Carbon in Environmental Engineering and Science

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It traditionally has been the organic carbon (along with pathogens) that was of the greatest concern in water pollution (Section 15.2.7), leading to the construction of wastewater treatment plants (Chapter 16) that focus on its removal. Management of wastewater treatment sludges often has stabilization of the organic material as a major objective. (Stabilization involves conversion of readily degradable materials to those that change only slowly; see later in this subsection). Municipal solid waste management also must stabilize the organic material (e.g., by incineration or composting), or else deal with the consequences (e.g., attraction of vermin, settling, and leachate and gas production during landfilling). Similarly, with soil and groundwater contamination, it is often organic carbon that is the target of remediation. Undesirable tastes and odors in drinking water, and the formation of cancer-causing compounds during disinfection, are traceable to organic compounds present in the water supply. Even air pollution control may involve organics, such as volatile organic compounds (VOCs) and soot (which includes organic particles), as important contaminants. Most individual toxic compounds of concern in water, soil, and air are also organics.

Thus, much of environmental engineering and science is directed at control of organic carbon or an understanding of its fate and effects in the environment. In particular cases the emphasis may be on a single compound, a particular class of compounds (such as petroleum hydrocarbons or chlorinated solvents), some broad fraction (such as oil and grease, or oxygen demanding biodegradable compounds), or the total organics. One concern might be rapid biodegradation, leading to depletion of oxygen, while another material might pose a hazard because it is very resistant to degradation. Slow-to-degrade compounds may be bioaccumulated (Section 18.7.2) in organisms, perhaps leading to toxic effects even if they are present only at low concentrations in the environment. Other slowly degrading compounds, such as many plastics, may pose aesthetic problems, or perhaps injure wildlife that eat (discarded plastic bags that are mistaken for jellyfish) or become entangled (abandoned fishing nets or six-pack rings) in them.

In some water bodies, contamination with excess levels of nitrogen and/or phosphorus may lead to excessive growth of aquatic plants, algae, or cyanobacteria (eutrophication; Section 15.2.6). In this case organic carbon is not added directly, but instead, becomes problematic after it is formed through photosynthesis.

On the other hand, for heterotrophic organisms, it is typically the availability of organic material that limits growth. This is particularly true for water and soil microorganisms, where survival is often dependent on an ability to subsist on very low levels of organic substrates, or to grow quickly when high concentrations of substrate suddenly become available (e.g., through death of a plant or animal, or depositing of animal waste products), followed by persistence until the next such event.

Thus, it is clear that the carbon cycle is of major importance in environmental engineering and science, and the next two subsections deal with measurement of organics and their biodegradation. However, it is also worth noting the potentially fragile nature of this essential cycle. This was demonstrated on a small scale with the Biosphere II facility in Oracle, Arizona (Section 14.2.2). That attempt to build a closed ecosystem including humans was unable to keep the carbon cycle in balance, producing too much CO2 and too little O2. On a broader scale, global warming, largely as a result of increased atmospheric CO2 levels, represents another example of a carbon cycle that is not in balance, with potentially major effects. In considering such global change, it is perhaps instructive to keep in mind that Earth's atmosphere was not always as we know it today. In fact, the low CO2 (380 ppm) and high O2 (21%) that we consider "normal" is a result of a previous "imbalance" brought about by the advent of oxygen-producing photosynthesis (Section 10.1).

An important lesson to be learned from these observations is the crucial balance that comes into play with the life-sustaining biogeochemical cycling of an element such as carbon. Compared to our planet, the scale of Biosphere II was so small that it was very sensitive to an imbalance, and problems manifested themselves in a correspondingly short period of time. The biogeochemical cycles of our planet, on the other hand, were established over a far longer period (geologic time). Nonetheless, the success of the ecosystems that sustain us depends on a harmony in nature, one that has finite (if unknown) limits to the levels of human influence that it can tolerate.

Quantification of Organic Carbon Because organic carbon is often the contaminant of greatest concern in water pollution and waste treatment, a number of approaches to measuring it have been developed. In some cases, the concentration of individual constituents must be known, especially for toxic substances. In these instances, sophisticated instrumental techniques such as gas chromatography (GC) or high-performance liquid chromatography (HPLC) may be used to separate and quantitate the compounds of concern, followed by mass spectroscopy to identify them with a high degree of certainty.

Another reason to know the amount of a compound present is to estimate the potential oxygen demand it might exert, for example, in a stream or during wastewater treatment. For a pure compound the theoretical oxygen demand (ThOD) can be calculated based on the stoichiometry of its complete oxidation.

Example 13.1 What is the ThOD of glucose, C6H12O6?

Answer

Thus, 6 mol of oxygen is needed to oxidize each mole of glucose completely. Since 1 mol of oxygen is 32 g, 6 x 32 = 192g of oxygen is needed for 180 g (1 mol) of glucose, or 1.067 g of O2 per gram of glucose.

More generally, for a hydrocarbon or carbohydrate, the equation can be written as b

Hence, with MW representing the molecular weight of the compound,

ThOD

To calculate the ThOD of an organic compound containing other elements (besides C, H, and O), the final form of the elements after the reaction must be known. P can be considered to be in the form of phosphate and chlorine as chloride (Cl-). However, nitrogen might end up as ammonia or nitrate, and sulfur as sulfide or sulfate. Often, it will be assumed that the products are in the reduced form (NH3 and H2S) so that the carbonaceous oxygen demand is calculated; any additional oxygen demand for oxidation of the inorganics can then be determined separately.

For mixed contaminants, such as in municipal wastewater or sludge, it is often necessary or sufficient to have an estimate of the total amount of organic matter present, or some fraction thereof, rather than trying to quantify each of the individual components separately. One of the older of such aggregate measures, still commonly used for sludges, is volatile solids (VS). In this test a sample is dried and its mass found to determine the total solids (TS). It is then ignited at around 500 °C, which destroys the organic material. (Note that this is also one weakness of the method, in that some inorganics also may volatilize, leading to overestimation of the organic content.) The VS is the difference in mass between the TS and the residual ash. Rather than use the total solids, the sample also can be filtered first and the test performed on the suspended solids (SS) trapped on the filter, yielding the volatile suspended solids (VSS). Organisms and their associated materials and waste products, including wastewater treatment plant sludges, often have a volatile fraction of around 85%.

This approach is also used to estimate the carbon content of soils. The volatile fraction is determined, and then multiplied by a factor, often around 0.55, since this is the typical fraction of the soil organic material that actually consists of carbon.

Total organic carbon (TOC) is an instrumental analysis method that gives a more selective measure of organic matter. Using combustion or chemical oxidation, the organic matter in a sample is converted to CO2, which is then measured using infrared spectro-metry. Correction must be made for the inorganic carbon present, either by acidification (to turn it all to CO2) and purging (bubbling with nitrogen gas) to remove it before analysis (note that this also strips out volatile organic compounds, so that they are missed in the analysis), or by also measuring the CO2 present before oxidation (inorganic forms) and subtracting it from the value obtained after oxidation (which includes both the organic and inorganic sources).

The TOC of pure compounds can also be calculated from the chemical formula.

Example 13.2 What is the TOC of glucose, C6Hi2O6? Answer There are 6 mol of C per mole of glucose.

0.40 g TOC/g glucose

For complex mixtures of unknown composition, a ThOD cannot be calculated. However, the chemical oxygen demand (COD) laboratory test empirically estimates this value. It utilizes heat and strong oxidizing agents to convert most organics to CO2, then estimates the amount of oxygen that would have been necessary to carry out the same reaction. One limitation of the method is that despite the very vigorous conditions, a few organic materials are not oxidized completely.

However, the most common method for estimating the strength of the oxygen-demanding organic material in wastewaters—as required by many regulations—is the biochemical oxygen demand (BOD). This test determines empirically, under standard conditions, the amount of oxygen utilized during microbial oxidation of the organics present in the sample. Thus, it measures only the biodegradable portion of the total oxygen demand. However, this is often the fraction of interest in determining the impact of a discharge on a receiving water, or the amount of aeration capacity needed for an aerobic treatment process, since only the biodegradable organics are likely to be oxidized under such conditions.

Like many of the tests described above, the BOD is operationally defined; that is, the BOD is the number that results from conducting the test. Thus, it is important to use a standard protocol, including a specialized bottle (Figure 13.5), so that results will be reproducible. The sample is first oxygenated to bring the DO up to near saturation (9.1 mg/L), the DO is measured, and the bottle is then sealed. After incubation in the dark (to prevent photosynthetic production of oxygen) at 20°C, usually for 5 days, the DO is measured again. The difference represents the oxygen utilized biochemically.

The BOD of domestic sewage is typically around 200 mg/L, yet the initially aerated sample can hold no more than about 9 mg/L of DO. This means that substantial dilution is required (with a specified standard buffer), and the final oxygen depletion in the bottle is than multiplied by the dilution factor. To ensure that sufficient microorganisms are present initially, some samples are seeded (inoculated) with a small amount of wastewater treatment plant effluent or settled sewage, and the oxygen demand of this seed must be corrected for in the final BOD calculation.

Figure 13.6 shows a typical plot of the BOD exerted versus time for a sewage sample. The first surge of oxygen demand results from oxidation of organic materials, while the second, often starting at about day 7, is a result of the oxidation of ammonia (nitrification; see Section 13.3.2). The standard BOD test is run for 5 days (BOD5), so that usually it includes much of the carbonaceous oxygen demand (C-BOD), but little of the nitrogenous demand (N-BOD or NOD). However, this makes the procedure difficult to use for short-term decision-making purposes, as the results will not be known for at least 5 days. (This choice is also unfortunate for the practical reason that tests started on a Tuesday must be finished on a Sunday!) In some cases the N-BOD begins to be exerted much earlier, giving a BOD5 value that overestimates the C-BOD. If desired, the N-BOD can be suppressed by adding to the BOD bottle a compound that acts as a specific inhibitor of nitrification, preventing exertion of the N-BOD.

Figure 13.5 BOD bottle.

Time (days) Figure 13.6 BOD exerted vs. time.

TABLE 13.3 Comparison of BOD, COD, and TOC Tests

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