General Considerations

The volatile-acids determination is widely used in the control of anaerobic waste treatment processes. In the biochemical decomposition of organic matter that occurs, facultative and anaerobic bacteria of wide variety hydrolyze and convert the complex materials to low-molecular-weight compounds, as discussed in Sees. 6.7 to 6.11. Among the low-molecular-weight compounds formed, the short-chain fatty acids, such as acetic, propionic, butyric, and to a lesser extent isobutyric, valeric, isovaleric, and caproic, are important components. These low-molecular-weight fatty acids are termed volatile acids because they can be distilled at atmospheric pressure. An accumulation of volatile acids can have a disastrous effect upon anaerobic treatment if the buffering capacity of the system is exceeded and the pH falls to unfavorable levels.

In anaerobic digestion units that are operating in a stabilized condition, three groups of bacteria work in harmony to accomplish the destruction of organic matter. Following hydrolysis and fermentation to complex acids the acidogenic and dehy-drogenating organisms carry the degradation to acetic acid and hydrogen and then the methane-forming bacteria complete the conversion into methane and carbon dioxide (Fig. 32.1). When a sufficient population of methane-forming bacteria is present and environmental conditions are favorable, they utilize the end products produced by the acidogenic bacteria as fast as they are formed. As a result, acids do not accumulate beyond the neutralizing ability of the natural buffers present, and the pH remains in a favorable range for the methane bacteria. Under such conditions the volatile-acid content of digesting sludges or anaerobically treated wastewaters usually runs in the range of 50 to 250 mg/L, expressed as acetic acid.

Methane-forming bacteria are ubiquitous in nature, and some are always present in domestic wastewater and sludge derived therefrom. Their population, however, is very small compared with that of many of the fermentative and acidogenic bacteria. This disparity in numbers is the reason for the troubles encountered in starting anaerobic treatment units without benefit of "seeding." Untreated municipal wastewater sludges and many industrial wastewaters have a relatively low buffering capacity, and

Hydrolysis and fermentation

Acetogenesis and dehydrogenation Methanogenesis

Figure 32.1

Stages in the methane fermentation of complex wastes. Percentages represent conversion of waste COD by various routes. (After p. L. McCarty, One Hundred Years of Anaerobic Treatment, in "Anaerobic Digestion 1981," Hughes et at., eds., Elsevier Biomedical Press, Inc., B. V., Amsterdam, pp. 3-22,1981.)

pltfr t lillifci

Hydrolysis and fermentation

Acetogenesis and dehydrogenation Methanogenesis

Figure 32.1

Stages in the methane fermentation of complex wastes. Percentages represent conversion of waste COD by various routes. (After p. L. McCarty, One Hundred Years of Anaerobic Treatment, in "Anaerobic Digestion 1981," Hughes et at., eds., Elsevier Biomedical Press, Inc., B. V., Amsterdam, pp. 3-22,1981.)

when they are allowed to ferment anaerobically, volatile acids are produced so much faster than the few methane bacteria present can consume them that the buffers are soon spent and free acids exist to depress the pH. At pH values below 6.5, methane bacteria are seriously inhibited, but many fermentative and acidogenic bacteria are not until pH levels fall to about 5. Under such unbalanced conditions, the volatile-acids concentration continues to increase to levels of 2000 to 6000 mg/L or more, depending upon the solids content of the sludge. Active methane treatment may never develop in such mixtures unless the sludge or wastewater is diluted or neutralizing agents, such as lime, are added to produce a favorable pH for the methane bacteria. The volatile-acids determination, in conjunction with pH measurements, is valuable in control of environmental conditions during the initiation of anaerobic treatment.

Successful operation of anaerobic treatment units depends upon maintaining a satisfactory balance between the methane and acidogenic bacteria. The methane bacteria appear to be the most susceptible to changes in environmental conditions and food load. They are affected much more radically by changes in pH and temperature than the acidogenic bacteria. Inhibitions caused by changes in either of these factors result in a decreased rate of destruction of volatile acids; consequently, volatile acids begin to accumulate in the system. Many of the acidogenic bacteria are known to reproduce more rapidly than the methane bacteria. Under increased food loads, volatile acids may be formed faster than the slow-growing methane organisms can take caxe of them. This discrepancy results in an accumulation of volatile acids in the system. Sludge must be removed or transferred from anaerobic treatment units on occasion; however, removal of too large an amount will deplete the methane organism population to levels where volatile acids cannot be destroyed as fast as they are formed and accumulations will develop. Volatile-acids determinations are important in detecting the presence of unbalanced conditions in anaero bic treatment units caused by any of these factors. The onset of unfavorable conditions can be detected almost immediately, and usually several days in advance of other methods, such as through pH measurements.

Because of the importance of volatile-acid formation in anaerobic treatment, it is of value to provide more details of the somewhat complicated biological processes involved. Volatile acids are formed as intermediates during the anaerobic degradation of carbohydrates, proteins, and fats, as discussed in Sees. 6.7 to 6.11. Figure 32.1 illustrates the four major stages through which a complex waste such as domestic sludge must pass during its conversion to methane gas. The first two stages of hydrolysis and fermentation may be carried out by the same organisms. Here, complex organic materials such as carbohydrates, proteins, and fats and oils are hydrolyzed into basic components, which are then fermented to fatty acids, alcohols, carbon dioxide, ammonia, and some hydrogen. In the third stage of acetogenesis and dehy-drogenation, a more specialized group of anaerobic bacteria ferment the higher organic acids to form acetic acid, hydrogen, and perhaps formic acid. In the fourth stage, hydrogen, formate, and acetic acid are converted to methane by the anaerobic methane-forming bacteria. The third and fourth stages are closely coupled as here the third-stage acetic-acid-forming bacteria depend upon the methanogens to reduce the concentrations of hydrogen, formate, and acetic acid to sufficiently low levels for the third-stage conversion to be energetically favorable. Because of this close coupling, it was formerly thought that methanogens carried out both the third and fourth stages. Through painstaking studies to isolate the organisms involved, the two separate third and fourth stages are now known to exist.

Acetic acid is the most abundantly volatile acid produced, and is formed as an intermediate during the anaerobic treatment of almost all organic material. With a complex waste such as domestic sludge, about 72 percent of the organic matter based upon COD or electron equivalents is converted to acetic acid before finally being changed into methane gas. A similar high percentage is found with most industrial wastewaters as well.

The anaerobic biological degradation of wastes thus results in the production of large quantities of organic acids. If these acids are not converted to methane gas as rapidly as they are formed, their concentration will increase and will lower the pH. The major buffering material in anaerobic treatment that tends to prevent a drop in pH is bicarbonate, which, in equilibrium with carbonic acid, tends to regulate the hydrogen-ion concentration (Sec. 4.5):

Was this article helpful?

0 0
Healthy Chemistry For Optimal Health

Healthy Chemistry For Optimal Health

Thousands Have Used Chemicals To Improve Their Medical Condition. This Book Is one Of The Most Valuable Resources In The World When It Comes To Chemicals. Not All Chemicals Are Harmful For Your Body – Find Out Those That Helps To Maintain Your Health.

Get My Free Ebook


Post a comment