1,721,020 research outputs found
Validation of a new model for the sizing of denitrification reactors, by testing full scale plants
No full textA new deterministic model for the calculation of the Specific Denitrification Rate (SDNR), to be used for the design of pre-denitrification reactors, was tested on eight full scale activated sludge plants of different capacity. The model represents the SDNR at 20°C (SDNR20°C) as a function of the sludge loading in the denitrification reactor (F:MDEN) and the residual dissolved oxygen in the denitrification reactor (DODEN). The results proved the ability of the new model in calculating the SDNR20°C. The model shows a little lower degree of adaptability for small-size plants.. SDNR20°C proved a very strong sensitivity to DODEN, mainly in correspondence of low dissolved oxygen (DO) concentrations (less than 2 mg L-1). The sensitivity decreases at greater DO values, but with a progressively less marked gradient, up to become weak only at DO concentrations greater than 0.4-0.5 mg L-1, which are however rarely found in full-scale plants. DO concentrations measured in real-scale facilities are mostly in the range 0.20.4 mg L-1 and are mainly due to the mixed-liquor recycle. These concentrations cause severe adverse effects on the kinetics of nitrogen removal, and consequently on the denitrification performance. Thus, minimizing DO in the pre-denitrification reactor is relevant. The sensitivity of SDNR20°C to F:MDEN resulted much less important, as it is characterized by a growing linear behaviour with a very low slope
Predictive model of limestone scaling in ammonia stripping towers and its experimental validation on a treatment plant fed by MSW leachate-polluted groundwater
No full textGroundwater pollution by municipal solid waste (MSW) landfill leachate is a global concern. Stripping
towers are one of the most implemented techniques for the removal of ammonia pollution. This study
presents a predictive computational model to estimate calcium carbonate precipitation in ammonia
stripping towers. The model considers the Ca2+ super-saturation condition due to the water pH, temperature
and salinity. The results have been validated through experimental data obtained from a plant fed
with MSW landfill leachate-polluted groundwater. The plant consisted of two parallel lines composed of
a coagulation-flocculation stage at high pH followed by a stripping tower. Six combinations of water pH
and temperature conditions were tested. Maximum precipitation was 1,400 kgCaCO3 after a period of
120 days, observed at inlet pH and temperatures of 10.5 and 38 C The maximum removal efficiency
of ammonia was reported as 91%, 87% and 80% respectively. Finally, a good relationship between the loss
of efficiency in ammonia removal and the increase of precipitating CaCO3 to the tower plain area ratio,
valid for all water pH and temperatures, has been found
A modified biotrickling filter for nitrification-denitrification in the treatment of airborne ammonia
No full textA conventional biotrickling filter for airborne ammonia nitrification has been modified, by converting the liquid sump into a biological denitrifying reactor. The biotricking filter achieves an ammonia removal efficiency of 92.4% av., with an EBRT (Empty Bed Retention Time) = 36 s and an av. ammonia concentration of 54.7 mg Nm-3 in the raw air stream. The denitrification reactor converts ammonia into inert gas N2, in addition to other important advantages connected to the alkaline character of the biochemical pathway of the denitrifying bacteria. Firstly the trickling water crossing the denitrification reactor, underwent a notable increase in pH (from an average pH=7.3 to 8.0) which prevented the acidic inhibition of the nitrifying bacteria due to the build-up of nitric and nitrous acids. Secondly, the increase in pH created the ideal conditions for the autotrophic nitrifying bacteria. Our tests proved that an ammonia removal efficiency of above 90% can be achieved with an EBRT greater than 30 s and an ammonia volumetric load lower than 200 g ammonia d-1 m3. The results of the biofilm observation using a scanning confocal laser microscope, are reported together with the identification of degrading bacteria genera in the biotrickling filter. Overall, the efficiency of the plant and its excellent operational stability highlight the effectiveness of the synergistic action between the denitrification reactor and the biotrickling filter in removing airborne ammonia.A conventional biotrickling filter for airborne ammonia nitrification has been modified, by converting the liquid sump into a biological denitrifying reactor. The biotrickling filter achieves an average ammonia removal efficiency of 92.4 %, with an empty bed retention time (EBRT) equal to 36 s and an average ammonia concentration of 54.7 mg Nm(-3) in the raw air stream. The denitrification reactor converts ammonia into inert gas N-2, in addition to other important advantages connected to the alkaline character of the biochemical pathway of the denitrifying bacteria. Firstly, the trickling water crossing the denitrification reactor underwent a notable pH increase from 7.3 to 8.0 which prevented the acidic inhibition of the nitrifying bacteria due to the buildup of nitric and nitrous acids. Secondly, the pH increase created the ideal conditions for the autotrophic nitrifying bacteria. The tests proved that an ammonia removal efficiency of above 90 % can be achieved with an EBRT greater than 30 s and a volumetric load lower than 200 g NH3 m(-3) day(-1). The results of the biofilm observation by using a scanning confocal laser microscope are reported together with the identification of degrading bacteria genera in the biotrickling filter. The efficiency of the plant and its excellent operational stability highlight the effectiveness of the synergistic action between the denitrification reactor and the biotrickling filter in removing airborne ammonia
Indagine preliminare sulla miscelazione mediante getti turbolenti in ambienti confinati
No full textLa miscelazione di vasche di depurazione e reattori chimici è una problematica di notevole importanza per l’industria; infatti, il raggiungimento dell’omogeneità della concentrazione di reagenti all’interno di una vasca permette tempi di reazione ridotti con notevoli risparmi sia in termini economici (riduzione dell’utilizzo di
reagenti e dei tempi di contatto) sia in termini di qualità del prodotto finale (ad esempio, l’aumento delle rese di depurazione in un impianto di depurazione a fanghi attivi) Per raggiungere questo obiettivo, è indispensabile migliorare la miscelazione eliminando zone morte (zone dove la reazione attesa non avviene) e by-pass di portata (aliquota di portata che permane nel reattore un tempo inferiore rispetto alla durata della reazione).
L’obiettivo della presente memoria è quello di descrivere i risultati iniziali dell’indagine che si intende svolgere sull’idrodinamica di vasche e reattori biologici in cui la funzione di miscelazione è svolta da getti turbolenti; in particolare, verranno descritti i risultati di misure di velocità effettuate in un modello fisico di
reattore; i dati sperimentali verranno confrontati con i risultati della simulazione numerica al fine di valutarne l’affidabilità; inoltre, verranno mostrati risultati sulle caratteristiche di periodicità del getto all’interno della vasca al variare del tirante
idrico, in quanto utili per aumentare la turbolenza, quindi la miscelazione, all’interno della vasca
An Eulerian–Lagrangian method for the simulation of the oxygen concentration dissolved by a two-phase turbulent jet system
Full text linkMultiphase jet systems are used in environmental engineering to mix and provide oxygen in activated sludge plants for the aerobic digestion of degradable substrates.
Here a laboratory experiment, based on a lab-scale model where mixing is obtained by a turbulent multiphase jet of tap water and pure oxygen, is used to validate a multiscale numerical model of the oxygen concentration evolution based on a ‘‘one-way coupling’’ approach.
The velocity field inside the lab-model, measured by means of a 3D Acoustic Doppler Velocimeter, allows to check the data provided by a finite-volume RANS numerical model of the flow, where turbulent effects are taken into account by a RNG
k–epsilon turbulence model. The obtained numerical velocity field is
then used as input for an Eulerian–Lagrangian numerical model for the dissolved oxygen balance equation, where diffusion effects are computed by a random walk model based on the turbulent kinetic energy field. The bubble size distribution of the jet, determined by a photographic technique, is taken into account in the numerical model in order to calculate the mass transfer from the bubbles to the liquid.
Several mass transfer models are implemented and tested in order to perform a sensitivity analysis. The obtained numerical results are compared with experimental measurements of dissolved oxygen concentration inside the lab-scale model during a transient. Results show that the developed numerical tech-
nique yields an accurate reproduction of the measured oxygen concentration in time and can be applied to analyse the behaviour of jet systems for oxygenation and mixing in activated sludge plants
Study of the performance of disinfection with sodium hypochlorite on a full-scale sewage treatment plant
Full text linkA full-scale sewage treatment plant was investigated to assess the performance of the disinfection stage. Sodium hypochlorite was used as a disinfectant agent and the process efficiency was evaluated by E.coli removal. The research took place over a period of two years in order to evaluate the effect of retention time (t) and residual chlorine (Cr) under different seasonal conditions. The effectiveness of E.coli removal with sodium hypochlorite proved to be strictly dependent on the factor CR t (product of residual chlorine with the contact time). The regression line of the experimental points was, on the whole, well comparable with the model proposed by Collins, especially in the field of CRt lower than 30 mg L-1 min
Airborne toluene removal for minimizing occupational health exposure by means of a trickle-bed biofilter
Full text linkThe paper presents the experimental results on a biotrickling pilot plant, with a water scrubber as pre-treatment, finalised to the treatment of an airborne toluene stream in a working place. The air stream was characterized by a very high variability of the inlet concentrations of toluene (range: 4.35-68.20 mg Nm-3) with an average concentrations of 16.41 mg Nm-3. The pilot plant has proved its effectiveness in toluene removal, along a 90 days experimentation period, in steady-state conditions. The scrubbing pre-treatment has achieved an average removal efficiency of 69.9%, but in particular it has proven its suitability in the rough removal of the toluene peak concentrations, allowing a great stability to the following biological process. The biotrickling stage has achieved an additional average removal efficiency of 75.6%, confirming the good biodegradability of toluene. The biofilm observation by a scanning confocal laser microscope has evidenced a biofilm thickness of 650 μm fully penetrated by toluene degrading bacteria. Among the micro-population Pseudomonas putida resulted the dominant specie. This bacterium can therefore be considered the responsible for most of the toluene degradation. The whole experimented process has determined an average 92.7% for toluene removal efficiency. This result allows to meet the most stringent limits and recommendations for occupational safety, given by authoritative organizations in USA and EU; it also meets the odorous threshold concentration of 11.1 mg Nm-3
Numerical and experimental analysis of a confined turbulent multiphase jet
No full textMultiphase jet systems are used in environmental engineering to mix and provide oxygen in activated sludge plants for the aerobic digestion of sludge. In this frame, the possibility to forecast flow dead zones and by-passes by studying the reactor velocity field assumes strong importance. In this paper we present an experimental and numerical analysis of the velocity field and of the dissolved oxygen concentration in a lab-scale model of an activated sludge plant where mixing is obtained by a turbulent multiphase jet of water and pure oxygen. The velocity field inside the model is measured by means of a 3D Acoustic Doppler Velocimeter, allowing us to validate a finite-volume RANS numerical model of the flow, where turbulence effects are taken into account by a RNG k-ε turbulence model. The obtained numerical velocity field is used as input for a Eulerian-Lagrangian numerical model of the transport-diffusion of oxygen inside the water, leading to an estimate of the time evolution of oxygen concentration; dispersion effects on bubble motion are computed
by a random walk model based on the turbulent kinetic energy field. The oxygen bubble diameter distribution inside the multiphase jet needed as input by the numerical model is obtained by post-processing of high-resolution digital images
of the multiphase flow. The obtained numerical results are compared with experimental measurements of dissolved
oxygen concentration inside the lab-scale model during a transient, validating a numerical technique useful to analyse
the behaviour of jet systems for oxygenation and mixing in activated sludge plants
Formação e destruição de hidrocarbonetos aromáticos policíclicos (HAP) na combustão do biogás coletado de um aterro de resíduos de veículos desmantelados / Formation and destruction of Polycyclic Aromatic Hydrocarbons (PAHs) in the flaring of the biogas collected from an automotive shredded residues landfill
Full text linkThe paper shows the results of the combustion in an enclosed flare of the biogas collected from an automotive shredded residues landfill. The results demonstrate that at 1,000°C and long combustion, several synthesis reactions lead to the formation of 4 to 6 rings of PAHs. This formation also involves the formation of compounds such as benzo(g,h,i)perylene, indeno(1,2,3-cd)pyrene and dibenzo(a,h)anthracene not present in raw biogas. However, the compounds most likely to form in combustion are benzo (a) anthracene and benzo (b) fluoranthene. The only exception is chrysene which is significantly destroyed. The experience has proved the total lack of formation of PAHs with only 2 and 3 aromatic rings. © 2016, Institute for Environmental Research in Hydrographic Basins (IPABHi). All rights reserved
Simulazione numerica dell’evoluzione della concentrazione di ossigeno disciolto nel flusso indotto da un getto turbolento acqua-ossigeno in ambiente confinato
No full textLa non uniforme miscelazione e ossigenazione nelle vasche di ossidazione degli impianti a fanghi attivi è una problematica cruciale nella gestione degli impianti di trattamento delle acque reflue. L’evoluzione della concentrazione di ossigeno disciolto (OD) nel tempo è stata valutata mediante la simulazione numerica del flusso bifase (acqua e ossigeno in bolle) indotto da un getto circolare turbolento in un impianto a scala ridotta. Il campo di moto indotto dal getto è calcolato con un metodo Euleriano ed è assunto indipendente dal moto delle bolle di ossigeno; quest’ultimo è risolto con un metodo statistico Lagrangiano, attraverso la soluzione dell’equazione della dinamica delle bolle in cui è implementato un contributo di movimento casuale per tenere conto degli effetti legati alla turbolenza. La
risoluzione dell’equazione di bilancio della concentrazione di OD permette di valutare il trasferimento di massa dalle bolle alla fase liquida, a partire da una distribuzione dimensionale delle bolle determinata sperimentalmente mediante post processing di immagini ad alta definizione. I risultati ottenuti mostrano una buona concordanza con i valori misurati di OD ed evidenziano l’importanza della determinazione della dimensione delle bolle per la corretta valutazione del fenomeno in esame
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