Classification of Settling Tanks | Sedimentation | Waste Management

The settling tanks are generally made of reinforced cement concrete and may be rectangular or circular in shape.

The settling tanks can be classified in the following two ways:

1. Classification based on method of operation:

(a) Fill and draw type settling tanks

(b) Continuous flow type settling tanks

2. Classification based on location:

(a) Preliminary settling tanks (grit chambers)

(b) Primary settling tanks

(c) Secondary settling tanks

Fill and Draw Type Settling Tanks:

The fill and draw type settling tanks are also known as quiescent type or intermittent type settling tanks. These tanks are mostly of rectangular shape with horizontal bottom. Sewage is filled in the tank and it is retained there for a certain period during which sewage is held at rest in the tank. During the period of retention of sewage in the tank, the particles in suspension settle down and get deposited at the bottom of the tank.

At the end of the retention period the effluent is drawn off through the outlet from the tank and the tank is cleaned of the settled panicles. After cleaning, the tank is again filled with sewage and the same operation is repeated.

The usual period of retention of sewage in the tank to cause maximum settling of particles is about 24 hours or so. Thus if time required for filling, emptying and cleaning the tank (which usually varies from 6 to 12 hours) is added, a period of about 30 to 36 hours will be required between the two successive fillings of the tank.

This will necessitate the provision of at least two tanks and if one additional tank is to be provided as stand by, then minimum three tanks will be required for this type of working. However, the operation of fill and draw type tanks involves a lot of labour and time and hence these tanks are not being used these days.

Continuous Flow Type Settling Tanks:

In a continuous flow type settling tank sewage is continuously admitted into the tank and allowed to flow slowly in the tank during which the particles in suspension settle down and it flows out continuously from the tank. These tanks work on the principle that by reducing the velocity of flow of sewage a large amount of suspended particles present in sewage can be made to settle down.

The velocity of flow of sewage in these tanks is reduced by providing sufficient length of travel for sewage in the tank. Further the velocity of flow of sewage in these tanks is so adjusted that the time taken by a particle to move from inlet to outlet of the tank is slightly more than that required for settling of that particle.

The continuous flow type settling tanks may be rectangular, square or circular in shape.

These tanks may be classified on the basis of direction of flow of sewage in the tank into the following two types:

1. Horizontal flow settling tanks

2. Vertical flow settling tanks

1. Horizontal Flow Settling Tanks:

In a horizontal flow settling tank the direction of flow of sewage in the tank is substantially horizontal.

The horizontal flow settling tanks may be further classified into the following two types:

(i) Rectangular tanks with longitudinal flow

(ii) Circular tanks with radial flow

(i) Rectangular Tanks with Longitudinal Flow:

Fig. 12.5 shows a rectangular tank. Sewage enters the tank through an inlet provided on one side of the tank and after flowing slowly in horizontal direction in the tank, it passes out through an outlet provided on the opposite side of the tank. The inlet and outlet are provided at the top edge of the tank. Near the inlet and the outlet, baffles are provided to enable the flowing sewage to spread out evenly and thus prevent direct currents.

Rectangular tanks have maximum lengths and widths of 90 m and 30 m respectively with length to width ratios of 1.5:1 to 7.5:1 and length to depth ratios of 5:1 to 25:1. A minimum depth of 2.5 m in case of primary settling tanks and 3.5 m in case of secondary settling tanks is provided.

The floor of the tank is provided with a slope of 1% from the outlet end towards the inlet end where a sludge hopper with a sludge withdrawal pipe is provided. The side slopes of the sludge hopper ranges from 1.2:1 to 2:1 (vertical to horizontal). The maximum velocity of flow of sewage in these tanks is 1.5 mph.

The sludge (or settled material) deposited at the bottom of the tank is usually removed mechanically by a power-operated mechanism. It consists of two endless chains moving on sprocket wheels and supporting wooden cross-bars or flights, which periodically sweep the sludge towards the hopper provided near the inlet end, from which it is withdrawn under hydrostatic head or pumped into separate sludge-digestion tanks. The scum formed at the surface is removed by the mechanical scraper with the aid of a second blade called skimmer, through a scum trough.

(ii) Circular Tanks with Radial Flow:

The circular tanks with radial flow are of the following two types:

(a) Circular tank with central feed

(b) Circular tank with peripheral or rim feed

Fig 12.6 shows the schematic diagrams of these two types of circular tanks. In a circular tank with central feed, sewage enters the tank at the centre and leaves at its periphery. On the other hand in a circular tank with peripheral or rim feed water enters the tank from the periphery or rim and leaves at the centre. Both the types of circular tanks have been found to be satisfactory.

In a circular tank with central feed sewage is carried to the centre of the tank in a pipe suspended from a bridge, or encased in concrete beneath the tank floor. At the centre of the tank sewage enters a circular well and emanates from multiple ports of the well to flow radially outwards equally in all directions in the tank. The sewage flowing slowly in the radial direction approaches the periphery of the tank where the clarified sewage passes through a controlling weir and flows into an effluent channel (also called effluent launder) and finally into an effluent pipe.

In a circular tank with peripheral or rim feed, a suspended circular baffle a short distance from the tank wall forms an annular space into which sewage is discharged in a tangential direction. The sewage flows spirally around the tank and underneath the baffle, and the clarified sewage is skimmed off over weirs on both sides of a centrally located weir trough.

Diameters of circular tanks vary widely from 3 to 60 m although the most common range is 12 to 30 m. The depth of sewage in the tanks varies from 2 m for primary settling tanks to 3.5 m for secondary settling tanks. The floors of these tanks are provided with a slope in the range of 7.5 to 10% from periphery to centre to form an inverted cone at the bottom of the tank.

The sludge (or settled material) deposited at the bottom of the tank is continuously removed by a sludge removal mechanism provided in the tank. The tanks 3 to 9 m in diameter have the sludge-removal mechanism supported on beams spanning the tank. Tanks 10.5 in and more in diameter have a central pier that supports the sludge-removal mechanism and is reached by a walkway or bridge (see Fig. 12.7).

The sludge-removal mechanism consists of scraper blades mounted on two or four collector arms or raking arms which also support blades for scum removal. The collector arms are revolved by a driving unit at a slow speed such that one revolution is completed in about 30 to 40 minutes.

The slow movement of the arms avoids creation of undesirable currents and also requires little power for operation. As the collector arms revolve the sludge is scraped by scraper blades to a small hopper located near the centre of the tank from which it is removed by a pipe.

2. Vertical Flow Settling Tanks:

Vertical flow settling tanks are square or circular in plan. The sides of these tanks are vertical in the upper portion and inclined at 45° or more in the lower portion to form hopper bottom. Sewage enters at the bottom of the tank where flocculation takes place as particles cojoin into aggregates.

The upflow velocity decreases with increased cross-sectional area of the tank which helps flocculation of particles. The flocculated masses readily fall to the bottom of the tank forming a sludge blanket. The sewage entering the tank rises upwards through the sludge blanket and gets clarified.

Because of this phenomenon, these tanks are also known as up flow sludge blanket clarifiers. The clarified sewage passes through a circumferential or central weir located near the top of the tank and discharging into a draw off channel. The tank is so designed that the upward velocity of flow of sewage is less than the settling velocity of smallest particles to be removed.

Thus in these tanks sewage can be passed with a relatively higher velocity. Also for these tanks shorter detention times are required resulting in smaller capacities. The sludge is removed from these tanks under hydrostatic pressure or by pumping.