In this article we will discuss about the physical, chemical and biological characteristics of wastewater.
Physical Characteristics of Wastewater:
Following are important physical characteristics of wastewater:
(v) Solid contents (Total solids).
Fresh domestic sewage is grey, somewhat resembling a weak solution of soap. With the passage of time, as putrefaction starts, it begins to get black. The colour of septic sewage is more or less black or dark in colour. The colour of industrial wastewater depends upon the chemical process used in the industries. Industrial waste water, when mixed with domestic sewage, may also add colour to it.
Normal fresh sewage has a musty odour which is normally not offensive, but as it starts to get stale, it begins to give offensive odour. Within 3 or 4 hours, all the oxygen present in the sewage gets exhausted and it starts emitting offensive odour of hydrogen sulphide, gas and other sulphur compounds produced by anaerobic micro-organisms. Industrial wastewater may contain either process of wastewater treatment.
Offensive odours can be harmful in many ways such as- (i) reduction in appetite for food (ii) lowering in water consumption (iii) impaired respiration, nausea and vomiting and (iv) cause for mental perturbation. Due to this the elimination of odours has become a major consideration in the design and operation of wastewater collection, treatment and disposal facilities.
Generally, the temperature of wastewater is higher than that of the water supply, due to addition of warm water from the households and from industries. When the wastewater flows in closed circuits, its temperature rises further.
This results in the increase in the viscosity of water and also increase in its bacterial activity. Average temperature of wastewater in India is around 20°C, which is quite close to the ideal temperature for the biological activities.
The change in the temperature affects the wastewater in the following ways:
(a) As the temperature rises, its viscosity increases with a corresponding increase in its tendency to precipitate. Extremely low temperatures affects adversely the efficiency of sedimentation,
(b) The bacterial activity increases with increase in temperature, upto about 60°C. After this, it again falls. This characteristic of temperature thus effects the design in wastewater units and their efficiency.
(c) The solubility of gases in wastewater decreases with the increase in temperature. This results in the separation of dissolved oxygen and other gases from the wastewater, and consequent reduction in the self-purification of streams and increase in bacteriological growth. The increase in the rate of biochemical reactions that accompanies an increase in temperature, combined with the decrease in the quantity of oxygen present in surface waters, can cause serious depletion in dissolved oxygen concentrations—specially when large quantities of relatively warm wastewater are discharged into the receiving water,
(d) The increase in temperature of wastewater, when discharged into receiving waters, affects the aquatic life,
(e) Abnormally high temperatures can foster the growth of undesirable water plants and wastewater fungus in the receiving waters.
The turbidity of wastewater depends on the quantity of solid matters present in the suspension state. Turbidity is a measure of light-emitting properties of wastewater, and turbidity test is used to indicate the quality of waste discharges with respect to colloidal matter. The turbidity depends upon the strength of sewage or waste water. The stronger or more concentrated the sewage, the higher is its turbidity. Turbidity can be determined either by turbidity rod or by Jackson’s turbidimeter.
(v) Total Solids:
Sewage normally contains 99.9 per cent of water and 0.1 per cent of solids. Analytically, the total solids content (ST) of a wastewater is defined as all the matter that remains as residue upon evaporation to 103 to 105°C. Total solids in wastewater exist in three different forms (a) suspended solids (b) colloidal solids and (c) dissolved solids. Suspended solids (5s) are those which can be filtered out on an asbestos mat or filter paper, i.e., suspended solids are non-filterable solids.
The suspended solids may be further subdivided into (a) settleable solids and (b) non-settleable solids. Settleable solids are those that will settle to the bottom of a cone-shaped container, called Imhoff cone, in 2-hour, period. The Imhoff cone consists of a conical glass vessel, having a capacity of 1 litre and graduated upto about 50 ml.
Sewage is allowed to stand in the cone for a period of two hours and the quantity of settleable solids (Ss) in the cone is then read. However, to obtain the exact proportion of settleable solids, the liquid from the cone is decanted off and the settleable solids collected in the bottom of the cone is weighed. Non-settleable solids do not settle down by mere detention, but may be arrested by special laboratory filters.
The total solids in wastewater are further classified as (a) fixed solids (SF) and (b) volatile solids (SV). The fixed solids are generally classed as inorganic while the volatile solids represent the organic matter. In order to determine the quantity of volatile solids (SV), the dry residue (i.e. total solids), obtained by evaporating the wastewater at 105°C, is further heated and ignited. The loss of weight due to this operation represents the volatile solids (SV) or the strength or nuisance capacity of the wastewater.
When sewage is passed through laboratory filter, the filtrate is not clear but is turbid, because of presence of filterable solids. The filterable-solids fraction consists of colloidal solids as well as dissolved solids (SD). The colloidal fraction consists of the particulate matter of diameters ranging from 1 millimicron (mµ) to 1 micron (µ). The colloidal matter consists of finely divided particles of gels, emulsion and foam, and it cannot be removed by gravitational setting; they are generally removed by biological oxidation or coagulation, followed by sedimentation.
The dissolved solids consist of both organic and inorganic molecules and ions that are present in true solution in water. Turbidity test in used to indicate the quality of wastewaters with respect to colloidal matter. Colloidal matter scatters or absorbs light and thus prevents its transmission.
(a) Determination of Total Solids and Volatile Solids:
Procedure – (i) Take a tarred dish and ignite it to constant weight (W1). (ii) Take a known volume (V) of a well-mixed sample and transfer it to the above dish. (iii) Evaporate the sample to dryness at 103°C for 24 hours, in constant temperature oven. (iv) Cool the dish in a desiccator and determine its weight (W2). (v) Ignite the dish at 600°C in a muffle furnace, for 30 miniatures, (vi) Cool the dish in a desiccator and determine its weight (W3).
(b) Determination of Non-Filterable Solids (Suspended Solids) and Volatile Non-Filterable Solids:
Procedure – (i) Filter a suitable aliquote of sample (vol. V) through a tarred gooch crucible (ignited to constant weight W1), applying suction (ii) Dry the crucible along with the retained matter in a constant temperature oven maintained at 103°C, for 24 hours. (iii) Cool it in a desiccator and find its weight (W2). (iv) Ignite the crucible at 600°C in a muffle furnace, for 30 minutes, (v) Cool in a desiccator and record the weight (W3).
Chemical Characteristics of Wastewater and their Determination:
The following important chemical characteristics of wastewater which affect the selection and operation of various types of treatment processes are given below:
(i) pH value
(ii) Chloride content
(iii) Nitrogen content
(iv) Fats, grease and oil content
(v) Sulphides, sulphates and H2S gas
(vi) Dissolved oxygen (DO)
(vii) Chemical oxygen demand (COD)
(viii) Bio-chemical oxygen demand (BOD)
(ix) Stability and relative stability.
(i) pH Value:
The test for pH value of wastewater is carried out to determine whether it- is acidic or alkaline in nature. Fresh sewage is generally alkaline in nature, (its pH value between 7.3 to 7.5). However, as the time passes, pH value tends to fall due to production of acids by bacterial action, and the sewage tends to become acidic. However, after oxidation when it is relatively stable, it becomes alkaline again.
Properly oxidised effluent should have a pH value of about 7.3 or so. A high concentration of either an acid (pH ≪ 7) or alkali (pH ≫ 7) in wastewater is indicative of industrial wastes. The determination of pH value of sewage is important since certain treatment methods depend on proper pH value of the wastewater for their efficient workings. Sometimes, lime is added for creating alkaline condition.
(ii) Chlorides Content:
Chlorides are mineral salts and, therefore, are not affected by biological action of sewage. Chlorides in natural water result from the leaching of chloride-containing rocks and soils with which the water comes in contact. Chlorides found in domestic sewage are derived from kitchen wastes, human faeces and urinary discharges etc. Human excreta, for example, contain about 6 g of chlorides per person per day.
Water softeners also add large quantities of chlorides. Large amounts of chlorides may also enter in wastewaters from industries like ice cream plants, meat salting etc. Infiltration of ground water into sewers adjacent to saltwater is also a potential source of high chlorides. The chloride content of waste-water can be measured by titrating the sample of wastewater with standard silver nitrate solution, using potassium chromate as indicator.
(iii) Nitrogen Contents:
The presence of nitrogen in waste-water indicates the presence of organic matter in it. Nitrogen is essential to the growth of Protista and plants and as such is known as nutrient or bio-stimulant. Since nitrogen is an essential building element in the synthesis of protein, nitrogen data is required to evaluate the treatability of waste-water by biological processes.
Nitrogen appears in the following five different forms in waste-water:
(a) Ammonia nitrogen or free ammonia
(b) Organic nitrogen
(c) Albuminoid nitrogen
(d) Nitrites nitrogen and
(e) Nitrates nitrogen.
(a) Ammonia Nitrogen or Free Ammonia:
Ammonia nitrogen exists in aqueous solution as either the ammonium ion or ammonia, depending upon the pH of the solution, in accordance with the following equilibrium reaction –
NH3 + H2O ⇔ NH4+ + OH– … (4.5)
When pH > 7, the equilibrium is displaced to the left while for pH < 7, the ammonium ion is predominant. For measuring the free ammonia which is given off when sewage decomposes, it is distilled and collected in the distillate. The age of wastewater is indicated by the relative amount of ammonia that is present. The presence of considerable amount of free ammonia indicates stale or old sewage.
(b) Organic Nitrogen:
Organic nitrogen present in wastewater is determined by Kjeldahl method. In this method, ammonia is first driven off from the aqueous sample by boiling it. The sample is then digested, during which the organic nitrogen is converted to ammonia, and from the latter the quantity of nitrogen is calculated. Total Kjeldahl nitrogen (or the total organic nitrogen), which is the sum of organic and ammonia nitrogen, is determined in the same manner as the organic nitrogen, except that the ammonia is not driven off before the digestion step.
(c) Albuminoid Nitrogen:
Just as free ammonia indicates the very first stage of decomposition of organic matter; albuminoid nitrogen indicates quantity of nitrogen present in wastewater before the decomposition of organic matter is started. In other words, albuminoid nitrogen (or albuminoid ammonia) indicates the amount of under-composed nitrogenous material in the wastewater.
However, unlike free ammonia, this does not really exist in the wastewater. It is produced by first freeing from the sample all free ammonia and then treating the sample with the alkaline solution of potassium permanganate. The ammonia given out as a result of the chemical reaction is called albuminoid ammonia which is taken as a measure of the easily decomposable organic nitrogen present in the wastewater.
(d) Nitrites Nitrogen:
Nitrites indicate the presence of partly decomposed (not fully oxidised) organic matter. The presence of nitrites in effluent indicates that oxidation is in progress. Nitrite nitrogen is relatively unimportant in wastewater or water pollution studies because it is unstable and is easily oxidised to the nitrate form. The presence of nitrites show that the treatment given to wastewater is still incomplete, and the sewage is still stale.
Hence, it is an indicator of past pollution in the process of stabilisation. Its quantity seldom exceeds 1 mg/l in wastewater. Nitrite nitrogen is determined by colourimetric methods. The colour is developed by adding sulphonilic acid and naphthamine, which is then compared with standard colours of known concentration.
(e) Nitrates Nitrogen:
Nitrates indicate the presence of fully oxidised organic matter. They indicate the most stable form of nitrogenous matter contained in wastewater, thus indicating the well oxidised and treated wastewater. Increase in proportion of nitrates during the process of the sewage treatment serves as a guide for measuring the progress achieved in the wastewater treatment.
Nitrates may vary in concentration from 0 to 20 mg/l as nitrogen in wastewater effluents. The nitrate concentration is also usually determined by colourimetric methods. The colour is developed by adding phenoldisulphonic acid and potassium hydroxide, which is then compared with standard colours of known concentrations.
(iv) Fats, Grease and Oils:
Fats and oils are mainly contributed from kitchen wastes, because they are major components of food stuffs such as butter, lard, margarine, and vegetable oils and fats. Fats are also commonly found in meats, seeds, nuts and some fruits.
Grease and oils are also discharged from industries like garages, workshops, factories etc. Fats and oils are compounds (esters) of alcohol or glycerol (glycerine) with fatty acids. Such matters float on the top of sedimentation tanks, often choke pipes in the winter, and clog filters.
They thus interfere with functioning of normal treatment plants. The particles interfere with biological action and cause maintenance problems. Fats are among the more stable of organic compounds and are not easily decomposed by bacteria. It is therefore necessary to detect and remove these from wastewater.
Fats, grease, mineral oils and lubrication oils are soluble in hexane or ether. In order to determine their content, the sample of wastewater is first treated with dilute hydrochloric acid which results in liberation of fatty acids. The latter are then evaporated. The residue is then mixed with ether. When the ether is driven off, the ether-soluble matter, which remains behind, represent the fats, grease and oil content.
Surfactants come primarily from synthetic detergents. These are discharged from bathrooms, kitchens, washing machines etc. Surfactants (or surface-active agents) are large organic molecules which cause foaming in wastewater treatment. Due to this, aeration of wastewater is hindered. Alkyl-benzene-sulphonate (ABS), a type of surfactant commonly used in synthetic detergents, is more troublesome since it is not biodegradable.
Hence some countries have banned the use of ABS in detergents, and hence ABS has been replaced in detergents by linear-alkyl-sulphonate (LAS) which is biodegradable. The content of surfactants in wastewater is determined by measuring the colour change in standard solution of methylene blue dye.
(vi) Phenols, Pesticides and Agricultural Chemicals:
Phenols are mostly found in industrial wastewater. If such wastewaters are directly discharged into receiving streams, they cause serious taste problems in drinking water, specially when water is disinfected by chlorination. However, phenols can be biologically oxidized if the concentrations are upto 500 mg/l.
Pesticides, herbicides and other agricultural chemicals primarily result from surface runoff from agricultural, vacant and park lands, specially in a combined sewerage system. The concentration of these trace contaminants is measured by the carbon- chloroform extract method.
(vii) Toxic Compounds:
Copper, lead, silver, chromium, arsenic and boron are some of the cations which are toxic to micro-organisms resulting in the malfunctioning of the biological treatment plants. These results from industrial wastewaters. Some toxic anions, including cyanides and chromates, present in some industrial wastes also hinder the wastewater treatment facilities. Hence their presence should be taken into consideration in the design of biological treatment plants.
(viii) Sulphates, Sulphides and H2S Gas:
Sulphates and sulphides are formed due to decomposition of various sulphur containing substances present in wastewater. The sulphate ions (SO4) occur naturally in most water supplies and hence they are also present in wastewater. Sulphur, required in the synthesis of proteins is released in the degradation. Anaerobic bacteria chemically reduce sulphates to sulphides and to hydrogen sulphide, as indicated by the following equations –
The hydrogen-sulphide gas so produced cause bad smells and odours. Besides this, H2S gas gets oxidised biologically to sulphuric acid resulting in corrosion to sewer pipes. The biological process of sludge digesters is severely hindered when sulphates are reduced to sulphides, specially when their concentration exceeds 200 mg/l. Also, H2S gas, which is evolved and mixed with wastewater gas (CH4 + CO2), is corrosive to the gas piping.
Following are the gases that are commonly found in untreated wastewater:
(i) Nitrogen (N2) (ii) Oxygen (O2) (iii) Carbon-dioxide (CO2) (iv) Hydrogen sulphide (H2S) (v) Ammonia (NH3) (vi) Methane (CH4). While the first three are the gases of the atmosphere which are found in all waters (including wastewaters) exposed to air, the later three are as a result of decomposition of organic matter present in the wastewater.
Methane gas is the principal by-product of the anaerobic decomposition of the organic matter in wastewater. This gas is colourless, and odourless and is highly combustible. Since its explosion hazard is high, manholes, sewer junctions, junction chambers etc. should be kept well ventilated.
Oxygen in a sample of wastewater is reported in the following three ways:
(a) Oxygen consumed
(b) Dissolved oxygen and
(c) Oxygen demand.
(a) Oxygen Consumed:
Oxygen consumed is the oxygen required for the oxidation of carbonaceous matter. This quantity of oxygen is determined by adding standard amount of potassium permanganate with dilute sulphuric acid to a sample of wastewater. The reaction is allowed to take place for periods of 15 minutes and 4 hours, at a constant temperature of 18°C.
The potassium permanganate liberates oxygen which is consumed by the wastewater. This test is made to determine the relative strength of sewage (i.e., whether strong, medium or weak), instead of BOD test. However, this test does not give the total oxygen needed for the biological oxidation of all or the bulk of the organic matter.
(b) Dissolved Oxygen:
Dissolved oxygen (DO) is the amount of oxygen in the dissolved state in the wastewater. Though wastewater generally does not have DO, its presence in untreated wastewater indicates that the wastewater is fresh. Similarly, its presence in treated wastewater/effluent indicates that considerable oxidation has been accomplished during the treatment stages.
While discharging the treated wastewater into receiving waters, it is essential to ensure that atleast 4 p.p.m. of DO is present in it. If DO is less, the aquatic animals like fish etc. are likely to be killed near the vicinity of disposal. The presence of DO in wastewater is desirable because it prevents the formation of noxious odours.
It is essential to determine the DO content of wastewater before it is subjected to treatment, so as to select proper treatment method. The small DO content of fresh wastewater is soon depleted due to aerobic decomposition.
The actual quantity of DO is governed by (i) solubility of oxygen (ii) partial pressure of oxygen in atmosphere (iii) the temperature, and (iv) purity (salinity, suspended solids etc.) of the water. DO of the wastewater decreases as the temperature increases. The solubility of DO in wastewater is only 95% of that in distilled water. DO content of wastewater may be determined by Winkler’s method.
(c) Oxygen Demand:
The presence of oxygen is essential for the livelihood of organisms. The aerobic action continues only till the oxygen is present in wastewater, and after that anaerobic action begins resulting in putrefaction. Thus, oxygen is demanded in wastewater for the oxidation of both inorganic as well as organic matter.
Thus demand of oxygen may be expressed in the following ways:
(i) Biochemical oxygen demand (BOD)
(ii) Chemical oxygen demand (COD)
(iii) Total oxygen demand (TOD)
(iv) Theoretical oxygen demand (Th. OD).
In addition to these, the amount of organic matter present may also be determined by the total organic carbon (TOC) test.
(i) Biochemical Oxygen Demand (BOD):
The biochemical oxygen demand (BOD) is a measure of the oxygen required to oxidise the organic matter present in a sample, through the action of microorganisms contained in a sample of wastewater. It is the most widely used parameter of organic pollution applied to both wastewater as well as surface water.
The BOD may be defined as the oxygen required for the micro-organisms to carry out biological decomposition of dissolved solids or organic matter in the wastewater under aerobic conditions at standard temperature.
The BOD test results are used for the following purposes:
(i) Determination of approximate quantity of oxygen required for the biological stabilisation of organic matter present in the wastewater.
(ii) Determination of size of wastewater treatment facilities.
(iii) Measurement of efficiency of some treatment processes.
(iv) Determination of strength of sewage.
(v) Determination of amount of clear water required for the efficient disposal of wastewater by dilution.
The organic matter present in wastewater may belong to two groups:
(i) Carbonaceous matter and
(ii) Nitrogenous matter.
The ultimate carbonaceous BOD of a liquid waste is the amount of oxygen necessary for the micro-organisms in the sample to decompose the carbonaceous materials that are subject of microbial decomposition. This is the first stage of oxidation and the corresponding BOD is also sometimes called the first stage demand. In the second stage, the nitrogenous matter is oxidised, and the corresponding BOD is known as second stage BOD or nitrification demand. In fact, pollution waters will continue to absorb oxygen for a long time.
Biochemical oxidation is a slow process and theoretically takes an infinite time to go to completion, though the ultimate first stage BOD of a given wastewater is equal to the initial oxygen equivalent of the organic matter present.
Generally, a 5 day period is chosen for standard BOD test, during which oxidation is about 60 to 70 per cent complete, while within 20 days period, the oxidation is about 95 to 99 per cent complete. A constant temperature of 20°C is maintained during the incubation. The BOD value of 5-day incubation period is commonly written as BOD5 or 5-day BOD.
(ii) Chemical Oxygen Demand (COD):
The BOD test takes a minimum of 5 days’ time, and due to this, it is not useful in the control of treatment processes. An alternative test is the COD test, which can be used to measure content of organic matter of both wastewater as well as natural waters. COD can be determined only in 3 hours in contrast to 5 days of BOD test. In COD test, a strong chemical oxidising agent is used in an acidic medium to measure the oxygen equivalent of organic matter that can be oxidised.
The COD test involves an acidic oxidation with potassium dichromate. A measured amount of potassium dichromate is added to the sample. The acidified sample is then boiled for 2 hours, cooled and the amount of dichromate remaining is measured by titration with ferrous ammonium sulphate.
To accelerate the oxidation of certain types of organic compounds, a catalyst, usually silver sulphate, is required to aid the oxidation. When dichromate is used as oxidising agent, the principal reaction may be represented in a general way by the following unbalanced equation.
The COD test is specifically, more suitable to measure organic matter present in industrial wastes having compounds that are toxic to biological life. However, COD results are generally higher than BOD values since the test will oxidise materials such as fats and lignins which are only slowly biodegradable.
No clear correlation exists between BOD and COD in general, but at specific treatment plants, a correlation is possible. When once a correlation has been established, the COD measurements, which can be concluded within 3 hours, can be used to good advantage for the control and operation of those treatment plants.
For typical untreated domestic wastes, the ratio COD/BOD5 is found to vary from 1.25 to 2.5. A higher value of the ratio indicates that the wastewater is difficult to biodegrade. For non-biodegradable wastewater, the ratio exceeds 10. The limiting value of COD of wastewater, generally specified by the authorities is 250 mg/l.
Total Organic Carbon (TOC):
One approach used to evaluate the amount of organic matter present in the wastewater is to determine the amount of organic carbon in the wastewater. The total organic carbon test (TOC test) is specially applicable to small concentrations of organic matter. The TOC test consists of acidification of the wastewater sample to convert inorganic carbon to CO2 which is then stripped.
The sample is then injected into a high temperature furnace where it is oxidised in the presence of a catalyst. The CO2 that is produced is quantitatively measured by means of an infrared analyser, and converted instrumentally to original organic carbon content. The error due to the presence of inorganic carbon can be eliminated by acidification and aeration of the sample prior to the analysis.
The Carbonaceous Analyser, provides a measure of TOC of an aqueous sample in approximately two minutes. The test is rapid, accurate, and correlates moderately well with BOD. However, certain organic compounds may not be oxidised, and the measured TOC will be slightly less than the actual amount present in the sample.
The major obstacle to widespread use of TOC is the cost of the equipment and the skill necessary in its operation. For typical untreated domestic wastewater, the BOD5/TOC ratio varies from 1.0 to 1.6.
(iii) Total Oxygen Demand (TOD):
This is yet another instrumental method to measure the organic content of wastewater. The TOD method is based on the quantitative measurement of the amount of oxygen used to bum the organic substances and to a minor extent, inorganic substances. It is thus a direct measure of the oxygen demand of the sample. The test is conducted in a platinum-catalysed combustion chamber.
The oxidisable components into a liquid sample introduced into the combustion chamber are converted to their stable oxides by a reaction that disturbs the oxygen equilibrium in the nitrogen carrier gas stream. The momentary depletion in the oxygen concentration in the carrier gas is detected by an oxygen recorder. The TOD for the sample is obtained by comparing this peak height to the peak height of standard TOD calibration solutions. This test can be carried out rapidly and the results have been correlated with the COD.
(iv) Theoretical Oxygen Demand (ThOD):
This is a theoretical method of computing the oxygen demand of various constituents of the organic matter present in wastewater. The organic matter present in the wastewater may be of animal or vegetable origin, consisting of principal groups such as carbohydrates, protein, fats and products of their decomposition. Each one of these is a typical combination of carbon, hydrogen, oxygen and nitrogen, based on its chemical formula.
Hence, if the chemical formulae of the constituents of the organic matter are known, ThOD can be easily computed. For example, glycine, commonly present in wastewater has a chemical formula [CH2 (NH2) COOH].
Its ThOD can be analytically determined by assuming the following steps in the reactions:
(i) In the first step, carbon is converted to CO2 and nitrogen is converted to ammonia.
(ii) In the second step, ammonia is oxidised to nitrite.
(iii) In the third step, nitrite is oxidised to nitrate.
The ThOD will then be the sum of the oxygen required for all the three steps of reactions.
Relative stability of wastewater is defined as the ratio of available oxygen to the required oxygen satisfying first stage BOD. The available oxygen will include dissolved oxygen (DO) as well as oxygen present as nitrite or nitrate. It is generally expressed as percentage of total oxygen required.
The test for relative stability is carried out in the following steps:
(i) The wastewater sample is filled in a glass-stoppered bottle and a small quantity of methylene blue is added to it.
(ii) The mixture is then incubated either at a temperature of 20°C or at 37°C. In countries like India, a temperature of 37°C is preferred.
(iii) During the incubation period, the anaerobic bacteria start their function, the available DO is consumed and H2S is produced which decolourises the mixture. The time t (in days) required for bleaching the blue colour is noted.
The relative stability (SR) is worked out from the following expressions:
where t20 and t37 are the number of days of incubation at 20°C and 37°C respectively.
The relative stability test is not suitable for raw sewage because the colour is precipitated out due to the presence of some dissolved and colloidal solids. However, the test is quite suitable for studying the quality of effluents coming out of the wastewater treatment plants. If the value of t comes out to be less, it will indicate that availability of oxygen is less and therefore the wastewater is less stable.
If the decolourisation during the test takes place in a time lesser than 4 days, the effluent may be taken as relatively unstable (with its relativity less than about 60%) and the effluent cannot be safely discharged into the receiving waters (i.e. streams etc.). However, if t > 5, the effluent is relatively stable and hence it can be safely discharged into the receiving water. Table 4.7 gives the values of relative stability at 20°C and 37°C required for decolourisation.
The wastewater carried by a sewer consists mainly of domestic sewage and the industrial wastewater. Since the contribution of solids to sewage should be nearly constant on a per capita basis, the BOD contribution (expressed in grams/person per day) should also be constant.
Generally, BOD contribution per capita per day may be taken as 80 g/day (or 0.08 kg/day). Industrial wastewaters are generally compared with per capita domestic sewage, through the concept of population equivalent (PE) using per capita BOD value as the basis. Thus, we have –
Similarly, if at the point of measurement, we have a combined wastewater, consisting of domestic sewage as well as industrial wastewater, the population equivalent of the combined wastewater can be found by dividing BOD5 of the combined wastewater by the per capita/day BOD value.
Generally, the population equivalent is used to indicate the strength of industrial wastewater required for (i) estimating the treatment required at the common treatment plant (municipal treatment plant) and (ii) charging appropriate levy on the industries to meet the proportionate cost of treating waste in the municipal treatment plant.
Biological Characteristics of Wastewater:
Domestic sewage, by its nature, contains enormous quantities of micro-organisms. The biological characteristics of sewage are related to the presence of these microorganisms. The sanitary engineer must have considerable knowledge of (i) principal groups of microorganisms found in water and wastewater (ii) pathogenic organisms in wastewater, and (iii) organisms used as indicators of pollution.
Excremental matter contains myriads of micro-organisms – as many as 320 billions in the sewage per head per day. Most of them are not only harmless, but are friendly to mankind, since they help in the biological treatment of sewage to render it harmless.
A micro-organism that requires living tissues to grow is called a pathogen and is a parasite which must depend upon host organism as a proper environment for growth and reproduction. Pathogenic microorganisms are harmful to man. Micro-organisms can be either plants or animals.
When the assemblages of aquatic organisms drift more or less passively with waves and current, they constitute the plankton. If however, they change their position or location due to their own effort, they are called nekton. If they maintain their stability due to surface tension forces, they are referred to as neuston.
The various micro-organisms found in water or wastewater may be broadly classified under three categories:
(i) Aquatic plants
(ii) Aquatic animals
(iii) Aquatic moulds, bacteria and viruses.
(i) Aquatic Plants:
Under this category, the following are included:
(a) Spermophyta – Water weeds.
(b) Bryophyta – Mosses and lever words.
(c) Pteridophyta – Ferns and horsetails.
(d) Thallophyta – Algae.
Out of these, water weeds and algae are of practical importance to sanitary engineers. If favourable conditions of light, temperature and nutrition exist, water weeds grow in dense form. Their decay during autumn add organic matter to water. Algae are considered as simple, photo-synthetic plants with unicellular organs of reproduction. They are, thus 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 their effluents discharged into receiving waters do not encourage growth of algae and other aquatic plants. This solution may lie in the removal of carbon, various forms of nitrogen and phosphorus.
(ii) Aquatic Animals:
They include the following:
(a) Vertebrate – Fish and amphibians.
(b) Mollusca – Mussels, snails, slugs, limplets, cocklets.
(c) Arthopoda – Crustacea, insects, spiders, mites.
(d) Worms – Aquatic earthworms, thread worms, rotifera.
(e) Metazoa –Hydra, polyzoa.
(f) Protozoa – Endameba histolytica etc.
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 simplest forms of animal life. One form of protozoa – Endemeba histolytica causes amoebic dysentery. If forms cysts which are excreted in the bowel discharges of infected persons and which will live for long periods in water.
The protozoa are bacteria eaters and survive in dilute organic wastes by eating bacteria and thus destroy the pathogens. When the concentration of organic waste is sufficiently great, the protozoa can utilise 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.
(iii) Aquatic Moulds, Bacteria and Viruses:
Strictly speaking, moulds (or fungi), bacteria and viruses come under the category of aquatic plant, but because of their special importance, they are generally kept in a separate category.
Fungi are unicellular, 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 as well. 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.
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 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 several other 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 animals. Enteric viruses include coxsackie viruses, infectious hepatitis viruses, poliviruses, reoviruses etc.
Bacteria are single-celled micro-organisms with rigid cell walls that take in soluble food and convert it to new cells. 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 beneficial to man, and obtain organic matter in solution from dead and decaying tissue in plants and animals. Parasitic bacteria live and multiply on or within the body of a living organism of a higher type.
Pathogenic bacteria and pathogenic organisms are capable of causing disease within the living organisms on which they subsist. According to oxygen need, bacteria can be classified as (i) aerobic bacteria which need oxygen to live (ii) anaerobic bacteria which survive in the absence of oxygen and (iii) faculative bacteria which can live and multiply with or without oxygen.
According to temperature of which they flourish, bacteria can be (a) psychrophilic bacteria which can survive between 10° to 20°C (b) mesophilic bacteria which can survive between 20° to 40°C and (c) thermophilic bacteria which can survive between 40° to 65° C. Bacteria are less sensitive to cold than heat. At low temperatures, they are essentially dormant and can survive for long periods of time. Some bacteria tolerate low pH while others tolerate high pH, the optimum being between 6.5 and 7.5.
The bacteria of excretal origin may be three types:
(a) The coli-aerogenes group bacteria
(b) Clostridium welchii
(c) Faecal streptococci.
(a) Coli-Aerogeneous Group:
The intestines of men and warm blooded animals contain certain harmless bacteria which are excreted with faeces. These are numerous in sewage. Formerly, these organisms were known as colon bacilli but at present they are known as coliforms. The number of coliform organisms in human faeces is estimated to lie between 1011 and 1013 per capita daily.
The coliform group includes all of the aerobic and facultative anaerobic non-sporeforming, Gram-negative rod shaped bacteria that ferment lactose (milk sugar) with production at 35°C of gas within 48 hours. The coliform group is comprised of two important species. Escherichi coli (E-coli) and Aerobacteria aerogenes. They are of advantage as indicators of water contamination.
(b) Clostridium Welchii:
It is a stout Gram-positive rod having elliptical sub-terminal spores. The organism is found in cultivated soils, sewage and polluted water. Intestines are its natural habitat where it causes non harm but assists digestion.
(c) Faecal Streptococci:
They are found in human intestines. It is a Gram-positive coccus occurring in pairs or short chains. It grows in presence of bile salt and is capable of growing at temperatures as high as 45°C. However, they are not as numberous as E-coli in all normal cases. Hence test for faecal streptococci offers no advantage over the E-coli test except in case of doubt.
The coliforms and the E-coli are normally used as indicator organisms since they are present in large numbers than pathogens. They also lend themselves to numerical evaluation as well as qualitative distinction. They are also identifiable by relatively simple analytical procedures, providing information quickly and economically.
The usual procedure for determining the presence of coliforms consists of- (i) presumptive (ii) the confirmed tests, and (iii) completed test. There are two accepted methods for obtaining the number of coliform organisms in the specimen – (i) the coliform index and (ii) the most probable number (M.P.N.).
The most probable number (M.P.N.) of coliform is defined as that bacterial density, which if it had been actually present in the sample under examination, would more frequently than any other, have given the observed analytical, result. Thus, MPN is not the absolute concentration of organisms that are present but only a statistical estimate of that concentration.
The work done by anaerobic bacteria, viz. decomposition of organic matter is called putrefaction and the result is called liquefaction, as the solid organic matter is dissolved by enzymes. Anaerobic bacteria oxidise organic matter utilising electron acceptors other than oxygen. In carrying out their metabolic processes, they produce CO2, H2O, H2S, CH4, NH3, N2, reduced organics and more bacteria.
A large part of the available energy appears in the form of end products, and hence cell production is low. Treatment units which work on putrefaction alone are septic tanks, Imhoff tanks, and sludge digestion tanks. The end products of an anaerobic fermentation are likely to be odourous. The production of a stable effluent is unlikely since wastes do not usually contain sufficient electron acceptors to permit complete oxidation.
The work of the aerobic bacteria, i.e. combination with oxygen is called oxidation. Aerobic bacteria utilise free oxygen as an electron acceptor. The end products of aerobic activity are CO2, H2O, SO4, NO3, NH3 and more bacteria. The bulk of the available energy finds its way into cell mass or heat, yielding a stable effluent which will not undergo further decomposition.
Though each of the above two processes work in the opposite direction—the former by splitting up and the latter by, building up, there is co-ordination between the two. In the first stage, the anaerobic bacteria decompose complex organic matter into simple organic compounds while in the second stage, the aerobic bacteria oxidise them to form stable compounds.