FEATURE
Monitorina with carbon analyzers For water effluents, on-line COD (chemical oxygen demand) measurements are accurate and faster than BOD (biochemical oxygen demand) measurements
M. Louis Arin lonics, Inc., Watertown, Mass. 021 72 Unquestionably, the first water pollution monitoring devices were man's senses of sight, smell, and taste. If the water looked, smelled, or tasted bad, the water was not consumed. However, a more objective measurement of the purity of water was needed if pollution control or the results of pollution control were to be measured.
BOD /CO D One measure of the polluting nature of a material is the determination of the amount of dissolved oxygen that will be consumed by microorganisms in the process of oxidizing the material to a stable form under aerobic conditions. This phenomenon is called biochemical oxygen demand-the robbing of dissolved oxygen from the marine life to oxidize the material. The biochemical oxygen demand (BOD) of an effluent is an important property that should be monitored before allowing it to enter the ecosystem of a river or ocean. Such a criterion for water purity was first established by the English Royal Commission on Sewage Disposal in the late 1800's. The test involved placing a sample of diluted wastewater in a constant temperature environment and measuring the oxygen content of the sample over a period of time. Initially, a 20-day period was selected. The time period was eventually shortened to a minimum of 5 days to allow for the proper control of a sewage treatment plant. BOD measurements not only suffer from lengthy minimum time but from other limitations: 0 separation of insoluble inorganics (fats, oils, hydrocarbons) from the liquid phase that cause low BOD values 0 oxidation of inorganics, such as ferrous iron, sulfide, and sulfite, that cause BOD values 0 presence of toxic materials that cause low BOD values unless properly adapted by a seed organism 0 presence of lignin (major contaminant of wood pulping wastes) that causes low BOD values. Because of these considerations and a reproducibility of only =t20%, other on-line pollution monitoring methods were sought. The chemical oxygen demand (COD) measurement was the next stage in the search for a more rapid and accurate measurement of the level of water pollution. A powerful oxidizing agent, such as potassium dichromate, is substituted for the bacteria which carry out the oxidation in the BOD determination. In the COD determination, the amount of C o n evolved or, more conveniently, the 898
Environmental Science & Technology
amount of dichromate reacted is measured. Because of the more severe oxidizing conditions of the COD determination, the results of COD tests are usually higher than those of the BOD tests. Some organic materials completely oxidized in the COD test are only slowly decomposed by the microorganisms in the BOD test. Like the BOD analysis, COD measurement has limitations including: 0 length of test period-&3 hr 0 slow oxidation of pollutants, such as pyridine, benzene, ammonia, and acetic acid, leading to low results inapplicability of the method to straight chain and aromatic hydrocarbon wastes, such as n-hexane and nheptane, that remain on surfaces and yield low results 0 presence of chlorides that produce high results unless complexed by the addition of mercuric sulfate to the sample before refluxing 0 special handling required of hazardous chemicals 0 reproducibility of 4 ~ 8 % . Many laboratory techniques for measuring the carbon content of water samples have been developed (Hill, H. N., "Carbon Analysis," 23rd Annual ISA Conf., New York, N.Y., October 28, 1968). I t was not until the early 1960's that Van Hall (Van Hall, C. E., et al., Anal. Chern.. 35, 315, 1963) at Dow Chemical and workers at Union Carbide reported the development of simple rapid combustion methods for the determination of organic substances in aqueous solution. These workers initiated a new generation of pollution monitoring instruments capable of realtime pollution monitoring.
Carbon measurements By definition, the most general characteristic of the organic compounds is the presence of carbon atoms in their structures. Consequently, carbon analyzers have been used extensively for the nonspecific detection of organic compounds in water samples. The common carbon [Ox Prod], where [C] oxidation reaction is [C] 4- [O] represents the carbon atoms in the organic compound to be measured; [O] represents an oxidizing compound (oxygen, chromate, water) ; [Ox Prod] represents products of the oxidation reaction (carbon dioxide, carbon monoxide). The range of oxidizing temperatures may vary from near room temperatures for biological oxidation to 1000°C in a catalytic furnace. In the pollution monitoring carbon analyzers, a catalytic furnace is used so that the oxidative process may be performed in minutes instead of days.
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Figure 1
Generalized carbon analyzer Sample injection device
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Detector
Flow controller
L
Reactor tube
A typical analysis (Fig. 1 ) involves the injection of a sample onto the catalyst bed where oxidation takes place. The carrier gas carries the products of the oxidation reaction through a sample conditioner and into the detector. The analytical instruments, however, differ in design and with regard to the nature of the component monitored. A total carbon (TC) analysis measures the sum of organic and inorganic carbon present in a water sample. A sample is injected onto an oxidizing catalyst bed and the carbon dioxide generated from the carbon present in the sample is measured. Although this method has the advantage of being a sensitive simple measurement, it does not distinguish between organic and inorganic carbon. The TC analyzer manufactured by lonics, Inc., oxidizes organic compounds via the reforming reaction of water on a fixed bed of palladium metal at 950°C, whereas Envir0 Control, Inc., carries out the oxidation reaction of water on a fluidized bed of aluminum oxide in air at about 900°C. Another manufacturer, Astro Ecology, Inc., has a fixed bed reactor composed of nonprecious metal materials at about 900°C.
A total organic carbon (TOC) analysis measures the difference between TC and total inorganic carbon (TIC). If we assume that the major inorganic carbon contribution is due to the presence of carbonate, there are two procedures for measuring TOC. One common procedure is to eliminate the carbonate from the sample before measuring its total carbon content. The other procedure is to measure the amount of carbonate present and subtract its contribution from the measured total carbon content. Methods used to eliminate carbonate ion from water solutions include the precipitation of carbonate with barium hydroxide and the acidification of the solution, followed by expulsion of the evolved CO2 by boiling or purging with inert gas. Van Hall & Stenger's method (Van Hall, C. E., Stenger, V. A,, Anal. Chem., 39, 503, 1967) for the direct determination of carbonate consists of introducing a 10-50 pl sample containing carbonate ion onto a catalyst bed at 150°C and measuring the CQ2 evolved. The catalyst bed is composed of quartz chips (6-12 mesh) coated with 85% phosphoric acid. A similar direct analytical method using a catalytically active salt has been developed by Arin at lonics, Inc. Astro Ecology, Inc., Delta Scientific, and lonics, Inc., have developed the acidification and purge technique. Though reliable, it suffers from the disadvantage of eliminating volatile hydrocarbons from the sample during the purge step. To circumvent this difficulty, lonics, Inc., has developed a TOC analyzer utilizing the direct determination of carbonate method. The two most common detectors used in total carbon analyzers are the nondispersive infrared (NDIR) analyzer sensitized to carbon dioxide and the flame ionization detector ( F I D ) . Van Hall, in studying possible interferences with the total carbon measurement using an infrared analyzer, found substantially no interferences except for a slight interference from strong brines and certain acids. These solutions produced fogs that were counted as carbon dioxide by the infrared analyzer. The presence of a
Process an a Iyze rs Instrument type
Manufacturer
Injection method
Total carbon/ Astro Ecology Sample metered, 4 0 0 p particutotal organic Corp. (Houston, lates Tex.) carbon Samples metered, Total carbon/ Delta Scientific 4 0 0 0 p particuCorp. (Lindentotal organic lates hurst, N.Y.) carbon Total carbon lonics, Inc. Slide plate valve, (W a tertown , solids capacity: Mass.)
