Aerobic Respiration

A generalized overall reaction for aerobic respiration of organic material can be expressed as organic matter + O2 ! CO2 + H2O + new biomass + energy

Analogous reactions can be developed for inorganic substrates such as ammonia and hydrogen sulfide, which are used by lithotrophic organisms. The rate of change of each of the six quantities in this equation in theory could be used to measure the rate of aerobic microbial activity. This is typically done by making repetitive measurements of disappearance or accumulation over time. However, such repeated sampling may inadvertently affect the system itself. Alternatively, multiple replicate systems can be established initially and one or more sacrificed at each different sampling time. A disadvantage of this approach is the inherent variability between "replicate" systems.

Organic Matter Disappearance In some situations the microbial activity on a particular compound is of interest. If this compound can be measured, its disappearance over time will be a direct indication of activity. However, proper controls are necessary to ensure that the activity is biological rather than the result of other mechanisms, such as volatilization, adsorption, or chemical transformation. Killed controls, in which any organisms present or added are destroyed by heat, ultraviolet radiation, or chemical means, are often best for such techniques. Also, disappearance of the parent compound does not necessarily mean that it is being completely biodegraded; some compounds are merely transformed to metabolic products that then persist and accumulate (see "Biodegradation" in Section 13.1.3).

Alternatively, some general measure of organic material might be used, and its disappearance monitored. Biochemical oxygen demand (BOD), chemical oxygen demand (COD), and total organic carbon (TOC) are three such measures commonly used for this purpose and are discussed further in the section "Quantification of Organic Carbon" in Section 13.1.3.

Measurements of organic material are often of great importance for measuring extent of disappearance but typically are not convenient for measuring rates. Good rate measurements usually require frequent sampling, which may be disruptive to the system or demand large numbers of replicate systems for sacrifice. In most cases the need for chemical analysis also places practical limits on the number of samples that can be processed.

Oxygen Utilization In the headspace of a closed system of fixed volume and constant temperature, oxygen removed by respiration is replaced by an almost equal molar amount of carbon dioxide produced. However, carbon dioxide, a weak acid, is easily removed by absorption with an alkali, such as KOH solution. The pressure in the system will then drop as oxygen is consumed. Alternatively, if the pressure and temperature of the system are held constant, the volume will decrease. In both cases, frequent measurements can be taken without the need for expensive or disruptive analyses. Such methods are referred to as respirometric techniques.

Early respirometers typically required manual measurements of changes in pressure. Many modern systems can collect data automatically every few seconds, if desired, allowing careful monitoring of the pattern of oxygen utilization (Figure 11.15). In one device,

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Figure 11.15 Example of respirometry data. Oxygen uptake with various grades (reagent, technical, lean, and rich) of monoethanol amine (MEA). (Data courtesy of Y. Lam, R. M. Cowan, and P. F. Strom.)

for example, oxygen is replaced automatically each time a slight pressure change occurs, and the amount of oxygen supplied is recorded.

With the development of dependable electrochemical dissolved oxygen (DO) probes, it has also become common to measure the oxygen uptake rate (OUR) directly in liquid culture. The liquid is first aerated, if needed, and then the drop in DO over time is measured and reported in milligrams of DO uptake per liter per minute (or hour). This may be further normalized to the biomass present, usually measured as SS or VSS, to give a specific OUR (SOUR), in units such as milligrams of DO uptake per minute per gram of biomass.

In systems in which the air flows through continuously, it may be possible to determine oxygen utilization by measuring the difference in the entrance and exit concentrations of oxygen in the airstream. Similarly, in continuous-flow liquid systems it may be possible to measure oxygen uptake by measuring changes in DO concentrations.

Production of Carbon Dioxide The carbon dioxide produced by respiration might be measured directly (e.g., with an infrared spectrophotometer) or absorbed in an alkali and measured by titration with acid. For an aerated system, the exit gas might be bubbled through KOH solution in several flasks in series to be sure that it is all removed. Special biometer flasks (also known as Bartha flasks after their inventor, Richard Bartha) with sidearms containing KOH can also be used (Figure 11.16). Measurements of CO2 can be particularly useful because they indicate mineralization of organic matter. Especially if appropriately 14C-radiolabeled test compounds are available, 14CO2 production gives clear evidence of their complete biodegradation.

Production of Water In most aqueous systems the amount of metabolic water produced is too small to measure compared to the amount of water already in the system.

New Biomass Many of the methods described in Section 11.4 might be used to determine biomass over time. Alternatively, counts (Sections 11.5.1 and 11.5.2) might be used and converted (Section 11.5.3), if necessary, to biomass. However, the relationships

Figure 11.15 Example of respirometry data. Oxygen uptake with various grades (reagent, technical, lean, and rich) of monoethanol amine (MEA). (Data courtesy of Y. Lam, R. M. Cowan, and P. F. Strom.)

stopcock

CO2 trap stopcock

Bartha Biometer Flasks

needle

KOH solution

Figure 11.16 Biometer flasks.

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KOH solution

Figure 11.16 Biometer flasks.

between new biomass produced and other aspects of microbial activity may be complex. These are discussed more in Section 11.7.

Energy Some of the energy released by metabolic activities is captured and used for growth and other cell functions; the rest is given off as heat. Special instruments known as calorimeters can sometimes be used to measure this release of heat experimentally. The amount of heat released per mass of oxygen utilized is ^14,000 J/g O2 for oxidation of a wide variety of organic compounds. Also, in some systems of interest to environmental engineers and scientists, such as composting, sufficient heat is released that it may be possible to measure temperature changes or other signs of heat production as a means of determining the rate of activity [see the discussion of equation (16.5)].

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