Handbook Biological Wastewater Treatment - Design of Activated Sludge Systems

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Anaerobic Digestion

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When activated sludge is kept in an anaerobic environment, specialised bacteria will develop that use the excess sludge as a source of organic matter for fermentative metabolic processes. The end products of the fermentation are mainly methane and carbon dioxide. The overall conversion process of complex organic matter into methane and carbon dioxide can be divided into four steps as shown in Figure 8.12: hydrolysis, acidification, acetogenesis and methanogenesis. The following items will be discussed in this section:

(1) Theory and stoichiometrics of anaerobic sludge digestion

Figure 8.12 Schematic representation of the decomposition of excess activated sludge by anaerobic digestion
Figure 8.12
Schematic representation of the decomposition pathways of excess activated sludge (and other organic material) by anaerobic digestion

The four metabolic steps (hydrolysis, acidogenenis, acetogenenis and methanogenesis) as shown in Figure 8.12 will be discussed in more detail, including the stoichiometry of the different reactions and the effect on alkalinity and pH. Calculations are shown in Example 8.4. To download this section, click here

(2) Anaerobic sludge digestion configurations

In this section several anaerobic digester configurations are discussed:

  • The low-rate digester, which combines the two functions of sludge stabilisation by anaerobic digestion and separation of the digested solids from a supernatant substantially free of suspended solids
  • The high rate digester, in which active mixing occurs. Solids/liquid separation takes place either in a second stage digester or in a specific dewatering unit.

Several methods are discussed that can be used to stimulate the performance of the anaerobic digester. To download this section, click here.

(3) Operational parameters of the anaerobic sludge digestion process

Among the various operational parameters that influence the anaerobic digestion process, the most important ones are the digester temperature, the pH value and -stability, the presence of nutrients in the digester and finally the presence- and concentration of toxic materials. To download this section, click here.

(4) Anaerobic sludge digester performance

The volatile solids removal efficiency in an anaerobic digester depends basically on three factors: the digestion temperature, the applied retention time in the digester and the nature of the excess sludge to be digested. Equations are developed which predict solids removal efficiency and biogas production, as is shown in Example 8.5. To download this section, click here.

(5) Energy generation from sludge digestion

The potential for energy production during anaerobic digestion in the form of methane is of significant practical importance. It is interesting to compare the potential energy production from sludge digestion to the energy demand for aeration in the main activated sludge system, as is done in Example 8.6. To download this section, click here.

(6) Design and optimisation of anaerobic sludge digesters

The most important design parameter of an anaerobic sludge digester is the operational temperature. The retention time in the digester depends on this parameter, whereas the volatile solids removal efficiency is also affected. A procedure is presented to optimize an activated sludge system consisting of an aeration tank, final settler, sludge thickener and anaerobic digester. Refer to Example 8.7, while detailed design examples are also given in the section on Integrated Design. To download this section, click here.

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