Mechanical Aeration in Activated Sludge Process | Waste Management

Various systems of mechanical aeration have been developed to achieve the required Agitation, most of which are patented.

Some of these systems of mechanical aeration which are commonly used are as indicated below: 1. Hartley System or Birmingham Bio-Flocculation System 2. Haworth System or Sheffield Aeration System 3. Link-Belt System 4. Kessener System 5. Simplex System.

1. Hartley System or Birmingham Bio-Flocculation System:

Hartley system is adopted at Birmingham in England. In this system the aeration tank is divided into a series of long and relatively narrow parallel interconnected channels by means of thin dividing walls as shown in Fig. 13.6.

At the bends at one end of each pair of channels partially submerged vertical paddles are provided. The paddles are not truly vertical but they are slightly inclined from the vertical and are so arranged that they rotate about vertical shafts.

The rotating paddles give a forward movement to the mixture of sewage and returned activated sludge (or mixed liquor) in the channels and also set up a wave action due to which spiral flow of the mixed liquor is caused which brings the mixed liquor in intimate contact with the atmospheric oxygen.

In each channel at regular intervals diagonal flat baffle plates are provided to reduce the velocity of flow of the mixed liquor, maintain spiral flow of the mixed liquor, prevent settling of sludge and effect aeration of the mixed liquor and thus increase the efficiency of aeration. The depth of the aeration tank is about 1.2 m and width of each channel is about 1.2 to 2 m. The paddles are rotated at a speed of about 15 r.p.m. The aeration period in this case is quite long being about 15 hours or so.

2. Haworth System or Sheffield Aeration System:

Haworth system is adopted at Sheffield in England and it is similar to the Hartely system. As shown in Fig. 13.7 in the Haworth system also the aeration tank is divided into a series of long and relatively narrow parallel interconnected channels by means of thin dividing walls.

Two rows of paddles arranged to rotate about horizontal shafts crossing the channels are provided. The paddles are arranged in staggered fashion and these are placed in the middle of each channel. Due to the rotating paddles sufficient velocity of flow is maintained to keep the sludge in suspension and at the same time a wave action is set up due to which spiral flow of the mixture of sewage and returned activated sludge (or mixed liquor) is caused which brings the mixed liquor in intimate contact with the atmospheric oxygen.

Further in this case as shown in Fig. 13.7 the positions of the inlet and outlet are such that they are cross connected due to which a part of the flow is recirculated within the tank which has an advantage of improving the efficiency of aeration. The depth of the aeration tank in this case is also about 1.2 m and width of each channel is about 1.2 to 2 m. The paddles are rotated at a speed of about 15 r.p.m and the aeration period in this case is also quite long being about 15 hours or so.

3. Link-Belt System:

In this system the aeration tank is about 2.5 to 3 m deep and about 3 to 4 m wide. The stirring and circulation of the mixture of sewage and returned activated sludge (or mixed liquor) are accomplished by a rotating steel paddle wheel placed near the top of the tank on one side and extending for the full length of the tank as shown in Fig. 13.8.

The diameter of the steel paddle wheel is about 75 cm and its speed of rotation is about 45 to 50 r.p.m., with 48 r.p.m being the normal value. A longitudinal vertical baffle wall is provided at a distance of about 45 cm from the tank wall supporting the above indicated mechanism for aeration, with an opening at the bottom, thus forming a sort of lift channel.

The baffle wall carries a narrow steel trough at right angles at the top as shown in Fig. 13.8. When the paddle wheel is rotated, it pushes the mixed liquor down in the tank which then passes through the opening at the bottom of the baffle wall and rises through the lift channel behind the baffle wall to the trough and is forced across the surface.

The circulation of the mixed liquor so set up in the aeration tank produces a wave action at the surface and spiral motion inside the tank which brings the mixed liquor in intimate contact with the atmospheric oxygen.

4. Kessener System:

Kessener system is adopted for small sewage treatment plants in Holland. It consists of a long tank with an agitating device in the form of a partially submerged horizontal brush located at one side of the tank at the top as shown in Fig. 13.9. Originally the brushes were ordinary street cleaning brushes but they are now made of stainless steel needles.

The brush is fixed longitudinally and is partially submerged to a depth of about 5 to 40 mm, and it is rotated at a speed of about 40 to 50 r.p.m. The rotation of the brush causes wave action in the mixture of sewage and returned activated sludge (or mixed liquor) to bring about necessary aeration.

5. Simplex System:

The simplex system also known as bioaeration system, consists of deep hopper bottom vertical flow tanks, generally of square shape as shown in Fig. 13.10. The rectangular tanks are also sometimes adopted, but in that case they are suitably divided into square units. At the centre of each tank, a hollow uptake tube or draft tube is suspended from the top, keeping it about 15 cm above the tank floor.

A cone with spiral vanes is provided at the top of the uptake tube and the cone is rotated by a motor placed at the top of the tank. The speed of rotation of the cone is about 60 r.p.m. Rotation of the cone causes a suction effect and the mixture of sewage and returned activated sludge (or mixed liquor) is sucked through the uptake tube, and it falls on the top surface in the form of spray which brings about the satisfactory aeration of the mixed liquor. Also the sewage gets thoroughly mixed up with the returned activated sludge during its downward movement which is in the form of a spiral due to the rotation of the cone.