For the purpose of designing, the continuous flow basin may be divided into following four zones:
(a) An inlet zone,
(b) A settling zone,
(c) Sludge zone, and
(d) An outlet zone.
Before starting design work the following simple assumptions could be made:
(i) The water in the settling zone acts as quiescent without any disturbance and the sedimentation takes place exactly as in a quiescent container of the same depth.
(ii) The flow of water is steady and all the suspended particles are uniformly distributed on the full cross-section at right angle to flow, just after entering the settling zone.
(iii) Any particle which enters the sludge zone settles and stays till removed.
Design of Inlet Zone:
The inlet zone should be designed in such a way that incoming water is uniformly distributed on the full width of the tank, and it enters the settling zone without causing any disturbance to the settling particles. Fig. 11.1 shows some common types of inlets which are mostly used.
Design of Settling Zone:
When any particle enters the settling zone in continuous flow settling tank, it is acted upon by the two forces. The horizontal flow of water gives it horizontal movement, while gravitational force causes it to move downward. The particle moves under the influence of both these forces and traces out a parabolic path as shown in Fig. 11.2. If the particle reaches the sludge zone, it is removed otherwise it will go out from the outlet zone.
The condition for entering the sludge zone by the particle is,
Where, L = Length of settling zone.
H = Depth of water in settling zone,
v = Horizontal velocity of flow of water.
Vs = Settling velocity of particle.
Therefore, water should be detained in the settling zone for such a period that all the desired suspended particles may reach the sludge zone and are removed. The time for which a water is detained in the settling tank is called detention time.
But in practice some water directly goes from the inlet to outlet due to short-circuiting, therefore the detention time which is taken for design purposes is always greater than the theoretical one. Generally detention period of 2 hours to 4 hours is adopted for mechanically cleaned tanks and 4-8 hours for ordinary settling tanks.
Flow of water per unit surface area of the settling tank is called surface loading. The settling velocity of suspended particles is always numerically equal to the surface loading and it is totally independent to the depth of the tank.
On the basis of the above, the settling zone is designed so that suspended particles of desired shape and size may settle in the bottom of the tank.
Design of Sludge Zone:
The sludge zone of the sedimentation tanks is designed in such a way that all the settled particles may be collected in it, and can be conveniently removed whenever desired without causing and disturbance to the water of settling zone. Generally the bottom floors of the tanks are made sloping towards one side or towards centre of the tank.
Cast Iron sludge pipes with gate valves are provided at the lowest point of the floor, from where sludge is removed under hydrostatic pressure. Figs. 11.3 and 11.4 show typical cross-sections of the lower portion of the sedimentation tanks, showing sludge zones.
Design of Outlet Zone:
The outlet zone should also be designed in the same way as inlet zone, so that water may be taken out from the tank without causing any disturbance to the water of settling zone. Few types of outlets are shown in Fig. 11.5.