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Lead and chromium as limiting factors in recycling textile industrial sludge as an amendment in crop production
Abstract
Textile and dyeing industry is one of the leading industries in Tamilnadu state of India. The coagulant process followed for treating the wastewater generated from this industry leads to sludge accumulation with a production of daily average of 10 tonnes in each Common Effluent Treatment Plant (CETP). As the sludge contained 17.2 per cent of Ca, its ameliorative potential as an amendment alternate to gypsum was assessed in a pot experiment with groundnut (Arachis hypogea) as a test crop. However, the sludge also contained high content of Pb (95.2 mg kg-1) and Cr (89.5 mg kg-1) and hence their accumulation pattern in both plant and soil was also assessed. Unlike Cr, there was no DTPA extractable Pb detected in soil. Surprisingly, there was no decrease in the total extractable Pb, in spite of increased uptake of Pb with increase in the levels of applied sludge. The uptake of nutrients in different parts of crop indicated a general trend of highest uptake in haulms. The uptake of Cr was least in kernel and also low content compared to that of Pb. Though the content of Pb and Cr in the edible part namely kernel is low on comparison with haulm, yet the content itself is a concern when health hazards are considered.
Introduction
Tiruppur city in Tamilnadu state of India is famous for hosiery and textile business. The effluent discharged from these industries contains high TDS, appreciable amount of toxic constituents like heavy metals and dyes and thereby led extensive contamination of land and water. Hence to overcome such problems, 8 CETPs (Common Effluent Treatment Plant) were set up in Tiruppur city alone. One of the treatment processes is coagulation wherein the use of lime and ferrous sulphate led to accumulation of sludge, the disposal of which is a great risk compared to effluent discharge. With this view, a study was attempted, since, once the value of sludge is explored, there will be demand to utilize it and thereby such solid waste pollution can be overcome. Land application of solid wastes provides an effective and environmentally acceptable option of waste disposal, which also recycles valuable nutrients in to the soil and plant system. The usefulness of phospho-gypsum (PG), a solid waste containing gypsum content of 96.24 % generated in phosphoric acid plant has been internationally (Richcigl, 1995), nationally (Tandon, 1995) and regionally (Kalyana Sundaram et al., 1996) well documented.
Sludge containing heavy metals may affect crop growth, quality of crop produce and cause health hazards to organisms subsequently through food chain. Heavy metals associated with land application of sludge significantly reduced rhizobial diversity (Hirsch, 1996) and reduced nitrogen fixation (Graham and Vance, 2000). In the present study, while evaluating the efficiency of CETP sludge as source of Ca and S and as an amendment, the transformation of Pb and Cr in soil and their uptake by crop were also studied.
Materials and Methods
A pot culture experiment was conducted with groundnut (variety VRI 2) as test crop to assess the potentiality of CETP sludge as a nutrient source and also an ameliorant. Based on the Ca content in the common recommendation of gypsum which is 400 kg ha-1 for groundnut crop, 100 % sludge was calculated on equating its Ca content with that of gypsum. As sludge is a complex mixture and also contained heavy metals, it was tried at various levels i.e. 0, 50, 75, 100, 125 and 150 % (denoted as S0, S50, S75, S100, S125 and S150, respectively) and compared with gypsum application(Gp). The experiment was conducted in a design of CRD with treatments replicated thrice.
The initial and post harvest soils (PHS) were analysed for DTPA and total extractable Cr and Pb and other parameters. Similarly, the plant parts namely haulm and shell collected at harvest stage were analyzed for content of Cr and Pb and also their uptake by plant.