*4.4.6. Rotating biological contactors*

*4.4.5. Biological filters*

**Figure 27.** Step aeration plant [3].

24 Wastewater Treatment Engineering

The main systems of operation of biological filters are: (a) single filtration, (b) recirculation, (c) ADF, and (d) two-stage filtration with high-rate primary biotower (Figure 29). There are several types of biological filters, for example, submerged aerated filters that are widely known as biological aerated filters (BAFs) and are the commonly implemented design (Figure 30), and the percolating (trickling) filters (Figure 31). The BAFs implement either the sunken granular

**Figure 28.** Food pyramid illustrating the feeding relationships within the activated sludge process [1].

The rotating biological contactors (RBC) system (Figure 33) can be implemented to amend and improve the available treatment processes as the secondary or tertiary treatment processes. The RBC is successfully implemented in all three steps of the biological treatment, which are BOD5 removal, nitrification, and denitrification. The process is a fixed-biofilm of either aerobic or anaerobic biological treatment system for removal of nitrogenous and carbonaceous compounds from wastewater (Figure 34). The RBC installations (Figure 35) were designed for removal of BOD5 or ammonia nitrogen (NH3-N), or both, from wastewater [1, 2].

(c)

**Figure 30.** Biological aerated filters [1].

Figure 30. Biological aerated filters [1].

(a)

(b)

(c)

Figure 30. Biological aerated filters [1].

**Figure 30.** Biological aerated filters [1].

26 Wastewater Treatment Engineering

**Figure 31.** Relationship between the natural bacterial populations in rivers and the development of (A) trickling (per‐ colating) filter and (B) activated sludge system [1].

**Figure 32.** Comparison of the food chain pyramids for biological filters and activated sludge systems [1].

**Figure 33.** Schematic diagram of air-drive RBC [2].

The RBC consists of media, shaft, drive, bearings, and cover (Figure 34). The RBC hardware consists of a large diameter and closely spaced circular plastic media that is mounted on a horizontal shaft supported by bearings and is slowly rotated by an electric motor. The plastic media are made of corrugated polystyrene or polyethylene material with different designs, dimensions, and densities. The model designs are based on increasing surface area and firmness, allowing a winding wastewater flow path and stimulating air turbulence [1, 2].
