In most textbooks of microbiology, respiration of organic matter is explained by Eq. 1, with glucose used as a model substance. Except for an exact reaction stoichi-ometry of the oxidative metabolism, mass and energy dissipation, if mentioned at all, are not quantified. Both parameters are, however, very important for activated sludge treatment plants. The surplus sludge formed during wastewater stabilization requires further treatment, causes disposal costs, and - in the long run - may be an environmental risk, and heat evolution during unevenly high-loaded aerobic treatment may shift the population toward more thermotolerant or thermophilic species and thus, at least for some time, may decrease the process efficiency.
1 mol C6H12O6 + 6 mol O2 ^ 6 mol CO2 + 6 mol H2O + heat energy (1)
Table 1.1 Carbon flow during (A) aerobic degradation in an activated sludge system under (a) saturating and (b) limiting substrate supplya and during (B) anaerobic degradation.
(A) Aerobic degradation:
(a) Saturating substrate supply = high-load conditions
1 unit substrate carbon ^ 0.5 units CO2 carbon + 0.5 units cell carbon
(b) Limiting substrate supply = low-load conditions
1 unit substrate carbon ^ 0.7 units CO2 carbon + 0.3 units cell carbon
(B) Anaerobic degradation:
1 unit substrate carbon ^ 0.95 units (CO2 + CH4) carbon + 0.05 units cell carbon a Estimated from surplus sludge formation in different wastewater treatment plants.
If 1 mol of glucose (MW=180 g) is degraded in an activated sludge system at a high BOD loading rate (e.g., >0.6 kg m-3 d-1 BOD), approximately 0.5 mol (90 g) is respired to CO2 and water, with consumption of 3 mol of O2 (96 g), releasing 19 mol of ATP (Fig. 1.1). The other 0.5 mol of glucose (90 g) is converted to pyruvate by one of three glycolytic pathways, accompanied by the formation of 0.5-1 mol ATP. Pyruvate or its subsequent metabolic products, e.g., acetate or dicarboxylic acids, are directly used as carbon substrates for cell multiplication and surplus biomass formation. A maximum amount of 20 mol ATP is thus available for growth and maintenance (Fig. 1.1). At a pH of 7, about 44 kJ of energy is available for growth per mol of ATP hydrolyzed to ADP and inorganic phosphate (Thauer et al., 1977). For an average molar growth yield of aerobes of 4.75 g per mol ATP (Lui, 1998), 90 g biomass can be generated from 180 g glucose. If the combustion energy per g of cell dry mass is 22 kJ, about 890 kJ (2870-980 kJ) is lost as heat during respiration (Fig. 1.1). The energy loss is the sum of heat losses during respiration and cell growth.
At a low BOD loading rate, the proportion of glucose respired in relation to the proportion of glucose fixed as surplus biomass can shift. Up to 0.7 mol (126 g) of glucose can be oxidized to CO2, requiring 4.2 mol of oxygen (134.4 g O2). Thus, for respiration of 1 mol of glucose, different amounts of oxygen may be consumed, depending on the loading rate of the wastewater treatment system and the different amounts of carbon dioxide and of surplus sludge formed (Fig. 1.1, Table 1.1).
The energy and carbon balance deduced above can be analogously applied to aerobic stabilization of raw sewage sludge. If the initial dry matter content is around 36g L-1 (average organic dry matter content of sewage sludge) and if a biodegradability of 50% within the residence time in the sludge reactor is obtained, about 9 g L-1 of new biomass is formed, and thus 27 g L-1 (36 - 18 + 9) remains in the effluent.
The released heat energy is approximately 89 kJ L-1 of reactor content. To estimate the theoretical temperature rise, this amount of heat energy must be divided by 4.185 kJ (specific energy requirement for heating 1 L of H2O from 14.5-15.5 °C). Thus, by respiration of 18 g L-1 organic dry matter, the reactor temperature increases by 21.3°C within the residence time required for degradation (<16 h), provided that no heat energy is lost. A great proportion of the heat energy is, however, transferred via the liquid phase to the aeration gas and stripped out, whereas a smaller proportion is lost through irradiation from the reactor walls. Since air, containing almost 80% nitrogen, is normally used as an oxygen source in aeration ponds or activated sludge reactors, the heat transfer capacity of the off-gas is high enough to prevent a significant increase in the wastewater temperature. Thus, ambient or at least mesophilic temperatures can be maintained. An increasing temperature of several degrees Celsius would lead to a shift in the population in the reactor and - at least temporarily - would result in reduced process stability, but an only slightly increased temperature of a few degrees Celsius might simply stimulate the metabolic activity of the prevalent mesophilic population. In practice, in activated sewage sludge systems no self-heating is observed because degradability is only about 50% and complete heat transfer to the atmosphere occurs via the off-gas at a retention time of more than 0.5 d. If, however, wastewater from a dairy plant or a brewery with a similar COD concentration, but with almost 100% biodegradable constituents, is stabilized with pure oxygen, twice as much heat evolves, leading to a theoretical temperature rise of 57 °C. Self-heating is observed, since there is much less offgas and the heat loss is thus significantly lower. In addition, due to higher reaction rates than with sewage sludge, the heat is generated during a shorter time span (shorter retention time).
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