Team:UCL/QWERTYtest

Challenges in the textile industry

The global production of dyestuff amounts to over millions of tons per year. Azodyes represent two thirds of this value, a majority of which find their way to wastewater effluent streams. Characterized by the presence of one or more azo group (more), this type of organic colorant is also found in cosmetics, pharmaceuticals and food industries. While azodyes are a dye-class of choice in the textile industry, their global consumption is taking a toll on the environment.

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Conventional textile effluent treatment process

The considerable structural diversity and recalcitrant nature of Azodyes make traditional wastewater treatment technologies markedly ineffective. Hence there exists an array of methods that deal with the removal of synthetic dyes from dyestuff-rich effluents, in order to reduce their environmental impact. These include large-scale physio-chemical processes outlined in the flow sheet below and a variety of organic/inorganic-support based adsorption and photocatalytic and oxidative decolorization. The latter are however more recent methods that are currently too expensive and not scalable to production scales.

General process flowsheet for a wastewater treatment plant

The unit operations involved are:
Screening – ‘separation of particles on the basis of size i.e. removing dyeing process debris which may damage equipement’
Equalization – ‘Reducing the variability in composition of textile waste prior to treatment’
Neutralization: pH control – ‘Reduce downstream consumption of chemicals for used in the physiochemical stages i.e. coagulation and flocculation’
Coagulation – ‘Used to remove waste materials in suspended or colloidal form’
Flocculation – ‘Converts finely divided suspended solids into larger particles so that efficient, rapid settling can occur’
Primary treatment – ‘gravity seperation/clarification/sedimentation unit to separate larger solid particles
Secondary treatment – ‘removing/reducing concentration of organic and inorganic compounds through microbial decomposition’

With azodyes The contamination of natural habitats surrounding textile factories by coloured (azodye-rich) effluents is a real problem (more). This is because the enzymatic breakdown products of azodyes i.e. aromatic amines, are carcinogenic when ingested. These can not only build up within local ecosystems but can also be a hazard to humans through bio-accumulation in the food chain. With a large section of dyehouse effluents consisting of dyes that have half-lives spanning over decades, the latter remain in the environment for long periods of time.

With current technologies in the textile industry, exorbitant volumes of water are used for processing (around 90%), the rest being used for heat exchange purposes. Unfortunately most of the water used for processing is discharged as waste, resulting in highly diluted azodye effluent streams. Secondly, the recalcitrant nature of azodyes hikes the inherent costs of large-scale physical separation systems. As a result, industrial processes used to deal with such soluble hazardous wastes would not be a feasible option to deal with azodye effluents.

By using whole cell biocatalysis as the workhorse for detoxification, this process will yield lucrative byproducts such as quinones, that can then be separated from the process stream and sold off.
With Immobilization The following outlines the general consensus on the benefits of using the immobilized biocatalyst format, with respect to free-floating systems.
Catalyst Retention – A huge decrease in the losses of valuable catalyst into product streams. These losses are exponential in ‘free biocatalysts’ systems, and economically unfeasible when using costly enzymes.

Minimized Contamination of product streams, eliminating the requirement for subsequent protein deactivation and further downstream processing. This is especially important for containment of our recombinant organisms.

Flow-rates are not limited by a threshold critical value for “biomass washout”, and only impact substrate-catalyst contact time. Catalyst concentrations can remain steady through independence of the dilution rate (see figure 1.3). High volumetric flow-rates can enhance mass transfer and speed up the removal of inhibitors (e.g. Azo-dye metabolites) from the system.

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Bioremediation process: Breakdown of the main engineering considerations
1. Process flow sheet
2. Bioreactor design
3. Module operation

Process Flowsheet

The overview diagram below presents the proposed layout for the plant, using an E. coli biofilm as the ‘immobilisation method’, one of the process alternatives we are considering. The synthetic E. coli immobilisation mechanism would take the same format i.e. longitudinal plates, however, we will also consider beads of the synthetic immobilising agent in a packed bed format.

Key Features of Our System

- Fermentation stage is where optimized growth will take place by controlling mixing and oxygen supply. At the end of this stage, viable E. coli cells expressing the enzymes of interest will be present in a broth.
- Module- see cross section of a single system. Continuous flow system with flow rates and residence times based on mass transfer kinetics, specific to E. Coli
- Module 1 designed to capture the bulk of the azodyes, module 2 is a polishing step
- Both anaerobic and aerobic reactions take place at the same time in both the modules, design based on gas supply (nitrogen vs. oxygen)
- Cleaning operation using biodegradable chemical at high flow rate (from holding tank 2)
- Continuous recycle system for maximal active and diffusive uptake.
- Filter modules- exploring the use of disposable low cost agricultural waste for filtration
- Further processing- based on the commercial value of the breakdown products, investments could be made into higher-tier technology such as chromatography columns to separate the breakdown products individually.

This versatile and simple process offers a wide range of future developments into various chemical producing sectors. It would be possible to use this technology in parallel with different industries as a form of platform technology using different synthetic biology anchors, in order to detoxify various effluent polluting chemicals.

Bioreactor Design

Using the estimates for the required E. coli biomass, this section will qualify optimal sizing and operation of the bioreactor required for the microbial fermentation stage. The underlying assumptions on dyeing efficiencies and mass transfer kinetics are hence incorporated in the design.

Module operation

After the fermentation stage, the E. coli biomass is dispersed in a liquor also containing various byproducts. A concentration step could be beneficial to reduce volumes in the next stage. However, capital costs of such unit operations would not be attractive to potential dyeing companies deciding to acquire the entire system. The subsequent modules are equipped to handle large volumes and operate in continuous-flow mode with intermittent discharges. By controlling residence time and operating flow rates, it will be possible to achieve a cell recovery deemed efficient. These will be then immobilized onto the surface of the plates within the modules. There exists a wide range of immobilization strategies used for biological wastewater treatment and this is what gives the unit its modular character. By supporting a number of immobilization methods without changing the hardware, the module allows for the enzymatic breakdown of a wide range of recalcitrant chemicals that might be financially and environmentally costly to treat using conventional methods. (more to come)

Design of immobilization unit

Top view of the module with Azodye feed pipe (red) and aeration inlets for the plates (green).

In the textile industry today, the global production of dyestuff amounts to over millions of tonnes per year. Azodyes represent two thirds of this value, a majority of which find their way to wastewater effluent streams. Characterized by the presence of one or more azo group (more on chemistry), this type of organic colorant is also found in cosmetics, pharmaceuticals and food industries. While the desirable properties of azodyes i.e. chemical stability, high molar extinction coefficient and fastness make them a dye-class of choice, their widespread use in countries such as India and China make them a dye to die for—literally. This is because, in parallel to being aesthetically intrusive to ecosystems, azodye breakdown products have been found to be mutagenic and carcinogenic. With such a high worldwide consumption, the benefits in developing and integrating a sustainable strategy for dealing with such effluent streams is clear. It is worth to note that the ‘azodye problem’ is exacerbated by the high costs, both financial (economic) and environmental, of current physio-chemical and biological methods of treatment (more on current treatment). This year, we are looking into the processing options that are relevant to tackling the problem of azodye discharges. In order to assess the feasibility and determine key engineering parameters for each option, the most important dyestuff sector will be used as a case study: textiles and dyeing industry.

Industrial Consultation

A major part of our project involved engaging with key industrial experts to better understand their wants and needs. We identified the pigment manufacturing and waste water disposal sectors as the two major players who would benefit from our work. By meeting with these leading corporations we have been able to tune our research towards the assenbly of a process that would be most attractive for industry to utilise.

Meeting with ETAD - Ecological and Toxicological Association of Dye and Pigment Manufacturers

ETAD - an association based in Basel represents over 35 different pigment and dyeing corporations interna