Treatment of textile liquid waste by chlorination and evaluation of possible trihalomethane (THM) formation

Abstract

The rise in the number of industries in Bangladesh, including textile-dyeing operations, has seriously increased water pollution in the country. The effluent treatment plants (ETPs) are based on a combination of two or more treatment methods: physical, chemical and biological.The aim of this study is to assess the treatment efficiency of textile waste by chlorination and evaluation of possible THM formation. The influent and effluent characteristics and efficiency of ten chlorine based treatment plants were investigated in this study. The concentrations of untreated effluent was found ranged between BOD5:96 -242mg/l, COD: 225-800mg/l, TDS: 228-2040mg/l, TSS: 15-110mg/l, Color 382- 205 (Pt-Co unit). All the treated effluents do not meet the standards set as per Schedule 12-B of the Environmental Conservation Rules -1997 (ECR-1997, Bangladesh). Chlorine oxidation of untreated sample of textile waste water by commercially available bleaching powder solution as the source of chlorine was done in the laboratory. The effect of chlorine dose was observed. It is found that 1 to 1.5 kg chlorine is required for 1 kg BOD removal. The effect of contact time was also assessed and it is evident that 20 min contact time is enough for chlorination process. The concentrations of Trihalomethane (THM) in the treated effluent were assessed since these disinfection byproducts are suspected to cause cancer, liver and kidney damage in human fetus. The concentration of THM was measured and the presence of THMs was confirmed in this study. However, the concentration of THMs in treated water sample was found below the WHO concentration where the effluents contain a lower value of DOC. The effect of pH and dissolved organic carbon (DOC) and chlorine dose on the concentration of THMs was assessed. It is assessed that trihalomethanes formation increase significantly with increasing pH but it is independent of THM when the concentration of THM is 150ppb and above. The pH value is 8.5 at 150 ppb concentration of THM. Finally curves have been generated in Matlab software to find out the concentration of THM for different chlorine dose, pH and DOC values. The concentrations of THM in the outlet stream of different ETPs can be found out using these curves. The results are very close to the practical measured values.