Evaluation of Removal Efficiencies of Various Coagulants and Disposal of Sludge in Treatment of Arsenic Contaminated Ground Water Sources of West Bengal, India

摘要

Due to the sporadic incidence of arsenic related health consequences, there has been a global concern to initiate formulation of rules, regulations and acts to control inadvertent and advertent contamination of arsenic in water resources intended for human consumption. USEPA is evaluating the occurrence of arsenic, the cancer and non-cancer health risks associated with ingestion of inorganic arsenic, the essentiality (i.e. health benefit) of arsenic, practical issues concerning the routine analysis of arsenic by utilities, the availability and the costs of treatment technologies for removal of arsenic from source waters for developing the new MCL. The World Health Organization (WHO) revised the guideline value in 1993 (WHO, Guideline for Drinking Water Quality, Recommendations, 1993) and set a new provisional guideline value for arsenic as 10 μg/L. It is the most contentious and costly to implement it in most of the developing countries due to less developed technologies for removing arsenic from contaminated water. In the backdrop of serious health effects due to arsenic poisoning and lowering of guideline value for arsenic by various regulatory agencies an urgent need was felt to develop simple, efficient and cost effective treatment for removal of arsenic from drinking water. The paper presents extensive treatability studies on arsenic contaminated ground water samples from West Bengal, India. Arsenic removal studies using arsenic contaminated ground water from West Bengal and various coagulants such as ferric chloride, ferric sulphate, alum and poly aluminium chloride (PAC Ac/190) were carried out. The studies revealed that the dose of ferric chloride required for removal of arsenic from ground water is less than that of ferric sulphate. In case of coagulation with alum and PAC, higher removal efficiencies were observed with increasing dose of alum whereas for the same initial arsenic concentration, the PAC dose was half that of alum for complete removal of arsenic. The sludge generated in arsenic removal treatment process viz. coagulation (by ferric salt), flocculation, sedimentation and filtration requires further treatment before its disposal. Arsenic bearing sludge was obtained from Water Treatment Plant in the arsenic affected areas of West Bengal. The solidification/stabilization process was selected for disposal of arsenic bearing sludge in the environment. The main objectives of the sludge disposal included characteristics of the arsenic bearing sludge, solidification of sludge using various binding materials, stabilization of sludge for reducing mobility and solubility of contaminants from the solidified/stabilized sludge, determination of compressive strength (UCS) of S/S cubes, examination of leaching of arsenic through S/S cubes through Toxicity Characteristic Leaching Procedure (TCLP) Test as per the protocol stipulated by USEPA, effect of pH on leaching of arsenic in the S/S cubes and optimization of water/binder and binder/sludge ratios. Dynamic, static and semi-dynamic leach tests were performed to assess the equilibrium of the solidified/stabilized cubes and the leaching medium. With all the test cubes, unconfined compressive strength (UCS) was found greater than 4 Kg/cm2 after 28 days of curing and satisfied the minimum guideline value of 50 psi as stipulated by USEPA.