Indicators for technological, environmental and economic sustainability of ozone contactors

Residence time and the concentration of microorganism in the ozone contactor: a CFD simulation on chamber deflectors

Disinfection is an important step in deep drinking water treatment technology. This study applies computational fluid dynamics to investigate and optimize the hydrodynamics inside the ozone contactor. ANSYS Fluent was used to solve all the control equations. A step method is used to simulate the residence time distribution. The mean residence time is simulated under the Eulerian framework. The deflectors are installed in chambers to direct flow. The deflectors allow for a more uniform flow and a longer mean residence time within the contactor. The baffling factor showed that the deflectors could reduce the short-circuit effect in the contactor and improve the disinfection efficiency by 34.6% compared to the original reactor. The Morrill factor coefficient is improved by 22.8% compared to the original reactor. According to the Aral-Demirel index, contactors with deflectors are significantly better than other baffle-type contactors. The presence of the deflectors increased the microbial inactivation efficiency from 95.3 to 96.5%. The optimal deflector height should be controlled between 30 and 60 mm.

Perforated Baffles for the Optimization of Disinfection Treatment

Water disinfection is one of the main treatments aimed at maintaining human health. Traditionally, the treatment takes place inside multichamber tanks that facilitate the contact between disinfectant and pathogenic microorganisms to be removed. However, the traditional contact tanks used for disinfection have geometric characteristics causing the formation of dead or recirculation zones that reduce treatment efficiency with potentially harmful effects on human health. This study proposes the creation of holes in the baffles that divide the various chambers in order to increase the mixing inside the reactor. In particular, various configurations with holes of different sizes were considered. The results obtained through fluid dynamics simulations based on the LES (large eddy simulation) model show that the jet emerging from the holes penetrates the recirculation zones, transforming them into areas of active mixing. The analysis of the hydraulic mixing indices traditionally used to evaluate the performance of these tanks shows that the presence of the holes allows a significant increase in the mixing efficiency by reducing the short-circuit phenomena and the entrapment of the disinfectant inside the dead zones. Parameters of fundamental importance are the size of the holes, the arrangement of the holes within the baffles and the perforation percentage.

Ultra-sonication for controlling the formation of disinfection by-products in the ClO2 pre-oxidation of water containing high concentrations of algae

Eutrophication has become great concern in recent years due to the fact that rivers, lakes, and reservoirs are the main drinking water source. Studies have been performed to enhance the removal of algae with ClO₂ pre-oxidation, but there was high potential in the formation of chlorite and chlorate. In this study, ultra-sonication was employed to assist algae removal and control disinfection by-products formation in ClO₂ pre-oxidation processes. It was found that solo ultra-sonication for 10 min (algae removal 86.11 ± 2.16%) could achieve similar algae removal efficiency as that with solo ClO₂ (0.5 mg/L) pre-oxidation for 10 min (algae removal 87.10 ± 3.50%). In addition, no formations of chlorite and chlorate were detected in solo ultra-sonication process. Five-minutes ultra-sonication followed by 5-min 0.5 mg/L ClO₂ treatment (total treatment time 10 min; algae removal 93.55 ± 3.22%) provided a better performance on algae removal compared to the solo ClO₂ (0.5 mg/L) pre-oxidation for 10 min. Moreover, chlorite was undetectable. It suggests that the utilization of ultra-sonication in ClO₂ pretreatment for algae removal has highly prevented the formations of chlorite and chlorate.

Efficiency Enhancement of Chlorine Contact Tanks in Water Treatment Plants: A Full-Scale Application

The mixing and disinfection performance of a full-scale chlorine contact tank (CCT) is thoroughly investigated by means of numerical simulations for seasonal water supply variations in the water treatment plant (WTP) of Eskisehir in Turkey. Velocity measurements and tracer studies are carried out on a 1:10 scale laboratory model of the CCT to validate the numerical model. A good agreement between numerical and experimental results shows that the numerical model developed can be reliably used for the simulation of turbulent flow and solute transport in the full-scale CCT. Tracer studies indicate that the hydraulic performance of the CCT is classified as “average” according to the baffling factor, while the Morrill, Aral-Demirel (AD), and dispersion indexes indicate low mixing due to the recirculating and short-circuiting effects inside the chambers of the CCT. With respect to the first order modeling of chlorine decay and pathogen inactivation, chlorine concentrations are found to be significantly distinct for seasonal variations in water supply to maintain 3-log inactivation of Giardia cysts. A recently developed and patented slot-baffle design (SBD) is then applied to the full-scale CCT. It is found that the hydraulic efficiency of the CCT is improved to “high” and the Morrill index approaches 2, which identifies the system as a perfect mixing tank. Using the SBD, the chlorine demand has been successfully decreased by 19% while providing equivalent inactivation level. The novel SBD design also reduces energy loses in the turbulent flow through the tank and increases the energy efficiency of the CCT by 62%, which is significant for energy considerations in modern cities.

Ozone and Photocatalytic Processes for Pathogens Removal from Water: A Review

The search for alternative water sources is pushing to the reuse of treated water coming from municipal wastewater treatment plants. However, this requires that tightened standards be fulfilled. Among them is the microbiological safety of reused water. Although chlorination is the mostly applied disinfection system, it presents several disadvantages, such as the high doses required and the possibility of formation of dangerous by-products. Moreover, the threat of antibiotic resistance genes (ARGs) spread throughout poorly treated water is requiring the implementation of more efficient disinfection systems. Ozone and photo assisted disinfection technologies are being given special attention to reach treated water with higher quality. Still, much must be done to optimize the processes so that cost-effective systems may be obtained. This review paper gives a critical overview on the application of ozone and photo-based disinfection systems, bearing in mind their advantages and disadvantages when applied to water and municipal wastewater. Also, the possibility of integrated disinfection systems is considered.

Rotating packed bed as a novel disinfection contactor for the inactivation of E. coli by ozone

The intensification of mass transfer and mixing in ozone-based disinfection processes is very important in order to achieve a certain disinfection effect at a reasonable cost. This study employed rotating packed bed (RPB), an efficient process intensification device, as an ozone-disinfection contactor, and its performance on disinfection and mass transfer was evaluated by the inactivation of Escherichia coli (E. coli) and mass transfer coefficient (k_La). The ozone exposure (Ct value) and the log inactivation of E. coli in the RPB reached 0.0008-0.0014 min mg L^-1 and 6.8-7.3 in phosphate buffered saline buffer, respectively. And the k_La in RPB rose from 0.030 to 0.186 s^-1 with the increase of liquid flow rate from 10 to 60 L h^-1. The increase of rotation speed of the RPB enhanced the log inactivation and k_La simultaneously. It was also noted that a higher gaseous ozone concentration at the identical applied ozone dosage is favorable for the inactivation of E. coli. The calculation of Hatta number indicated that the inactivation of E. coli in RPB is a diffusion-controlled process for which RPB is well suited. Compared to the conventional bubble reactor, the achievement of high disinfection efficacy at a low Ct value in RPB revealed that RPB is an efficient ozone disinfection contactor for the inactivation of microorganisms in water.

Computational fluid dynamics analysis of the hydraulic (filtration) efficiency of a residential swimming pool

Hydraulic or filtration efficiency of residential swimming pools, quantified in terms of residence time characteristics, is critical to disinfection and thus important to public health. In this study, a three-dimensional computational fluid dynamics model together with Eulerian and Lagrangian-based techniques are used for investigating the residence time characteristics of a passive tracer and particles in the water, representative of chemicals and pathogens, respectively. The flow pattern in the pool is found to be characterized by dead zone regions where water constituents may be retained for extended periods of times, thereby potentially decreasing the pool hydraulic efficiency. Two return-jet configurations are studied in order to understand the effect of return-jet location and intensity on the hydraulic efficiency of the pool. A two-jet configuration is found to perform on par with a three-jet configuration in removing dissolved constituents but the former is more efficient than the latter in removing or flushing particles. The latter result suggests that return-jet location and associated flow circulation pattern have an important impact on hydraulic efficiency. Thus return-jet configuration should be incorporated as a key parameter in the design of swimming pools complementing current design standards.

Effect of Porous Baffles on the Energy Performance of Contact Tanks in Water Treatment

Three-dimensional numerical simulations are performed to evaluate the effect of porous baffles on the efficiency of water treatment contact tanks. A second-order accurate numerical model is employed for the solutions of unsteady flow and tracer transport through the porous baffles. The flow through the porous medium is characterized while using the Darcy-Forchheimer relationship. Large Eddy Simulation (LES) model is used to simulate the instantaneous mixing of the tracer in the chambers of the contact tank. Three different porosities are considered to evaluate the effect of porosity on the hydraulic and mixing efficiencies of the contact tank. Simulated time-averaged flow field shows that porous baffles that are placed at the entrance of each chamber could successfully mitigate short-circuiting and yield plug-flow conditions through the system for low porosities. Flow in the contact tank becomes laminar as the flow velocities decrease due to viscous effects and inertial resistance in the porous zone. For this case, the tracer is transported with bulk flow through the system and leaves the contact tank with a high peak seen in the Residence Time Distribution (RTD) plot. Porous layer increases the hydraulic efficiency of the conventional design from “poor” to “good” according to the baffling factor and increases the overall efficiency from “compromising” to “good” according to the AD index. Comparison of the performance of the porous layer with the previously developed slot-baffle design shows that the slot-baffle design increases the efficiency of the tank with increasing dispersion effects, whereas the porous design increases hydraulic efficiency and reduces the dispersion effects. While the porous design reduces energy efficiency by 33% due to a drastic increase in drag in the flow through porous zone, the slot-baffle design increases the energy efficiency of the conventional design by 67%.