Sequential Anaerobic/Aerobic Treatment of Dye-Containing Wastewaters: Colour and COD Removals, and Ecotoxicity Tests

Abstract

Colour and COD removals of the azo dyes Congo Red (CR) and Reactive Black 5 (RB5) were individually evaluated in a sequential anaerobic/aerobic treatment system. Additionally, dye toxicity was assessed by using acute ecotoxicity tests with Daphnia magna as the indicator-organism. The anaerobic reactor was operated at approximately 27 °C and with hydraulic retention times of 12 and 24 h. The aerobic reactor was operated in batch mode with a total cycle of 24 h. During anaerobic step, high colour removals were obtained, 96.3% for CR (400 mg/L) and 75% for RB5 (200 mg/L). During the aerobic phase, COD effluent was considerably reduced, with an average removal efficiency of 52% for CR and 85% for RB5, which resulted in an overall COD removal of 88% for both dyes. Ecotoxicity tests with CR revealed that the anaerobic effluent presented a higher toxicity compared with the influent, and an aerobic post-treatment was not efficient in reducing toxicity. However, the results with RB5 showed that both anaerobic and aerobic steps could decrease dye toxicity, especially the aerobic phase, which removed completely the toxicity in D. magna. Therefore, the anaerobic/aerobic treatment is not always effective in detoxifying dye-containing wastewaters, sometimes even increasing dye toxicity.

Destruction of azo dyes by anaerobic–aerobic sequential biological treatment: a review

“…Azo compounds with a hydroxyl or amino groups are more likely to be degraded than those with methyl, methoxy, sulpho, or nitro groups (Supaka et al 2004). It has been reported that dyes with simple structures and low molecular weights exhibit higher rates of decolourization than those with high molecular weight and high substitutions (Solis et al 2012;Da Silva et al 2012 The lesser decolourization of RB5 was attributed to its high molecular weight and highly substituted chemical structure as compared to that of CR (see Fig. 1).…”

“…Thus, the mixed microbial consortia can carry out tasks better than or equivalent to that of an individual pure culture without any precautions to prevent contamination. Different reactor configurations, such as the widely used upflow anaerobic sludge blanket (UASB), expanded granular sludge bed (EGSB), anaerobic/aerobic sequential batch reactor (SBR), activated sludge process (ASP), have been reported for efficient dye decolourization using mixed microbial consortia (Dos Santos et al 2007;Karatas et al 2010;Yasar et al 2012;Da Silva et al 2012). The biodegradation of azo dyes occurs in a two-step process (Rai et al 2005;Vanderzee et al 2001).…”

“…Extent of anaerobic biological colour removal has been reported to be higher than 80 % and in some cases even 100 % for a variety of azo dyes (Karatas et al 2010;Yasar et al 2012;Da Silva et al 2012). The extent and rate of anaerobic decolourization of azo dyes are influenced by various parameters such as dye structure, dye concentration, supplementation with different carbon and nitrogen sources, electron donor, and redox mediator (Vanderzee and Villaverde 2005;Da Silva et al 2012;Johnstrup et al 2011;Koupaie et al 2011;Mohanty et al 2006;Albuquerque et al 2005). The influence of each parameter is discussed as below.…”

“…There is a direct relationship between the HRT of the anaerobic biological treatment unit and the colour removal efficiencies (Johnstrup et al 2011;Karatas et al 2010;Da Silva et al 2012;Oh et al 2004;Spagni et al 2010). For instance, Johnstrup et al (2011) reported increase in the extent of decolourization of Remazol Red RR (initial concentration: 100 mg/L) from 85 to 98 % when HRT of anaerobic batch reactor was increased from 0.5 to 3 days.…”

“…Anaerobic decolourization by cleavage of azo bond generates aromatic amines, which are in some cases more toxic than the parent dyes. Anaerobic decolourization followed by aerobic biodegradation may or may not reduce toxicity (Puvaneshwari et al 2006;Da Silva et al 2012;Montano et al 2008). Various authors have employed different types of toxicity tests to evaluate the toxicity of dye wastewater at different stages of biological treatments.…”