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Title: Laboratory study of chemical coagulation as a means of treatment for dredged material
Authors: Wang, Chun-Ching
Chen, Kenneth Y.
Keywords: Coagulation
Soils--Trace element content
Contaminated sediments
Dredging spoil
Dredged material
Dredged Material Research Program (U.S.)
Publisher: U.S. Army Engineer Waterways Experiment Station
Series/Report no.: Technical Report (Dredged Material Research Program (U.S.)) ; no. Technical Report D-77-39
Abstract: Trace metals, chlorinated hydrocarbons, and biostimulants are found to be enriched in fine particulates of contaminated sediments, When different types of sediments are resuspended, it has been found that the extent of redissolution is very small. However, contaminants attached to the particulates can be transported along the waterways downstream from the disturbed areas. In confined areas, proper treatment and rapid removal of those suspended particulates from the water column will not only lessen the oxygen demand, but will also reduce the input of contaminants into the receiving water. With the exception of sandy types of sediments, total concentrations of trace metals and biostimulants from the resuspension oi contaminated sediments after two hours of sedimentation (without any pretreatment) are mostly in the ppm range. In some cases, these concentrations may exceed the allowable discharged requirement imposed by regulatory agencies. In this study, extensive laboratory experiments were carried out to screen commercially available polymers and conventional coagulants, such as alum and ferric sulfate, for the removal, of suspended particles. It has been found that these conventional coagulants are unsuitable for this use due to the large dosages (in the range of 30 to 40 ppm) and pH control that are required to achieve acceptable effluent quality. There is also the problem of carry-over of trace metals from these conventional coagulants. Furthermore, polymers affect pH to a minor extent, thereby eliminating the need for pH adjustment of the treated effluent. Among fifty types of polymers evaluated in this study, some high molecular weight cationic polymers (such as Betz 1160 and Hereofloc 815.3 and 849) and anionic polymers (such as Calgon WT- 3000 and Betz 1120) were found to be very effective for the treatment of the resuspensions tested in the laboratory. Rapid removal of contaminants was observed immediately after flocculation. When a detention time of a few minutes is provided, the concentration of contaminants in the treated effluent can be drastically reduced from the ppm range to the ppb range. Pertinent factors such as salinity and initial turbidity level of resuspensions were also studied in relation to the optimum dosage of Betz 1160 polymer. It was found that the optimum dosage of a polymer is closely related to the level of salinity and initial turbidity in the resuspensions. A suspension with high salinity and low initial turbidity usually requires less polymer. However, a suspension with higher turbidity is easier to clarify; i.e., fast floc formation. Parameters of gross sediment content such as COD, TOC, and particle size of sediments <5 µm were also found to be well correlated with optimum dosage requirements of Calgon WT-3000, a high molecular anionic polymer. Correlation coefficients of 0.93, 0.95, and 0.97, respectively, were found in this study. Therefore, parameters of contaminated sediment content such as TOC, COD, and particle size of sediments <5 µm could be useful in the initial selection of the optimum polymer dosage. There is no significant difference in the physical properties of resettled sediments with and without polymer treatment in terms of plastic limit, liquid limit, and plasticity index. However, there is a slight increase in the coefficient of permeability for polymer-treated sediments. An assessment of long-term mobilization of chemical constituents from polymer-flocculated particles was also conducted to gain additional insight concerning the possibility of release of contaminants to the water column. In most cases, the polymer-treated particles do not show a significant difference in the release of contaminants and stimulants from that of the untreated sample.
Description: Technical Report
Gov't Doc #: Technical Report D-77-39
Rights: Approved for Public Release; Distribution is Unlimited
Appears in Collections:Technical Report

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