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Title: Chemical clarification methods for confined dredged material disposal
Authors: Dredging Operations Technical Support Program (U.S.)
Schroeder, P. R. (Paul R.)
Keywords: Chemicals
Dredging spoil
Dredged material
Dredged material disposal
Water quality
Water purification
Publisher: Environmental Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
Description: Technical Report
Abstract: This report presents the results of verification studies on chemical clarification methods for dredged material disposal developed during the Dredged Material Research Program and provides laboratory procedures and design guidelines for designing treatment systems to clarify the effluent from a dredged material containment area. Guidelines are also presented on operating the systems and estimating the treatment costs. Thirty-five chemicals were screened in laboratory tests to determine which types of polymer were most effective in clarifying suspensions of fine-grained, freshwater sediments. Moderately high molecular weight, highly cationic polyamines performed well on the suspensions tested, which ranged in solids concentration from 0.21 to 2.1 g/l. The required polymer dosage increased with increasing solids concentration. The required dosage decreased as the mixing conditions were improved. Three field demonstrations were performed to determine the technical and economic feasibility of clarification by flocculants and to develop a treatment system that is simple, economical, and adapted to the disposal operation. The treatment system that was developed disperses a concentrated, low viscosity liquid polymer in the effluent at the weir of the primary containment area. The polymer is diluted in-line with supernatant from the containment area as it is being pumped to the weir. The weir structure provides the required mixing for flocculation. A small secondary containment area provides settling time and storage volume for the treated material. Field tests showed that the treatment system was very effective. The solids concentration was reduced from about 2 g/l to 50 mg/l using a polymer dosage of 10 mg/l. The treatment cost for the demonstrations, excluding the cost of constructing a secondary containment area, was about $0.20/yd³ of in situ sediment dredged (1981 dollars). A preliminary laboratory and field investigation was performed to examine the polymer dosage requirements to treat dredged material slurries by injecting polymer into the dredge pipeline. The required polymer dosages were proportional to the concentration of nonsettleable fines in the slurry; that is, the required polymer loading in milligrams of polymer per 1000 Nephelometric Turbidity Units (NTU) of nonsettleable turbidity was the same for treating an influent slurry as for treating a containment area effluent. Therefore, the required dosage, in milligrams per litre, is much higher than for treating effluents. Laboratory jar test procedures are presented to select an effective polymer (including dosage and feed concentration), to determine the mixing requirements, and to determine the relationship between the influent solids concentration or turbidity and the required polymer dosage. Guidelines are presented for designing the polymer feed system, the weir and discharge culvert for mixing, and the secondary containment area for settling and storage. The polymer feed system is sized to deliver the required polymer dosage at the recommended feed concentration for the range of possible flow rates and influent solids concentration. The weir and discharge culvert are designed to maximize the mixing available for a given head loss through the culvert. The secondary basin is designed to provide a sufficient depth and volume of ponding for good settling and also to provide sufficient storage volume whi le minimizing the required area. The object of the design is to use the available depth effectively for mixing, settling, and dredged material storage. The main costs of treatment are for polymer, labor, and construction. The treatment costs are dependent on the specifics of each project: effluent requirements, treatment system design, production rate, polymer selection, and required dosage. The treatment cost, excluding the costs of constructing a secondary cell, would probably range from $0.08 to $0.25/yd³ of in situ material dredged. Higher dredge production rates and lower chemical dosages would lower unit costs. NOTE: This file is large. Allow your browser several minutes to download the file.
Rights: Approved for public release; distribution is unlimited.
Appears in Collections:Technical Report

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