Solids precipitation, effect disposal

Heavy metals often can be removed effectively by chemical precipitation in the form of carbonates, hydroxides, or sulfides. Sodium carbonate, sodium bisulfite, sodium hydroxide, and calcium oxide are all used as precipitation agents. The solids precipitate as a floe containing a large amount of water in the structure. The precipitated solids need to be separated by thickening or filtration and recycled if possible. If recycling is not possible, then the solids are usually disposed of to a landfill. 

Disposal of brine in subsurface wells is probably the most widely used control method, especially in the western and southern oil and gas producing states [23]. For this to be an effective disposal option, two conditions must be met the natural aquifer must be naturally saline and must not leak to freshwater aquifers, and the reinjection pressure must not exceed the fracture pressure of the formation [9]. Produced water is usually pretreated to prevent equipment from being corroded and to prevent plugging of the sand at the base of the well. Pretreatment may include the removal of oils and floating material, suspended solids, biological growth, dissolved gases, precipitable ions, acidity, or alkalinity [27]. A typical system is shown in Fig. 12. 

Radioactive metal wastes from the nuclear industry are of increasing concern as the amount of waste to be disposed of increases. Current treatment of nuclear wastewater involves the addition of lime, which is effective in precipitating most metals out of solution with the exception of radium (Tsezos Keller, 1983). Barium chloride (BaCl2) is used to precipitate radium from sulfur-rich effluents as barium-radium sulfate. Other treatment methods include incineration for some solid wastes, and filtration, adsorption and crystallization for liquid wastes (Godbee Kibbey, 1981). 

Compodtion of Waste Stream. The filtrate from the Mn precipitation is the waste stream resulting from this process. It has a pH of 9 and solid content of 17 g/1 of Na2S04. Its impurity content is given in Table XI, which lists the 10 highest contaminants. The concentrations are given in ppm no other impurity was detected at a level greater than 50 ppb. The impurity content of this waste stream is quite low and, if not directly disposable, compatible with standard waste water treatment procedures. The low concentration of the transition metal ions, particularly Fe and Mn, which were major constituents of the feed stream attests to the effectiveness of the separation and recovery processes. 

Organic solvents and solution (separated as halogenated and nonhalogenated) should be disposed in accordance with the locally effective laboratory regulations. Solids obtained in the above procedure may be disposed as nontoxic waste. Fluoride-containing solutions should not be treated with acids. The fluoride content can be precipitated by treatment of the solution with Ca(OH)2 and disposed as solid chemical waste. 

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