Wastewater Management in Cotton Wet Processing in Thika Cloth Mills in Kiambu County, Kenya
Waithaka, Alice Wambaire
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Wastewaters generated from textile industry are usually variable in composition, difficult to biodegrade and have been classified as potentially harmful to the environment. Research on the physico-chemical characteristics and concentration levels of the potentially hazardous substances in these wastewaters is apparently lacking. This study evaluated the physico-chemical characteristics of polluting substances in raw wastewater from cotton wet processing and pretreated wastewater in Thika cloth mills, in comparison with National Environment Management Authority (NEMA) discharge limits to the sewer.The key pollution indicator parameters were; colour, dissolved oxygen, temperature, turbidity, pH, conductivity, biochemical oxygen demand, total suspended solids, chemical oxygen demand, total dissolved solids, nitrates, phosphates, phenols, sulphides,chromium, manganese, aluminium, cadmium, lead, Zinc and copper. Two sampling techniques namely purposive and professional judgment were used in this study. Samples were collected at two weeks intervals for a period of three months, between May and August 2013. Field measurements were taken for each of the 33 samples collected from wet processes namely; scouring, desizing, bleaching, washing, mercerizing, dyeing, printing, finishing and municipal sewer. Analysis of the 22 indicator parameters was carried out at Kenya Industrial Research and Development Institute analytical laboratory using APHA standard methods of testing. Descriptive analysis of the data for means and standard deviations was done with the aid of Statistical Package of Social Sciences and Excel. Results, showed that, the untreated wastewater discharged from the cotton wet processes streams was characterised by alkaline pH (9.992),high; temperature (43.406oc),COD (5231.83mg/l),BOD (2160.13), TSS(2979.7 mg/l),TDS 24339.78mg/l),colour (1219.71 hazens) and sulphides (81.233mg/l) against Nema standards at pH(6-9),temperature(20oc-35oc),COD(1000mg/l), BOD(500mg/l),TSS (250mg/l),TDS (2000mg/l), colour (<40 hazens) and sulphides (2mg/l). Other parameters including zinc, lead, nitrates, phenols, phosphates, chromium, cadmium and copper were within the Nema standards. The study hypotheses were tested using the Mann-Whitney U test for two independent groups. There was a significant difference in concentration of polluting substances in the dyeing wastewater as compared to Nema standards where Mann-Whiney U value Z (-0.226) was less than α (0.05). Pre-treated wastewater exhibited TSS (423.5 mg/l), TDS (3842mg/l), color (200 hazens) and sulphides (19mg/l) in excess of Nema limits, while other parameters were within the discharge limits. When compared to the untreated wastewater under Mann-Whitney U test, the pretreated wastewater exhibited a low concentration of polluting substance whereby its mean rank (16.21) was lower than (22.79) for untreated wastewaters. This was an indication of the effectiveness of the pretreatment process. Similarly, the null hypothesis that there was no difference in concentration levels of the polluting substances in pretreated wastewater compared to Nema standard was rejected where Mann-Whitney U value Z (-0.829) was less than α (0.05) and the mean ranks (15.30) for pretreated and (17.88) for Nema standards. This was an indication of suitability of the pretreated wastewater for discharge to the sewer. However, there is need for advanced research, to determine the volumes and the toxicity levels of the textile effluent. Other effluent streams discharging to the sewer need to be studied to determine their potential to pollute the municipal water resources.