Characterization, in situ treatment of sewage effluents and their impact on soil micronutrient and heavy metal concentrations

G.Ramesh, M.V Shantaram, K.L. Sharma* and J.Kusuma Grace *

Acharya N.G Ranga Agricultural University, Hyderabad, India

*Central Research Institute for Dryland Agriculture, Hyderabad, India

Increasing industrialization essential for sustaining economic and financial health of countries has accentuated the demand for water.  Because of this, the quantum of water available for human beings and agriculture is dwindling day by day leads to water scarcity. Added to this, the trend towards commercial agriculture has further aggravated water scarcity in agriculture. This has necessitated the need to reuse and recycle both domestic sewage and industrial wastewaters in agriculture.  If we use these wastewaters for agriculture without treatment may affect soil, crops finally human and animal health. Hence, study was under taken to investigate the characteristics of sewage effluents generated in Hyderabad city, India, its in situ treatment and their impact on plant and soil trace elements and heavy metal concentrations. Sewage effluent samples were collected from main sewage canal in duplicate at weekly intervals over a period of 12 months i.e all three seasons and analyzed for various chemical characteristics including pollution causing parameters by the standard methods of analysis. The impact of sewage water on vegetable, fodder and crop plants was studied. In situ sewage treatment technology was developed by construction of filter beds to reduce the pollution load of sewage water.  Impact of leachate collected from filter beds and sewage water on vegetables, fodder crop and on soils was investigated.

Characterization of sewage effluents revealed its own distinct characteristics, reflecting the kinds of pollutants that are entering into its stream from domestic as well as industrial units. The sewage water generated at Hyderabad were within safer limits for irrigation in case of pH, EC, Dissolved Oxygen (DO), Biochemical Oxygen Demand (BOD), Total Solids (TS) and B, but Total Dissolved Solids (TDS), Total Suspended Solids (TSS), Cl^-, SO₄^-2, Na, Ca and Mg were beyond the permissible limits prescribed by Bureau of Indian Standards (BIS).  Among trace elements and heavy metals except Mn, Cd and Cr all others constituents were within critical limits and the characteristics were varied among seasons.  In general, pollution causing parameters observed during summer were high. Plant uptake studies showed high amounts of heavy metals in fodder grasses compared to vegetables and crop plants, which were cultivated for periods of time (> 30 years) without replanting.   Analysis of leachates collected after in situ treatment of sewage water revealed a significant reduction in pollution causing parameters.  The BOD (raw sewage: 373 mgl^-1, leachate: 76.8 mgl^-1) COD (raw sewage: 714 mgl^-1, leachate: 143.2 mgl^-1) trace elements and heavy metal contents were decreased significantly with corresponding increase in DO (raw sewage: 4.8 mgl^-1, leachate: 7.0 mgl^-1) as compared to raw sewage.  Among crops tested on filter beds, paragrass (Brachiaria mutica) was found to be better in case of uptake of trace elements and heavy metals with high dry mater yields compared to panicum (Panicum maximum) and maize (Zea mays).  Analysis of trace elements and heavy metals in crops grown with leachate and raw sewage water showed that crops grown with leachate from the filter beds had less of trace elements and heavy metals compared to raw sewage irrigated crops.  The trace elements and heavy metals accumulate more in roots than in foliage and stems except in radish. Soil analysis after irrigation with leachate and raw sewage water clearly indicated decreased accumulation of trace elements (Fe: 86.3, Cu: 4.21, Mn: 81.3 and Zn: 9.67 mg kg^-1) and heavy metals  (Pb: 5.66, Ni: 1.64, Cd:0. 142 and Cr:0. 361 mg kg^-1) in leachate irrigated soils compared to raw sewage irrigated soils (Fe: 93.6, Cu: 4.78, Mn: 87.2 and Zn: 11.74 mg kg^-1) (Pb: 6.28, Ni: 1.73, Cd:0..147and Cr:0. 393 mg kg^-1). Overall there is need to treat the raw sewage water to reduce the pollution hazards of its use for irrigation in the long run.