In this article we will discuss about the process of sedimentation in the treatment of sewage.
The sewage is passed through plain sedimentation before the biological treatment and by sedimentation with coagulation after the biological treatment. The process of sedimentation with coagulation is also known as ‘Chemical Precipitation’. Therefore, sedimentation is a very important process in the treatment of sewage. In most of the sewage work, it is done as primary treatment while in some cases, it is done with activated sludge plants.
The purpose of the sedimentation of sewage is to separate the settleable solids so that the settled sewage, if discharged into water courses, does not form sludge banks and when used for irrigation, does not lead to excessive organic loading. This process also reduces the organic load on secondary treatment method.
Screening and skimming can remove large suspended matters, oils, grease, fats etc. Fine suspended matters cannot be removed by screening or skimming. These suspended matters are removed by sedimentation process, by allowing the sewage to remain quiescent in sedimentation basins.
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The sedimentation tanks, known as plain sedimentation or primary settling tanks are located after screens and grit chambers. Secondary sedimentation or secondary settling tanks find use in settling of the effluents from secondary treatment operations like trickling filter or activated sludge where the flocculated solids produced by the biological treatment are removed.
Characteristics of Settleable Solids:
The settleable solids to be removed in sedimentation tanks are mainly organic in nature, dispersed or flocculated. The specific gravity of the suspended solids may vary from 1.01 to 1.20. Usually raw sewage is a dilute heterogeneous suspension of low specific gravity solids ranging from fully dispersed to completely flocculated ones.
In primary sedimentation, the bulk of the solids reaching the tank is in a completely flocculated stage or susceptible to flocculation due to fluid motion within the tank. Therefore, the laws of sedimentation governing discrete and non-flocculent particles cannot be directly applied for the purpose of design. Since the particles are subjected to flocculation, the sedimentation tank cannot be designed on the basis of surface area alone, but displacement or detention time is also to be taken into account.
Theory of Sewage Sedimentation:
The settling velocity of the particles in any liquid is given by formula (11.6) (known as Stroke’s law) which states.
Where v = settling velocity in cm/sec.
g = 981 cm/sec2
P3 = specific gravity of the particle
P = specific gravity of fluid
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d = diameter of the settling particle in cm.
= viscosity of water in centi-stokes.
The settling velocity from the Newton’s Law is,
The rates of settling of suspended solids through quiescent water at 10°Care given in Table 11.2.
The settling value of the suspended solids depends on the following:
(a) Specific gravity of the particle:
The settling velocity increases with the specific gravity of the solid particles.
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(b) Size of the particle:
The settling velocity increases with the size of the particle.
(c) Temperature:
As the viscosity of the water reduces with the temperature and the settling velocity increases with the reduction in the fluid viscosity. Therefore, the settling velocity increases with the rise in temperature.
Following factors must be considered while using the formulae (14.1) and (14.2) given above:
(i) The sewage contains solid particles of various sizes and different specific gravities, therefore they do not settle as discrete particles but enmass a horizontal blanket of sewage sludge.
(ii) The sewage is in continuous motion inside the settling tanks, whereas the formula are based on the quiescent condition of water. Therefore, the eddies caused at the entrance, around the baffle walls and exit should be considered. Other forces which disturb the steady flow of sewages should also be considered.
Classification of Sedimentation Tanks:
On the basis of their purpose and design, the sedimentation tanks can be classified as follows:
(i) Grit chambers for removal of sand and grit.
(ii) Plain sedimentation tanks for removal of nearly all settleable solids.
(iii) Chemical precipitation tanks for removal of very fine suspended matters, by adding chemical coagulants in sewage.
(iv) Septic tanks for doing sedimentation and sludge digestion in the same compartment.
(v) Imhoff tanks also known as two storey tanks, in which sedimentation and sludge digestions are done in different compartments.
(vi) Secondary settling tanks – in these tanks the effluent from activated sludge process or trickling filter is allowed to settle.
The sedimentation tanks can also be classified depending upon the direction of How of sewage in them as:
(a) Horizontal flow settling tanks.
(b) Vertical or upward flow settling tanks.
(c) Radial flow settling tanks.
Design of Sedimentation Tanks:
For convenience in design work, a continuous-flow basin can be divided into following four zones:
(i) An inlet zone, in which influent flow and suspended matter disperse over the cross-section at right angle to flow.
(ii) A settling zone in which the suspended solids settle in the following sewage.
(iii) A bottom zone in which the settled solids are collected, from where they can be removed under hydrostatic pressure.
(iv) An outlet zone in which the sewage effluent together with remaining suspended particles are carried out from the tank.
If L, B and D be the length, breadth and the effective depth of the settling tank,
The cross-sectional area Ac = B × D and the surface area = A = B × L
If Q be the Rate of flow of sewage in the tank and I be the horizontal velocity of flow.
Q = AC V = B.D. V … (14.3)
Now if T be the detention time for which the sewage is detained in the settling tank, the capacity of the lank will be equal to QT
in other words T = L/V ……………..(14.4)
If V be the settling velocity of the suspended solid, this particle must settle down in the bed of the tank before the sewage reaches the outlet.
T = D/V ….. (14.5)
From equations (14.4) and (14.5)
= surface loading of the settling tank.
Therefore, the above expression gives that the settling velocity should be equal to the surface loading on the settling tank. This also indicates that the surface area is more important than the depth of the tank. The efficiency of the tank directly increases with the surface area, more the surface area, it will be able to remove more fine particles.
Types of Sedimentation Tanks:
Horizontal flow and vertical flow sedimentation tanks are generally constructed in sewage treatment work. Figs. 14.2 to 14.5 show various types of sedimentation tanks.
These tanks may be square, circular or rectangular in plan, the depth varying from 2.3 to 5 m. The diameter of circular tanks may be upto 40 m. The side of square tanks may be 25m. The length of rectangular tanks may be upto 100m but to avoid the currents due to wind, the length is limited upto 40m.
The width of these tanks may be 10m. The slope of sludge hoppers in these tanks is generally 2:1 (vertical: horizontal).The steepness is necessary so that solids may slide to the bottom themselves.
Sludge Removal:
After some time when the sludge is accumulated in the bed of sedimentation tank, the cleaning of the tanks is required. In one method, first the tank is cut out of service and un-watered. The accumulated solids are washed into a sump, from where they are removed by pumping or gravity.
Sometimes this sludge is removed under hydrostatic pressure after filling the tanks. But the above method is intermittent in operation and not suitable for sewage sludge, which is more in quantity and foul in nature.
The sewage sedimentation tanks are mostly cleaned more or less continuously by mechanical devices. Mechanical scrapers are installed in these tanks which continuously scrap the sludge and collect it in a sump from where they are removed under hydrostatic pressure by operating the valves.
Method of Obtaining Uniform Flow in Sedimentation Tanks:
Figs. 14.6 and 14.7 show the structural details of inlet and outlet in the tanks, for obtaining uniform How in the tanks.
For high efficiency of sedimentation tanks, the inlet should distribute the sewage flow with the suspended matter as far as possible equally into batteries of tank and also within individual tanks. For this, the dividing flow must face equal frictional resistances.
By regulating the outflow the level of sewage in all the tanks is kept at the same level. In the tank of Fig. 14.8 the whole sewage is evenly distributed between each battery of the tank. In the tank of Fig. 14.6, the flow at the inlet is sub-divided in such a manner that equal quantity of sewage should enter in each tank.
Standard Design Loading:
Experience obtained from the existing settling units shows that the effective depth of 2.5 to 3.5 m is satisfactory. During the design of the sedimentation tanks the surface loading is adjusted in such a way that it becomes numerically equal to the settling velocity of the suspended particles, which are to be removed from the sewage.
The surface loading in sewage settling tanks is kept between 40 to 50 m3/m2/day. Now after fixing the depth and the surface area the capacity of the settling tank is determined. Choosing a suitable ratio between the length and the breadth, the length and breadth are determined.
The length of the weir at the inlet and at the outlet is designed by keeping the loading between 225 to 300 cu.m./day. The weir loading is much smaller in case of circular sedimentation tanks than the rectangular tanks.
The velocity of flow of sewage in the settling tanks is kept as low as possible, to prevent the lifting up of the settling particles. The velocity v is usually kept between 30-60 cm/minute. At this velocity the suspended solid particles having diameter greater than 0.005 mm are not effective and settle in the bed without any obstruction.
Detention Period:
The detention period to be provided depends on the size of the suspended particle to be removed in the settling tank. Usually the detention period is kept between 1 to 3 hour. The percentage of removal of suspended solids depends on the detention period, more the detention period, more will be percentage removal of the particles.
The graph plotted between the detention period and B.O.D. and the percentage removal of the suspended solids is shown in Fig. 14.1.
The study of this graph will show that:
(i) The percentage rate of removal of the suspended solids and the B.O.D. is much higher in the beginning and goes on reducing with the detention period. This reveals that there is no use of providing the longer detention period, because longer detention period will increase the size and the overall cost of the settling tanks too high without giving much benefit.
If much longer detention period is provided, the anaerobic action may take place and the sewage will become septic causing smell nuisance and uplift of the settling particles due to the production of rising gases.
(ii) The percentage removal of the B.O. D. is small as compared with that of suspended solids. The main reason behind this is that the suspended solids responsible for B.O.D. are non-settleable and some are dissolved in sewage, and escape with the effluent leaving the settling tank.
(iii) If the sewage consists of high quantity of suspended solids, the percentage removal shall be more. On the other hand if the quantity is low, the percentage removal shall be low.
This indicates that the efficiency of the settling tank becomes higher with the concentration of the suspended solids in the sewage.
Settling Efficiency of Particles:
The settling efficiency of suspended matter in the sedimentation tanks is reduced by:
(i) Eddy currents, caused at the inlet by the incoming sewage.
(ii) Surface currents, caused due to the wind at the surface of the open tanks.
(iii) Vertical convection currents – caused due to difference in temperature at the bottom and top of the sewage in the tank.
(iv) Density current, caused due to difference in the density of sewage at various depths.
Grit Chambers:
The sewage contains large amount of gritty substance in addition to other matters. The main sources of grit are industrial wastes kitchens storm water runoff, pumping from excavations and ground water seepage.
Grit must be removed before pumping of sewage to other treatment units, otherwise it will cause an obstruction to the flow, will wear the pumping machinery and cause unsightly heavy accumulation of inert matter in the settling tanks, sludge-digestion tanks and inverted siphons.
The grit from the sewage is generally removed by Grit Chambers. Fig. 14.8 shows the essential of a grit chamber which is most commonly employed. It consists of 10-17 metres long narrow open channel. The sewage flows in this chamber horizontally with a constant velocity between 0.24 metre to 0.3 metre/second so that larger and heavier grit particles may settle down in the bottom but lighter organic solids can escape out with the sewage.
The depth of liquid in these chambers is usually kept between 1-1.3 metre. Weir type outlet is provided along with proportional flow weir and venturi devices. If the quantity of sewage is much more than one such chambers are constructed in parallel to each other in the shape of a battery.
Mostly grit chambers are designed to capture particles with specific gravity of 2.65 and a diameter of 2 × 10-2 cm. Near the outlet weir space is provided to store the grit, in the invert of the chamber.
The efficiency of grit chamber can be increased even during heavy fluctuations in the quantity of sewage reaching the chamber by the following methods:
(i) Adjusting the chamber according to the quantity of flow by breaking it up into a series of parallel channels.
(ii) Passing compressed air in grit channel so that settled organic solids may return to the sewage and escape out of the chamber.
(iii) Combining sedimentation of wanted particles with hydraulic rather than pneumatic scour or re-suspension of unwanted particles.
For hydraulic control there should be sufficient provision of adequate surface area and maintenance of adequate displacement velocity. Even during fluctuation of sewage flow, the value of Q/A (or surface loading) should be kept constant.
The cross-section of the chamber at right angles to the direction of flow should kept uniform. The displacement velocity should be held substantially constant at all depths of flow by placing flow control devices such as proportional-flow weir, a vertical throat or a standing-wave flume at the end of the chamber.
Two outlet control devices as shown in Fig. 14.8, a proportional flow weir and an adjustable throat should be provided in small grit chamber.
In small grit chambers, the accumulated solids are removed by hand or by flushing the grit into a disposal area. In large plants, the accumulated grit can be removed by mechanical grit conveyor.
Design of Grit Chambers:
Grit chamber can be designed on the rational basis, by considering it as a sedimentation tank, where discrete particles settle with their own settling velocity. The settling velocity is governed by the size and specific gravities of grit particles to be separated and the viscosity of the sewage.
The size of separation is based on the minimum size of the grit to be removed is 0.20 mm. But in case of sewage carrying ashes the minimum size may be taken as 0.15 mm. For design purpose the specific gravity of particles may be taken as 2.65, although it may be even 2.4. Formula 14.1 also holds good for it, which is,
Where SS = ρs/ρ = specific gravity
g = µ/ρ = kinematic viscosity in centistrokes.
Note:
I centipoise = 10 -2 poise or 10-2 (dynes)
(sec)/cm -2 = 10 -2 gm mass/(cw)(sec)
1 centistroke = 10-2 stroke or 10 -2 cm2/sec
Strokes law holds good for settling of particles of diameters less than 0.1 mm, in which case the viscous force dominates over inertial force. This is called ‘stream line’ settling. When the settling particles are larger than 1.0 mm, the nature of settling is called ‘turbulent’ and is governed by the Newton’s equation, which is:
Generally the grit particles lie between 0.1 -1.0 mm. The zone of settling corresponding to this range of particles is called ‘transition zone’ of settling. The relationship between settling velocity, size and density of particles, density and temperature of liquid medium in this transition zone is given by Hazen’s modified equation.
Where, t = temperature of liquid in °C.
If the specific gravity of grit is 2.65 and that of liquid is 1.0 respectively.
Overflow rate:
The efficiency of grit can be expressed as the percentage removal of grit above a specified particle size. A grit chamber designed to remove 100% of grit particles of smallest size would also remove all grit particles larger than this. For obtaining a 100% removal of the smallest size particles, it would be theoretically necessary for the detention time in the tank to equal the time required for the minimum sized particles to reach the tank bottom.
Table 14.1 gives settling velocities of different size particles of specific gravity 2.65 (mineral matter) and 1.20 (organic matter) and corresponding overflow rates for 100% removal of these particles based on Hazen’s modified equation.
The values given in table 14.1 should be corrected for any other temperature by the factor 3T + 70 / 100 usually value of two thirds to one half are used in design depending on the type of grit chamber. These values are much higher than those needed for organic solids of spec. gr. 1.2. The surface area of the grit chamber may be worked out on the basis of reduced loading rates.
Detention period:
Usually a detention period of 60 sec. is adopted.
Bottom scour and flow through velocity. The efficiency of the grit chamber is greatly affected by bottom scour. The scouring process itself determines the optimum velocity of flow through the unit. Actually there is a ‘critical’ velocity of flow ‘v’ beyond which particles of a certain size and density once settled, may be again placed in motion and reintroduced into the stream of flow.
The critical velocity for scour can be calculated by the following Schield’s formula:
For grit particle of 02 mm size, formula 14.10 gives critical velocity values of 17.1 to 25.6 cm/sec. In practice a horizontal velocity of 15.30 cm/sec. is used at peak hours. The horizontal velocity of flow should be maintained constant as other flow rates also to ensure that only organic solids and not the grit are scoured from the bottom.
Proportional flow weir:
The proportional flow weir is a combination of a weir and an artifice (Fig. 14.8). It maintains a nearly constant velocity in the grit channels by varying the cross-sectional area of flow through the weir so that the depth proportional to flow. The sides are so curved that the area decreases as the three half power of the increasing depth of flow over the weir.
Discharge Q in l.p.s. over the weir is given by:
Where, w = width of the opening at height h in m
C = discharge coefficient.
Loss of head:
The loss of head in a grit chamber varies from 0.06 to 0.6 m. depending on the device adopted for velocity control.
Disposal of Grit:
Clean grit is without odours. Washed grit may resemble particles of sand and gravel, interspersed with particles of egg shell, and other similar relatively inert materials from the households. Grit washing machines should be employed when the detention time is more and the flow velocity is less. Unless washed, it may contain considerable amount of organic matter. This attracts rodents and insects and also is unsightly and odorous.
The grit should be disposed of by dumping or burying or by sanitary land fill of low lying areas. The method used for the disposal of grit mainly depends on the characteristics of the grit, availability of the land for dumping, filling, or burial. If the grit is not washed, it should by disposed of by burial method only.
Detritus Tanks:
These are rectangular or square continuous flow settling tanks used for removing grit and fine sand at one time. This combined setting is done by giving slightly longer detention period of about 3-4 minutes and maintaining 0.3 metre/sec constant velocity. The settled solids are removed continuously by mechanical scrapers.
The light organic matter can be washed out from the detritus (mixture of grit and organic solids) by any one of the following ways:
(i) Passing compressed air through the deposited detritus and re-suspending the light solids.
(ii) After removing the detritus from the tank, it is washed with water, the wash water being sent with the effluent of detritus tank.
(iii) By placing the detritus on-a conveyor passing through the water, in such a way that organic solids are flushed back into the flowing sewage.