Sludge toxicity, reduction

Wong and Maroney [49] reported on a pilot plant comparison of PACT and extended aeration (activated sludge) for treating petroleum rehnery wastewater. Results indicated that although both processes performed similarly in COD removal, only the PACT system yielded an effluent meeting the discharge requirements for whole effluent toxicity reduction. Similar results in toxicity reduction have been reported for wastewaters from other industries [50]. 

Asano, T., ed. 1998. Wastewater Reclamation and Reuse. Lancaster, PA Technomic Publications. Experts from around the world contributed to this useful and unique text that analyzes and reviews aspects of wastewater reclamation, recycling, and reuse in countries around the world. This is volume 10 of an 11-volume series, the Water Quality Management Library, which thoroughly addresses issues in wastewater treatment, sludge, nonpoint pollution, toxicity reduction, and groundwater remediation. 

Adequate ventilation is necessary for aH process lines to ensure worker safety. Electroless copper baths must have good ventilation to remove toxic formaldehyde vapors and caustic mist generated by the hydrogen evolution reactions and air sparging. Electroless nickels need adequate ventilation to remove nickel and ammonia vapors. Some states and municipalities requite the removal of ammonia from wastewaters. A discussion of printed circuit board environmental issues and some sludge reduction techniques is avaHable (25). 

As the mere disappearance of the parent pollutants cannot be considered as complete degradation and in some opportunities degradation-intermediates can be even more toxic than their parent compounds, a global estimation of toxicity was necessary to determine the reduction of toxic potential in the fungal-treated sludge. The standardized tests of acute immobilization of Daphnia magna and bioluminescence inhibition of Vibrio fischeri were accordingly applied. 

Anaerobic bioreactors have been used since the 1880s to treat wastewaters with large amounts of suspended solids. However, anaerobic reactors are sensitive to toxic pollutants and vulnerable to process upsets, and have been used mainly for municipal sludge digestion. For methane production the sequential metabolism of the anaerobic consortia must be balanced, and the methanogens in particular are vulnerable to process upsets. Recently, anaerobic-aerobic processes (Figure 1.1) have been developed for the mineralization of xenobiotics. These processes take advantage of an anaerobic reactor for the initial reductive dechlorination of polychlorinated compounds or the reduction of nitro substituents to amino substituents. If the reduced compounds are more readily mineralized in an aerobic reactor, an anaerobic-aerobic process is feasible. 

Many different chemical treatment systems have been developed to reduce the leachability of lead and cadmium compounds in flue dust. These systems usually rely on stabilization/solidification, adsorption, chemical reduction, or pH control. Chemical reduction employing the use of metallic iron has been successful in reducing the leachability of lead to below EP-Toxicity levels. Adding a 5 percent by weight dose of iron filings to cupola furnace emissions control sludge, for instance, reduced lead leaching from 28.6 mg/1 to less than 0.1 mg/1 (Stephens 1984). 

While hexavalent chromium is reduced to its trivalent form in treatment systems mainly so that the metal can be precipitated, this also lowers its toxicity by a factor of 1000. Ferrous sulfate can be used for this reduction, but is not popular due to its inefficiency, high sludge generation rate, and expense. Sulfur dioxide gas is a far more economical reducing agent, although it is efficient only at low pH, preferably below 2. There can also be problems with atmospheric emission of S02 in this process. 

Another study examined the PAC-activated sludge process (14). The addition of powdered carbon directly into mixed liquor in an activated sludge plant aeration basin is referred to as the PACT process. The benefits attributed to this system are numerous, such as improved BOD and COD removal by adsorption, adsorption of color and toxic agents, reduction of aerator and effluent foam, and so on. The goal of the PACT process is to improve organic removal or to improve sludge settling characteristics. 

The economics of powdered carhon addition are difficult to assess. While the use of carhon might appear to increase material cost, reduction in total expense may, in some cases, be realized through savings on coagulants, sludge handhng chemicals, and so on. In other cases, the cost of carhon can he justified if it provides treatment protection from hydraulic or toxic overloads. 

The removal of six-valent chromium from an electrolyte is a special problem. Its toxic compounds (e.g. CrC>42 ) remain in the dissolved state, though a fraction of chromium is adsorbed by the sludge. To transform dissolved chromium into sludge, substances converting Cr(VI) into Cr(III) are added to the spent electrolyte. Na2SC>3, NaHSC>3, and FeS04 are commonly used as the reductants. By introducing barium salts into the electrolyte, Cr(VI) can be directly precipitated in the form of insoluble BaCrC>4. A fraction of Cr(VI) is reduced in the electrolyte by the reaction with Fe2+ and N02- yielded by the cathodic reaction (5). 

The sludge contains POPs that were not digested in the initial aerobic treatment. The sludge is pumped to a second tank where it is subjected to heavy treatment with ozone. The ozone is toxic to the aerobic bacteria, but it attacks the POPs and converts them to biodegradable compounds. After ozone, treatment the sludge is held until all the ozone reacts or decomposes. When free from ozone the sludge is returned to the primary aerobic digestion tank for further reduction. 

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