*4.4.2.1. Anaerobic digesters*

Samer [9] elucidated and illustrated the structures and constructions of the anaerobic digesters and the used building materials. Samer [10] developed an expert system for planning and designing biogas plants. Figures 8 to 13 show different types of anaerobic digesters. While Figures 14 and 15 show some industrial applications. Table 1 shows the advantages and disadvantages of anaerobic treatment compared to aerobic treatment.

**Figure 8.** Most commonly used anaerobic reactor types: (A) Completely mixed anaerobic digester, (B) UASB reactor, (C) AFB or EGSB reactor, and (D) Upflow AF [8].

**Figure 9.** Single-stage conventional anaerobic digester [3].

treatment demonstrating the possibility of installing collective anaerobic and aerobic units in

Samer [9] elucidated and illustrated the structures and constructions of the anaerobic digesters and the used building materials. Samer [10] developed an expert system for planning and designing biogas plants. Figures 8 to 13 show different types of anaerobic digesters. While Figures 14 and 15 show some industrial applications. Table 1 shows the advantages and

**Figure 8.** Most commonly used anaerobic reactor types: (A) Completely mixed anaerobic digester, (B) UASB reactor,

disadvantages of anaerobic treatment compared to aerobic treatment.

series [1].

*4.4.2.1. Anaerobic digesters*

12 Wastewater Treatment Engineering

**Figure 7.** Major steps in anaerobic decomposition [1].

(C) AFB or EGSB reactor, and (D) Upflow AF [8].

**Figure 10.** Dual-stage high rate digester [3].

**Figure 11.** Schematic representation of digester types. Flow-through (A–B) and contact systems (C–F) [1].

**Figure 12.** The upper scheme shows a two-stage anaerobic sludge digester, while the lower scheme shows the conven‐ tional sludge digestion plant [1].

**Figure 13.** Primary digestion tank with screw mixing pump and external heater [1].

**Figure 14.** Wastewater treatment plant for corn processing industry [8].

**Figure 11.** Schematic representation of digester types. Flow-through (A–B) and contact systems (C–F) [1].

**Figure 12.** The upper scheme shows a two-stage anaerobic sludge digester, while the lower scheme shows the conven‐

tional sludge digestion plant [1].

14 Wastewater Treatment Engineering

**Figure 15.** Mass balance study for a wastewater treatment plant of the baker's yeast industry [8].

By definition, the anaerobic treatment is conducted without oxygen. It is different from an anoxic process, which is a reduced environment in contrast to an environment without oxygen. Both processes are anoxic, but anaerobic is an environment beyond anoxic where the oxidation reduction potential (ORP) values are highly negative. In the anaerobic process, nitrate is reduced to ammonia and nitrogen gas, and sulfate (SO3 2-) is reduced to hydrogen sulfide (H2S). Phosphate is also reduced because it is often transformed through the ADP–ATP chain [3].


**Table 1.** The advantages and disadvantages of anaerobic treatment compared to aerobic treatment [1].
