Sludge disposal can represent up to 40% of the overall cost of wastewater treatment. In addition, sludge contributes significantly to the environmental impact of the wastewater treatment plant, not only in terms of greenhouse gas emissions, but also for the effects that may arise from sludge disposal. The reduction of sludge therefore contributes to reducing both costs and the associated environmental impact. This work describes two reduction modes, one of which (ozonolysis) acts on the biological process of the wastewater treatment line and the other (electro-assisted dehydration) acts on mechanical dewatering. Sludge ozonolysis is a chemical treatment that acts on the biomass growth mechanisms, on which bacterial selection operates, coupling this action with the solubilization of the particulate and part of the biomass itself present in the biological reactor. The process considered in this report concerned ozonolysis on a recirculation flow of biological sludge at the San Giuliano Milanese Ovest plant (CAP Group). The result of the experiment, conducted with an ozone dosage equal to 6 gO3 per kgTSS wasted in the excess sludge, made it possible to evaluate a 30% reduction in the production of sludge to be disposed of. The reduced sludge production was attributed, by means of respirometric tests and a model simulation, to the variation of the cell yield coefficient (decreased from 0.68 to 0.53), to a lower cell growth constant (from 5.96 to 4.53 d-1) and a higher rate of cellular decay (from 0.88 to 1.38 d-1), in addition to the solubilization (and consequent removal by oxidation) of part of the organic substrate (COD) in particulate form. The biological process simulated with the ASM1 model on the WEST software platform made it possible to identify the biological parameters on which ozonolysis had the greatest effect. In particular, the decrease in cell yield has the greatest impact, followed by the solubilization of the particulate COD. In addition to the ozonolysis process, the electro-dewatering process on a laboratory scale was also studied at Politecnico di Milano. It was also considered a prototype installed at the CAP plant in Locate Triulzi, which, however, could not function due to problems with the supply of electromechanical components. Laboratory tests have shown that EDW is able to achieve dry solid levels about 3% higher than exclusively mechanical dewatering, but the tests indicate that the results could be improved by making some modifications. The electro-dewatering process reduced the sludge resistance to filtration (SRF), which decreased in each test carried out both without conditioning and with a dosage of 4 g/kgTSS of conditioner; with higher dosages, the effect decreases due to the supply of liquid associated with the dosage of the conditioning solution and the larger size of the flakes. The prototype, although functioning without the application of the electric field, achieved values of 15 - 18% of dryness by weight, regardless of the initial water content. The results of increased dryness level due to the electric field application allow similar increases forecasts on the prototype. Economic convenience was assessed for both technologies. Ozonolysis is an economically convenient treatment with savings that increase as the population served increases, therefore for greater flow rates treated: a 4% saving was recorded in the report for the pilot plant in San Giuliano Milanese Ovest. Instead, the application of EDW is advantageous if the cost of disposal remains greater than 40 €/tFU or, assuming a fixed cost for disposal equal to 90 €/tFU, if the cost of electricity is less than or equal to 0.30 €/kWh. Anyway, the prototype needs a minimum dryness level equal to 30% to be convenient respect the actual mechanism of dewatering in the Locate Triulzi plant.
Lo smaltimento dei fanghi può rappresentare fino al 40% del costo complessivo di gestione del processo. Inoltre, contribuiscono in misura determinante all’impatto ambientale del servizio di depurazione, non solo in termini di emissioni di gas serra, ma anche per gli effetti che possono derivare dal suo smaltimento. La riduzione dei fanghi contribuisce quindi a ridurre sia i costi, sia l’impatto ambientale ad essi connesso. Il presente lavoro descrive due modalità di riduzione, di cui una (ozonolisi) che agisce sul processo biologico della linea di trattamento delle acque di rifiuto e l’altro (disidratazione elettro-assistita) sulla disidratazione meccanica. L’ozonolisi dei fanghi è un trattamento chimico che agisce sui meccanismi di crescita della biomassa, su cui opera una selezione batterica, accoppiando a questa azione anche la solubilizzazione del particolato e di parte della biomassa stessa presenti nel reattore biologico. Tale processo, oggetto di questo elaborato, ha riguardato l’ozonolisi su un flusso di ricircolo del fango biologico presso l’impianto di San Giuliano Milanese Ovest (Gruppo CAP). Il risultato della sperimentazione, condotta con dosaggio di ozono pari a 6 gO3 per kgSST spurgato nei fanghi di supero, ha consentito di valutare una riduzione del 30% della produzione dei fanghi da smaltire. La ridotta produzione di fanghi è stata attribuita, mediante prove respirometriche e una simulazione di modello, alla variazione del coefficiente di resa cellulare (diminuito da 0.68 a 0.53), a una minore costante di crescita cellulare (da 5.96 a 4.53 d-1) e un maggiore tasso di decadimento cellulare (da 0.88 a 1.38 d-1), oltre alla solubilizzazione (e conseguente rimozione per ossidazione) di parte del substrato organico (COD) in forma particolata. Il processo biologico simulato con il modello ASM1 su piattaforma software WEST ha consentito di identificare i parametri biologici sui quali ha maggiormente avuto effetto l’ozonolisi. In particolare, la diminuzione della resa cellulare ha l’impatto maggiore, seguita dalla solubilizzazione del COD particolato. Oltre al processo di ozonolisi, è stato studiato anche il processo di elettro-disidratazione a scala di laboratorio presso il Politecnico di Milano. Si è anche considerato un prototipo installato presso l’impianto CAP di Locate Triulzi, il quale tuttavia non ha potuto funzionare per problemi di fornitura di componenti elettromeccaniche. Le prove in laboratorio hanno dimostrato che l’EDW è in grado di raggiungere tenori di secco mediamente superiori del 3% rispetto ad una disidratazione esclusivamente meccanica, ma le prove indicano che i risultati potrebbero essere migliori apportando alcune migliorie. Il processo di elettro-disidratazione ha infatti ridotto la resistenza alla filtrazione del fango (SRF), che è diminuito in ogni prova effettuata sia senza condizionante, sia con dosaggio di 4 g/kgTSS; con dosaggi maggiori, l’effetto diminuisce a causa dell’apporto di liquido associato al dosaggio di soluzione condizionante e della maggiore dimensione dei fiocchi. Il prototipo, sia pure funzionante senza applicazione del campo elettrico, ha conseguito valori del 15 – 18 % di secco, in peso, indipendentemente dalle condizioni di umidità iniziali. Le ipotesi di incremento del tenore di secco conseguente all’applicazione del campo elettrico valutate in laboratorio consentono previsioni di incremento analoghe sul prototipo. Per entrambe le tecnologie è stata valutata la convenienza economica. L’ozonolisi è un trattamento economicamente conveniente con risparmi crescenti all’aumentare della popolazione servita, quindi per maggiori portate trattate: nell’elaborato si è registrato un risparmio del 4% per l’impianto pilota a San Giuliano Milanese Ovest. Invece, l’applicazione dell’EDW risulta vantaggiosa se il costo di smaltimento rimane maggiore di 40€/tFU oppure, ipotizzando un costo fisso per lo smaltimento pari a 90 €/tFU, se il costo dell’energia elettrica è minore o uguale a 0.30 €/kWh. Il calcolo mostra che il prototipo deve raggiungere un tenore di secco minimo del 30% per essere economicamente conveniente rispetto all’attuale sistema di disidratazione presente a Locate Triulzi.
Minimizzazione dei fanghi di depurazione mediante ozonolisi in linea acque e disidratazione elettro-assistita
Palmitessa, Massimiliano
2020/2021
Abstract
Sludge disposal can represent up to 40% of the overall cost of wastewater treatment. In addition, sludge contributes significantly to the environmental impact of the wastewater treatment plant, not only in terms of greenhouse gas emissions, but also for the effects that may arise from sludge disposal. The reduction of sludge therefore contributes to reducing both costs and the associated environmental impact. This work describes two reduction modes, one of which (ozonolysis) acts on the biological process of the wastewater treatment line and the other (electro-assisted dehydration) acts on mechanical dewatering. Sludge ozonolysis is a chemical treatment that acts on the biomass growth mechanisms, on which bacterial selection operates, coupling this action with the solubilization of the particulate and part of the biomass itself present in the biological reactor. The process considered in this report concerned ozonolysis on a recirculation flow of biological sludge at the San Giuliano Milanese Ovest plant (CAP Group). The result of the experiment, conducted with an ozone dosage equal to 6 gO3 per kgTSS wasted in the excess sludge, made it possible to evaluate a 30% reduction in the production of sludge to be disposed of. The reduced sludge production was attributed, by means of respirometric tests and a model simulation, to the variation of the cell yield coefficient (decreased from 0.68 to 0.53), to a lower cell growth constant (from 5.96 to 4.53 d-1) and a higher rate of cellular decay (from 0.88 to 1.38 d-1), in addition to the solubilization (and consequent removal by oxidation) of part of the organic substrate (COD) in particulate form. The biological process simulated with the ASM1 model on the WEST software platform made it possible to identify the biological parameters on which ozonolysis had the greatest effect. In particular, the decrease in cell yield has the greatest impact, followed by the solubilization of the particulate COD. In addition to the ozonolysis process, the electro-dewatering process on a laboratory scale was also studied at Politecnico di Milano. It was also considered a prototype installed at the CAP plant in Locate Triulzi, which, however, could not function due to problems with the supply of electromechanical components. Laboratory tests have shown that EDW is able to achieve dry solid levels about 3% higher than exclusively mechanical dewatering, but the tests indicate that the results could be improved by making some modifications. The electro-dewatering process reduced the sludge resistance to filtration (SRF), which decreased in each test carried out both without conditioning and with a dosage of 4 g/kgTSS of conditioner; with higher dosages, the effect decreases due to the supply of liquid associated with the dosage of the conditioning solution and the larger size of the flakes. The prototype, although functioning without the application of the electric field, achieved values of 15 - 18% of dryness by weight, regardless of the initial water content. The results of increased dryness level due to the electric field application allow similar increases forecasts on the prototype. Economic convenience was assessed for both technologies. Ozonolysis is an economically convenient treatment with savings that increase as the population served increases, therefore for greater flow rates treated: a 4% saving was recorded in the report for the pilot plant in San Giuliano Milanese Ovest. Instead, the application of EDW is advantageous if the cost of disposal remains greater than 40 €/tFU or, assuming a fixed cost for disposal equal to 90 €/tFU, if the cost of electricity is less than or equal to 0.30 €/kWh. Anyway, the prototype needs a minimum dryness level equal to 30% to be convenient respect the actual mechanism of dewatering in the Locate Triulzi plant.| File | Dimensione | Formato | |
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Massimiliano Palmitessa 920819.pdf
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https://hdl.handle.net/10589/179449