e più graduali nel tempo che portando, per tutti i carboni testati, al raggiungimento del breakthrough in tempi maggiori.
Rimozione per adsorbimento della sostanza organica come precursore di sottoprodotti di disinfezione
Zungri, Simone Giuseppe Francesco
2023/2024
Abstract
Natural Organic Matter (NOM) is an element to which particular attention must be paid in the water treatment sector, particularly in disinfection processes. The presence of NOM, that gives negative organoleptic characteristics to water, leads to the formation of disinfection by-products (DBPs) in processes that use chlorine-derived disinfectants, of which the most widespread is sodium hypochlorite (NaClO), and chlorine dioxide (ClO2). Trihalomethanes (THMs) and haloacetic acids (HAAs) are formed in the case of chlorine-derived disinfectants and chlorite (ClO2-) and chlorate (ClO3-) in the case of chlorine dioxide, not resulting from the direct reaction of the disinfectant with NOM but rather from the decomposition of ClO2, which can be favored by the presence of NOM. The presence of DBPs represents a source of danger for humans, being THMs and HAAs carcinogenic and ClO2- and ClO3-, although not carcinogenic, impacting human health. The use of ClO2 is of interest, given the lower amount of THMs formed. The removal of NOM by adsorption on activated carbon represents a valid strategy for reducing the formation of DBPs. In this thesis work, the formation of DBPs produced by ClO2 in different aqueous matrices was studied as a function of the concentration and type of NOM. 3 activated carbons, with a different porosity, were also tested to evaluate the efficiency of NOM removal by adsorption: Rapid Small-Scale Column Test (RSSCT) tests were carried out, varying the Empty Bed Contact Time (EBCT) and the diffusivity model (proportional and constant). NOM was evaluated by absorbance and fluorescence spectroscopic methods. From the disinfection tests, results related to the quantity of DBPs formed using ClO2, and further information related to the residual disinfectant in water after different contact times, were obtained. A greater generation of THMs, in particular chloroform (CHCl3), and chlorite was highlighted for waters with a higher NOM content and longer contact times. From the RSSCT tests, mesoporous activated carbon proved to be the most efficient in NOM removal with longer operating times of the activated carbon bed compared to microporous and mesoporous activated carbons. Furthermore, the constant diffusivity model, compared to the proportional diffusivity model, leads to breakthrough curves that are widespread and more gradual over time leading, for all the activated carbons tested, to breakthrough reached in longer times.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/218693