The following points highlight the four methods adopted for on-site disposal of sewage. The methods are: 1. Septic Tanks 2. Leaching Cesspools 3. Imhoff Tanks 4. Clarigester.
Method # 1. Septic Tanks:
A septic tank is a combined sedimentation and digestion tank where sewage is held for one to two days. During this period, the suspended solids settle down to the bottom. This is accompanied by anaerobic digestion of settled solids (sludge) resulting in reasonable reduction in the volume of sludge, reduction in biodegradable organic matter and release of gases like carbon dioxide, methane and hydrogen sulphide.
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The anaerobic decomposition gives rise to septicity or septic condition, and hence these tanks are known as septic tanks. Since foul gases such as hydrogen sulphide, methane and carbon dioxide are evolved during the digestion process, the tank is completely covered at the top, with a provision of a high vertical vent shaft for the escape of these gases.
The effluent from the septic tank although clarified to a large extent, is still sufficiently foul in nature, containing considerable amount of dissolved and putrescible organic solids and pathogens. Therefore the septic tank effluent disposal needs careful consideration.
Because of the unsatisfactory quality of the effluent and also the difficulty in providing a proper effluent disposal system, septic tanks are recommended only for individual homes and small communities and institutions whose contributory population does not exceed 300. For larger communities, septic tanks may be adopted with appropriate effluent treatment and disposal facilities.
Advantages and Disadvantages of Septic Tanks:
The advantages of septic tanks are as follows:
(i) Septic tanks can be easily constructed. No skilled supervision is required during their construction.
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(ii) There is no maintenance problem (except periodical cleaning) as there are no moving parts.
(iii) The cost is less and within the reach of private householders.
(iv) The sludge produced is relatively small, most of it being liquefied and digested. As compared to sludge of plain sedimentation tank, it is about 60% less in volume and about 30% less in weight.
(v) Performance of a properly constructed septic tank is very good. It can remove about 90% of BOD and about 80% suspended solids.
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(vi) Sludge, effluent and scum obtained from the septic tanks can be disposed of easily without causing serious nuisance.
(vii) When once installed they give long carefree service.
The disadvantages of septic tanks are as follows:
(i) The size required is large and uneconomical when serving more persons.
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(ii) If the tank is not properly functioning, the effluent is dark and foul smelling, and it is even worse than the influent.
(iii) Leakage of gases through the top of septic tanks may cause bad smell and air pollution,
(iv) Occasional removal of sludge adds to the maintenance cost and it is a tedious job.
(v) Working of septic tanks is unpredictable and non-uniform.
Method # 2. Leaching Cesspools:
A leaching cesspool or cesspool is a circular or rectangular underground chamber or tank, in which the top portion acts as an absorption field and the bottom portion acts as a septic tank. This is made possible by providing in the upper portion just below the inlet an open jointed lining of dry brickwork backed by 15 cm of coarse aggregate and 30 cm outer casing of coarse sand as shown in Fig. 17.8.
The sewage influent will pass through these linings and easily disperse into the surrounding soil without disturbing it. The bottom portion of the cesspool is, however, made watertight so as to retain the heavier sludge for septic action/digestion.
This is so made when the bottom of the cesspool is above the highest watertable. But when the bottom of the cesspool is below the watertable, it may not be made watertight except for the side walls made of brickwork with cement mortar joints.
Method # 3. Imhoff Tanks:
An Imhoff tank, designed and developed by Karl Imhoff of Germany, is an improved septic tank which provides for both sedimentation and sludge digestion. As shown in Fig. 17.9, the Imhoff tank is a two storey tank having two chambers. The upper chamber is called the sedimentation chamber or flowing-through chamber, through which sewage flows at a very low velocity so that sedimentation takes place.
The lower chamber is called the digestion chamber in which anaerobic or septic decomposition occurs. In general the process that takes place in an Imhoff tank is similar to that in a septic tank except that the Imhoff tank is so designed that the flowing-through upper chamber is separated from the lower digestion chamber with the result that the incoming sewage or influent is not allowed to get mixed with the sludge produced, and the outgoing sewage or effluent is not allowed to carry with it large amount of the suspended matter as in the case of a septic tank.
The solids of the slow moving sewage settling down to the bottom of the sedimentation chamber are made to fall in the digestion chamber through sloping bottom walls (slope about 60°) and an entrance slot at the lowest point.
The slot (with minimum width as 15 cm) is trapped or overlapped in such a way that the gases generated in the digestion chamber do not enter the sedimentation chamber, and thus avoids direct contact of sewage with the foul gases and its consequent pollution.
For the escape of gases a gas vent also called scum chamber is provided above the digestion chamber alongside the sedimentation chamber (see Fig. 17.9). Some other configurations of Imhoff tanks are shown in Fig. 17.10.
In order to prevent the sludge from entering the sedimentation chamber, the sludge must be maintained at a distance of at least 45 cm below the slot. This clear zone of 45 cm depth (minimum) is called the neutral zone. Further in order to prevent the scum from entering the sedimentation chamber the scum must be maintained at a distance of at least 45 cm above the slot.
The digestion chamber is divided into a number of (usually three to four) interconnected compartments. The bottom of each compartment is made up in the form of an inverted cone or hopper with sides sloping at 1:1 so as to concentrate the sludge at the bottom of the hopper.
In order to ensure uniform distribution of solids settling in different hoppers, the flow of sewage in the sedimentation chamber is reversed intermittently as shown in Fig. 17.9. The digested sludge is removed periodically through sludge removal pipe provided in each compartment.
Minimum diameter of sludge removal pipes is about 15 to 20 cm. Sludge may be removed either under hydrostatic pressure or by pumping. The entire sludge is not removed and only the sludge in the bottom layers which is completely digested is withdrawn, leaving behind some sludge loaded with anaerobic bacteria to act as seed sludge. The removed sludge may be disposed of by some suitable method of sludge disposal.
Design Considerations:
In designing Imhoff tanks, following considerations are made-
(i) Sedimentation Chamber:
It is rectangular in shape with the following specifications:
(a) Detention Period:
2 to 4 hours, usually 2 hours
(b) Flowing through Velocity:
It should not exceed 30 cm per minute
(c) Surface Loading:
It should not exceed 30 000 litres per m2 per day. It may, however/ be increased to about 45 000 litres per m2 per day for the effluent coming from activated sludge plant or where recirculation is adopted.
(d) Length and Width:
Length should preferably not exceed 30 m, so as to provide good sludge distribution. Length to width ratio between 3:1 to 5:1.
(e) Depth:
To be kept shallow to permit particles falling to the slot before reaching the end of the sedimentation chamber. General depth to be 2.5 to 3.5 m so as to limit the total depth of the tank to 9 to 10.5 m.
(f) Free Board- 45 cm
(ii) Digestion Chamber:
(a) Capacity:
This chamber is designed for a minimum capacity of 0.057 m3 per capita. However, for warm climates since shorter periods between successive sludge withdrawls are possible the capacity may be reduced to about 0.035 to 0.040 m3 per capita. Further this chamber is generally designed to store sludge for 1 to 6 months, common value being 2 months or so.
(b) Gas Vent or Scum Chamber:
Surface area of gas vent or scum chamber should be about 25 to 30 per cent of the area of horizontal projection of the top of the digestion chamber. Minimum width of vent should be 60 cm.
Advantages and Disadvantages of Imhoff Tanks:
The advantages of Imhoff tanks are as follows:
(i) They are quite economical in operation.
(ii) They do not require skilled supervision during operation.
(iii) There are no moving parts.
(iv) They require only preliminary treatment.
(v) The results obtained are good, with about 60% removal of the suspended solids and about 30% removal of the BOD.
(vi) There is no difficulty in sludge removal. Also the weight and volume of sludge are less.
The disadvantages of Imhoff tanks are as follows:
(i) The depth of these tanks is more and this aspect may make their construction uneconomical in hard soils.
(ii) These tanks may give out offensive odours when improperly, operated.
(iii) These tanks have a tendency to foam or boil. This may cause the scum to go up to the top of the tank and it may also force the sludge particles to enter the sedimentation chamber through the slot. The foaming may thus adversely affect the quality of effluent.
(iv) Imhoff tanks are unsuitable and do not function properly where sewage is highly acidic in character.
(v) There is no adequate control over the operation of Imhoff tanks. This makes them unsuitable for use in large treatment plants where separate sludge digestion tanks are preferred in addition to sedimentation tanks.
Imhoff tanks are, therefore, useful only for small cities and institutions, where it is not possible or economical to install separate sludge digestion tanks. However, the use of these tanks has greatly decreased and they have become obsolete these days.
Method # 4. Clarigester:
A clarigester is a modified Imhoff type tank. As shown in Fig. 17.11 it is a circular double storey tank, without hopper bottom and fitted with mechanical sludge and scum breaking equipment. The sewage after preliminary treatment enters the influent well through the inlet pipe and gets distributed in the sedimentation chamber or clarifier in the upper portion of the tank.
The settled sewage flows over the weir and passes into the effluent channel located along the periphery of the tank. The effluent is taken for the secondary treatment. The scum collected on the surface of the liquid in the sedimentation chamber is removed by the skimmer. The sludge gets settled on the sloping bottom of the sedimentation chamber.
This sludge is collected in a hollow at the centre of the chamber by the scraping blades. The sludge from the hollow falls into a sludge-digestion chamber below. The sludge in the digestion chamber is stirred and agitated by the moving arms provided at top and bottom of the chamber.
When the arms rotate slowly by the driving unit located on the top of the clarigester, the upper arms break the scum whereas the lower arms scrape the digested sludge at the bottom of the chamber to the central sludge pocket. The fully digested sludge is removed through the sludge outlet pipe under hydrostatic pressure.
The sludge may be heated for thorough digestion by means of heating coils provided in the digestion chamber. The sludge gas produced is collected in a gas dome and utilized for various purposes.
The clarigesters may be used for treating limited sewage from small and isolated colonies, institutions, hospitals, etc. The results obtained from clarigesters are same as those obtained from Imhoff tanks. Dorr-Oliver Co. of N.Y., manufactures equipment required for clarigesters of diameters 4 to 12 m to treat the sewage for a population of 1500 to 20 000 at the rate of 115 litres per capita per day.
The advantages of clarigesters are as follows:
(i) Lesser depth is required because of the elimination of hopper bottom.
(ii) Costs of construction and maintenance are low.
(iii) Operation is Continuous and uniform.
(iv) Mechanical handling of sludge reduces the labour requirements,
(v) Digestion of sludge may be done at a high temperature by heating if necessary,
(vi) Digestion of sludge is effective, rapid and satisfactory
(vii) Stirring of sludge results in the formation of more gas
(viii) No scum problem and no foaming problem,
(ix) No special attention is required as all the operations are done by mechanical means.