UNIT IV

CHAPTER - 9

CHARACTERISTICS OF WASTEWATER9.1 SEWAGE CHARACTERISTICS

Characterization of wastes is essential for an effective and economical waste management programme. It helps in the choice of treatment methods deciding the extent of treatment, assessing the beneficial uses of wastes and utilizing the waste purification capacity of natural bodies of water in a planned and controlled manner. While analysis of waste in each particular case is advisable and the data from other cities may be utilized during initial stage of planning.

Domestic sewage comprises spent water from kitchen, bathroom, lavatory, etc. The factors which contribute to variations in characteristics of the domestic sewage are daily per capita use of water, quality of water supply and the type, condition and extent of sewerage system, and habits of the people. Municipal sewage, which contains both domestic and industrial wastewater, may differ from place to place depending upon the type of industries and industrial establishment. The important characteristics of sewage are discussed here.

9.1.1 TemperatureThe observations of temperature of sewage are useful in indicating solubility of oxygen, which affects transfer capacity of aeration equipment in aerobic systems, and rate of biological activity. Extremely low temperature affects adversely on the efficiency of biological treatment systems and on efficiency of sedimentation. In general, under Indian condition the temperature of the raw sewage was observed to be between 15 to 350C at various places in different seasons.

9.1.2 The pHThe hydrogen ion concentration expressed as pH, is a valuable parameter in the operation ofbiological units. The pH of the fresh sewage is slightly more than the water supplied to the community. However, decomposition of organic matter may lower the pH, while the presence of industrial wastewater may produce extreme fluctuations. Generally the pH of raw sewage is in the range 5.5 to 8.0.

9.1.3 Colour and OdourFresh domestic sewage has a slightly soapy and cloudy appearance depending upon its concentration. As time passes the sewage becomes stale, darkening in colour with a pronounced smell due to microbial activity.

9.1.4 SolidsThough sewage contains only about 0.1 percent solids, the rest being water, still the nuisancecaused by the solids cannot be overlooked, as these solids are highly putrescible and therefore

1

need proper disposal. The sewage solids may be classified into dissolved solids, suspended solids and volatile suspended solids. Knowledge of the volatile or organic fraction of solid, which decomposes, becomes necessary, as this constitutes the load on biological treatment units or oxygen resources of a stream when sewage is disposed of by dilution. The estimation of suspended solids, both organic and inorganic, gives a general picture of the load on sedimentation and grit removal system during sewage treatment. Dissolved inorganic fraction is to be considered when sewage is used for land irrigation or any other reuse is planned.

9.1.5 Nitrogen and PhosphorusThe principal nitrogen compounds in domestic sewage are proteins, amines, amino acids, andurea. Ammonia nitrogen in sewage results from the bacterial decomposition of these organic constituents. Nitrogen being an essential component of biological protoplasm, its concentration is important for proper functioning of biological treatment systems and disposal on land. Generally, the domestic sewage contains sufficient nitrogen, to take care of the needs of the biological treatment. For industrial wastewater if sufficient nitrogen is not present it is required to be added externally. Generally nitrogen content in the untreated sewage is observed to be in the range of 20 to 50 mg/L measured as TKN. Phosphorus is contributing to domestic sewage from food residues containing phosphorus and their breakdown products. The use of increased quantities of synthetic detergents adds substantially to the phosphorus content of sewage. Phosphorus is also an essential nutrient forthe biological processes. The concentration of phosphorus in domestic sewage is generally adequate to support aerobic biological wastewater treatment. However, it will be matter of concerned when the treated effluent is to be reused. The concentration of PO4 in raw sewage is generally observed in the range of 5 to 10 mg/L.

9.1.6 ChloridesConcentration of chlorides in sewage is greater than the normal chloride content of water supply. The chloride concentration in excess than the water supplied can be used as an index of the strength of the sewage. The daily contribution of chlorides averages to about 8 gm per person. Based on an average sewage flow of 150 LPCD, this would result in the chloride content of sewage being 50 mg/L higher than that of the water supplied. Any abnormal increase should indicate discharge of chloride bearing wastes or saline ground water infiltration, the latter adding to the sulphates, which may lead to excessive generation of hydrogen sulphide.

9.1.7 Organic MaterialOrganic compound present in sewage are of particular interest for sanitary engineering. A large variety of microorganisms (that may be present in the sewage or in the receiving water body) interact with the organic material by using it as an energy or material source. The utilization of the organic material by microorganisms is called metabolism. The conversion of organic material by microorganism to obtain energy is called catabolism and the incorporation of organic material in the cellular material is called anabolism. To describe the metabolism of organic material, it is necessary to characterize quantitatively its concentration. In view of the enormous variety of organic compounds in sewage it is totally

2

unpractical (if not possible) to determine these individually. Thus a parameter must be used that characterizes a property that all these have in common. In practice two properties of almost all organic compounds can be used: (1) organic compound can be oxidized; and (2) organic compounds contain organic carbon.

In sanitary engineering there are two standard tests based on the oxidation of organic material: 1) the Biochemical Oxygen Demand (BOD) and 2) the Chemical Oxygen Demand (COD) tests. In both tests, the organic material concentration is measured during the test. The essential differences between the COD and the BOD tests are in the oxidant utilized and the operational conditions imposed during the test such as biochemical oxidation and chemical oxidation. The other method for measuring organic material is the development of the Total Organic Carbon (TOC) test as an alternative to quantify the concentration of the organic material.

9.1.8Biochemical Oxygen Demand (BOD): The BOD of the sewage is the amount of oxygen required for the biochemical decomposition of biodegradable organic matter under aerobic conditions. The oxygen consumed in the process is related to the amount of decomposable organic matter. The general range of BOD observed for raw sewage is 100 to 400 mg/L.

9.1.9Chemical Oxygen Demand (COD): The COD gives the measure of the oxygen required for chemical oxidation. It does not differentiate between biological oxidisable and non oxidisable material. However, the ratio of the COD to BOD does not change significantly for particular waste and hence this test could be used conveniently for interpreting performance efficiencies of the treatment units. In general, the COD of raw sewage at various places is reported to be in the range 200 to 700 mg/L.

In COD test, the oxidation of organic matter is essentially complete within two hours, whereas, biochemical oxidation of organic matter takes several weeks. In case of wastewaters with a large range of organic compounds, an extra difficulty in using BOD as a quantitative parameter is that the rate of oxidation of organic compounds depends on the nature and size of its molecules. Small molecules are readily available for use by bacteria, but large molecules, colloidal and suspended matter can only be metabolized after preparatory steps of hydrolysis. It is therefore not possible to establish a general relationship between the experimental five-day BOD and the ultimate BOD of a sample, i.e., the oxygen consumption after several weeks. For sewage (with k=0.23 d-1 at 20oC) the BOD5 is 0.68 times ultimate BOD, and ultimate BOD is 87% of the COD. Hence, the COD /BOD ratio for the sewage is around 1.7.

9.1.10 Toxic Metals and CompoundsSome heavy metals and compounds such as chromium, copper, cyanide, which are toxic mayfind their way into municipal sewage through industrial discharges. The concentration of these compounds is important if the sewage is to treat by biological treatment methods or disposed off in stream or on land. In general these compounds are within toxic limits in sanitary sewage however, with receipt of industrial discharges they may cross the limits in municipal wastewaters.

3

CHAPTER – 10DISPOSAL OF SEWAGE

10.1 Disposal of Sewage on LandDisposal of Sewage Effluents on Land for Irrigation:

In this method, the sewage effluent (treated or diluted) is generally disposed of by applying it on land. The percolating water may either join the water table or it is collected below by a stream of under drains. This method can be used for irrigating crops.

The following three methods are normally used in broad irrigation:a) Sub Surface Irrigation b) Sprinkler Irrigation andc) Surface Irrigation.a) Sub Surface Irrigation: In this system, underground drains pipes are laid and

sewage is applied directly to the root zone of plants. This system of sewage disposal is a costly affair and is not used.

b) Sprinkler Irrigation: In this system, the sewage is applied under pressure through nozzle and is termed as sprinkler irrigation.

c) Surface Irrigation: this method is useful in sloping areas; the sewage is applied to the drain by a distributaries channel. In this method, sewage is applied on the surface of land is called surface irrigation.

10.2 Sewage Farming:

When the sewage is used for growing crops it is called sewage farming. The fertilizing elements of sewage are consumed by the root of crops. The mineral salts of sewage such as nitrates, sulphates and phosphates are the main fertilizing constituents of sewage. By the sewage farming a good income can be done which is always profitable.

The nutrients of sewage like nitrogen, phosphorous and potassium (N, K, P) along with micronutrients as well as organic matter present in it could be advantageously employed for sewage farming to add to the fertility of the fields along with the irrigation potential of the water content. Sometimes in sewered areas where treatment facilities are absent, then sewage farming can be done. Even the application of the treated effluent to land should be carried out with certain precautions because it is completely free from risk. The sewage farming should be done such that hygienic safety to the staff is protected against the infection by pathogenic organisms. Though the sewage after primary treatment can be applied to the farm, the main reason of providing only after primary treatment and eliminating secondary treatment merely depends on cost circumstances raw sewage should be applied to the farms directly.

10.3 Sewage Sickness:

When the sewage is applied continuously on a piece of land, the soil pores or voids may get filled up and clogged with sewage matter retained in them. The time taken for such clogging will depend upon the type of the soil and the load present in sewage. But once these voids gets clogged, free circulation of air will be prevented and anaerobic conditions will develop

4

within the pores. Decomposition of organic matter would take place under anaerobic conditions with evolution of foul gases like H2S, CO2 and methane. This phenomenon of soil getting clogged is known as sewage sickness.

10.4 Self Purification of Stream Please refer to the additional supplement attached to this notes of unit IV.

10.5 Zones of pollution in a river or Stream A polluted stream undergoing self-purification can be divided into following four zones• Zone of degradation• Zone of active decomposition• Zone of recovery• Zone of clearer water

These zones are explained below:

1. Zone of DegradationThis zone is found for a certain length just ahead of the point where sewage is discharged

into the river. This Zone is characterized by water becoming dark and turbid with formation of sludge deposits at the bottom. Bacteria will start oxidation of the organic matter the DO is reduced to about 40 % of the saturation valve 9 at 30 0C it is 7.6 mg/l and at 20 0C it is 9.17 mg/l). There is an increase in carbon dioxide content. Oxygenation occurs but is slower than deoxygenating.

5

These conditions are unfavorable to the development of aquatic life; and as such, algae dies out, but certain type of fish may be present feeding on fresh organic matter. Moreover, certain typical bottom worms such as limondrilus and Tubifex appear with sewage fungi.

2. Zone of Active Decomposition

This zone is marked by heavy pollution. It is characterized by water becoming grayish and darker than in the previous zone. DO concentration falls down to zero, an anaerobic conditions may set in with the evolution of gases like methane, carbon dioxide, hydrogen sulphide, etc.. bubbling to the surface, with masses of sludge forming ugly scum layer at the surface. As the organic decomposition slackens due to stabilization of organic matter, rate of deoxygenating becomes lesser than the re-aeration rate and DO again rises to the original level (i.e. above 40 %)

In this region, the bacteria flora will flourish. At the upper end, anaerobic bacteria will replace aerobic bacteria, while at the lower end, the position will replace aerobic bacteria, while at the lower end the position will be reversed. Protozoa and fungi first disappear then reappear. Fish life will be absent. Algae and tubifex will also be absent. Maggots and Psychoda (Sewage fly) larvae will, however, be present in all but the most septic sewage.

3. Zone of Recovery

In this zone, the river stream tries to recover from its degraded condition to its former appearance. The water becomes clearer, and so the algae reappear while fungi decrease. BOD decreases and DO content rises above 40 % of the saturation value; Protozoa, Rotifers, Crustaceans and large plants like sponges, Bryozoans, etc. The organic material will be mineralized to form nitrate, phosphate, carbonates, etc.

4. Zone of Clearer Water

In this zone, the river attains its original condition with DO rising up to the saturation value. Water becomes attractive in appearance, fish (which requires at least 4 to 5 mg /l of DO) and usual aquatic life prevails. Some pathogenic organisms may still, however survive and remain present, which confirms the fact that “once the river water has been polluted, it will not be safe for drinking, unless it is properly treated”.

10.6 The Oxygen Sag or Oxygen Deficit of a Polluted River.

• The oxygen deficit D at any time in a polluted river is the difference between the actual D.O. Content at the water temperature

• Oxygen Deficit (D)= Saturation DO- Actual DO

• In order to maintain clean conditions in a river, the oxygen deficit must be nil, and this can be found out by knowing the rates of de-oxygenation and re-oxygenation.

6

10.6.1 De- Oxygenation Curve:

In a polluted stream, the D.O. content goes on reducing due to decomposition of volatile organic matter. The rate of de oxygenation depends upon the amount of the organic matter remaining to be oxidised at the given time (i.e. Lt) as well as on the temperature of reaction (i.e. t). Hence, at a given temperature, the curve showing depletion of D.O. with time, i.e. de-oxygenation curve is similar to the first stage BOD curve. It can also be expressed mathematically as per equation of BOD.

10.6.2 Re-Oxygenation Curve

• Oxygen which is consumed by the bacteria in oxidation of organic matter is replenished from the atmosphere, this process is called re-oxygenation. The rate at which the oxygen is supplied by the atmosphere to the polluted water depends upon:

• Velocity of the stream and the resulting turbulence created, more the velocity more is the rate.

• The depth of the receiving water, lesser is depth more is the rate.

• Temperature of water, rate decreases with increase in temperature.

• The saturation deficit or the oxygen deficit below the saturation valve.

• Depending upon these factors, the rate of de-oxygenation can be expressed mathematically and plotted in the form of a curve called re-oxygenation curve.

10.6.3. Oxygen Sag Curve or Oxygen Deficit Curve

• In running polluted stream exposed to atmosphere, the de oxygenation and re-oxygenation process occurs simultaneously. If the de-oxygenation is more rapid than the re-oxygenation, an oxygen deficit occurs.

• The amount of the resultant oxygen deficit can be obtained by algebraically adding the de-oxygenation and re-oxygenation curve. The resultant curve so obtained is called the oxygen sag curve or oxygen deficit curve. From the curve, the oxygen deficit D and oxygenation balance (100-D) % in a stream after a certain lapse of time can be found out.

• It can be seen that when de-oxygenation rate exceeds the re oxygenation rate, the oxygen sag curve shows increasing deficit of oxygen; but when both rates become equal, the critical point is reached, and finally when the rate of de-oxygenation falls below that of re-oxygenation, the oxygen deficit goes on decreasing till becoming zero.

7

8