Performance assessment of recycled alum sludge in the treatment of textile industry effluent in South Africa

Abstract

The textile industry is considered one of the most polluting sectors in terms of the large volume and toxic composition of the effluent that is generated. For example, the effluent contains dyes, which represent an environmental hazard when discharged without proper treatment. This study aimed to assess the use of recycled alum sludge (RAS) as an alternative treatment for the reduction of colour from dye based synthetic textile industry wastewater. To determine treatment efficiency, the colour, chemical oxygen demand (COD), total dissolved solids (TDS), and pH before and after treatment were monitored. The pH at which optimal removal rates were achieved was also determined. Coagulation/flocculation experiments were conducted on five batches of synthetic wastewater containing disperse dye with an average colour, COD and TDS of 133 ± 13 mg/l (range 115-145 mg/l), 38 ± 4 mg/l (range 32-43 mg/l) and 779 ± 18 mg/l (range 754-804 mg/l), respectively, using a coagulant of alum:RAS mixed in ratios of 1:0, 0:1. 1:1, and 1:2. An average removal efficiency of 89 ± 2% (range: 81-96%) for colour, 29 ± 3% (range: 19-41%) for COD, and 36 ± 4% (range 19-59%) of TDS was recorded during treatment with fresh alum (unmixed). The average removal efficiencies for treatment with RAS (ratio 1:0, i.e. unmixed) were 78 ± 3% (range 67-88%), 22 ± 3% (range 14-34%) and 32 ± 1% (range 29-35%) for colour, COD and TDS, respectively. When fresh alum sludge was mixed with RAS at a ratio of 1:1, average colour and TDS removal efficiencies of 86 ± 3% (range 83-88%), 37 ± 5% (range 30-50%), respectively were achieved, while at ratios of 1:2, the average colour and TDS removal efficiencies were 82 ± 2% (range: 80-84%) and 30 ± 5% (range 22-35%), respectively. Increases in the COD concentrations were observed when fresh alum sludge was mixed with RAS in 1:1 and 1:2 ratios. However, the initial COD concentrations in the synthetic wastewater were low [38 ± 4 mg/l (range 32-43 mg/l)] and increases after treatment were marginal (3 ± 7% and 9 ± 3%, respectively). A second method was applied for colour removal from the synthetic wastewater, namely adsorption with corn cobs. Results were inconclusive due to high levels of turbidity in the treated effluent caused by leaching of components from the shredded corn cobs. This study intended to present alternative means or methods for the treatment of textile wastewater containing dye. The findings of this study compared well with previous laboratory studies conducted with synthetic textile wastewater containing dye. The coagulant of fresh alum and RAS mix ratio of 1:1 offered the best alternative to fresh alum in the treatment of synthetic textile wastewater in terms of reduction of disperse dye from the synthetic textile wastewater. The use of RAS could reduce the volume of waste to be discarded as well as the amount of fresh coagulant necessary for the daily operation.