ABSTRACT
810Under sponsorship of Water Environment Federation, the standard procedure for chemical oxygen demand (COD) determination as outlined in the APHA/AWWA/WPCF Standard Methods for the Examination of Water and Wastewater was simplified for use by the waste treatment plant operators and researchers. Specifically, the principles, analytical procedures, and waste management processes of dichromate reflux method for rapid COD determination in laboratories are presented and discussed. Proper handling and treatment of spent COD liquid wastes for safe disposal are discussed and emphasized in this chapter. The spent COD testing wastewater contains high concentrations of sulfuric acid, mercuric ions (Hg2+), silver ions (Ag+), and dichromate salts, which are considered to be hazardous wastes and must be handled properly by the operators, chemists, and engineers in the laboratories. Normally a laboratory is a conditionally exempt small quantity generator if it generates no more than 100 kg of hazardous waste in a calender month. Such facilities may be allowed to treat hazardous wastes on-site. This chapter discusses several on-site treatment processes for possible adoption by small laboratories. It is important to note that even if federal regulations allow on-site treatment of COD testing wastewater, the State or local regulations may prohibit it. Hazardous sulfuric acid in the spent COD waste can be neutralized by sodium hydroxide or equivalent alkaline chemicals, such as potassium hydroxide; sodium hydroxide is cheaper. Before neutralization of COD wastewater (18 M hydrogen ion) by concentrated sodium hydroxide solution, the wastewater should be diluted with water by at least sixfold. With the sixfold dilution and adequate mixing, neutralization process increases wastewater temperature to about 40°C when sodium hydroxide is added to acidic COD wastewater carefully over a 5-min period to avoid boiling. The COD wastewater must be diluted by a factor of 6 also to avoid crystallization and precipitation
of Na2SO4–10H2O. When Na2SO4–10H2O does precipitate, a larger dilution factor is needed. In case the laboratory hesitates to process the COD testing wastewater by neutralization with sodium hydroxide, the authors recommend that the laboratory manager store all COD testing wastewater in a 55-gallon drum for off-site treatment by a licensed commercial hazardous-waste handler instead. The recent price for commercial disposal of a 55-gallon (208-liter) drum of COD wastewater is US$7,250. If the laboratory manager has confidence to conduct his/her own on-site neutralization process, then the following heavy metal removal processes for removal of chromium, mercury, and silver can be carried out by the laboratory personnel for significant cost saving. The dichromate
Cr2O7 2- in the COD reagent oxidizes most organic substances nearly quantitatively under high temperature and in the presence of concentrated acid. If final detection of COD is done by titration, all chromium should be in the form of Cr3+. The trivalent chromium ions can easily be precipitated as insoluble chromium hydroxide by the above chemical neutralization process using sodium hydroxide (but not calcium hydroxide, nor calcium oxide) at pH >7. The precipitated chromium hydroxide sludge can be removed by filtration. If final COD detection is by colorimetry, there will still be
unknown amount of Cr2O7 2- ions left. The hexavalent chromium in Cr2O7 2- can be reduced to trivalent chromium Cr3+ with any reducing agents (such as industrial grade ferrous sulfate, or steelmaking by-product ferrous chloride) at pH below 3, that is, before the COD wastewater is neutralized with sodium hydroxide. After chromium reduction process is complete, ferrous ions are converted to ferric ions at the same time. Both trivalent chromium ions and trivalent ferric ions can then be easily precipitated by sodium hydroxide as chromium hydroxide and ferric hydroxide, respectively 811(brown sludge), at pH >7. The chemical reduction and neutralization process can reduce chromium with >99.99% efficiency. The effluent soluble chromium concentration after chemical precipitation, sedimentation, and filtration can be <100 μg/L. The pH range for minimum chromium solubility is 7–11. Neutralizing the sulfuric acid with sodium hydroxide and adjusting the pH to >5 causes the precipitation of mercury oxide (HgO), which removes 96% of the Hg. HgO precipitation may be useful as either pretreatment or the first step of a multistep processing method to treat COD testing wastewater. Neutralizing the sulfuric acid and precipitating most of the mercury may lower the price charged by a hazardous waste service to dispose of the treated wastewater. Straight-chain aliphatic compounds are oxidized more effectively when silver sulfate (Ag2SO4) is added as a catalyst in the reagent. Accordingly there are toxic Ag+ in the COD wastewater. Silver has been removed from the United States Environmental Protection Agency (USEPA) priority-pollutant list, but it is still regulated by many local governments. The Urbana-Champaign Sanitary District (UCSD), IL, for instance, has a silver discharge standard of 0.3 mg/L. It is known that Ag+ can be precipitated by chloride ions (or any halides), and thus, its catalyzing effect in COD tests can be significantly reduced. For chloride concentrations up to 2000 mg/L, the chloride interference in silver ion’s catalyzing effect can be overcome by adding a large excess of Hg2+ to complex the chloride. Both Ag+
and Hg2+ can be removed together with sodium sulfide (Na2S) or equivalent. Regardless of the final COD detection method being used (titration or colorimetry), toxic soluble mercury and silver in the COD wastewater can be effectively removed by sulfide precipitation, sedimentation, and filtration. In the wastewater treatment process system, excess Na2S is added to precipitate soluble mercury and soluble silver as HgS and Ag2S, respectively, and then excess zinc ion (Zn2+) is added to precipitate the excess sulfide ion (S2-). Addition of Zn2+ reduces concentrations of H2S at low pH values and concentrations of Hg and Ag at high pH values to very low levels. Addition of Zn2+ (such as ZnCl2) after Na2S allows complete reaction of Hg2+ and Ag+ with HS- before ZnS precipitation. The final mercury concentration in the filtrate is reduced to <3 μg/L μg/L (which is the mercury discharge standard for the UCSD, IL), representing a 99.999% reduction. The UCSD silver discharge standard of 0.3 mg/L is also met. Chemical precipitation of Cr(OH)3, HgS and Ag2S sludge is a rapid process. The precipitated and settled wastewater can be filtered within minutes of pH adjustment or chemical addition. The treated wastewater can be discharged to the laboratory sewer (with permit,
if required), and the very small amount of Cr(OH)3, HgS, and Ag2S sludge can be delivered to a licensed hazardous-waste handler for further processing or recovery.
