Microbe-Mediated Degradation of Synthetic Dyes in Wastewater - Microbial Degradation of Synthetic Dyes in Wastewaters - page 219

2000 ). Recently, several attempts have been made to increase the photocatalytic
ef ciency of TiO 2 which include noble metal deposition, ion doping, addition of

inorganic co-adsorbent, coupling of catalysts, use of nanoporous

lms and so on.
Apart from that, new catalysts, such as polymeric metalloporphyrins, have been
reported to be easily excited by violet or visible light, whereas utilization of solar
energy by commonly used TiO 2 is only about 3 % (Chen et al. 2004 ).

2.3.2 Photochemical/Electrochemical

In electrochemical treatments, oxidation is achieved by means of electrodes where a
determined difference of potential is applied. On this principle, several different
processes have been developed as cathodic and anodic processes-direct and indirect
electrochemical oxidation, electrocoagulation, electrodialysis, electromembrane
processes and electrochemical ion exchange (Chen 2004 ). Occasionally, a combi-
nation of electrochemical technology and photocatalysis has been adopted to yield
some unique advantages. For instance, chemical synergism of photocatalysis and
electrochemical processes may yield enhanced decoloration and COD removal (An
et al. 2002 ) and added advantage may be derived from existence of salt in solution,
which is detrimental for sole photocatalysis (Zhang et al. 2003 ). Conversely,
electro-Fenton process requires no addition of chemical other than catalytic quantity
of Fe +2 , since H 2 O 2 is produced in situ, thereby avoiding transport of this hazardous
oxidant (Guivarch et al. 2003 ; Neyens and Baeyens 2003 ).

2.4 Combination: AOPs and Other Physico-Chemical
Processes

Many studies have focused on different combinations among physicochemical
systems for treatment of textile and dye wastewaters. Combinations of conventional
physicochemical techniques with the AOPs have been an attractive option.

2.4.1 Coagulation Based Combinations

Coagulation/ occulation/precipitation processes have been used intensively for

decolorizing wastewater. For the pretreatment of raw wastewater before discharging
to publicly owned treatment plants, these processes may be satisfactory with respect
to COD reduction, and partial decolorization. Their alone application in treating
textile/dye waste is, however, found to be relatively ineffective (Hao et al. 2000 ;
Papic et al. 2004 ). For example, only 50 % removal was achieved using either alum
or ferrous sulfate for an azo reactive yellow dye (Hao et al. 2000 ). In the coagu-
lation process, it is difcult to remove highly water-soluble dyes, and even more