Heavy Metals and Pathogens Pollution of Soils and Crops Irrigated With Domestic Waste Water

Abstract

Heavy metals in terrestrial ecosystems are associated with adverse environmental impacts. The extent of heavy metals in domestic waste waters, their effects on soil, crops, and soil biology were investigated. Waste water analysis revealed pH (10.3), electrical conductivity (ECe; 3.87 dSm-1), sodium adsorption ratio (SAR; 36.2), biological oxygen demand (BOD; 850 mg L-1), and chemical oxygen demand (COD; 1590 mg L-1) were well above the standard limits of Food and Agriculture Organization (FAO) and National Environmental Quality Standards (NEQS). In contained, iron (Fe; 15.4 mg L-1), zinc (Zn; 5.83 mg L-1), copper (Cu), nickel (Ni), manganese (Mn), lead (Pb), cadmium (Cd) and chromium (Cr) exceeded standard limits. In tube-well water, concentrations of Fe, Zn and Mn were below the standard limits, while Cu, Pb, Cd, Cr and Ni were undetectable. Waste water from irrigated soil showed greater concentrations of Zn (59.8 mg kg-1) and Cd (7.11 mg kg-1), while tube-well irrigated soil lacked Zn contents and had Cd below standards. With the exception of Zn and Cd, all of them were within the limits. The bacterial counts of Escherichia coli (590) and Salmonella species (630) were higher than the allowed limit. Plant analysis data showed the highest (395 mg kg-1) and the lowest (21.1 mg kg-1) Fe content in the shoots of eggplant and lettuce, respectively. Maximum Cu (254 mg kg-1), Pb (184 mg kg-1) and Zn (207 mg kg-1) were noted in the shoot of maize (Zea mays) and the minimum in the shoot of lettuce. Likewise, 13.4 mg kg-1 Cd and 2.48 mg kg-1 Cr were noted in berseem shoot and the lowest Cd and Cr, in the shoot of spinach and wild oat, respectively. But these were slightly above the limits in spinach and wild oat. The highest Mn (12.1 mg kg-1) and Ni (9.12 mg kg-1) were noted in the shoot of maize, while the lowest and within the limits were values noted in the shoot of lettuce and ladyfinger respectively. Moreover, the maximum (171 mg kg 1) and the minimum (21.1 mg kg 1) values of Fe were recorded in the root of maize and lettuce roots, respectively. These values were much lower than the standards. Highest concentrations of Cu (209 mg kg-1), Pb (157 mg kg-1) and Zn (179 mg kg-1) were noted in the root of maize while lowest values of Cu and Pb were found in the root of lettuce, and Zn in the root of berseem. Pb contents of 23.3 mg kg 1 were noted to be higher whereas, Cu (19.1 mg kg 1) and Zn (21.9 mg kg 1) were found within the limits. Highest Cd (13.0 mg kg 1) in ridged gourd root and lowest Cd (0.03 mg kg 1) in spinach root was observed. Similarly highest (Cr 1.97 mg kg-1) in the root of ridged gourd and lowest and within the limits was Cr (0.09 mg kg-1) in the wild oat root. Highest Mn contents (i.e. 3.67 mg kg 1 and 13.1 mg kg 1) and lowest (i.e. 0.51 mg kg-1 and 13.0 mg kg 1) were noted in cauliflower and spinach root, respectively. Similarly highest (3.85 mg kg 1) and lowest (0.87 mg kg 1) Ni concentrations were noted in the root of maize and spinach, respectively. In the second phase, a two-year pot experiment was conducted to observe the effects of tube-well water and different concentrations of waste water viz., 20%, 40%, 80% and 100% on Brassica napus. A decline of 61.5% occurred in the number of pods per plant when 100% waste water was used (T4). Similarly, pod length (59.7%); number of seeds per pod (42.5%); number of seeds per plant (82%); seed weight per plant (88%); 1000 seed weight (19.5) and straw yield (54.2%) were decreased in T4. There was 82% and 88% decrease in the number of seeds and seed weight per plant, respectively compared to control. Over all, a 60% decrease in dry matter in the first year and 4.83% decrease in the second year occurred. In third phase, Zea mays and Brassica napus were used for phyto-extraction of Cu, Pb and Zn from soil. A pot experiment was conducted in which treatments included tube-well water (T1), waste water (T2), and 300 mg kg-1 and 600 mg kg-1 soil each of Cu, Pb and Zn (T3), and T4 respectively. Compared to Brassica napus, maize (T2) retained maximum Cu (164 mg kg-1), Pb (259 mg kg-1) and Zn (271 mg kg-1) in shoot and 26.8 mg kg-1, 130.5 mg kg-1 and 61.1 mg kg-1 in root, respectively. Brassica napus absorbed Cu (117 mg kg-1), Pb (187 mg kg-1) and Zn (180 mg kg-1) in shoot and 40.0 mg kg-1, 81.7 mg kg-1 and 45.4 mg kg-1 in root respectively for treatment T2. Maize and Brassica napus had lowest dry matter yield of 66.3 and 31.6 g, respectively, for treatment T4. Finally, waste water was filtered through sorption media, viz., sawdust (T1), lime powder (T2), wood charcoal (T3) and sand (T4) placed within PVC pipe. Highly significant differences with respect to sorption of heavy metals Cd, Cr, Fe, Ni, Pb and Zn were noted. Also non-significant differences were observed for Cu and Mn when filtered through sawdust, lime powder, wood charcoal and sand. Sawdust retained maximum Cd (0.27 mg L-1), Cr (0.807 mg L-1), Fe (6.47 mg L-1), Ni (1.753 mg L-1), Pb (1.65 mg L-1) and Zn (2.680 mg L-1), respectively. Lime gave similar results as with sawdust and retained Cd, (0.237 mg L-1), Cr, (0.66 mg L-1), Fe, (5.75 mg L-1), Ni (1.70 mg L-1) and Pb (1.72 mg L-1), respectively. Wood charcoal, with minimum sorption efficiency, retained lesser quantity of almost all the heavy metals. It can be concluded that waste water could be used as a second option for irrigation where no canal water or Tube-Well water is available. Prolonged waste water irrigation can raise the magnitude of heavy metal pollutants in soil and crops. For the purpose of remediation, although bioremediation is a time consuming but is the cheapest method compared to physical removal of the pollutants from soil and it can be suggested that other crops which are grown for commercial purposes and neither used food nor feed can best be trialed. In the last various media used for the treatment of waste water though on small scale, proved effectiveness and the results were encouraging but it can be suggested that other locally available organic materials should be tested.