Hydrodynamic and kinetic study of a hybrid detoxification process with zero liquid discharge system in an industrial wastewater treatment
© Abid et al.; licensee BioMed Central. 2014
Received: 18 September 2013
Accepted: 6 December 2014
Published: 24 December 2014
This work focused on the degradation of toxic organic compounds such as methyl violet dye (MV) in water, using a combined photocatalysis/low pressure reverse osmosis (LPRO) system. The performance of the hybrid system was investigated in terms of the degradation efficiency of MV, COD and membrane separation of TiO2. The aim of the present study was to design a novel solar reactor and analyze its performance for removal of MV from water with titanium dioxide as the photocatalyst. Various operating parameters were studied to investigate the behavior of the designed reactor like initial dye concentration (C = 10-50 mg/L), loading of catalyst (CTiO2 = 200-800 mg/L), suspension flow rate (QL = 0.3-1.5 L/min), pH of suspension (5–10), and H2O2 concentration (CH2O2 = 200-1000 mg/L). The operating parameters were optimized to give higher efficiency to the reactor performance. Optimum parameters of the photocatalysis process were loading of catalyst (400 mg/L), suspension flow rate (0.5 L/min), H2O2 concentration (400 mg/L), and pH = 5. The designed reactor when operating at optimum conditions offered a degradation of MV up to 0.9527 within one hours of operation time, while a conversion of 0.9995 was obtained in three hours. The effluent from the photocatalytic reactor was fed to a LPRO separation system which produced permeate of turbidity value of 0.09 NTU which is closed to that of drinking water (i.e., 0.08 NTU). The product water was analyzed using UV-spectrophotometer and FTIR. The analysis results confirmed that the water from the Hybrid-System could be safely recycled and reuse. It was found that the kinetics of dye degradation was first order with respect to dye concentration and could be well described by Langmuir-Hinshelwood model. A power-law based empirical correlation was developed for the photocatalysis system, related the dye degradation (R) with studied operating conditions.
KeywordsSolar photocatalysis Membrane separation Hybrid system Zero liquid discharge Titanium dioxide photocatalyst Dye synthetic wastewater Falling-film slurry reactor
Pollution of ground water and rivers by organic pollutants is a constant concern and a common problem throughout the world. Some of these organic pollutants are; herbicides, pesticides, and fungicides used in agricultural activities, which can be carried away by rain and polluted water resources, hydrocarbons from wastewater discharges from oil production activities and synthetic dyes from textile industry’s waste processing fluids . Textile dyes and other commercial colorants have become as toxic organic compounds the focus of environmental remediation efforts because of their natural biodegradability is made increasingly difficult owing to the improved properties of dyestuffs . Color interferes with penetration of sunlight into the water, retards photosynthesis, inhibits the growth of aquatic biota and interferes with solubility in water bodies . Various physical, chemical and biological pre-treatment and post-treatment techniques have been developed over the last two decades to remove color from dye contaminated wastewater in order to cost effectively meet environmental regulatory requirements. Chemical and biological treatments have been conventionally followed till now but these treatment methods have their own disadvantages. The aerobic treatment process is associated with production and disposal of large amounts of biological sludge, while wastewater treated by, anaerobic treatment method does not bring down the pollution parameters the satisfactory level . As international environmental standards are becoming more stringent, (ISO 14001, 1996), a technological system for the removal of organic pollutants, such as dyes has to be developed. Heterogeneous photocatalysis is one of the advanced oxidation process (AOP) that has proven to be a promising method for the elimination of toxic and bio-resistant organic and inorganic compounds in wastewater by transforming them into innocuous species ,. Advanced oxidation process (AOP) is a chemical oxidative process, which can be applied to wastewater treatment to oxidize pollutants. It generates hydroxyl radicals which considered as the second strongest known oxidant (2.8 V vs. standard hydrogen electrode). It is able to oxidize and mineralize almost every organic molecule, yielding CO2 and inorganic ions . AOPs not only oxidized the organic compounds, but also a complete mineralization is achievable, and the processes are not specific and therefore are capable of destroying a broad range of organic compounds. The process is very powerful, and is immune to organic toxicity. In the eighties and nineties, water reuse started to become a popular means to reduce freshwater intake and reduce treatment costs. A concept that refers to closed circuits of water, such that disposal is eliminated. Advantages and disadvantages of zero discharge facilities are currently being seriously considered and discussed. Zero liquid discharge minimizes the consumption of freshwater to that of make-up; therefore, it should help relieve freshwater availability limitations in places where it is scarce or expensive . In photocatalytic degradation of dyes in wastewaters, the main operating parameters which affect the process are pH of the solution to be degraded, oxidizing agent, catalyst loading, and contaminant concentration . Photocatalytic reactions are the result of the interaction of photons having the appropriate wavelength with a solid semiconductor . Recent studies have been devoted to the use of photocatalysis in the removal of dyes from wastewaters, particularly, because of the ability of this method to completely mineralize the target pollutants .
The general mechanism of photocatalysis could be represented elsewhere -. Many semiconductors have been tested so far as photocatalysts, although only TiO2 in the anatase form seems to have the most interesting required attributes; such as high stability, good performance and low cost . In this respect, the photodecomposition power of TiO2, for a wide variety of organic compounds present in water, has been reported in the literature .
Photocatalytic reactors for water treatment can generally be classified into two main configurations, depending on the deployed state of the photocatalysts: (1) reactors with suspended photocatalyst particles and (2) reactors with photocatalyst immobilised onto continuous inert carrier . Various types of reactors have been used in the photocatalytic water treatment, including the annular slurry photoreactor , cascade photoreactor , downflow contactor reactor  and, etc. The disparity between these two main configurations is that the first one requires an additional downstream separation unit for the recovery of photocatalyst particles while the latter permits a continuous operation. Vincenzo et al.  carried out a photodegradation of two common and very stable azo-dyes, in aqueous suspensions of polycrystalline TiO2 irradiated by sunlight using a plug flow reactor in a total recirculation loop. They reported that Complete decolourization was obtained in few hours for both dyes but mineralisation occurred after longer times with the formation of CO2, nitrates and sulphates. Damszel et al.  investigated the possibility of application of the hybrid photocatalysis/membrane processes system for removal of azo dyes (Acid Red 18, Direct Green 99 and Acid Yellow 36) from water. The photocatalytic reactions were conducted in the flow reactor with immobilized photocatalyst bed and in the suspended system integrated with ultrafiltration (UF). They found that the solutions containing the model azo dyes could be successfully decolorized during the photocatalytic processes applied in the studies. The application of UF process results in separation of photocatalyst from the treated solutions whereas during the (NF) and membrane distillation (MD) high retention of degradation products was obtained.
The main objectives of this work were the evaluation and testing of a novel photocatalytic reactor for the degradation of methyl violet (MV) dye which is selected as a model organic toxic pollutant in water using a commercial TiO2 catalyst. And also to study the possibility to couple a membrane separation system with the reaction system, to remove TiO2 particles from product stream of the solar photocatalytic reactor to achieve a zero liquid discharge.
Materials and methods
Methyl violet 6B (MV) dye (commercial grade, C24H28N3Cl, λmax (nm) = 586, Sigma Aldrich Co., USA.), Titanium dioxide powder (antase type, ≥99.5% trace metals basis, particle size ~ 21 nm, specific surface area (35–65 m2/g), particle density (4.26 g/mL (at 25°C)), Sigma Aldrich Co.) were used as received. Reagent-grade hydrogen peroxide (H2O2) (50% v/v solution), was used as oxidant. Technical grade hydrochloric acid (35%) and sodium hydroxide (98% flakes) were used to adjust the pH of synthetic wastewater (to around 5–10). Distilled water (conductivity <10 μS cm−1, Cl− = 0.7–0.8 mg L−1, NO3− = 0.5 mg L−1, organic carbon <0.5 mg L−1).
Average daily incident solar radiation (kWh m −2 ) on 37° inclined surface at University of Technology-Baghdad
Average daily global incident solar radiation, kWh. m−2
Average daily UV incident solar radiation, kWh.m−2
Range of operating variables in photocatalytic experiment
Catalyst loading (TiO2), mg/L
200, 300, 400, 500, 800
Hydrogen peroxide concentration, mg/L
300, 400, 500, 800, 1000
Flow rate, L/min
0.3, 0.5, 1, 1.5
5, 6, 7, 9, 10
Dye concentration, mg/L
10,20, 30, 40, 50
Results and discussion
Effect of operating parameters on dye degradation and COD removal in photocatalytic reactor
Effect of pH
Effect of hydrogen peroxide (H2O2)
Effect of liquid flow rate
Effect of initial dye concentration
Effect of catalyst loading
The coefficients of the correlation depict the predominant effect of pH on other operating variables for dye degradation. This correlation could be used as a model to predict the behavior of the dye degradation within the operating range of the studied variables.
Variation of turbidity against C TiO2 before and after neutralization process
Turbidity input (NTU)
Residual turbidity (NTU)
Low pressure reverse osmosis (LPRO) membrane was used to separate TiO2 particles from product water. The experimental results indicated that separation efficiency of LPRO was nearly 100% which suggested the use of such type of membrane for the hyper detoxification process.
This work focused on the degradation of toxic organic compounds such as methyl violet dye (MV) in water, using a combined photocatalysis/low pressure reverse osmosis (LPRO) system. The performance of the hybrid system was investigated in terms of the degradation efficiency of MV, COD and membrane separation of TiO2. The following conclusions have been noticed:
Photocatalysis Degradation of Dye
It appears that the flow rate recirculation, irradiation time catalyst load, pH, H2O2 concentration, and concentration of dye mainly controls the rate of degradation for which optimum conditions for achieving maximum efficiency were established.
Regarding catalyst loading, the degradation increased with the mass of catalyst up to an optimum amount. After then the degradation decreased as the catalyst loading continued to increase attributed to the UV light penetration depth which is considerably smaller than in suspension containing optimum amount of catalysts.
The capacity of TiO2 towards dye degradation strongly depended on pH of suspension. At basic pH, degradation was smaller than that at acidic pH where degradation of anionic dye was fast indicating that the mechanism involving complete mineralization could be achievable.
The impact of the liquid Reynolds number on dye degradation shows a positively increasing trend to a point where all the surface of the photocataytic reactor was covered with a thin falling–film of synthetic wastewater, after then the dye degradation started to decrease with further increasing of liquid flow rate. This may be attributed to decreasing the residence time of the reactants.
The addition of H2O2 to TiO2 suspension resulted in an increase in the degradation ratio. The H2O2 acted as electron acceptors to make electron/hole recombination avoided and increased the concentration of •OH radicals.
Membrane separation system
To make the product water available for reuse, the TiO2 particles in suspension must be removed. LPRO membrane was selected to perform the task. The following conclusions can be drawn from the experimental runs.
The neutralization process of the reactor effluents seems to be feasible for reduction catalyst loading in the influent line of the membrane system.
Increasing the catalyst loading from (400 to 800 mg/L) results in reduction of permeate flow rate by 15%.
The TiO2 separation efficiency of membrane could be estimated by utilizing the turbidity values of influent and effluent solutions.
The LPRO membrane system has proved to be an efficient solution for the separation of TiO2 suspended particles.
Authors are thankful to the Department of Chemical Engineering-University of Technology for providing facilities and space where the present work was carried out. Thanks are also due to the Solar Energy Research Center-Baghdad for their assistance.
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