The Anaerobic Process

During studies with the anaerobic acetogenic clostridia, it was recognized that these bacteria can ferment sugars such as glucose, fructose, and xylose almost exclusively to acetic acid by the following reactions  

Organic Add and Solvent Production Acetic, Lactic, Gluconic, Succinic, and Polyhydroxyalkanoic Acids 

Organism Optimal growth temperature ( C) Growth substrates organic One carbon References 

Clostridium aceticum 30 Fructose, pyruvate, ethanol, and malate CO2/H2, HCOOH Adamse (1980) 

C formicoaceticum 37 Fructose, gluconate, galacturonate, glucuronate, lactate, and glycerol CH3OH/CO2 Andreesen et al. (1970) 

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There are two methods of biological treatment aerobic and anaerobic (217). The aerobic systems use free oxygen dissolved in the wastewater to convert wastes in the presence of microorganisms to more microorganisms, energy required for their existence, and carbon dioxide. The anaerobic process occurs in the absence of free oxygen and converts the waste to methane and carbon dioxide, generally in deep tanks or basins, and can produce odor problems when sulfides or sulfates are present in the wastewater. 

In sewer networks without considerable amounts of sediments, the anaerobic processes are dominated by the acidogenic production of VFA and C02 and by sulfate reduction (hydrogen sulfide production). The methanogenic phase can normally be excluded as being of minor importance. These facts were verified by Tanaka and Hvitved-Jacobsen (1999) and Tanaka (1998) under sewer conditions in a number of laboratory experiments and in the field. 

From a general point of view, but still related to sewer conditions, the anaerobic processes in wastewater are dealt with in Chapter 3, especially in Section 3.2.2. In the following, the sulfur cycle is focused on. A part of this cycle proceeds under anaerobic conditions, and another part is aerobic. In a sewer system with changing aerobic and anaerobic conditions, this combined cycle is of particular interest but, at the same time, also complex to deal with. The nature of the sulfur cycle in a sewer is further complicated because the processes proceed in and between the biofilm, the sewer sediments, the water phase, and the sewer atmosphere. 

The methods for determination of the parameters concerning the anaerobic processes in the aerobic-anaerobic process model are less structured compared with the procedures that have been described in Sections 7.2.1 through 7.2.4. The following points constitute a rather simple approach in this respect ... 

In the case where the anaerobic processes take place under conditions where consumption of Ss by the sulfate-reducing biomass and the fermenting biomass must be considered, Equation (7.10) expresses the total anaerobic hydrolysis rate. This equation is based on the assumption that methane formation in sewers without sediment normally can be neglected (Section 3.2.2). 

The three described groups of methodologies are experimental ways leading to the estimation of model parameters for the description of the anaerobic processes according to the aerobic-anaerobic conceptual model (Table 6.6). The determination of the remaining kinetic and stoichiometric parameters in this model, however, requires a calibration procedure, where the results of the above three described methodologies are used. Table 6.7 shows typical values of such parameters determined by the three methodologies folio wed by a model calibration. 

In our opinion the anaerobic process is not being optimised for animal wastes, and optimum digestion should be based on new technologies which reduce the retention time. 

Biodegradation. Halogenated solvents are degraded under aerobic and anaerobic conditions. The anaerobic process is typically a reductive dechlorination that progressively removes one halide at a time,... 

The aerobic pathway via the Krebs cycle yields 36 moles of ATP, while the anaerobic process utilizing glycolysis yields only 2-3 moles. Clearly, fish performing work of great capacity must cultivate oxidative metabolism... 

Given the complex stepwise nature of the anaerobic process, it is proposed that the rate-limiting step be defined as that step in the process which will cause process failure to occur under imposed conditions of kinetic stress. Kinetic stress is applied to the system by continually reducing the value of 6c until the limiting value of Oc—i.e., 6 = (/x" )"S is exceeded and washout of the microbial fiora results. The parameter 6 is the value of 6c obtained from Equation 12 when Si = Sq. The value of 6 approaches the value of nm when So > > K . 

Table IV shows [0c" ]iim values computed for the volatile acids using Equation 13 and [0c um values computed or estimated for other methane fermentations discussed in subsequent sections. At 35 °C the [ c ]iim values for acetic and propionic acids are essentially equal and differ from the values for long chain fatty acids by less than one day. Hence at 35°C the methane fermentations of long and short chain fatty acids are equally limiting in the anaerobic process. At lower temperatures, long chain fatty acid degradation appears to be the limiting phenomenon.

Briefly describe the anaerobic process and explain its difference from an aerobic process. 

Yeast cells ferment the sugars in fruits to produce ethanol and carbon dioxide. The anaerobic process must be carried out in tightly sealed containers such as these large tanks. One-way valves allow carbon dioxide to escape from the fermenting liquid without letting air in. ... 

A considerable proportion of the sludge dry matter is that ascribed to lipids (ether and petroleum-ether extractable substances), consisting of free fatty acids, their esters and substances incapable of undergoing saponification. The content of fatty acids in sludge is a value which is useful for characterizing the biochemical conditions in the anaerobic process of sludge stabilization. 

Aerobic metabolism is a highly efficient way for an organism to extract energy from nutrients. In eukaryotic cells, the aerobic processes (including conversion of pyruvate to acetyl-GoA, the citric acid cycle, and electron transport) all occur in the mitochondria, while the anaerobic process, glycolysis, takes place outside the mitochondria in the cytosol. We have not yet seen any reactions in which oxygen plays a part, but in this chapter we shall discuss the role of oxygen in metabolism as the final acceptor of electrons in the electron transport chain. The reactions of the electron transport chain take place in the inner mitochondrial membrane. 

Equation (25) describes a process of electron transfer from donor molecules to CO2 (for both oxygenic and anoxygenic photosynthesis). Unlike the anaerobic processes described in Section 3, in which the electron transfer is frequently extracellular (between cells), the electron transfer in photosynthesis is intracellular. As such, it can occur completely via biological electron carriers, and without need for free electron carriers such as H2. However, the presence of hydrogenase enzymes in representatives from both classes of phototrophs [2,24,25] makes H2 metabolism possible in both accessory and primary roles. 

Quality. Landfill gases, specifically methane gas, are naturd by-products of anaerobic microbial activity in the landfill. The anaerobic process requires water and the proper mix of nutrients to maintain optimal conditions. The quality of gas varies with time, and may be characterized by four distinct phases. In the first phase, which may last several weeks under optimum conditions, aerobic... 

Glycolysis The anaerobic process of breaking down glucose to form pyruvic acid or lactic acid and energy in the form of ATP. 

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