Sludge, economical disposal

Ocean Disposal. Disposal of raw or treated sludge by barging to sea was practiced for many years by some coastal cities, but today is highly controversial, and it appears that this method will be no longer economically feasible. Federal regulations require that sludge be taken to disposal sites about 160 km from the coast, whereas formerly, disposal sites were permitted within 20 km offshore. Transportation costs are expected to be so high that ocean disposal will be discontinued. 

Many of the waste streams from U.S. process industries are water containing small quantities of metal ions that the law requires be removed before the wastewater is disposed of There is an economic incentive to recoup at least some of the cost of wastewater treatment by recovering and selling the metal content instead of merely disposing of the metals as sludge. Because the waste streams are dilute in desired materials, research is needed to devise efficient extraction and separation processes. 

The rising cost of scrap and waste disposal, the scarcity of onsite landfill space, and potential environmental liabilities make it an economic necessity to recover iron units from dust and sludge. However, recycling to the blast furnace raises the hot metal phosphorus content to undesirable levels. 

Sludge formed during initial steps of the treatment process (passive oxidation and neutralization) may affect the overall performance of the plant, especially when they are leached through the biological cells. Therefore, their disposal, or their recovery, as they may have economic value (Hedin 2003), is an issue of concern in the management of the treatment plant. 

Recovery economics depend upon the characteristics of the filter cake, sludge generation rates, and present disposal costs. Small generators will best be served by central recovery plants. Larger generators will find it more economical to install their own recovery systems. 

The disposal of low-value waste metals at hazardous dumpsites continues, principally because there are currently no economically viable alternatives. Conventional precipitation and filtration techniques are widely used by metal fabricators to remove and concentrate metals from their waste water streams. In some cases, the sludges are subsequently dried to reduce weight before transportation and disposal, or to comply with solids content regulations. The concentrated wastes are shipped in drums, dumpsters or heavy plastic bags to dumpsites. 

Many electronic companies in the State of California create a substantial volume of arsenic waste through the processing or manufacture of gallium arsenide (GaAs) microchips. Since the concentration of arsenic in wastewaters or sludges usually exceeds disposal limits for sewer and municipal landfills, these wastes are quite often treated and disposed to a Class I hazardous landfill. This practice is undesirable environmentally and economically as a long-term mechanism for waste disposal. 

Each industrial chemical process has as its objective the economical production of a particular primary product. It is frequently true that, in attaining this objective, one or more by-products may become available. If these by-products can be disposed of at a profit, this serves to decrease the overall cost of operation and to permit the sale of the primary product at a lower, more favorable, competitive price. Thus, the cost of electrolytically refined copper is dependent on the recovery and sale of the by-products—silver, gold, platinum, and palladium. These precious metals are recovered in large quantities from accumulated anode sludges. Fully one-fourth of the total production of silver, about one-eighth of the gold, and lesser quantities of platinum and palladium are obtained as by-products of the electrolytic refining of copper. 

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