Anaerobic Fluidized Bed Reactor

An anaerobic fluidized bed reactor (AFBR) utilizes small particles such as sand and activated carbon as the media for microorganism growth. Wastewater flow enters the reactor through the bottom of the reactor. Media are kept in the fluidized state by controlling the up-flow velocity of the wastewater stream in such a way that the weight of the media particles equals the drag force exerted by the wastewater.21,22 The height of the fluidized bed is stabilized. On average, a packing size of 0.3-0.8 mm and up-flow wastewater velocities of 10-30 m/h can be used in order to provide 100% bed expansion. The depth of the fluidized bed ranges between 4 and 6 m (Figure 30.5).

However, in the AFBR process, due to the decrease in particle density of the media after the accumulation of biomass on the surface, particle density can drop. As a result, particles tend to wash

Influent

FIGURE 30.5 Anaerobic fluidized bed reactor (AFBR).

Influent

FIGURE 30.5 Anaerobic fluidized bed reactor (AFBR).

out from the reactor. Therefore, it is necessary to clean and remove the attached biomass from a portion of the media from time to time. After cleaning, the particles are returned to the reactor and the separated biomass is wasted as sludge.21,33,34 Due to high turbulence and the thin biofilms developed in the AFBR, biomass capture is relatively weak; therefore, an AFBR is better suited for wastewater with mainly soluble COD.21

Borja and Banks35 investigated an AFBR to treat ice-cream wastewater. The COD of the wastewater to be treated was 5.2 g/L. The operating temperature was 35°C and the duration for start-up was reported as 30 d. At steady state, COD removal efficiency was 94.4%. The HRT and organic loading rate, while achieving the above performance, were 8 h and 15.6 g COD/(L d), respectively. Moreover, temperature, pH, flow rate, and COD loading shocks deteriorated reactor performance. However, after restoring favorable conditions, the system regained its steady state after 6-16 h. The same authors reported the use of an AFBR for wastewater from the soft drink industry.36 Three different media (bentonite, saponite, and polyurethane) were used for the growth of microorganisms. The composition and parameters of the soft drink wastewater were total solids (TS) 3.7 g/L, TSS of 2.9 g/L, volatile suspended solids (VSS) 2.0 g/L, COD of 4.95 g/L, volatile acidity (acetic acid) 0.12 g/L, alkalinity 0.14 g CaCO3/L, ammonium 5 mg/L, phosphorus 12 mg/L, and pH 4.8. The average COD removal of reactors was 89.9%, 93.3%, and 91.9% when the media used in the reactor were bentonite, saponite, and polyurethane, respectively. The percentage of methane in biogas was 66%, 72%, and 69% for reactors with bentonite, saponite, and polyurethane, respectively. It has been reported that the production of biogas reduced with increasing HRT.

The design and performance of the anaerobic treatment processes that we have discussed are shown in Table 30.3. According to the table, the AFBR outperforms the UASB and the AF. An AFBR can achieve higher removal efficiency within a shorter HRT. The volumetric loading rate of the AFBR is comparatively higher than that of the UASB and AF. However, the choice of appropriate technology should consider factors such as the footprint, operational and maintenance cost, and the level of treatment required.

30.5.2 Aerobic Biological Treatment Systems

In aerobic biological treatment systems, oxygen has to be supplied through aeration. The biodegradable portion of organics in the wastewater converted into carbon dioxide, water, and other simple compounds. Usually, aerobic treatment systems are employed if the wastewater is of low strength. Compared to the anaerobic biological treatment processes, the quality of effluents from aerobic processes is higher. In this section, selected aerobic biological treatment systems that can be used in the food processing industry are discussed.

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