In order to get a fairly high percentage of removal of the suspended material in a continuous flow type settling tank, it is desirable that the tank is properly designed.
For this it is essential to study the various design considerations for these tanks which are as follows:
(i) Velocity of flow
(ii) Surface overflow rate and solids loading rate
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(iii) Removal efficiency of settling tanks Settling tank efficiency
(iv) Detention period
(v) Tank dimensions
(vi) Inlets and outlets
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(vii) Weir loading
(i) Velocity of Flow:
The velocity of flow of sewage in settling tank should be such that maximum settling of suspended particles is caused in the tank. It should remain uniform throughout the tank. Further it is essential that the velocity of flow should be kept low so that settled particles are not scoured from the bottom of the tank.
The critical velocity Vc at which the settled particles may be displaced is given by the following equation which was developed by Camp (1946) using the results from studies by Shields:
Where Ss is specific gravity of particles;
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d is diameter of panicles;
g is acceleration due to gravity;
f is Darcy-Weisbach friction factor; and
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β is constant which depends on type of material being scoured.
Typical nines of β are 0.04 for unigranular sand and 0.06 for sticky interlocking matter. The value of f depends on the characteristics of the surface over which flow is taking place and the Reynolds number. For settling tanks typical values of f are 0.02 to 0.03.
(ii) Surface Overflow Rate and Solids Loading Rate:
The plan surface area of primary settling tanks is determined using the surface overflow rate. For secondary settling tanks designed to remove bioflocculated solids, in addition to surface overflow rate, solids loading rate or solid flux is also an important design parameter.
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The solids loading rate or solid flux represents the solids loading per unit surface area of tank per unit time and is expressed as kg of suspended solids per m2 per day (kg (ss)/m2/d). Thus the plan surface area of secondary settling tanks is determined using both surface overflow rate and solids loading rate and the greater of the two is adopted in the design.
Surface overflow rates and solids loading rates must be checked both at average flow and peak flow. Table 12.3 gives values of surface overflow rates and solids loading rates to be adopted for the design of different settling tanks as recommended in the Manual on Sewerage and Sewage Treatment prepared by the Ministry of Urban Development (MUD), New Delhi.
(iii) Removal Efficiency of Settling Tanks – Settling Tank Efficiency:
The removal efficiency of discrete suspension in a settling tank is given by equation 12.12. However, equation 12.12 gives only theoretical efficiency of a settling tank, because in actual practice the efficiency of a settling tank is reduced by the following currents set up in the tank.
(i) Eddy Currents – set up by the inertia of the incoming sewage.
(ii) Surface Currents – induced due to wind in open tank.
(iii) Vertical Convection currents – set up due to thermal gradient along the depth of the tank.
(iv) Density Currents – set up due to cold or heavy sewage under-running the tank and warm or light sewage flowing across its surface.
The currents set up in the tank induces short circuiting for certain amount of sewage which results in reducing the efficiency of the tank in actual practice.
The efficiency of a real tank affected by current induced short circuiting may be mathematically expressed as:
(iv) Detention Period:
The detention period of a settling tank is the theoretical time sewage is detained in it. In case of a settling tank the detention period may be considered as the theoretical time taken by a particle of sewage to pass from the entry to the exit of the tank. A relation between the capacity of a settling tank and its detention period can be established as indicated below.
Let C be the capacity of a settling tank; and Q be the discharge or rate of flow of sewage through the tank; then the detention period t0 of the tank as given as:
Where S.O.R is surface overflow rate.
The rate of removal of biochemical oxygen demand (BOD) and suspended solids (SS) is maximum during the first 2 to 2½ hours of settling and thereafter decreases appreciably. Hence, increase in the detention time beyond 2 to 2½ hours will not increase the percentage removal of BOD or SS proportionately.
Moreover, longer detention period may affect the tank performance adversely due to setting in of septic conditions, particularly in tropical climates. Experience has shown that a detention period of 2 to 2½ hours for primary settling tanks and 1½ to 2 hours for secondary settling tanks will produce the optimum results. Longer detention periods in secondary settling tanks may result in de-nitrification which adversely affects the settling efficiency.
Thus primary settling tanks are designed for a detention period of 2 to 2½ hours and secondary settling tanks are designed for a detention period of 1½ to 2 hours.
(v) Tank Dimensions:
The recommended dimensions of the various settling tanks are indicated earlier and the same are summarized below:
(a) Rectangular Tanks:
Maximum length – 90 m
Maximum width – 30 m
Length of width ratio – 1.5 : 1 to 7.5 : 1
Length to depth ratio – 5 : 1 to 25 : 1
The floor of the tank is provided with a slope of 1% from the outlet end towards the inlet end where a sludge hopper is provided. The side slopes of the sludge hopper ranges from 1.2 : 1 to 2:1 (vertical to horizontal).
(b) Circular Tanks:
Diameter – 3 to 60 m with most common range being 12 to 30 m
The floor of the tank is provided with a slope in the range of 7.5 to 10% (normally 8.33% or 1 vertical to 12 horizontal) from periphery to centre to form an inverted cone at the bottom of the tank.
Depth:
The depth sets the detention time in the settling tank and also influences sludge thickening in secondary settling tanks of activated sludge plants. The depths recommended for horizontal flow settling tanks are given in Table 12.3.
It may be seen that in general a minimum depth of 2.5 m in the case of primary settling tanks and 3.5 m in the case of secondary settling tanks is provided. However, in the case of circular primary settling tanks a minimum depth of 2.0 m may be provided.
(vi) Inlets and Outlets:
Performance of settling tanks is very much influenced by inlet devices which are intended to distribute and draw the flow evenly across the tank. All inlets must be designed to keep down the entrance velocity to prevent formation of eddy or inertial currents in the tank to avoid short circuiting.
(1) Rectangular Tanks:
In horizontal flow rectangular settling tanks inlets and outlets are placed opposite each other separated by the length of the tank with the inlet perpendicular to the direction of flow.
In the design of inlets to rectangular tanks the following methods are used to distribute the flow uniformly across the tank:
(a) Multiple pipe inlets with baffle boards of depth 0.45 to 0.6 m in front of the inlets, 0.6 to 0.9 m away from it and with the top of baffle being 25 mm below sewage surface for the scum to pass over.
(b) Channel inlet with perforated baffle side wall between the tank and the channels.
(c) Inlet channel with submerged weirs discharging into tank followed by a baffle board inside the tank.
A stilling chamber is necessary ahead in inlets if the sewage is received under pressure from pumping mains.
Outlet is generally an overflow weir located near the effluent end, preferably adjustable for maintaining the weir at a constant level. V-notches are provided on the weir to provide for uniform distribution of flow at low heads of discharge over the weir. Weir lengths could be increased by placing outlet channel inside the tank with weirs on both sides. Scum baffles are provided ahead of outlet devices to prevent the escape of scum with the effluent.
(2) Circular Tanks:
In radial flow circular tanks the usual practice is to provide a central inlet and a peripheral outlet. The central inlet pipe may be either a submerged horizontal pipe from wall to centre or an inverted siphon laid beneath the tank floor. An inlet baffle is placed concentric to the pipe mouth generally with a diameter of 10-20% of the tank diameter and extending 1 to 2 m below sewage surface.
Where the inlet pipe discharges into a central hollow pillar, the top of the pillar is flared to provide adequate number of inlet diffusion ports through which sewage enters the tank with an entry velocity of 0.10 to 0.25 m/s through the ports. The entry ports are submerged 0.3 to 0.6 m below sewage surface.
Outlet is generally a peripheral weir discharging freely into a peripheral channel. The crest of the weir is provided with V-notches for uniform draw off at low flows. In all primary settling tanks a peripheral scum baffle extending 0.20 to 0.30 m below sewage surface is provided ahead of effluent weir. If the length of the peripheral weir is not adequate, a weir trough mounted on wall brackets near the periphery with adjustable overflow weir on both sides is provided to increase the length of weir.
(vii) Weir Loading:
Weir loading represents the discharge per unit length of outlet weir and it is expressed as m3/d/m. Weir loading influences the removal of solids in settling tank, particularly in secondary settling tanks where flocculated solids are settled. There is no positive evidence that weir loading has any significant effect on removal of solids in primary settling tanks.
However, certain loading rates based on practice are recommended both for primary as well as secondary settling tanks. For all primary, intermediate and secondary settling tanks, except in the case of secondary settling tanks for activated sludge process, weir loading for average flows should not exceed 125 m3/d/m. For secondary settling tank in activated sludge process or its modifications the weir loading should not exceed 185 m3/d/m.
The loading should, however, ensure uniform withdrawal over the entire periphery of the tank to avoid short circuiting or dead pockets. Performance of existing settling tanks can be improved by merely increasing their weir length.
The restriction in weir overflow rate requires special outlet weir design including a total weir length several times the tank width, for rectangular tanks and often two weirs with an outlet channel between them for circular tanks. Very long weirs cannot be maintained truly level over their full length, except perhaps at considerable expense, and satisfactory distribution of flow is more readily obtained by forming indentation at regular intervals such as shallow V-notches say 50 mm deep spaced 0.15 to 0.3 m apart. In addition to the head above the V-notches, a reasonable free fall of 0.05 to 0.15 m should be allowed for maximum flows depending in part, on the total head available.