The characteristics of sewage are classified under the following three heads: 1. Physical Characteristics 2. Chemical Characteristics 3. Biological Characteristics.
The physical characteristics are related to the physical properties of sewage. The chemical characteristics are related to the chemical constituents of sewage. The biological characteristics are related to the biological constituents of sewage. The physical properties and the chemical and biological constituents of sewage, and their sources, are listed in Table 8.2.
1. Physical Characteristics of Sewage:
The physical characteristics of sewage are:
(i) Specific gravity,
(iv) Temperature, and
(v) Total solids content-turbidity
(i) Specific Gravity:
Specific gravity of sewage is very nearly equal to that of water and, therefore, no modification of hydraulic formulae is necessary.
Fresh domestic sewage has an earthy or light brown colour. However, with the passage of time sewage undergoes decomposition due to which its colour changes. In about 6 hours, with all the dissolved for free oxygen present in the sewage being practically exhausted due to aerobic decomposition, the sewage becomes stale and its colour becomes light-to-medium grey.
As more time passes the sewage undergoes anaerobic decomposition due to which it becomes septic and its colour becomes dark grey or block. The blackening of sewage is often due to the formation of various sulphides, particularly ferrous sulphide. The results when hydrogen sulphide (H2S) produced during anaerobic decomposition combines with a divalent metal, such as iron, which may be present. Thus the colour or appearance of domestic sewage may be used to assess its general condition or age.
The colour of industrial sewage depends on the chemical process used in the industry. If industrial sewage of any other colour is mixed with domestic sewage, the colour of the resulting mixed sewage will be different.
Fresh domestic sewage is either odourless or it has a musty odour which is usually not offensive, but when organic matter present in sewage undergoes anaerobic decomposition a variety of compounds are formed which emit offensive odour. The main compound formed under anaerobic conditions is hydrogen sulphide (H2S) which has a smell of rotten eggs that is highly offensive.
The other compounds, such as indol, skatol, cadaverin and mercaptan, formed under anaerobic conditions may cause odours that are more offensive than that of hydrogen sulphide. Offensive odours are harmful because these can cause-
(a) Reduction in appetite for food,
(b) Lowered water consumption,
(c) Impaired respiration, nausea and vomiting, and
(d) Mental perturbation.
As such special care is needed to be taken in the design of treatment facilities to avoid conditions that will allow the development of offensive odours.
The temperature affects sewage in the following ways:
(a) Temperature affects the biological activity of bacteria present in sewage, which increases with increase in temperature up to about 60°C and then it decreases.
(b) It affects the solubility of oxygen and other gases in sewage. When the temperature of sewage is more the content of dissolved oxygen and other gases becomes less.
(c) Temperature affects viscosity of sewage which decreases with increase in temperature.
(d) Extremely low temperature affects adversely the efficiency of sedimentation.
Normally the temperature of domestic and municipal sewage is slightly higher than that of the water supply because of heat added during the utilization of water. The average temperature of sewage in our country is 20°C which is quite close to the ideal temperature for the biological activities.
(v) Total Solids Content-Turbidity:
Total solids present in sewage may be in three different forms:
(A) Suspended solids,
(B) Colloidal solids and
(C) Dissolved solids.
Suspended solids are those which are held in suspension in liquid, and these may be further subdivided as:
(a) Settleable solids and
(b) Non-settleable solids.
Settleable solids are those which settle out if sewage is allowed to remain undisturbed for a period of about 2 hours. Non-settleable solids are those which do not settle down by mere detention, but may be arrested by special laboratory filters.
Colloidal solids are finely divided solids held in suspension, which cannot be removed by settling or filtering, but they are generally removed by biological oxidation or coagulation, followed by sedimentation. Dissolved solids are those which remain dissolved in sewage just as salt in water.
When sewage is passed through laboratory filter the suspended solids will be filtered out i.e., they will be retained on the filter, while colloidal and dissolved solids will pass through the filter in the filtrate. Thus suspended solids are non-filtrable-solids while colloidal and dissolved solids are the filtrable-solids. The colloidal solids consist of particles with an approximate diameter range of from 1 millimicron (mμ) to 1 micron (μ). Fig. 8.1 shows the classification and size range of solids found in sewage.
Each of the categories of solids present in sewage may be further classified as-
(a) Organic or volatile solids, and
(b) Inorganic or non-volatile or fixed solids.
The organic solids account for about 45 per cent of the total solids and these can be grouped in the following three categories:
1. Carbohydrates such as cellulose, cotton and wool fibre, starch, sugar, etc.
2. Fats, oils and grease received from kitchens, laundries, garages, shops, filling stations, etc.
3. Nitrogenous compounds proteins and their decomposed products, animal and vegetable wastes, urea, amines, amino acids, fatty acids, hydrocarbons, alcohol, etc.
The inorganic solids account for the other 55 per cent of the total solids and these consist of minerals and salts such as sand, gravel, grit, clay and other debris, dissolved salts, chlorides, sulphates, etc.
The presence of suspended matter in sewage makes it turbid. Turbidity, a measure of the light- transmitting properties of water, is used to indicate the quality of sewage with respect to colloidal matter. Colloidal matter will scatter or absorb light and thus prevent its transmission. The turbidity depends on the strength of sewage and it increases as sewage becomes stronger.
2. Chemical Characteristics of Sewage:
The chemical characteristics indicate the state of sewage decomposition, its strength and type of treatment required. The chemical characteristics are dependent on the substances contained in sewage.
The significant chemical characteristics of sewage are:
(i) pH value (or Hydrogen ion concentration),
(ii) Chloride content,
(iii) Nitrogen content,
(iv) Phosphorus content,
(v) Fats, oils and grease content,
(vi) Sulphates, sulphides and H2S gas,
(ix) Pesticides and agricultural chemicals,
(x) Toxic compounds,
(xi) Gases—Dissolved oxygen, Hydrogen sulphide, and Methane.
(i) pH Values:
The pH value of sewage indicates whether it is acidic or alkaline in nature. The pH value of fresh domestic sewage is slightly more than that of the water supply to the community. Thus fresh domestic sewage is alkaline in nature having pH value between 7.3 and 7.5, which is good for bacterial action.
However, as time passes the pH value of sewage tends to fall due to production of acids by bacterial action and the sewage tends to become acidic. Thus septic sewage is acidic in nature which is difficult to be treated efficiently. A high concentration of either an acid (pH<<7) or an alkali (pH >>7) in sewage is indicative of industrial sewage being mixed with domestic sewage. The determination of pH value of sewage is necessary because certain methods of sewage treatment for their efficient working require sewage of pH value in a particular range. Sometimes lime is added for creating alkaline condition.
(ii) Chloride Content:
Chlorides found in domestic sewage are derived from kitchen wastes, human faeces, urinary discharges, etc. For example human excreta contributes about 6 to 8 gm of chlorides per person per day. Thus based on an average sewage flow of 150 Ipcd this would result in the chloride content of 40 to 50 mg/1 being added to sewage by human excreta.
Water softeners also add large quantities of chlorides to sewage. Large amounts of chlorides may also be contributed from industrial sewage. Infiltration of saline groundwater is also a potential source of chlorides in sewage. Chlorides are mineral salts and, therefore, these are not affected by the biological action of sewage.
(iii) Nitrogen Contents:
The nitrogen content of sewage is in the form of nitrogenous compounds present in it. The principal nitrogenous compounds present in domestic sewage are proteins, amines, amino-acids and urea which are organic compounds, and ammonium salts which are inorganic compounds. The nitrogenous organic compounds (or nitrogenous organic matter) present in sewage undergo decomposition or oxidation.
Depending on the state of decomposition or oxidation of nitrogenous organic matter nitrogen appears in sewage in the following forms:
(a) Albuminoid nitrogen or albuminoid ammonia;
(b) Ammonia nitrogen or free nitrogen;
(c) Nitrites or Nitrite nitrogen; and
(d) Nitrates or Nitrate nitrogen.
(a) Albuminoid Nitrogen or Albuminoid Ammonia:
Albuminoid nitrogen or albuminoid ammonia indicates the quantity of nitrogen present in sewage before the decomposition of nitrogenous organic matter is started. In other words albuminoid nitrogen or albuminoid ammonia indicates the amount of undecomposed nitrogenous organic matter present in sewage.
(b) Ammonia Nitrogen or Free Ammonia:
In sewage ammonia nitrogen or free ammonia results from the bacterial decomposition of the nitrogenous organic matter.
It exists in sewage as either ammonium ion (NH4+) or ammonia (NH3) depending on the pH value of the sewage, in accordance with the following equilibrium reaction:
When pH > 7, the equilibrium is displaced to the left, thus it exists as ammonia, while for pH < 7, it exists as ammonium ion. The age of sewage is indicated by the relative amount of ammonia that is present. The presence of considerable amount of ammonia nitrogen or free ammonia indicates stale or old sewage. In an aerobic environment bacteria can oxidize the ammonia nitrogen to nitrites and nitrates.
Nitrites indicate the presence of partly decomposed (or partly oxidized) nitrogenous organic matter in sewage. The presence of nitrites in sewage indicates that decomposition or oxidation of nitrogenous organic matter is in progress. Hence nitrites indicate the intermediate stage of conversion of nitrogenous organic matter of sewage into stable form.
When sewage is undergoing treatment the presence of nitrites shows that the treatment is still incomplete and the sewage is still stale. In other words nitrites will predominate in stale sewage. Thus the presence of nitrites is an indicator of past pollution in the process of stabilization. In sewage the amount of nitrites seldom exceeds 1 mg/1. Nitrites are, however, unstable and are easily oxidized to nitrate form, and hence they are relatively unimportant in sewage or water-pollution studies.
Nitrates are the final products of decomposition (or oxidation) or nitrogenous organic matter present in sewage. As such the presence of nitrates indicates fully oxidized and the most stable form of nitrogenous organic matter contained in sewage thereby indicating the well oxidized and treated sewage.
Increase in proportion of nitrates during the process of sewage treatment serves as a guide for measuring the progress achieved in the sewage treatment. Nitrates may vary in concentration from 0 to 20 mg/1 as nitrogen in sewage.
In fresh sewage nitrites and nitrates are usually not present or they may be present in very small amount.
Nitrogen being an essential component of biological protoplasm, its determination in sewage is necessary for proper biological treatment or its use for land irrigation. Where nitrogen content in sewage is inadequate, it becomes necessary to supplement with addition of salts containing nitrogen. Generally domestic sewage contains sufficient nitrogen to take care of the needs of the biological treatment.
(iv) Phosphorus Content:
Phosphorus is contributed to domestic sewage from food residues containing phosphorus and their breakdown products. The use of increased quantities of synthetic detergents add substantially to the phosphorus content of sewage. Industrial sewage also contributes phosphorus. Phosphorus just as nitrogen, is an essential nutrient for biological processes. Generally domestic sewage contains adequate quantities of phosphorus.
(v) Fats, Oils and Grease Content:
Fats and oils are the major components of food stuffs such as butter, lard, margarine and vegetable fats and oils. Fats are also commonly found in meats, seeds, nuts and some fruits. Thus fats and oils are contributed to domestic sewage from kitchen wastes which contain these food stuffs. Grease and other oils such as kerosene, lubricating and road oils are contributed from shops, garages, workshops and industries.
Fats and oils are compounds (esters) of alcohol or glycerol (glycerine) with fatty acids. Fats are among the more stable of organic compounds and are not easily decomposed by bacteria. The term ‘grease’ as commonly used, includes the fats, oils, waxes and other related constituents found in sewage.
The grease content of sewage can cause many problems in both sewers and sewage treatment plant. The grease content floats on the top of sedimentation tanks, often chokes sewers in the winter and clogs filters thus interferes with normal functioning of treatment plant. Although for the most part grease content floats on the sewage, a portion of it is carried into the sludge on settling solids, which interferes with biological action and cause maintenance problems.
Moreover, if grease content is not removed before sewage is discharged into a water body, it can interfere with the biological life in the water body and create unsightly floating matter and films. It is, therefore, necessary to remove grease content from sewage.
(vi) Sulphates, Sulphides and H2S Gas:
Sulphates and sulphides are formed due to decomposition of various sulphur containing substances present in sewage. Further sulphate ions (SO4) occurs naturally in most water supplies and hence these are also present in sewage. Sulphur is required in the synthesis of proteins and is released in their degradation.
Anaerobic bacteria chemically reduce sulphates to sulphides and to hydrogen sulphide (H2S) as indicated by the following equations:
The hydrogen sulphide gas so produced cause bad smells or odours. Besides this, H2S gas gets oxidized biologically to sulphuric acid (H2SO4) which is corrosive to sewer pipes.
Sulphates are reduced to sulphides in sludge digesters and may upset the biological process if the sulphide concentration exceeds 200 mg/l. However, such concentrations of sulphides are rare. Further H2S gas which is evolved and mixed with the sewage or wastewater gas (CH4 + CO2) is corrosive to the gas piping.
Surfactants (or surface active agents) come primarily from synthetic detergents. Thus surfactants are contributed to domestic sewage from bathroom wastes, kitchen wastes, wastes from washing machines, etc. Surfactants are large organic molecules that are slightly soluble in water and cause foaming in sewage treatment plants and in water bodies into which the sewage is discharged. Surfactants tend to collect at the air-water interface.
During aeration of sewage, these compounds collect on the surface of the air bubbles and thus create a very stable foam due to which aeration of sewage is hindered. Alkyl-benzene- sulfonate (ABS) is a type of surfactant commonly used in synthetic detergents, but it is more troublesome because it is not biodegradable (i.e., it resists break down by biological means). As such some countries have banned the use of ABS in detergents, and ABS has been replaced in detergents by linear-alkyl- sulfonate (LAS) which is biodegradable.
Phenols are mostly found in industrial sewage. If such a sewage is directly discharged into a surface source of water supply then drinking water may also contain phenols. However, the presence of phenols in drinking water is not desirable because phenols cause taste problems in drinking water, particularly when the water is chlorinated. Phenols present in sewage can be biologically oxidized if their concentrations are up to 500 mg/l.
(ix) Pesticides and Agricultural Chemicals:
Sewage may contain pesticides, herbicides and other agricultural chemicals which result primarily from surface runoff from agricultural, vacant and park lands. If sewage containing these chemicals is discharged in a water body it can result in fish kills, in contamination of the flesh of fish that decreases their value as a source of food, and in impairment of water supplies.
(x) Toxic Compounds:
Sewage may contain certain compounds which are toxic to micro-organisms. Such compounds may be contributed by industrial sewage. The industrial sewage may contain certain cations and anions which are toxic to micro-organisms. Copper, lead, silver, chromium, arsenic and boron are some of the cations which are toxic in varying degrees to micro-organisms.
For instance, in sludge digestors, copper is toxic at a concentration of 100 mg/L, chromium and nickel are toxic at concentrations of 500 mg/l, and sodium is also toxic at high concentrations. Other toxic cations include potassium and ammonium at concentrations of 4000 mg/l.
The toxic anions include cyanides and chromates which are found particularly in metal-plating wastes. It has been found that due to the presence of toxic ions the micro-organisms are killed and the treatment ceases. As such the presence of toxic ions should be taken into consideration in the design of biological treatment plants.
Besides solids sewage also contains gases.
The gases that are commonly found in untreated sewage include:
(i) Nitrogen (N2),
(ii) Oxygen (O2),
(iii) Carbon dioxide (CO2),
(iv) Hydrogen sulphide (H2S),
(v) Ammonia (NH3), and
(vi) methane (CH4).
The first three are common gases of the atmosphere and are found in all waters (including sewage) exposed to air. The later three are derived from the decomposition of the organic matter present in sewage. The presence of nitrogen and ammonia in untreated sewage is discussed earlier.
The other gases of interest in untreated sewage are oxygen, hydrogen sulphide and methane and the same are discussed below:
i. Dissolved Oxygen:
Dissolved oxygen (DO) is the amount of oxygen present in sewage in dissolved state. The presence of dissolved oxygen in untreated sewage indicates that the sewage is fresh. However, oxygen is only slightly soluble in water.
The actual quantity of dissolved oxygen that can be present in water is governed by:
(i) Solubility of oxygen,
(ii) Partial pressure of oxygen in atmosphere,
(iii) Temperature, and
(iv) Purity (salinity, suspended solids, etc.) of water.
The solubility of oxygen in sewage is only about 95% of that in distilled water. Further as indicated earlier the quantity of dissolved oxygen in sewage decreases as the temperature increases.
Dissolved oxygen is required for the respiration of aerobic micro-organisms as well as all other aerobic life forms present in sewage. Thus the presence of dissolved oxygen in sewage is desirable. The dissolved oxygen content of fresh sewage is soon depleted due to aerobic decomposition.
When the entire quantity of dissolved oxygen is exhausted, the aerobic decomposition ceases and anaerobic decomposition commences which results in the development of noxious odours. As such the presence of dissolved oxygen in sewage is also desirable because it prevents the development of noxious odours.
It is necessary to determine the dissolved oxygen content of sewage before it is subjected to treatment, so as to select proper method of treatment.
ii. Hydrogen Sulphide:
As indicated earlier hydrogen sulphide is formed from the decomposition of organic matter containing sulphur or from the reduction of mineral sulphites and sulphates. It is not formed in the presence of an abundant supply of oxygen. Hydrogen sulphide gas is a colourless, inflammable compound having the characteristic odour of rotten eggs.
The blackening of sewage and sludge usually results from the formation of hydrogen sulphide that has combined with the iron present to form ferrous sulphide (FeS). Although hydrogen sulphide is the most important gas formed from the standpoint of odours, other volatile compounds such as indol, skatol and mercaptans, which may also be formed during anaerobic decomposition, may cause odours far more offensive than that of hydrogen sulphide.
Methane gas is the principal by-product of the anaerobic decomposition of the organic matter in sewage. Methane gas is a colourless, odourless, combustible hydrocarbon of high fuel value. Normally large quantities of methane are not encountered in sewage because even small amounts of oxygen tend to be toxic to the organisms responsible for the production of methane.
Since methane is highly combustible and the explosion hazard is high, manholes and sewer junctions or junction chambers where there is an opportunity for the gas to collect should be ventilated with a portable blower during and before the time required for men to work in them for inspection, repairs or renewals.
In treatment plants, notices should be posted about the plant warning of explosion hazards, and plant employees should be instructed in safety measures to be maintained while working in and about the structures where the gas may be present.
3. Biological Characteristics:
The biological characteristics depend on the groups of organisms found in sewage. The principal groups of organisms found in sewage are classified as protista, viruses, plants and animals.
Protista includes bacteria, fungi, protozoa and algae. These are the most important group of organisms with which the sanitary engineer must be familiar, especially the bacteria, algae and protozoa.
These are single cell micro-organisms which play an extensive and fundamental role of decomposition and stabilization of organic matter present in sewage, both in nature and in treatment plants.
Bacteria may be classified according to type as:
(i) Saprophytic bacteria,
(ii) Parasitic bacteria,
(iii) Pathogenic bacteria, and
(iv) Non-pathogenic bacteria.
Saprophytic bacteria are those which live on dead or decaying organic matter, thus obtain organic matter in solution from dead and decaying tissue in plants and animals, and hence these are beneficial to mankind. Parasitic bacteria live and multiply on or within the body of a living organism of a higher type.
Pathogenic bacteria are those which may cause diseases within the living organisms on which they subsist. Non-pathogenic bacteria are harmless bacteria and under certain conditions are beneficial to human beings, animals and crops.
According to oxygen requirements bacteria are classified as:
(i) Aerobic bacteria,
(ii) Anaerobic bacteria, and
(iii) Facultative bacteria.
These are those which require free oxygen for their survival, thus if present in sewage they consume dissolved oxygen from the sewage. Anaerobic bacteria are those which survive and flourish in the absence of free oxygen, and the oxygen needed by them is extracted from the oxygen radical of organic compounds and mineral substances such as nitrites (NO2), nitrates (NO3), sulphates (SO4). Facultative bacteria are those which can survive and flourish with or without free oxygen.
According to temperature at which they survive and flourish, bacteria can be classified as:
(i) Psychrophilic bacteria which can survive and flourish between 10° to 20°C,
(ii) Mesophilic bacteria which can survive and flourish between 20° to 40°C, and
(iii) Thermophilic bacteria which can survive and flourish between 40° to 65°C.
Pathogenic bacteria (or pathogens) found in sewage are discharged by human beings who are infected with disease or who are carriers of a particular disease. The usual pathogenic bacteria that may be excreted by man cause diseases of the gastrointestinal tract, such as typhoid and paratyphoid fever, dysentery, diarrhea, and cholera.
Since the identification of pathogenic bacteria in water as well as in sewage is extremely difficult, the presence of coliform bacteria is used as an indicator of the presence of pathogenic bacteria. In other words it is presumed that if coliform bacteria are present in sewage then it may also have pathogenic bacteria.
iii. Coliform Bacteria or Coliforms or B-Coli (Bacterium Coli):
These are rod-shaped, non-pathogenic, aerobic bacteria. The intestinal tract of man contains countless coliform bacteria. Each person discharges from 100 to 400 billion coliform bacteria per day, in addition to other kinds of bacteria.
Coliforms are harmless to man and are, in fact, useful in destroying organic matter in biological sewage treatment processes. Escherichia coli (or E-coli) are a type of coliform bacteria which inhabit the intestines of human beings and animals, and are thus excreted in large amount with their faeces. The usual procedure for determining the presence of coliform bacteria consists of the presumptive and the confirmed tests which can be seen in Author’s book entitled ‘Water Supply Engineering’.
Fungi are unicellular (single celled), non-photosynthetic plants capable of growing in low temperature and low pH environments. They flourish over a wide range of pH (4 to 10) and themselves modify the pH by producing organic acids and ammonia. The reproductive stage of fungi is a spore. Spores are transmitted long distances in the air by wind currents. Absence of suitable environment prevents most fungi spores from germinating.
Algae are considered as simple photosynthetic plants with unicellular (single celled) organs of reproduction. These are the organisms that are self-nourishing by deriving energy from simple inorganic substances with the aid of sunlight. One of the most important problems facing the sanitary engineers is how to treat wastes of various origins so that the effluents do not encourage the growth of algae and other aquatic plants. The solution may be in the removal of carbon, the removal of various forms of nitrogen and phosphorus, and possibly the removal of some of the trace elements, such as iron and cobalt.
Protozoa of importance to sanitary engineers include amoebas, flagellates, and free swimming and stalked ciliates. Protozoa are essentially unicellular (single celled) animals that reproduce by binary fission. They are the lowest and the simplest forms of animal life. One form of protozoa—Endameba histolytica causes amoebic dysentery.
It forms cysts which are carried in the bowel discharges of infected persons and which will live for long periods in water. The protozoa are bacteria eaters and survive on dilute organic wastes by eating bacteria and thus destroy the pathogens.
When the concentration of organic waste is sufficiently high, the protozoa can utilize the soluble organic compounds for their food. They are essential in the operation of biological treatment processes and in the purification of streams because they maintain a natural balance among the different groups of micro-organisms.
Viruses are infectious agents of both plant and animal cells. They are ultramicroscopic, obligate, intracellular parasites that manifest their presence by destruction or impairment of host cells. They can pass through an ultra-microscopic filter and they fall in the size range of 10 to 500 milli-microns.
Because of their small size, viruses lack the biochemical systems needed for normal metabolic cell functions and are essentially units organised solely for self-replications. A typical virus particle consists of an outer protein coat enclosing a core of nuclei acid. One group of viruses, the bacteriophages, are infectious agents of bacteria and are parasitic to bacteria.
It initiates infection by attaching itself by its tail to the wall of a bacterial cell. Out of the various forms of viruses, adenoviruses are associated with upper respiratory infections in children. Entero-viruses are found in gastro-intestinal tract and faeces of man and many lower animal. Enteric viruses include coxsackie viruses, infectious hepatitis viruses, polioviruses, reoviruses, etc.
Viruses that are excreted by human beings may become a major hazard to public health. For example – from experimental studies, it has been found that from 10000 to 100000 infectious doses of hepatitis virus are emitted from each gram of feces of a patient ill with this disease. It is known that some viruses will live as long as 41 days in water or sewage at 20°C and for 6 days in a normal river.
viii. Plants and Animals:
Plants and animals, which are of importance to sanitary engineers, range in size from microscopic rotifers and worms to macroscopic crustaceans. A knowledge of these organisms is helpful in evaluating the condition of streams and lakes, in determining the toxicity of sewage discharged to the environment, and in observing the effectiveness of biological life in the secondary treatment processes used to destroy organic wastes.