For the septic tank to function as an efficient suspended solids remover it should be of sufficient capacity with proper inlet and outlet arrangements. It should be designed in such a way that the sludge can settle at the bottom and the scum accumulates at the surface, while enough space is left in between, for the sewage to flow through without dislocating either the scum or the settled sludge.
As such a properly designed septic tank should provide for the following:
ADVERTISEMENTS:
(a) Space for sewage retention to allow sedimentation;
(b) Space for digestion of settled sludge; and
(c) Space for storage of digested sludge and scum accumulating in between successive cleanings or sludge withdrawals at intervals varying from 6 months or 2 years.
In order to provide for the above indicated requirements the following considerations are made in the design of septic tank:
1. Sewage Flow:
The maximum sewage flow into a septic tank is based on the number of plumbing fixtures discharging simultaneously. For this purpose various sanitation facilities are equated in terms of fixture units. A fixture unit is a quantity in terms of which the load producing effect of different plumbing fixtures on the plumbing system are expressed on some arbitrarily chosen scale. In the design of septic tank a fixture unit is taken as 9 litres per minute. Table 17.1 gives the equivalent fixture units of different sanitation facilities.
The estimated number of fixture units and the number of fixture units that contribute to the peak discharge in small installations serving upto 20 persons, for residential housing colonies upto 300 persons and for hostels and boarding schools are given in Tables 17.2, 17.3 and 17.4 respectively.
2. Detention Period:
In order to provide sufficiently quiescent conditions for effective sedimentation of the suspended solids, the minimum detention period for the sewage in a septic tank should be 24 hours. However, considering the volume required for sludge and scum accumulation, a septic tank is usually designed for a detention period of 24 to 48 hours.
3. Tank Capacity:
The capacity of a septic tank may be determined by the following methods-
ADVERTISEMENTS:
A. Rational method
B. Detention Period Method
A. Rational Method:
In this method the capacity of septic tank is determined on the basis of space required for sedimentation, sludge digestion and storage of digested sludge as indicated below:
ADVERTISEMENTS:
(a) Sedimentation:
For Indian conditions, at a temperature of 25°C the surface area of the tank required will be 0.92 m2 for every 10 litres per minute of peak flow rate. A minimum depth of sedimentation should be 25 to 30 cm.
(b) Sludge Digestion:
The suspended solids entering the tank may be taken as 70 gm per day per capita. The capacity required for sludge digestion is 0.033 m3 per capita at 25°C.
(c) Storage of Digested Sludge:
The volume of digested sludge is normally 0.00021 m3 per capita per day.
The total capacity of the tank will be equal to the sum of the above three requirements plus a free board of not less than 30 cm to provide additional capacity required for the storage of scum at the top. No additional capacity for seed sludge is provided if care is taken during desludging to leave a portion of the digested sludge at least 25 mm in depth at the tank bottom which acts as the seeding material for the fresh sludge.
Thus for example the total capacity of a septic tank to be provided for 10 persons will be as follows:
(i) Sedimentation:
Peak flow for 10 persons will be 18 1pm (see Table 17.2), and hence the required surface area of the tank = (0.92/10) x 18 = 1.656 m2. Keeping a depth of 30 cm for sedimentation, volume = (1.656 x 0.3) m3 = 0.4968 m3 ≃ 0.50 m3
(ii) Sludge Digestion:
Volume required for sludge digestion = (0.033 x 10) m3 = 0.33 m3
(iii) Storage of Digested Sludge:
For an interval of 1 year between successive cleanings, the volume required for storage of digested sludge = (0.00021 x 10 x 365) m3 = 0.77 m3
(iv) Free Board:
Volume corresponding to free board = (1.656 x 0.3) m3 = 0.4968 m3 ≃ 0.50 m3.
∴ Total capacity of the septic tank to be provided for 10 persons
= (0.50 + 0.33 +0.77 + 0.50) m3
= 2.10m3
The capacity of septic tank obtained by rational method provides a detention period of 24 to 48 hours based on an average daily flow of sewage.
B. Detention Period Method:
In this method average daily per capita rate of flow of sewage is estimated by first determining the daily per capita water consumption. The average daily per capita rate of flow of sewage is multiplied by the number of users and the detention period in days which gives the capacity of the tank required to store the sewage produced during the detention period.
In addition to this space is also required for the sludge accumulating in the tank during the period between successive cleanings. The space for accumulating sludge is usually kept at the rate of 15 to 45 litres per capita per year (with average value of 30 litres per capita per year). The total capacity of the tank will be equal to the sum of the above indicated two requirements plus a free board of not less than 30 cm to provide additional capacity required for the storage of scum at the top. As a thumb rule 1/6 to 1/5 m3 volume should be provided for every user.
4. Dimensions of Septic Tank:
The septic tanks are normally rectangular in shape with minimum width of 0.75 m and length 2 to 4 times the width. The liquid depth is 1 to 2 m with a minimum value of I m. The recommended sizes of septic tanks for individual households (upto 20 users), for housing colonies (upto 300 users) and for hostels and boarding schools are given in Tables 17.5, 17.6 and 17.7 respectively.
The sizes of the septic tanks given in Tables 17.5, 17.6 and 17.7 are based on the assumption of peak discharges indicated in Tables 17.2, 17.3 and 17.4 and while choosing the size of septic tank exact calculations should be made.
5. Sludge Withdrawal and Disposal:
Sludge may be withdrawn from a septic tank either half yearly or yearly. For small domestic tanks, for economic reasons, deslodging may be done once in 2 years provided the tank is not over loaded due to use by more than the number for which it is designed. Frequent desludging inhibits the anaerobic action in the tank.
Normally the tanks are cleaned when the sum of the depth of accumulated scum and sludge is observed to exceed half the depth of the tank. A portion of the sludge at least 25 mm in depth should be left behind in the tank bottom which acts as the seeding material for the fresh sludge.
The sludge from septic tanks is black in colour, and unless well digested by long storage, it is offensive because of the hydrogen sulphide and other gases it gives off. The digested sludge is withdrawn through a sludge pipe of at least 15 cm diameter under a hydrostatic pressure of at least 45 cm.
The sludge pipe which is installed at the bottom of the tank is provided with a delivery valve to draw the sludge as and when required and it delivers the sludge to a sump for subsequent disposal on land or sent for further treatment. The sludge can be dried on porous beds if spread out in thin layers.
Spreading of sludge on the ground in the vicinity should not be allowed. Portable pumps may also be used for deslodging in which case there will be no need for sludge pipe or sludge sump. As far as practicable manual handling of sludge should be avoided. If possible particularly in case of densely populated large cities, mechanical vacuum tankers should be used by the municipal authorities to empty the septic tanks.
6. Construction Details:
For smaller capacities (usually upto 2000 litres) a septic tank with a single compartment may be constructed, but for larger capacities, a two-compartment tank constructed with the partition wall at a distance of about two-thirds the length from the inlet gives a better performance than a single compartment tank.
The two compartments are interconnected above the sludge storage level by means of pipes or square openings of diameter or side length respectively of at least 75 mm. Typical sketches of single compartment and two compartment septic tanks are shown in Figs. 17.1, 17.2 and 17.3.
Fig. 17.3 Typical Sketch of Two Compartment Septic Tank for over 50 Users.
Septic tanks may be constructed either in brick work or stone masonry or cement concrete cast in situ or precast.
The floor of the tank is of cement concrete and it is provided with a minimum slope of 1 in 10 towards the sludge outlet to facilitate desludging. Both the floor and the side walls are plastered to rendered the surface smooth and to make them water tight.
Septic tanks are provided with watertight covers of adequate strength. Access manholes of adequate size are provided for the purposes of inspection and desludging of tanks.
Every septic tank is provided with a ventilation pipe of height extending at least 2 m above the top of the highest building within a radius of 20 m. The top of the ventilation pipe is covered with a suitable mosquito proof wire mesh.
The inlet and outlet are located at the two opposite ends of the tank and at different levels to avoid short circuiting. This is further ensured by using pipe tees with submerged ends as inlet and outlet (see Figs. 17.1 and 17.2) or by providing baffles in front of the inlet and before the outlet (see Fig. 17.3). The baffles should dip 25 to 30 cm into and project 15 cm above the liquid. The invert of the outlet pipe should be placed at a level 5 to 7 cm below the invert of inlet pipe.
7. Disposal of Septic Tank Effluent:
The septic tank effluent is malodorous, containing sizable portion of dissolved organic content and pathogenic organisms, and hence it should be disposed of carefully so as to avoid any nuisance or risk to the public health. The septic tank effluent is usually disposed off by land disposal methods or soil absorption methods which are described below.
Land Disposal Methods or Soil Absorption Methods:
In the land disposal methods the septic tank effluent is allowed to be soaked or absorbed by the surrounding soil. Satisfactory disposal therefore depends, to a great extent on porosity and percolation characteristics of the soil. In addition, other factors, such as level of subsoil water table, the climatic conditions, presence of vegetation, aeration of solids and concentration of suspended solids in the effluent also influence the application of these methods.
The land disposal methods are of two types:
(i) Soak pits or seepage pits, and
(ii) Dispersion trenches. Both these methods can be adopted in all porous soils where soak percolation rate is below 25 minutes per cm and the depth of water table is 2 m or more from the ground level.
Dispersion trenches are preferred in soils, with percolation rates between 12 and 25 minutes per cm if adequate land is available. Further in areas with higher water table dispersion trenches should be located partly or fully above ground level, in a mound.
The total subsurface soil area required for soak pits or dispersion trenches is given by the following empirical relation:
In calculating the effective leaching area required, only area of trench bottom in case of dispersion trenches and effective side wall area below the inlet level for soak pits should be taken into account.
1. Soak Pits or Seepage Pits:
Soak pits or seepage pits are cheap to construct and are extensively used. The pits may be of any regular shape, but circular shape is more common. These are preferred only when water table is sufficiently below ground level, land available is limited or when a porous layer underlies an impervious layer at the top, which permits easier vertical downward flow than horizontal spread out as in the case of dispersion trenches.
The minimum horizontal dimension of soak pit should be 1 m, and the depth below the invert level or inlet pipe being at least 1 m. The pit should be covered at the top and the top should be raised above the adjacent ground to prevent damage by flooding. The soak pits may be lined or unlined. The lined soak pits are kept empty while unlined soak pits are filled with rubble or brick bats.
2. Dispersion Trenches:
Dispersion trenches consist of relatively narrow and shallow trenches about 0.5 to 1 m deep and 0.3 to 1 m wide excavated to a slight gradient of about 0.25%. Open jointed earthenware or concrete pipes of 80 to 100 mm diameter are laid in the trenches over a bed of 15 to 25 cm of washed gravel or crushed stone.
The top of pipes is covered by coarse gravel or crushed stone to a minimum depth of 15 cm and the remaining depth of trench is filled with excavated earth and finished with a mound above the ground level to prevent direct flooding of trench during rains. The effluent from the septic tank is led into a small distribution box from which it is distributed to a number of trenches.
Usually one distribution box is provided for about 3 to 4 trenches. The total length of trench required is calculated by using equation 17.1 and the number of trenches worked out on the basis of a maximum length of 30 m for each trench and spaced not closer than 2 m apart.
In certain areas due to dense soil conditions, high water table and limited availability of land, disposal of the septic tank effluent by the land disposal methods or soil absorption methods may not be possible. In such cases the septic tank effluent may be disposed of after it is given a secondary treatment. The treated effluent being free from odour and other harmful effects can be safely disposed of in a water body or by spreading on land surface.
For secondary treatment of septic tank effluent the following two methods are adopted, which are described below:
(i) Upflow anaerobic filters, and
(ii) Biological filters,
(i) Upflow Anaerobic Filters:
An upflow anaerobic filter is a submerged filter with stone media, 0.6 to 1.2 m deep. The septic tank effluent is introduced from the bottom. The microbial growth is retained on the stone media making possible higher loading rates and efficient digestion. The capacity of unit is 0.04 to 0.05 m3 per capita or 1/3 to 1/2 the liquid capacity of the septic tank it serves.
The BOD removal is about 70%. The effluent obtained from the filter is clear and free from odour.
The unit has several advantages viz.:
(a) A high degree of stabilization;
(b) Little sludge production;
(c) Low operating and capital cost; and
(d) Low loss of head in the filter (about 10 to 15 cm) in normal operation.
The upflow anaerobic filter can either be a separate unit or constructed as an extended part of septic tank.
(ii) Biological Filters:
These filters are similar to trickling filters. The septic tank effluent is sprayed on the filter through a rotary distributor. The filter media consists of coarse stones and is about 0.9 to 1.4 m in thickness. A microbial film is formed around the stone media which requires ample ventilation to get required oxygen for bio-oxidation. To fulfil this requirement the underdrains are connected to air vents situated atleast 0.15 m above the ground level.
The volume of the filter is calculated on the basis of the following data:
