Color, removal

The BAT is defined as the level of treatment beyond BPCTCA that has been proven feasible in laboratory and pUot studies and that is, in some cases, in fuU-scale operation. BAT in the pulp and paper industry may include such processes as filtration, coagulation for color removal, and improved in-plant control to reduce the wasteload constituents. 

The treatment units used for color removal are the same as those used for turbidity removal. However, the pH must be increased prior to filtration so that the metal hydroxides are removed by the filters. At low pH values, metal ions or their soluble complexes readily pass through the filters and form insoluble species in storage tanks and in the distribution system. For iron salts, it is important that the pH be greater than 6 as the oxidation of iron(II) to iron(III) occurs rapidly above this pH in the presence of dissolved oxygen or other strong oxidants (18). 

The stoichiometric relationship between chlorine dioxide added and color removed during bleaching is nonlinear, but it is independent of temperature, pH, and pulp concentration under conditions normally used. Models used to explain the kinetics and stoichiometry show a strong dependence on chromophore concentration that probably results from differences in the reaction rates of the various chromophores present in the pulps (80). 

Foam Fractionation. An interesting experimental method that has been performed for wastewater treatment of disperse dyes is foam fractionation (88). This method is based on the phenomenon that surface-active solutes collect at gas—Hquid iaterfaces. The results were 86—96% color removal from a brown disperse dye solution and 75% color removal from a textile mill wastewater. Unfortunately, the necessary chemical costs make this method relatively expensive (see Foams). 

For color removal, ozonization has achieved the greatest practical importance as seen by the plethora of articles and patents on this method (147—163). Ozonization in combination with treatments such as coagulation, flocculation, carbon adsorption, uv irradiation, gamma radiation, and biodegradation significantly and successfully remove dye wastes and reduce costs (156,164—170). 

Sufficient color reduction is often achieved by recovery and purification methods. However, sometimes specific color removal is achieved by adsorption to, eg, activated carbon. 

A final are we should discuss is color removal. This is perhaps the most difficult impurity to remove from waters. In surface waters color is associated with dissolved or colloidal suspensions of decayed vegetation and other colloidal suspensions. The composition of this material is largely tannins and lignins, the components that hold together the cellulose cells in vegetation. In addition to their undesirable appearance in drinking water, these organics can cause serious problems in downstream water purification processes. For examples ... 

There are many ways of optimizing color removal in a clarifier. The three most common methods are ... 

Color removal is affected by pH. Generally, organics are less soluble at low pH. 

PAC is faster-acting than alum, even in cold water. It produces a compact, fast-settling floe and is effective over a wide range of pH levels (typically 4-9.5). It provides good color removal. 

Aluminum Chlorhydrate (ACH), A12(0H)5C1 ACH reacts very similarly to PAC and is available only as a solution. It is a very highly basic product (80%). Upon dilution, PAC and ACH solutions tend to act as highly cationic polymers. After dilution and with time, they also begin to degrade and act like alum. This period may be on the order of 1 to 10 minutes, depending on pH level and temperature, The dose rate is similar to that of PAC. Good turbidity and color removal are achieved with ACH. 

Krofta, M. and Wang, L.K., Potable Water Pretreatment for Turbidity and Color Removal by Dissolved Air Flotation and Filtration for the Town of Lenox, Massachusetts, Krofta Engineering Corporation, Lenox, MA, Report No. KEC-10-81/3, 84pp., October 1981. 

Sivakumar and Pandit [13] reported the use of hydrodynamic cavitation for decolorization of dye effluent stream. It has been observed that for same flow area, plate with larger number of small diameter holes gives higher extent of color removal as compared to the plate with smaller number of larger diameter holes. The observed results have been explained on the basis of frequency of turbulence for the two cases. The hydrodynamic cavitation reactor using multiple hole orifice plates has been found to give cavitational yields, which are two times higher than the best acoustic cavitation device (Dual frequency flow cell with capacity of 1.5 L). 

Escherichia coli N03 Reactive Red 22 After acclimation, time for 50% color removal lowered from 5.7 to 4.3 h [67]... 

Pseudomonas luteola Reactive azo dyes, Direct azo dyes and leather dyes The 59-99% color removal after 2-6 days static incubation, at dye concentration of 100 mg L 1, monoazo dyes showing fastest rate of decoloration [78]... 

Color removal of dye Reactive Red 22, Study notes competition among the degrader species affecting long term stability Bacterial community structure in the natural circulation bioreactor... 

SSCP The reactor inoculated with a microbial consortia obtained from a textile wastewater treatment plant Color removal and changes in bacterial community profile [168]... 

Acid Orange 7 Sludge originally collected from a pulp and paper wastewater treatment plants Color removal of 96% was achieved in the presence of liposomes that facilitated uptake of dyes by anaerobic biomass, leading to a fast decolorization. Amines such as sulfanilic acid and aniline were mineralized by inocula with high microbiological diversity, even with domestic effluent. Orthanilic and metanilic acids and 1-amino-2-naphtol were persistent under tested conditions [176]... 

Acid Orange 7 Mixed and methanogenic cultures The culture exhibited 94% color removal. Color removal was faster in mixed cultures than in methanogenic culture. Addition of electron donor stimulated reductive cleavage of azo bond [179]... 

Reactive Black 5 Activated sludge batch reactor Color removal occurred under anaerobic environment, while a slight attenuation was noticed under the aerobic condition [136]... 

Remazol Brilliant Sequencing batch About 90% color removal was [188, 189]... 

Remazol inoculated with Violet 5R and 75% color removal ... 

Simulated textile Inclined Tubular A 78% color removal by anaerobic [190]... 

Procion Red (upflow better color removal than ... 

Factors Effecting Anaerobic Color Removal Efficiency. 62... 

Anaerobic stages of SBR studies have been shown efficient with color removal rates mostly higher than 70% [3, 18-20], Meanwhile, COD removal efficiency of anaerobic phase of SBR was found to depend on dyestuff type, amount of initial COD concentration, anaerobic cycle time, etc. Nevertheless, there are also reports about no efficient COD removal in anaerobic cycle of SBR [21, 22],... 

Though cycle time plays an important role in the SBR for the decolorization process, not many reports are found in the literature. The long retention times are often applied in the anaerobic phase of the reactor studies, such as 18 and 21 h. In several studies, it was reported that there is a positive correlation between the anaerobic cycle time and the color removal [30, 31]. Indeed, in combined anaerobic-aerobic SBRs, since bacteria shifted from aerobic to anaerobic conditions, or vice versa, anaerobic azo reductase enzyme can be adversely affected by aerobic conditions, which is essential for aromatic amine removal, thereby resulting in insufficient color removal rate. To investigate the effect of cycle time on biodegradation of azo dyes, inar et al. [20] operated SBR in three different total cycle times (48-, 24- and 12-h), fed with a synthetic textile wastewater. The results indicated that with a... 

The sludge retention time (SRT) is known as a very important operational parameter for color removal in SBR system. To obtain efficient color removal rate, adequate microbial population is desired. It was reported that 10 days SRT remained insufficient to obtain adequate population, and to ensure the color removal, SRT was increased to 15 day [2]. 

Since anaerobic azo dye reduction is an oxidation-reduction reaction, a liable electron donor is essential to achieve effective color removal rates. It is known that most of the bond reductions occurred during active bacterial growth [48], Therefore, anaerobic azo dye reduction is extremely depended on the type of primary electron donor. It was reported that ethanol, glucose, H2/CO2, and formate are effective electron donors contrarily, acetate and other volatile fatty acids are normally known as poor electron donors [42, 49, 50]. So far, because of the substrate itself or the microorganisms involved, with some primary substrates better color removal rates have been obtained, but with others no effective decolorization have been observed [31]. Electron donor concentration is also important to achieve... 

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