The disposal of sewage without treatment or after treatment may be carried out by the following two methods: 1. Dilution or the Disposal of Sewage in Water 2. Land Treatment or the Disposal of Sewage on Land.
Method # 1. Disposal by Dilution:
In this method, the raw sewage or the treated sewage (or the effluent from treatment plants) is discharged into natural water bodies such as streams or rivers, lakes, sea, etc., having large quantity of water. The discharged sewage, in due course of time, is purified by what is known as self-purification process of natural water bodies.
Sewage discharged into a natural water body is mainly got rid of by its dilution or dispersion into the body of water with large volumes of water contained in it. The main action involved in this process of purification is due to the forces of purification operating in the sewage polluted waters. As such disposal by dilution may be referred to as the treatment by natural purification in water.
In order to achieve better efficiency of purification the amount of raw sewage to be discharged into water body may have to be controlled, and also the raw sewage may be given partial treatment before being discharged into water body.
The limit of discharge of raw sewage into water body and the degree of treatment to be given to raw sewage before being discharged into water body will depend not only on the quality of raw sewage but also on the self-purification capacity of the water body as well as the intended use of its water on the downstream side.
Conditions Favouring Disposal of Raw or Untreated Sewage by Dilution:
The following conditions are favourable for the disposal of raw or untreated sewage by dilution:
(i) Where sewage is comparatively fresh, i.e., it is discharged within 4 to 5 hours of its production.
(ii) Where the floating matter and the settle able solids have been removed from the sewage to be discharged.
(iii) Where the water body has large volume of water in comparison to the volume of the sewage to be discharged.
(iv) Where it is possible to thoroughly mix or diffuse sewage through diluting water.
(v) Where diluting water has high content of dissolved oxygen (DO).
(vi) Where swift forward currents are available in diluting water so that sewage is easily carried away to the point of unlimited dilution. On the other hand, slow back currents tend to cause sedimentation, resulting in large sludge deposits.
(vii) Where the natural water body having large volumes of water is available in near vicinity.
(viii) Where the water body is not to be used as source of water supply for at least some reasonable distance on the downstream from the point of sewage disposal.
(ix) Where sewage does not contain industrial sewage having toxic substances.
(x) Where the water body is not to be used for the purpose of navigation for at least some reasonable distance on the downstream from the point of sewage disposal.
Conditions Necessitating Treatment of Sewage before Disposal by Dilution:
The following conditions necessitate the treatment of sewage before its disposal by dilution:
(i) Where sewage contains such substances which are detrimental to the aquatic life in the receiving water body.
(ii) Where sewage contains industrial sewage containing toxic substances, and where industrial sewage is quite warm.
(iii) Where the volume of diluting water is insufficient,
(iv) Where the receiving water body is to be used for navigation,
(v) Where the receiving water body is to be used as a source of water supply,
(vi) Where sewage is not fresh but is stale.
(vii) Where sewage is not likely to be dispersed easily due to tides, winds, cross currents, etc.
Standards of Dilution:
The ratio of the quantity of diluting water to that of sewage is known as dilution factor or dilution ratio. Depending upon the value of dilution factor, the Royal Commission on Sewage Disposal has laid down certain standards of purification required or degrees of treatment required to be given to sewage before its disposal into water body. These standards are indicated in Table 9.1.
The standards given in Table 9.1 have been operative in England since 1912 and have also been followed in our country without much variance. However, with the increasing hazard of pollution of surface water bodies due to indiscriminate discharges of industrial sewage into them, there has been a growing need to evolve suitable standards which may enable the enforcement of suitable regulations against water pollution.
Bureau of Indian Standards [formerly known as Indian Standards Institution (ISI)] has, therefore, framed standards which lay down the tolerance limits, for discharging domestic and industrial sewage effluents into inland surface waters, of such characteristics as total solids, hydrogen ion concentration, BOD, oils and grease, cyanides, sulphides, radioactive materials and various minerals such as arsenic, barium, cadmium, zinc, chromium, etc.
These standards are as indicated below:
(i) Tolerance limits for sewage effluents discharged into inland surface waters.
(ii) Tolerance limits for industrial effluents discharged into inland surface waters, Part 1 General limits; and Parts 2 to 10 Limits for different industries.
The tolerance limits for sewage effluents discharged into inland surface waters are as indicated in Table 9.2.
The tolerance limits for odour have not been prescribed in this standard but it is recommended that as far as practicable, unpleasant odours should not be present in the sewage effluent. Further no limits have been laid down in this standard for pathogenic bacteria, but it is expected that if the limits for total suspended solids and BOD are satisfied, the bacterial load of the sewage effluent will be reduced.
The tolerance limits for industrial effluents discharged into inland surface waters are as indicated in Table 9.3
The tolerance limits for colour and odour have not been prescribed in this standard but it is recommended that as far as practicable, colour and unpleasant odours should not be present in the industrial effluents.
Besides these standards, Bureau of Indian Standards has also framed the following standard.
Tolerance limits for inland surface water subject to pollution.
This standard prescribes the tolerance limits for inland surface waters subject to pollution due to discharge of industrial and sewage effluents into inland surface waters which are used for the following purposes:
(a) Drinking water source without conventional treatment followed by disinfection;
(b) Outdoor bathing;
(c) Drinking water source with conventional treatment followed by disinfection;
(d) Fish culture and wild life propagation; and
(e) Irrigation, industrial cooling and controlled waste disposal.
Types of Natural Water Bodies:
Various natural water bodies into which sewage can be discharged for dilution are as follows:
(3) Ground waters;
(5) Ocean or sea;
(6) Perennial rivers or streams.
A creek is a narrow inlet or bay on sea coast. It may not have dry weather flow during some period of the year, and hence disposal of sewage into such a creek should be done with great care.
The wide lower tidal mouth of a river is known as estuary. The dilution of sewage in estuaries is affected by ocean or sea water in addition to river water. Hence the process of dilution of sewage in estuaries is generally satisfactory.
The sewage, when applied on land, ultimately filters out through different layers of soil and it meets groundwaters at great depth. If groundwater flows through favourable strata of soil, the dilution of sewage is satisfactory.
A lake is an enclosed body of water which may be used for the purpose of dilution of sewage. In some cases, lakes are used for dual purposes, namely, for the supply of water and for the disposal of sewage. In such cases, the location of sewage discharge point should be carefully decided so as not to affect the water supply intake. Various characteristics of lake such as its size, shape, nature of surrounding area, volume of fresh water flow in it, etc., should be carefully studied before using it for disposal of sewage.
(5) Ocean or Sea:
Ocean or sea has water in abundance and hence its capacity to dilute sewage is practically unlimited. Moreover, sewage of any quality can be diluted in ocean or sea. Since the saturation concentration of dissolved oxygen (DO) in water decreases with increase in salt content, the saturation concentration of dissolved oxygen (DO) in sea water is approximately about 80% of that in ordinary water.
In addition to this deficiency, the temperature of sea water is lower than the sewage temperature, and its specific gravity is higher. Due to these reasons, when sewage is discharged into sea water, the lighter and warmer sewage rises up to the surface, resulting in spreading of the sewage at the top surface of sea in a thin film or sleek.
Further sea water contains a large amount of dissolved matter which chemically react with the sewage solids, resulting in the precipitation of some of the sewage solids, giving a milky appearance to the sea water and also in the formation of sludge banks. These sludge banks and thin milky layer formed at the top surface of sea water produce offensive hydrogen sulphide (H2S) gas by reacting with sulphate rich water of the sea.
Hence, extreme care should be taken while discharging sewage in sea and following points should be noted:
(i) There should be sufficient depth of water at the point of sewage discharge into the sea.
(ii) The sewage should be discharged deep into the sea and at a distance of 1 to 1.5 km away from the shore so as not to cause any nuisance at the sea shore.
(iii) The sea outfall for sewage should be placed on a firm rocky foundation.
(iv) The outfall should be so designed that proper dilution of sewage with sea water is accomplished before it tries to rise to the surface. This is accomplished by providing a diffuser at the end section. At the end of the outfall sewage is released in a simple stream or jetted through a manifold or multiple-point diffuser.
(v) The sewage should be discharged below low water level and only at the time of low tides. This is accomplished by holding sewage into large sized tanks constructed near the sea shore during the high tides and releasing the same during the low tides.
(vi) While deciding the position of sea outfall, the ocean current and direction of velocity of wind should be taken into consideration. The point of sewage discharge should be such that sewage is carried away from the shore and there is no nuisance from the blowing of wind.
(6) Perennial Rivers or Streams:
Perennial rivers or streams possess some flow throughout the year. However, the flow rate varies considerably from a minimum flow during summer to a maximum flow during rainy season. As such during summer the dilution factor is low, and also high temperature of water results in low solubility of oxygen. The sewage, under such circumstances, should be properly treated before dilution.
Table 9.4 gives a comparison of sewage dilution by ocean or sea and river or stream-
Method # 2. Disposal by Land Treatment:
In this method, the raw sewage or the partly treated sewage is applied on land.
The disposal of sewage by land treatment may be accomplished in the following three ways:
(1) Irrigation or sewage farming;
(2) Overland flow; and
(3) Rapid infiltration or Infiltration-percolation.
(1) Irrigation or Sewage Farming:
Irrigation involves the controlled discharge of sewage to the land to support plant growth (Fig. 9.4a). Besides the disposal of sewage, this method may help to increase crop yield. It is found that it is possible to have 33 per cent or so more yield under sewage irrigated crops than under the canal or well irrigation.
This is so because sewage generally contains a lot of fertilising elements such as nitrogen, phosphate, potash, etc., which add to the fertility of the soil. Crops such as cotton, sugarcane, plantain, potatoes and grass can be profitably grown on sewage irrigated lands. However, the sewage effluent before being used for irrigation must be made safe.
Sewage can be applied by the following three methods of irrigation:
(a) Sprinkler or spray irrigation;
(b) Sub-surface irrigation; and
(c) Surface irrigation.
(a) Sprinkler or Spray Irrigation:
In this method, sewage is spread over the land through nozzles which are fitted at the tips of pipes. Sewage is sprinkled under pressure and the process can also be used for watering gardens and lawns. Hydrants for the supply of sewage under pressure may be located at suitable intervals so as to cover the entire field. This method of sewage application is useful for sandy soils and also for hilly land having steep slopes.
(b) Sub-Surface Irrigation:
In this method, sewage is supplied directly to the root zone of plants through a system of underground pipes with open joints. Sewage, as it flows through these pipes, percolates through the open joints, and it is distributed in the surrounding area through the capillary action.
This method is useful for places where rain fall is poor and demand for irrigation is high and subsoil water level is low. It entails less loss of water due to evaporation and absorption. However, this method is costly and it gives less yield of crops. Hence it is not generally adopted in practice.
(c) Surface Irrigation:
In this method, sewage is applied directly on the land.
This method is widely adopted in practice and its different modes of application are as follows:
(i) Basin method;
(ii) Flooding method; and
(iii) Furrow method
The selection of the method of sewage application will depend on crops to be raised, characteristics of soil, topography of country and quantity of available sewage.
(i) Basin Method:
In this method, basins are constructed around the plants and they are filled by sewage. The sewage slowly percolates to the root zone of plants and maintains the root zone in moist or damp condition. This method is useful for orchards or gardens of fruit trees.
(ii) Flooding Method:
In this method the land is divided into rectangular plots of convenient dimensions. Sewage is distributed over these plots to a depth of 30 cm to 60 cm. Subsoil drain pipes are provided to supply air to the soil and to remove the percolated effluent through the soil.
(iii) Furrow Method:
In this method, furrows and ridges are formed. Furrows are very small ditches having depth of about 30 to 50 cm and width of about 120 to 150 cm. Ridges have length of about 15 to 30 m and width of about 120 to 250 cm.
Furrows are filled with sewage to about two-third of their depth. Sewage from two adjoining furrows percolates from their sides and beds and thus cause saturation of root zones of plants which are grown on the ridges. Subsoil drain pipes are provided to collect percolated effluent and lead it to nearby natural waters for disposal. This method is useful when sewage is not to be kept in contact with beds of crops.
The surface irrigation is also known as broad irrigation or effluent irrigation. Broad irrigation is similar to sewage farming, but the purposes of the two are different. In sewage farming prime consideration is the successful growing of the crops while in broad irrigation the prime consideration is the successful disposal of sewage.
However, in both the cases the ultimate result is the same i.e., crop is raised and at the same time sewage is disposed of by land application. In sewage farming raw sewage is not used and it is necessary to remove the ingredients which may prove harmful and toxic to the crops grown.
On the other hand in broad irrigation raw or settled sewage is applied on vacant land which is provided underneath with a system of properly laid under drains. These under drains, usually consist of 15 to 20 cm diameter, porous pipes laid with open joints at a spacing of 12 to 30 cm. The effluent collected in these drains after getting filtered through the soil pores is generally small (as a large quantity gets evaporated) and well stabilized, and can be easily disposed of into a natural water body without any further treatment.
However, in general, for all practical purposes, both the terms viz., broad irrigation and sewage farming are used as synonyms, and both means use of sewage effluents for irrigating crops.
When raw or partly treated sewage is applied on land, a part of it evaporates, and the remaining part percolates into the ground. While percolating through the soil, the suspended solids present in sewage are held in the soil voids. If in the soil voids proper aeration is maintained, the organic sewage solids held in these voids get oxidized by aerobic process.
Such aeration and aerobic conditions will more likely prevail, if the soil is sufficiently porous and permeable such as sands and porous loams. On the other hand if the land is made up of heavy, sticky and fine grained soils such as clay and clay loam, the void spaces will soon get clogged, thus resulting in non-aeration of these voids.
This will lead to the developing of non-aerobic decomposition of organic matter, and evolution of foul gases. Moreover, excessive clogging of voids may also result in ugly ponding of sewage over the farm land, where mosquitoes may breed in large number, causing further nuisance.
Application of too strong or too heavy load of sewage will also result in early development of anaerobic conditions. This is so because greater is the sewage load, more likely it will be for the soil to get clogged early. As such if the sewage load is reduced either by diluting it or by pre-treating it, it may be possible to avoid the clogging of the soil pores.
The degree of treatment required will, however, depend on the type of soil of the land. Thus if the soil of the land to be irrigated is sandy and porous, the sewage effluents may contain more solids thereby requiring lesser treatment as compared to the case where the soil is less porous and sticky.
When industrial sewage effluents are applied on land for irrigation it is imperative that soils on which the effluents are applied are studied periodically from the viewpoint of physico-chemical characteristics to ensure that the soils are not damaged and the ground waters are not polluted.
The Indian Standard IS: 3307- 1977 recommends the hydraulic loading rates or dosage of settled industrial sewage effluents applicable for different types of soils, which are given in Table 9.7. These loading rates should also take into account the nature of crop and its water requirements, climatic conditions and frequency of application.
The extent of land area required for disposing of a certain volume of sewage effluent can be worked out from the values given in Table 9.7.
Further in order to regulate disposal of industrial sewage effluents on land for irrigation it is necessary to limit certain constituents in effluents, especially those considered toxic, so that the effluent may comply with normally accepted irrigation water quality. With this objective in view Bureau of Indian Standards (formerly known as Indian Standards Institution (ISI)) has laid down certain tolerance limits for industrial effluents discharged on land for irrigation purposes in the Indian Standard IS: 3307-1977 and the same are indicated in Table 9.8.
(2) Overland Flow:
Overland flow involves the controlled discharge of sewage onto land having a slope of 2 to 8 per cent, where it flows in a thin layer down the grade and appears as runoff (Fig. 9.4b) which is collected and disposed of. This method is adopted when the land is relatively impermeable. The land is generally planted with a grass cover crop to provide a habitat for micro-organisms, to serve as a living filter, and to prevent erosion.
(3) Rapid Infiltration or Infiltration-Percolation:
Rapid infiltration or infiltration-percolation involves the application of sewage to spreading basins, where it is allowed to percolate down to the groundwater (Fig. 9.4c). Thus besides sewage disposal this method is useful for groundwater recharge. To be effective, the underlying soils must be highly permeable. In order to maintain an adequate infiltration capacity, the basins are operated on an intermittent basis. The operating and resting period may vary from a few days up to six months.
Conditions Favourable for Disposal of Sewage by Land Treatment:
The conditions favourable for the disposal of sewage by land treatment are as follows:
1. When natural water bodies such as rivers or streams, ponds, etc., are not available in near vicinity, the land treatment is the only alternative left, and has to be adopted.
2. When large areas of open land with sandy, loamy or alluvial soil overlying soft murram are available, land treatment is favoured. Such soils are easily aerated and it is easy to maintain aerobic conditions in them.
3. When irrigation water is scarce, the use of sewage for irrigating crops is a good alternative.
5. Land treatment is favoured when subsoil water table is low even during the wet season.
6. When large open areas in the surrounding locality are available for practising broad irrigation by sewage.
7. When there is demand for cash crops which can be easily grown on sewage farms.
8. When large areas of land with suitable soil conditions are available which will permit the use of sewage for groundwater recharge.
When sewage is applied continuously on a piece of land, pores or voids of the soil get filled up or clogged, thereby free circulation of air is prevented and anaerobic conditions develop. At this stage, the land is unable to take any further sewage load, and due to anaerobic decomposition of organic matter foul smelling noxious gases are produced. This phenomenon of soil is known as sewage sickness of land. Sewage sickness of land can be prevented by adopting the following measures.
(i) Pretreatment of Sewage:
Sewage should be applied on land only after giving primary treatment such as screening, grit removal and sedimentation. This will help in removing settle able solids and reducing BOD load by 30% or so, and hence soil pores will not get clogged quickly.
(ii) Provision of Extra Land:
There should be ample provision of extra land so that the land with sewage sickness can be given the desired rest. However, if extra land is not available then there should be an alternative arrangement for the disposal of sewage when sewage farms are taking rest.
(iii) Drainage of Soil:
Subsoil drain pipes with open joints should be laid to collect the percolating effluent. This will minimize the possibility of sewage sickness.
(iv) Proper Choice of Land:
The land chosen for this purpose should be sandy or loamy, having higher permeability. Clayey soil should be avoided.
(v) Rotation of Crops:
Sewage sickness can be reduced by growing different crops in rotation instead of growing single type of crop. This will help in utilizing different fertilizing elements of sewage and help in aeration of soil.
(vi) Shallow Depth Application:
Sewage should be applied in shallow depths. If sewage is applied in greater depths, chances of sewage sickness are increased. The depth of sewage on land should be carefully decided by keeping in view the climatic conditions, drainage facilities, nature of crops and characteristics of soil.
(vii) Intermittent Application:
Sewage should be applied on land at intervals. The period between successive applications depends on general working of sewage farm and the permeability of soil. Depending on the nature of soil, this period between successive applications varies from few hours to few weeks.
(viii) Treatment of land:
The land affected by sewage sickness should be properly treated before it is put up in use again. Clogged soil should be broken up by suitable equipment.