Modelling And Design Of Water Treatment Processes Biology Essay

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Dissolved organic pollutants such as dyes and pigments are considered one of the problematic groups of pollutants as they are discharged into wastewaters from industrial operations such as dye manufacturing, leather tanning, carpet, paper, food technology and the textile industry. Many of these dyes are toxic and can be carcinogenic (McKay et al., 1985). Therefore, it is necessary to remove them from liquid wastes to below the concentrations accepted by national and international regulatory agencies before the wastes are discharged to the environment. Removal of dye compounds can be difficult and there are a number of processes used to reduce the concentration of dyes to the limits recommended by the World Health Organization (WHO) including adsorption (Walker and Weatherley, 2000), filtration (Mohan et al., 2002), chemical coagulation (Vandevivere et al., 1998) and photo degradation (Chu and Ma, 2000). These processes can be very effective for the removal of organic pollutants such as dyes, but have the disadvantage that they produce secondary wastes.

Adsorption processes are an attractive approach for water treatment, particularly if the adsorbent is cheap, does not require a pre-treatment step before its application and is easy to regenerate (Wang et al., 2005). For many applications this process has proven to be superior to other techniques for a variety of reasons (Sanghi and Bhattacharya, 2002); (Meshko et al., 2001); (Bulut and Aydin, 2006), including the simplicity of design, low cost, high removal efficiency, ease of operation and availability. One of the most attractive processes is adsorption onto activated carbon as very low concentrations at the outflow can be achieved and high loadings of pollutant are possible on these adsorbents. Activated carbon adsorption has been widely investigated as the adsorbent material to remove dyes from wastewater (Tunali et al., 2006); (Daifullah and Girgis, 1998); and (McKay et al., 1985). For example, Thinakaran et al., studied the removal of AV 17 from aqueous solutions by adsorption onto activated carbon prepared from sunflower seed hull. The adsorption capacity was found to be 116.27 mg/g. (Thinakaran et al., 2008). Adsorption processes are normally operated using a batch of adsorbent with sufficient capacity to operate for many months before reaching saturation. Once loaded the adsorbent must be disposed of or regenerated. The most environmentally acceptable and cost effective approach is thermal regeneration (San Miguel et al., 2001). However, analysis of the whole life costs of adsorption processes indicates that most of the treatment costs are associated with regeneration (EPA, 1989). In spite of this, most of the studies on adsorption have focused on the development of adsorbents with high capacity and very few on developing adsorbents that can be easily regenerated.

A search of the Science Citation Index for the last 20 years using the keywords 'adsorbent' and 'capacity' yields 3,654 references, while the keywords 'adsorbent' and 'regeneration' yields only 665 references.

1.2 Motivation

Recent work has shown that Nyex®, a graphite intercalation compound (GIC), which is the subject of this study, is an effective adsorbent (albeit with relatively low capacity) that can be electrochemically regenerated very rapidly and cheaply (Brown et al., 2004a), (Brown et al., 2004b), (Brown et al., 2004c), (Brown, 2005). Adsorption on Nyex® is rapid as it is non-porous, which eliminates intra-particle diffusion. GICs have high electrical conductivity, associated with their graphitic nature, so that the energy consumption during electrochemical regeneration is low. Based on these findings, a continuous treatment process using Nyex® has been developed whereby continuous adsorption and electrochemical regeneration occur within the same device (Brown et al., 2007).

GICs are well known materials and their properties have been investigated (Enoki et al., 2003). In GICs the intercalated molecules form layers in the Van der Waals gaps of the graphite matrix. The use of such material for adsorption significantly reduces both the time required to reach equilibrium, and the electrochemical regeneration time (Brown et al., 2004a).

In this thesis, a GIC adsorbent, Nyex®1000, has been studied for the removal of a dye (Acid Violet 17, AV17) from aqueous solution by both a batch and a continuous process using adsorption and electrochemical regeneration (the ARVIA® process, described in section 1.3). A chemical engineering model of the process has been developed for batch and continuous mode and these have been validated in a sequential batch cell and a prototype device treating simulated waste water contaminated with an organic dye, AV 17, respectively. The design of any flow reactor depends upon two factors which are the rate equations and backmixing or dispersion. In this case the first requires proper mathematical representation of the amount of the contaminated adsorbate on the adsorbent material at equilibrium and also requires a study of the kinetics of adsorption to provide information about the mechanism of adsorption, which is important for the efficiency of the process. The last factor is used to represent the combined action of all phenomena, namely molecular diffusion, turbulent mixing, and non-uniform velocities, which give rise to a distribution of residence time in the reactor (the residence time distribution or RTD). The RTD can also be used to determine the mean residence time and whether undesirable stagnant zones and / or by-pass routes occur within the reactor (Levenspiel, 1999). A 'tank in series' or dispersion coefficient model are usually used to express a mathematical description of RTD (Martin, 2000). Both a steady state and a dynamic mathematical model for designing the ARVIA® treatment process, which is a continuous stirred tank reactor (CSTR), have been developed for the process performance of adsorption and electrochemical regeneration in continuous mode and also in a plug flow reactor (PFR). A sensitivity study has been carried out for the PFR and this is compared with CSTR model at steady state. Also, a model of multi-stage batch adsorption and regeneration has been developed and validated with experimental data.

1.3 Introduction to the ARVIA® process

A prototype wastewater treatment system has recently been developed in Manchester. And the University of Manchester spin-out ARVIA Technology Ltd has been established to commercialize this process. The process combines a novel adsorbent (Nyex®) and an innovative electrochemical regeneration process. Figure 1.1, shows how the adsorbent is circulated through adsorption and regeneration zones. The effluent is fed into the adsorption zone and air is injected in order to fluidize the adsorbent and generate intense mixing. The air is disengaged at the top of the adsorption zone and the adsorbent and treated water flow into a settlement zone, where the adsorbent settle into the regeneration zone and the treated water overflows out of the process. The adsorbent forms a moving bed which gradually slides down into the electrochemical regeneration cell. The bed is in contact with an anode and is separated from the cathode by a microporous membrane. The anode and cathode are connected to a DC power supply to regenerate the adsorbent. Once the adsorbent reaches the bottom of the electrochemical cell it is fully regenerated and ready for reuse.




Air disengagement



Treated Effluent







Adsorption Zone

Figure 1. Continuous adsorption and electrochemical regeneration of the ARVIA® process for water treatment.

1.4 Scope of the work

This thesis focuses on development of a mathematical model of the batch and continuous adsorption occurring in the ARVIA® process, which is a new development in wastewater treatment based on the adsorption of organic pollutants (dyes) onto an adsorbent material (Nyex®1000) and subsequent electrochemical regeneration of adsorbent loaded with pollutant.

Accordingly, the main objectives of this thesis can be summarized as follows:

Investigate equations which describe the adsorption of a typical organic dye, including both the kinetic and equilibrium behaviour.

Determine kinetic and equilibrium parameters using batch adsorption experiments.

Examine the effect of different parameters, such as temperature and pH on the adsorption of dye.

Investigate the electrochemical regeneration of Nyex®1000 loaded with dye and the effect of the regeneration conditions on performance.

Develop a design model for the treatment of water contaminated with dye using multi-stage batch adsorption and electrochemical regeneration.

Investigate the mixing behaviour in the continuous ARVIA® process using the residence time distribution technique.

Develop steady state and dynamic models for the continuous water treatment by adsorption and electrochemical regeneration occurring in the ARVIA® process.

Develop a design model of water treatment by adsorption and electrochemical regeneration with co-current plug flow of the adsorbent and water.

Investigate the effect of operating conditions and key parameters on the process performance using the model.

This thesis is organized into five Chapters as follows:

Chapter 1 serves to introduce the problem and the objectives of the work.

Chapter 2 provides a review of the literature pertinent to this study, focussing on studies of adsorption, adsorbent regeneration, and application to water treatment. Specific topics covered include adsorption processes and regeneration methods with particular focus on electrochemical regeneration.

Chapter 3- discusses water treatment by batch adsorption and electrochemical regeneration. Previous work in this area is discussed, including the theory related to the kinetics and equilibrium of adsorption. The materials and methodology used are described and experimental results for batch adsorption and electrochemical regeneration are presented and discussed. A mathematical model of multi-stage adsorption and regeneration is developed and validated.

Chapter 4- focuses on water treatment by continuous adsorption and electrochemical regeneration. Relevant literature is reviewed, the behaviour of airlift reactors is discussed and residence time distributions are explained. The experimental apparatus and procedures for studying the continuous treatment process are described, including the method used for measurement of the residence time distribution.

The methodology and the results of validation studies for modelling of continuous water treatment by adsorption, and electrochemical regeneration are discussed. Sensitivity studies to evaluate the effect of key parameters on performance are presented and discussed.

Chapter 5- outlines the conclusions that can be drawn from this work and includes suggestions and recommendations for future work.

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