Environmental Applications of Fungal and Plant Systems: Decolourisation of Textile Wastewater and Related Dyestuffs

7. Conclusion

Wastewater treatment plants, such as activated sludge and methanogenic reactors, are not the natural habitat of WRF, since these organisms prefer solid substrates and well-aerated environments. The fact, that constructed wetlands (e.g. sub-surface flow systems with rooted emergent macrophytes), are transitional environments, i.e. are intermediate between terrestrial and aquatic ecosystems, can be an advantage in the treatment of polluted effluents. The wetlands system treats wastewater by physical, chemical and biotic processes, in a close association of appropriated plants, microorganisms, macro-organisms and substrates. Macrophytes enhance physical filtration, prevent clogging in vertical flow systems, mediate oxygen transfer to the rhizosphere and favour microorganism colonisation (Brix et al. 1996; Brix 1997). In sub-surface systems, there is an oxygen gradient, with high partial pressures near the plant roots, to be replaced progressively by anaerobic and anoxic environments. The mixture of aerobic, anoxic and anaerobic zones stimulates different microbial communities that can degrade complex organic substances (such as azo dyes) almost to mineralisation. The extent of dyes biodegradation must be evaluated, since the formation of intermediate compounds can enhance toxicity (Sweeney et al. 1994). The use of constructed wetlands is a low cost technique, with low maintenance needs (Schwitzguébel et al. 2002; Susarla et al. 2002). It is able to tolerate high fluctuations in flow, temperature (Winthrop et al. 2002) and the composition and/or concentration of pollutants in wastewater. Finally, it is likely to find widespread acceptance with the public for its obvious technological and aesthetic qualities.