Activated Sludge Systems

In an activated sludge treatment system, an acclimatized, mixed, biological growth of microorganisms (sludge) interacts with organic materials in the wastewater in the presence of excess dissolved oxygen and nutrients (nitrogen and phosphorus). The microorganisms convert the soluble organic compounds to carbon dioxide and cellular materials. Oxygen is obtained from applied air, which also maintains adequate mixing. The effluent is settled to separate

Table 2 Factors Affecting the Choice of Aerobic Processes

(A) Operating characteristics

(A) Operating characteristics

Table 2 Factors Affecting the Choice of Aerobic Processes

Resistance to shock loads of

Sensitivity to intermittent

Degree of skill

System

organics or toxics

operations

needed

Lagoons

Maximum

Minimum

Minimum

Trickling

Moderate

Moderate

Moderate

filters

Activated

Minimum

Maximum

Maximum

(B) Cost considerations

System

Land needed

Initial costs

Operating costs

Lagoons

Maximum

Minimum

Minimum

Trickling

Moderate

Moderate

Moderate

filters

Activated

Minimum

Maximum

Maximum

biological solids and a portion of the sludge is recycled; the excess is wasted for further treatment such as dewatering. These systems originated in England in the early 1900s. The layout of a typical activated sludge system is shown in Figure 8.

Most of the activated sludge systems utilized in the seafood-processing industry are of the extended aeration types: that is, they combine long aeration times with low applied organic loadings. The detention times are 1 to 2 days. The suspended solids concentrations are maintained at moderate levels to facilitate treatment of the low-strength wastes, which usually have a BOD5 of less than 800 mg/L.

It is usually necessary to provide primary treatment and flow equalization prior to the activated sludge process, to ensure optimum operation. A BOD5 and suspended solids removals in the range of 95-98% can be achieved. However, pilot- or laboratory-scale studies are required to determine organic loadings, oxygen requirements, sludge yields, sludge settling rates, and so on, for these high-strength wastes.

In contrast to other food-processing wastewaters, seafood wastes appear to require higher oxygen availability to stabilize them. Whereas dairy, fruit, and vegetable wastes require approximately 1.3 kg of oxygen per kg of BOD5, seafood wastes may demand as much as 3 kg of oxygen per kg of BOD5 applied to the extended aeration system [2].

The most common types of activated sludge process are the conventional and the continuous flow stiffed tanks, as shown in Figure 8, in which the contents are fully mixed. In the conventional process, the wastewater is circulated along the aeration tank, with the flow being arranged by baffles in plug flow mode. This arrangement demands a maximum amount of oxygen and organic load concentration at the inlet. A typical conventional activated sludge process is shown in Figure 9. Unlike the conventional activated sludge process, the inflow streams in the completely mixed process are usually introduced at several points to facilitate the homogeneity of the mixing such that the properties are constant throughout the reactor if the mixing is completed. This configuration is inherently more stable in terms of perturbations because mixing causes dilution of the incoming stream into the tank. In seafood-processing wastewaters the perturbations that may appear are peaks of concentration of organic load or flow peaks. Flow peaks can be damped in the primary treatment tanks. The conventional configurations would require less reactor volume if smooth plug flow could be assured, which usually does not occur.

Figure 8 Diagram of a simple activated sludge system.
Figure 9 Diagram of a conventional activated sludge process.

In activated sludge systems, the cells are separated from the liquid and partially returned to the system; the relatively high concentration of cells then degrades the organic load in a relatively short time. Therefore there are two different resident times that characterize the systems: one is the hydraulic residence time (0H) given by the ratio of reactor volume ( Vr) to flow of wastewater (Qr):

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