Assessing the Suitability of Sewage-Sludge Produced in Kolkata, India for Their Agricultural Use

In this study, the suitability of sludges produced in Kolkata, India for their agricultural usage has been assessed by analyzing the plant nutrients availability, heavy metals loads, and their mobility, in terms of the rate of release and potential negative effect on soil and plants by using sequential chemical extraction procedures. Results indicated that the sludge produced at different sewage treatment plants (STPs) of Kolkata were rich in organic C (9.75-15.88%) and major nutrients mainly N (2.05-3.87%) and P (1.62-2.47%); but the K content (0.98-1.96%) was much lower. The concentrations of Zn (1332.1-2584.6 mg kg(-1)), Pb (309.6-608.5 mg kg(-1)), Ni (125.5-388.5 mg kg(-1)), Cu (136.6-451.7 mg kg(-1)) (excepting Bhatpara and Chandannagar) and Cd (3.22-10.09 mg kg(-1)) (excepting Kamarhati and Garden Reach) in sludges of all the sites exceeded permissible standards as recommended by the Central Pollution Control Board (CPCB), India and should be restricted for use in agriculture. Speciation study also revealed that a considerable portion of Zn, Cd, Pb and Ni were distributed in the exchangeable, MnO2 and sesquioxide bound fractions, indicating high bioavailability and potential toxicity to the environment, while Cu was primarily distributed in the organically bound fraction. This study therefore suggests that the sludges produced in Kolkata, should not be used directly in agricultural purposes unless subjected to bioremediation or chemical remediation for reducing heavy metals load to a permissible level.