Activated sludge treatment step takes place into aeration tanks (activated sludge tanks), whose footprint shape has to be defined according to the aeration devices to be installed (see figure 6). Rectangular tanks have to be realised when diffused aeration devices are installed: the ratio width/height ranges between 1 and 2, the lowest values in the case of diffusors installed along only one of the tank’s sides; all the edges have to be round shaped in order to avoid dead zones into the tank.

Figure 6: Example for an activated sludge process

The activated sludge process uses microorganisms to feed on organic contaminants in wastewater, producing a purified effluent. The basic principle behind all activated sludge processes is that as microorganisms grow within metabolizing soluted organic material. They form particles that clump together. These particles (flocks) in most cases are able to settle, so that they can separated with a simple settling process, which works according to the same principle as the pre-settling. Wastewater supply is mixed with return of activates sludge (see figure 6) containing a high proportion of organisms taken from the final sedimentation. This mixture is stirred and injected with large quantities of air, to provide the oxygen demand of microorganisms and keep solids in suspension. After a period of time, mixed liquor flows to a clarifier, which is in most cases a settling tank. In special cases also a flotation tank or membranes can be used to separate microorganisms. Partially cleaned water flows on for further treatment if needed. The resulting settled solids, the activated sludge, are returned to the first tank to begin the process again. Due to the fact, that during the process microorganisms grow, the excess sludge has to be removed out of the system to held the microorganisms concentration nearly constant.

When mechanical aeration devices are installed, circular shapes can be chosen as well, especially in the case of small WWTPs. In such cases, the ratio between the width (or the diameter) and the height can range from 1,5 to 5 according to the size of the mechanical aerator(s). A good rule is to split the whole reaction volume into different units (except for very small sized plants). Rectangular shaped tanks, when different units are placed side by side, offer the advantage to allow a lower footprint occupation.

Water level into the tanks is fixed through adjustable weirs (level excursion of about 10 cm, to be expected especially in case of mechanical aerators) to the secondary settling unit.

The tanks’ bottom should have a slight slope to one or more shafts where submerged pumps aimed to the periodic tanks’ empty for maintenance can be installed. Diffused aeration systems consist of submerged diffusors, air pipes and blowers. Each system has to be chosen according to the air bubbles size and to the air immission depth. Both these parameters affect the technical and the economical design in a very noticeable way: fine bubbles devices can increase the aeration efficiency due to the higher water-air contact surface but at the same time they are more expensive and an additional air pre-treatment is always required (air filtration and oil removal); moreover, the higher is the air immission depth the higher is the oxygen transfer rate, but the blower size increases as well.

During operation breaks, sludge settling can cause air blowers clogging, therefore the system should allow an easy removal of the air diffusors for maintenance. Usually, air diffusors are installed on the tanks’ bottom, sometime only along one side in order to increase the turbolence inside the mixed liquor. Coarse bubbles devices consist of perforated pipes (5 -10 mm diameters) or coarse diffusors.

Aeration and Mixing mechanical devices consist of equipment that allow a deeper contact between air and mixed liquor into the tank. Basically, mechanical aerators can be classified into two types:

• Mechanical aerators with vertical axis

• Mechanical aerators with horizontal axis

Both of them can be classified into submerged and superficial ones.

Basic parameters that characterize the activated sludge process are:

• HRT, Hydraulic Retention Time into the aeration tank

• TSS into the mixed liquor

• Organic Load referred to the biomass

• Volumetric Organic Load

• SRT, Sludge Retention Time

• Recycle Ratio

• Type of flow into the tank (completely stirred, plug flow)

• Aeration System

Typical operational data for different activated sludge process are presented in table 3.4.1.

In table 3.4.2 a typical aeration basin operation sheet is reported; the reporting frequency depends on the WWTP size; details on routine inspections, lubrication and adjustment, performed by the operator, should be reported as well.

Tab. 3.4.1 Typical operational data for different activated sludge process

Tab 3.4.2 Aeration basin operation sheet