The following points highlight the five main ways adopted for purifying water. The ways are: 1. Self-Purification 2. Biochemical Oxidation in Streams 3. Atmospheric Re-Aeration 4. Sludge Deposits 5. Bacterial Self-Purification.
1. Self-Purification:
Agencies of Self-Purification:
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The discharge of sewage and wastes sets in motion a complex chain of physical, chemical, biological and biochemical events, the net result of which is the ultimate elimination of the pollution. The process is collectively designated as self-purification.
In the course of years and as a result of numerous investigations, a general concept of the nature of the self-purification process resulting from sewage pollution has been obtained. The detailed nature of the reactions and interactions and the definite rates of self-purification in streams of various characteristics are still not entirely understood.
Physiological Activity of Bacteria:
The most important phase of self-purification is the result of bacterial action upon the pollution material. It is therefore necessary to understand the general nature of bacterial processes and metabolism. Bacteria bring about profound changes in the physical and chemical nature of their environment, but are also affected in turn by them. The changes brought about by bacteria in their environment are the result of their life activity, which is collectively referred to as metabolism.
Metabolism consists of the related processes catabolism or dissimilation and anabolism, synthesis or assimilation. The catabolic process takes place after the food enters the cell and material is broken down with the liberation of energy.
Where the food is of such a nature that it cannot diffuse and enter the cell bacteria excrete enzymes to hydrolyze it and convert it to more readily diffusible form. The hydrolytic reactions taking place outside the cell by means of exoenzymes liberate only small amounts of energy which in no way can be made use of directly in the cellular metabolism.
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Metabolism starts, therefore, after the food materials enter the cell. The materials are broken down by means of endoenzymes with the liberation of energy. Certain fractions of the catabolic products are then used for the synthetic reactions of the bacteria, building up their cellular materials by utilizing the energy liberated in the catabolic processes. It follows that the energy liberated in the catabolic processes must be greater than the amount of energy required for synthetic processes for the building of the complex materials from the building blocks produced in the catabolic processes.
If the energy produced is in excess of that required for synthetic purposes, the balance may be dissipated into the surrounding environment as heat, which under certain favorable conditions may be registered as a rise in the temperature of the surrounding medium. Such conditions prevail when an excess of highly oxidizable organic matter is placed in open, well ventilated piles, such as in manure, hay or partially dehydrated sludge and garbage heaps.
Temperature under such favourable conditions may be raised to as high as 140 to 180°F. The breakdown of food and the growth of bacteria are closely related to each other. There is no growth without food and there is a relationship between that amount of food utilized and the quantity of growth made.
Catabolic dissimilation of food may take place either under anaerobic or aerobic conditions. Under anaerobic conditions the food materials are broken down incompletely to various intermediate compounds in which most of the energy of the initial material still remains. It follows, therefore, that the quantity of growth made under anaerobic conditions will be smaller than under aerobic conditions. Of greater importance is the nature of the intermediate products formed.
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Various organic acids such as acetic, butyric, proprionic, valeric, lactic as well as lower alcohols-ethyl, butyl, propyl, etc., are formed from carbohydrates, fats and proteins. The products of the degradation of nitrogenous materials are amino acids and ammonia. The gases formed are primarily methane hydrogen, carbon dioxide and hydrogen sulfide.
Under aerobic conditions the decomposition of food materials is complete; all the energy locked up in the food materials is liberated resulting in greater bacterial growth. The end products are carbon dioxide, water, ammonia and nitrates, none of which create offensive conditions.
Energy liberation under both aerobic and anaerobic conditions is the result of a series of steps involving the dehydrogenation of the organic materials. Under aerobic conditions oxygen serves as a hydrogen acceptor.
Under anaerobic conditions energy liberation as a result of dehydrogenation of the organic materials may take place by (a) intermolecular respiration in which oxygen rich compounds such as nitrates, sulfates and carbon dioxide serve as hydrogen acceptors, or (b) intramolecular respiration in which the hydrogen donor and acceptor are within the same molecule and hydrogenation proceeds by rearrangement of hydrogen atoms, where by one part of the molecule is oxidized (dehydrogenated) and another part, which receives the hydrogen atoms is reduced.
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All three types of energy-liberating reactions take place in the stream. In the presence of dissolved oxygen in the flowing part of the stream decomposition of food or impurities takes place by a straight forward dehydrogenation of the organic material and hydrogenation or reduction of oxygen.
The reaction proceeds by intra or intermolecular respiration in the flowing part of the stream when oxygen becomes depleted and in the sludge deposits at all times, except in the superficial layer of sludge in contact with oxygenated water.
Which of these two types of reactions will take place in the absence of oxygen depends, among other things, on whether or not internal hydrogen acceptors are present. Nitrates may be present in the stream as a result of (a) surface wash carrying the leaching from the soil, (b) discharge of nitrified effluents from sewage treatment plants, and (c) nitrates produced in situ in the well aerated unpolluted section of the stream. In the absence of dissolved oxygen, the oxygen in the nitrates will be utilized by bacteria for the oxidation of the organic matter.
The products of nitrate reduction may be nitrates, ammonia or nitrogen gas. Sulfates are present in the stream water naturally or may be added by sewage or industrial wastes. After the nitrites have disappeared and only after that, will the organisms attack the sulfates in the oxidation of the organic matter. The product of the reduction of sulfate is hydrogen sulfide. In addition sulfide may be produced from the sludge deposits either from sulfates or from organic sulfur compounds.
The utilization of carbon-dioxide as a hydrogen acceptor is associated with methane fermentation. This type of fermentation takes place in the sludge deposits of streams. Carbon-dioxide may be present in the water as such, or as bicarbonates, or may be produced as a result of other types of bacterial fermentations. The carbon dioxide supply is ample for the progress of methane fermentation under favorable conditions of temperature and seeding.
There are many other types of bacterial fermentations which do not require any external hydrogen acceptors. Examples of this type of reaction are lactic and alcohol fermentation. All these hydrogenation and dehydrogenation reactions are catalyzed by enzymes which are produced by the bacteria. In addition, there are a great number of enzymes which perform hydrolytic or digestive functions outside the cell, as well as intracellular enzymes which carry on synthetic reactions.
The bacterial cell contains C, H, O, N, S, P and smaller quantities of other elements. It is essential for all of these elements to be present in available form in the medium so that synthetic reactions may proceed and structural requirements be fulfilled. Ordinarily, in the presence of organic material there will be no deficiency of essential food requirements.
With the exception of nitrogen and phosphorus, all the other elements are present either in the food material or in the water. In streams polluted with sewage the nitrogen and phosphorus will not be deficient, but with certain industrial wastes of highly carbonaceous nature, the- nitrogen and possible phosphorus may become deficient and thereby restrict the rapid decomposition and stabilization of the organic materials.
The bacterial metabolism associated with the degradation of organic materials is termed heterotrophic metabolism. It is characterized by the utilization of organic materials as a source of energy as well as a source of carbon.
It is sharply differentiated from autotrophic metabolism in which the energy is derived from the oxidation of simple inorganic compounds such as ammonia, nitrates, hydrogen sulfide, sulfur, reduced iron, etc., and the carbon is derived from carbon dioxide or bicarbonates.
These organisms constitute a small group of highly specialized bacteria which can live in the absence of organic matter and produce organic growths from purely inorganic materials. Autotrophic metabolism can take place only under strict aerobic conditions. The conversion of ammonia to nitrite is brought about autotrophically by a few nitrifying bacteria such as Nitrococcus and nitrosamonas, while Nitrobacter only oxidize nitrite to nitrate.
Sulfur bacteria comprise a larger group of organisms, varying from the filamentous forms such as Beggiotoa to unicellular forms such as Thiothrix and Thiobacillus thiooxidans. The complete oxidation of the reduced sulfur compounds results in the production of sulfuric acid.
Thus, the production of nitrates and sulfates in the reverse of the reduction reactions of these compounds brought about by two entirely different groups of organisms under different environmental conditions in respect to air supply.
The oxidation of reduced iron compounds is brought about primarily by a group of filamentous bacteria such as Crenothrix, Leptothrix, Spirophyllum and Galionella. The action of these organisms under favourable conditions results in the conversion of soluble iron to ferric hydrate.
Environmental conditions play a very important role in the biochemical reactions, affecting both the nature and rate of the reactions. The far-reaching effects of a change in the oxygen relationship of the environment on the character of the biochemical reactions and the nature of products formed have been discussed above.
The pH value of the surrounding medium has a profound effect on the biological activities. Bacteria cannot grow below a certain pH value. This is known as the minimum pH value which varies with different organisms, some tolerating a greater degree of acidity than others.
Generally a pH value of 4.0 is the minimum for most bacteria, although Thiobacillus thiooxidans, the sulfur-oxidizing organism mentioned above, is known not only to tolerate but actually prefer pH values below this. It lowers the pH value under favorable conditions to 1.0 as a result of the production of large quantities of sulfuric acid.
As the pH value is increased from the minimum point toward neutrality the rate of bacterial reactions is increased in most cases. The points at which the rate is at a maximum is known as the optimum pH, which for most bacteria will be at or slightly above neutrality. With increasing pH values beyond the optimum the rate of reactions will again drop until it reaches a minimum beyond which no growth can take place. The maximum pH value that can be tolerated for most organisms is 9.0.
Self-purification can thus be directly affected by the discharge of alkaline or acid industrial wastes to the streams. The outstanding example in the country is the situation in the upper Ohio River system where large quantities of acids are discharged as a result of acid mine drainage.
A number of investigations have shown that the natural purification process is seriously affected; biological life, including plankton and sewage bacteria, is greatly reduced. The inhibition, due to the presence of acids, results in “embalming” the river for that particular stretch and transferring the zone of self-purification further downstream where the acids are neutralized or diluted. Other examples of this nature exist in different localities.
Temperature greatly affects the rate of biological activity. Just as in the case of pH values minimum, maximum and optimum temperature ranges can be recognized. These temperature ranges are not necessarily the same for all bacteria.
For example, the cryophilic (cold loving) bacteria have lower temperature requirements for growth than the mesophilic and thermophilic (heat loving) organisms. The latter, with an optimum of 50 to 60°C, are of little importance in streams.
The cryophilic organisms, with a temperature optimum of 18 to 20°C, may be found in streams and lakes. The mesophilic organisms have an optimum at a temperature of 20 to 40°C. The rate of activity of these organisms is greatly affected by changes of temperature. The temperature coefficients for biochemical oxidation in streams have been worked out. For every degree rise in temperature within the growth range, the rate of biochemical oxidation increases in the ratio of 1.047.
As a result of increased temperature, self-purification and biochemical oxidation of the impurities in a stream are completed in a shorter time because biochemical activities are greater in summer than in winter. With the same quantity of pollution and flow of dilution water in the stream, nuisance conditions will be pronounced in the summer but may not be created in the winter.
Bacteria and their activities are also greatly affected by a number of chemicals such as copper, mercury, chromium, arsenic, phenol, formaldehyde, etc. If these chemicals are present in sufficient quantity the bacteria will be killed and the polluting material will not be biochemically attached, e.g., self-purification will cease. With lower concentration of these chemicals the rate of activity may be partially reduced or may be temporarily inhibited. Many industrial wastes contain toxic materials which will affect the stream purification.
Fungi:
There are in addition to free-swimming unicellular bacteria a number of filamentous bacteria such as sphaerotilus which because of their filamentous nature are sometimes classified as fungi. Associated with sphaerotilus are a number of filamentous true fungi such as leptomitus. These form dense masses covering stones and are attached at one end to submerged objects while swaying with the current.
They look like cotton wool, but are usually gray in colour and are found in-polluted sections below sewer outfalls. They disappear in septic environment when the dissolved oxygen becomes too low. Their method of nutrition is similar to solved oxygen becomes too low. Their method of nutrition is similar to bacteria in so far as they break down complex organic materials to simpler forms.
Photosynthetic Organisms with Pigments:
They are a great number and variety of organisms in water with either chlorophyll or other similar photosynthetic pigments. These may be unicellular free-floating, or filamentous and attached forms. Among these are the blue-green and grass-green algae and diatoms.
These organisms are able, by means of their pigments, to utilize the sun’s energy in synthesizing organic materials from carbon dioxide, water and minerals and form oxygen. This process is referred to as photosynthesis and is diametrically opposed to bacterial metabolism in which only the energy contained in chemical compounds is available as a source of energy.
Whereas photosynthesis is purely synthetic and results in the formation of organic from inorganic substances, bacterial nutrition is primarily dissimilative and only to a small extent assimilative and therefore requires complex food.
The importance of pigmented organisms in self-purification of streams lies in the fact that under favourable conditions of sunlight and penetration of light into the water they add to the oxygen supply, at times creating a condition of super-saturation. It is difficult to evaluate and depend upon this source of oxygen, because the production is affected by many variables.
Protozoa:
Next to bacteria probably the most important group of organisms contributing the self- purification of streams are the protozoa. These comprise a group of unicellular animals of a variety of forms such as the amoebae, flagellates and ciliates and may be either free-floating or attached. Most of them can be more readily differentiated microscopically than bacteria because of their morphological characteristics.
The group is also varied physiologically and embraces types with methods of nutrition similar to bacteria and organisms with mouth organs. The latter are able to ingest whole particles like small pieces of suspended organic matter either living (bacteria) or dead. The ingestion of bacteria by some of the protozoa results in appreciable decreases in the numbers of bacteria but not necessarily in a decrease in the rate of activity of bacteria.
On the contrary, the remaining bacteria multiply and transform food materials at a greater rate until a maximum population level is reached. The pollutional organic material which was transformed once by the agency of bacteria and partially synthesized into bacterial cells is in turn ingested and digested by the protozoa. These in turn have their enemies in the form of rotifers, water fleas and crustaceans.
As the polluted water passes downstream and is purified the organic matter originally present in the pollution is successively attacked and gives rise to the growth of different forms of life until it becomes food for small and large fish The succession of biological forms helps to differentiate the various zones of pollution and degrees of self-purification.
A different group of organisms are found in the bottom deposits which are perhaps more dependable indicators of stream conditions. Rat-tail maggots can exist in the septic zone and are a sign of extremely polluted conditions. Sludge worms, Tubifex and blood worm (chironomous) larvae are found in sludge deposits where they burrow, ingest and digest large quantities of organic matter and help to stabilize the sludge.
Dilution:
Dilution of pollutional wastes might be considered as a phase of self-purification. In certain locations during dry-weather flow the amount of dilution may be inadequate and insignificant. In such cases wastes have to be treated to a higher degree in order to prevent nuisances.
There is a very important relationship between the amount of permissible pollutional material discharged and the amount of dilution water available under dry-weather conditions. Proper dispersion of the waste with the dilution water is highly important.
Sedimentation:
Sedimentation of suspended matter in wastes results in removal of the part of the pollutional material and its transfer to the bottom deposits where it is acted upon under a different set of conditions.
The magnitude of purification obtained by this means will depend upon the percentage of the pollutional material present in suspended form and the velocity of the stream. Additional removals may be obtained as a results of flocculation and coagulation of finely dispersed materials by the chemical constituents of the surface water.
2. Biochemical Oxidation in Streams:
Assay Methods:
The biochemical oxygen demand (BOD) test is very valuable from the stand point of stream pollution and self-purification. It is an empirical test carried on under standardized laboratory conditions simulating the biochemical oxidation in streams. A standard oxygenated dilution water is used for diluting the sewage or waste samples to be tested so that there will be an appreciable and yet not a complete utilization of the dissolved oxygen at the end of a definite incubation period (usually 5 days).
The bottles containing the diluted samples are tightly stoppered in order to prevent the solution of additional oxygen from the air during the incubation period. A sufficient quantity of buffer and nutrient elements is added to the dilution water. The organisms, if absent in the sample, are added to the dilution water. If the sample is acid or alkaline it is adjusted to near neutrality before making the dilutions.
Where toxic materials are present which interfere with biological life it is not possible to determine the biochemical oxidation and obtain a true value of BOD. In other words, the conditions necessary for proper biochemical oxidations in streams, are established. The quantity of dissolved oxygen utilized is measured.
The quantity of oxygen utilized is a measure of the amount of oxidizable organic matter present in the sample. The rate of oxidation can be determined by measuring the oxygen utilised over a period of 10 to 20 days. A great number of such rate studies have shown that oxidation can be represented by a monomolecular formula as Lt/L = 10–Kt in which L is the amount of oxidizable matter present initially and Lt represents the corresponding value at time t and k is a constant.
According to this formulation the oxidation of organic matter is a function of the amount remaining to be oxidized and the rate of oxidation is constant in terms of material remaining to be oxidized. The K1 value most commonly used is 0.1 at 20°C, although appreciable variations from the value are obtained.
The reaction is between the oxygen in solution and the organic matter, brought about by biological agencies and the results can be expressed interchangeably either in terms of oxygen utilized or in terms of the amount of oxidizable matter.
The influence of other variables such as temperature, pH, seeding and mineral requirements has been eliminated by the standardized conditions of the test. The greatest amount of oxygen is utilized in the first unit of time and quantity decreases progressively with each succeeding unit of time until it reaches a constant value. Expressed differently, the oxygen utilized plotted on a semi-logarithmic scale against time is a straight line.
After about 10 days of incubation, however, an increase in the rate of oxygen utilization takes place which signifies the start of the nitrification stage (the conversion of ammonia to nitrates) – the stage prior to this indicates the oxidation of the organic matter. It is necessary to differentiate between these two stages as far as possible because the oxygen utilized for nitrification is still available in the form of nitrates for oxidation of carbonaceous matter in case the dissolved oxygen disappears.
The monomolecular formulation will not apply if nitrification has taken place because the velocity constants of these two stages are entirely different. The BOD values obtained will be higher than for the carbonaceous stage along and it will be difficult to predict the value for any other time from the observed values or the ultimate demand (L value).
Ordinarily, with sewage, nitrification does not interfere during a standard 5-days incubation period, but with materials which have already entered the nitrification stage before the test, oxidation will be primarily of ammonia rather than carbonaceous materials.
The results obtained from the laboratory incubation tests can be used to predict the de-oxygenation rates in the stream fairly accurately. There are some variables which make the application hazardous under certain conditions.
These are as follows:
(i) The chemical composition of the river water is different than the artificial dilution water used in the laboratory test, although the latter is designed to furnish the necessary nutrient salts for bacterial development and represents the normal ingredients present in surface waters.
The concentration of inorganic ingredients in streams varies in different localities. This has led certain workers to use the unpolluted stream water for dilution to determine the specific effect of a waste on a particular stream but the results cannot be compared with other streams.
(ii) The influence of proper seeding is difficult to evaluate. Assuming that sewage used as seed has generally the same types of organisms in different localities, it does not follow necessarily that it corresponds to the flora and fauna which are active in biochemical oxidation in a stream receiving a particular waste such as, for example, sulfite liquor or cellulose fiber. It is conceivable that a stream polluted with such wastes develops a highly specialized flora which brings about the oxidation at a higher rate than can be accomplished in the laboratory with un-acclimatized seed.
(iii) There is in a stream, in addition biochemical oxidation, removal of impurities by sedimentation and absorption which is normally not taken into account in the laboratory de-oxygenation test and, therefore, the laboratory de-oxygenation rates do not correspond to the decrease of BOD in a stream between two points with a time of passage equal to the incubation period in a laboratory test.
Even if the test is run after removal of settleable solids, additional removals of solids and BOD take place as a result of flocculation and sedimentation and adsorption of finely divided material on slimy growths occurring in the stream, it has been shown by kittrell that higher de-oxygenation constants are obtained in shallow turbulent streams in which the volume of water in relation to the channel area is small as compared with a deep sluggish stream. The difference may be due, in part, to absorption.
A second method of determining de-oxygenation and self-purification is based on the rate of decrease of BOD in a stream at successive points separated by known times of flow and assuming that the reduction observed is a measure of oxidation accomplished in the river. This method is subject to the same errors as the projection of the laboratory de-oxygenation rates to stream conditions.
A third method is derived by estimating the de-oxygenation in a stream on the basis of the algebraic difference between known rates of re-aeration and observed rates of change in the dissolved oxygen content of the stream water.
This method, commonly referred to as the oxygen sag method, does not involve the direct application of the BOD test and is therefore free from the objection of the two previous methods. It has a disadvantage in that it is difficult to estimate the actual re-aeration value of different streams and of different stretches of the same stream. In all these methods the pollution of dilution contributed by tributary inflow has to be taken into account.
3. Atmospheric Re-Aeration:
The supply of oxygen in the atmosphere is unlimited but the quantity of the oxygen that water can hold in solution is very small. The maximum amount of oxygen that a water can dissolve is referred to as the saturation value which decreases with increasing temperature. Salinity of the water also affects the solubility of the oxygen. Fresh water as 20°C can hold only 9.2 ppm of oxygen.
As the dissolved oxygen content decreases below the saturation point more can dissolve and the rate at which is goes in solution increases as the degree of under-saturation increases and is in direct proportion with the deficit. The solution of oxygen in the surface film of an under-saturated water is instantaneous. Distribution from the oxygen rich layer to the lower layers under quiescent conditions can only take place by diffusion, which is an extremely slow process.
Therefore, water under, absolutely quiescent conditions, with not even thermal currents, would re-aerate very slowly. This slow diffusion rate is increased in nature by a number of factors such as wind action, velocity of flow, thermal currents and the slope and roughness of the stream bed which cause greater surface agitation. Because the surface film is extremely thin the amount of oxygen that can be supplied at any given time to a deep layer of water, even with mixing and agitation, is extremely small.
Thus, re-aeration is a function of time and increases with time at a decreasing rate. It is apparent that the re-aeration rate in a shallow turbulent stream is higher than in a deep sluggish stream. The rate of re-aeration of the underlying body of water is governed primarily by the rate of absorption at the surface of the water, which in turn is determined by the saturation deficit and secondarily by the temperature, depth and degree of agitation or turbulence.
The measurement of the extent of re-aeration in a stream is complicated by many factors. Theoretically, if the actual utilization or de-oxygenation between two points in a stream is known, the change in the quantity of dissolved oxygen can be attributed to re-aeration. The time of passage of water between the two points, as well as the temperature, must be known. If there is additional pollution or dilution entering between the points, this should be taken into account.
The re-aeration constant (K2) is not constant at a given temperature and even after it is reduced to uniform temperature widely divergent values are obtained for a given section under varying flow conditions.
Re-aeration constants for any given river stretch can be calculated from the formula K2 = Cvn/H2, in which c and n are constants associated with the velocity of flow, v is the mean velocity in feet per second and H is the mean depth of water in feet. If v and H are known, c and n can be obtained by substitution in the formula.
4. Sludge Deposits:
Sludge deposits are formed by the deposition of settleable solids discharged in sewage and wastes, augmented by flocculated finely divided materials in the stream and secondary sludge. Most of the solids are deposited near outlets where the velocity in the stream becomes insufficient to keep them in motion. Decreasing quantities are spread over wider stretches. The deposits are not static. During freshets and high flows they may be lifted and carried some distance before they are re-deposited in sluggish coves.
Organic deposits are generally mixed with clay and silt and usually have a lower volatile matter content than suspended organic materials. There is a tendency for compaction and consolidation with time. Anaerobic conditions prevail throughout the depths except at the extreme surface of the deposit in contact with the oxygenated flowing water.
As a result of physical consolidation, gas evolution and the movement of burrowing animals such as worms, etc., which abound in such an environment, a strict separation between the anaerobic and aerobic zones is not obtained.
Instead, some of the solid particles are brought from the lower levels to the surface. Soluble and colloidal products of anaerobic digestion either diffuse or are brought to the surface of the deposit by various forces. The over-all biological reaction in the bottom deposits, including the anaerobic and aerobic phases, is referred to as benthal decomposition.
The oxygen demand exerted by the thin aerobic zone is of the same order and rate as an equal amount of the material kept in suspension in the flowing portion of the water, except that it is augmented by the materials coming to the aerobic zone from the lower sections of the deposit.
In other words, if all the deposits were spread in a very thin layer on the river bed so that the sludge was stabilized by purely aerobic processes, the oxygen demand exerted would be equivalent to the same material kept in suspension.
The anaerobic decomposition in the lower sections does not exert an oxygen demand except by diffusion and transport of soluble and colloidal material to the aerobic zone. Under conditions of active fermentation the deposits may float to the surface of the stream and after releasing entrained gas may sink to the bottom again.
Additional oxygen from the stream itself may be utilized by this method of gas lifting of sludge and consequent elutriation of the products of anaerobic decomposition into the flowing portion of the water. Transportation of sludge deposits during flood flows has a similar effect except that during this period the demand exerted will be negligible in relation to the supply of oxygen available in the stream water.
With these exceptions, anaerobic decomposition of sludge deposits may be considered of benefit to the oxygen economy of the stream to the extent that the organic matter is converted to methane and hydrogen, which escape to the atmosphere without being oxidized to carbon dioxide and water.
As the depth of the deposits increases from a thin aerobic carpet to a thicker layer the proportion of the organic matter stabilized anaerobically increases, exerting a beneficial influence on the oxygen economy of the stream as a whole. However, since the same amount of organic matter is concentrated in smaller area the over-all localized effect will be worse, since the total oxygen demand per unit area will increase.
5. Bacterial Self-Purification:
Two general types of bacteria may be differentiated in a stream – namely, the native water population and the pollutional forms, such as the coliform organisms. In the biochemical oxidation of the pollution, the coliform organisms play only a minor role. The native water forms are most active in this respect. It is to be expected that the numbers of water forms increase as a result of pollution and during the period when active biochemical oxidation is taking place.
With the decrease in the quantity of oxidizable materials and the reduction of biochemical activity, their numbers decrease. The changes in the numbers of native bacterial population cannot be readily demonstrated since most of them cannot be grown on the usual culture media used. What passes for a total bacterial count includes only the pollutional forms and some of the more readily cultivated organisms of the non-pollutional type.
Observations on the numbers of bacteria in a polluted stream either by plate counts or of the coliform organisms, therefore, indicate the same general trend. A number of investigations have shown that bacteria and coliform organisms increase slightly below a sewer outlet. The exact cause for this increase is not understood, although it has been ascribed to possible multiplication or dispersion of bacterial aggregates. The numbers thereafter decrease regular by until a minimum level is reached.
The decrease in bacterial numbers parallels roughly the decrease of organic materials by biochemical oxidation. Attempts to predict the changes in bacterial numbers in streams from laboratory studies are not as quantitatively applicable as in the case of de-oxygenation. The initial rise of bacteria in the bottles is higher and the decrease is slower than in the stream itself. Otherwise the same general trends prevail.
The forces that affect decrease of bacterial numbers in the streams are:
(i) Sedimentation:
Although bacteria settle at a slow rate because of their small size, they are usually found in aggregates or attached to larger particles which upon settling remove a considerable number of bacteria.
(ii) Protozoa:
Many of the ciliated protozoa ingest large numbers of bacteria as well as particulate matters. Such an action contributes to a significant decrease in bacterial numbers in the stream since bacteria feeding protozoa are found in considerable numbers.
(iii) Food Supply:
Food supply even in a polluted stream cannot be considered as abundant as judged by bacteriological standards of culture medium. Furthermore, the nature of the organic matter is such that it does not favour the growth of pollution organisms.
Coliform organisms will grow in dextrose peptone medium, but a stream polluted by sewage or most industrial wastes does not contain a sufficient concentration of such readily available organic materials as dextrose and peptone. Coliform organisms inoculated into sterile sewage will not multiply or transform organic materials. The biochemical oxidation of organic matter in a stream further reduces the limited supply of food.
(iv) Temperature:
The temperature of the stream water even during the summer is below the optimum for the pollutional bacteria. In general, the effect of higher temperature is to stimulate the growth of bacteria in the presence of abundant food supply and favourable environmental conditions.
Where the food supply is limited and conditions are not favourable for growth, the effect of increasing the temperature within the growth range is to cause a more rapid death rate. The decrease of bacterial numbers during summer is higher than during winter (Fig. 37.2).
(v) Sunlight:
Sunlight has bactericidal properties but the role played under actual stream conditions in decreasing the numbers of bacteria is probably insignificant because of the poor penetration of the ultraviolet rays to any great depths even in clear water. In turbid water the penetration is greatly reduced.
(vi) Bacteriophage:
The destruction of bacteria can take place under favorable conditions by the lytic action of virus-like bacteriophase which is parasitic on bacteria. However, the importance of bacteriophage in reducing the numbers of bacteria in streams is questionable because the action is dependent upon a delicate adjustment of environmental conditions. Bacterial lysis occurs only under conditions which favor a rapid multiplication of bacteria. This condition does not occur in the stream as far as the intestinal group of bacteria is concerned.
(vii) Industrial Wastes:
Industrial wastes seldom contribute appreciable numbers of pollutional bacteria to the stream. The discharge of organic non-toxic wastes can result in initial increase in bacterial numbers in a stream followed by a decrease as the food becomes exhausted by biochemical oxidation. If the discharged wastes have toxic materials the effect will be an immediate and sharp decrease of bacterial numbers.