Metal Removals From Contaminated Acidic Water by Using Physico-Chemical Processes

摘要

Trace metals in streams and potable waters are a matter of continuingrnconcern to the aquatic environment and public health. Protection of sensitive aquaticrnlife may require that metal concentrations be lower than those permitted in potablernwater. Accordingly, both effluent standards and stream standards have beenrnpromulgated for a number of metals. Industrial wastes are the most common andrnsignificant source of metal contamination in water bodies. This study investigated thernconventional physico-chemical treatment technologies for removing aluminum, arsenic,rncopper, mercury, and selenium from a highly acidic (pH 2.7) ground water which wasrncontaminated due to the past industrial activities. A bench-scale study was conductedrnto test pH adjustment technologies and metal removal processes. The processesrninvestigated include chemical coagulation/flocculation followed by sedimentation, dualrnmedia (sand/anthracite) filtration, and granular activated carbon (GAC) adsorption. Limernand caustic soda were used for pH alteration. Alum and Iron coagulants were jar testedrnfor their effectiveness in metal removals at different dosages and pH range. The testrndata indicates that lime addition is not only effective in pH adjustment but also inrnprecipitating out almost all the target metals. Caustic soda is shown to be as effective asrnlime for pH adjustment. However, caustic soda's cost and handling will be higher. Inrnaddition, lime provided added benefit of imparting alkalinity. The jar test result showedrnthat both Iron and Alum coagulants are effective in metal removals. However, forrnselenium removal further investigation is recommended. It was also found that chemicalrncoagulation/flocculation/sedimentation process alone is sufficient to reduce almost allrnthe target metals to the required effluent limits. The additional treatment processes,rndual filtration and adsorption, may not be needed for polishing the effluent. The existingrnwater treatment plant practices at the industrial facility were looked into and somernrecommendations were made for better and effective operation. Installation of a limerndose monitoring equipment and a proper settling tank with sludge removal mechanismrnmay be considered.