3 Characteristics of Sewage – Physical, Chemical and Biological |Wastewater|24th March 2018
Characteristics of Sewage
Sewage is a dilute mixture of the various types of wastes from the residential, public and industrial places. The characteristics of sewage play an important role in the design and construction of various treatment units.
The characteristics of sewage can be classified as:
- Physical characteristics
- Chemical characteristics
- Biological characteristics
Physical characteristics of Sewage
The sewage has the following physical characteristics:
l) Specific Gravity
The specific gravity of sewage is very nearly equal to that of water
The fresh sewage has a yellowish or grey or light brown colour. It becomes black or dark brown when the sewage attains septic stage. The colour of sewage can normally be detected by the necked eye.
Fresh sewage is practically odourless. But the stale sewage has an offensive odour of hydrogen sulphide and other sulphur compounds which is formed due to the decomposition of sewage. Odour is tested by osmoscope.
The normal temperature of sewage is generally slightly higher than the temperature of the water. The average temperature of sewage in India is 20°C.
The temperature has an effect on the biological activity of bacteria present in sewage. Biological activities in sewage are higher at a greater temperature.
Temperature also affects the solubility of gases in sewage. In addition, temperature also affects the viscosity of sewage, which in turn affects the sedimentation process in its treatment.
Sewage is normally turbid. The turbidity directly depends upon the quantity of suspended particles. The degree of turbidity can be measured and tested by turbidity rods or by turbidimeters.
Chemical Characteristics of Sewage
The sewage has the following chemical characteristics:
The sewage normally contains a very small amount of solids in relation to the huge quantity of water. It only contains about 0.05 to 0.1 % of total solid matters. The solid matters present the sewage may be in any of the four forms:
- Suspended solids
- Dissolved solids
- Colloidal solids
- And, settleable solids
Again the solids may be organic or inorganic.
It has been estimated that about 1000 kg of sewage contains about 0.45 kg of total solids, out of which 0.225 kg is in solution, 0.112 kg is in suspension, and 0.112 kg is settleable. Colloidal solids remain either in solution or in suspension.
Further, the solids in sewage comprise of both organic as well as inorganic solids. The organic matter is about 45% of the total solids and the remaining about 55% is the inorganic matter. Distribution solids present in 1000 kg of sewage can be present by the following diagram.
The total amount of solids present in given sewage can be determined by evaporating a known volume of sewage sample and weighing the dry residue left.
The quantity of suspended solids can be determined by passing a known volume of sewage sample through a glass-fibre filter apparatus and weighing the dry residue left.
The difference between the total solids and the suspend solids represents dissolved solids plus colloidal. The quantity of settleable solids can be determined with the help of the Imhoff cone.
2) pH Value
The pH value is defined as the logarithm of reciprocal of hydrogen ion concentration present in water. It is used to designate the acidity and alkalinity of water.
Thus, pH value – Log[H+]
Nature of fresh sewage and treated sewage is alkaline and the septic sewage is acidic in nature. The pH value of fresh and treated sewage is generally more than 7 & the pH value of septic sewage is less than 7.
The pH value can be measured quickly and automatically with the help of a potentiometer.
3) Chloride Contents
Chlorides are derived from the kitchen wastes, human excreta, and industrial discharge. The normal chloride content of domestic sewage is 120 mg/lit.
The high chloride content of sewage indicates the presence of industrial sewage or infiltration of seawater.
The chloride content can be measured by titrating the wastewater with standard silver nitrate solution, using potassium chromate as an indicator.
4) Nitrogen Contents
The presence of nitrogen in sewage indicates the presence of organic matter. It may occur in one or more of the following forms:
- a) Free ammonia
- b) Albuminoidnitrogen
- c) Nitrites
- d) Nitrates
Presence of free ammonia indicates the first stage of decomposition of organic matter. Albuminoidnitrogen indicates the quantity of nitrogen present in sewage before the decomposition of organic matter is started.
The nitrites indicate the presence of partly decomposed organic matter. Nitrates indicate the presence of fully oxidized organic matter.
The amount of free ammonia present in sewage can be easily measured by simply boiling and measuring the ammonia gas.
The amount of albuminoid nitrogen can be measured by adding a strongly alkaline solution of potassium permanganate to the already boiled sewage sample and again boiling the same.
The amount of nitrites or nitrates present in sewage sample can be measured by colour matching methods.
5) Presence of fats, greases and oils
These are derived in sewage from the discharges of animals, kitchens of hotels, industries, etc.
In order to determine the amount of fats, greases, etc. a sample of sewage is first evaporated.
The residual solids left are then mixed with ether and the solution is then evaporated, leaving behind the fat, grease as a residue.
6) Presence of sulphides, sulphates and Hydrogen sulphide gas
Sulphides, sulphates and hydrogen sulphide gas are formed due to the decomposition of various sulphur-containing substances present in sewage.
Their presence in sewage reflects aerobic, and/or anaerobic decomposition. The determination of sulphides, sulphates in sewage is rarely called for.
7) Dissolved oxygen (D.C)
This represents the amount of oxygen present in sewage in a dissolved state. Sewage has generally no dissolved oxygen. Its presence in raw sewage indicates that the sewage is fresh.
Its presence in the effluent after treatment indicates that considerable oxidation has been accomplished by the sewage treatment methods.
The D.O in fresh sewage depends upon temperature. If the temperature of the sewage is more, the D.O content will be less. The D.O content of sewage is generally determined by Winkler’s method.
Biological Characteristics of Sewage
The sewage contains the following bacteria and microorganisms.
Bacteria are microscopic unicellular organisms. They may be the following types:
a) Pathogenic bacteria: These are responsible for all water-borne diseases.
b) Non-pathogenic bacteria: These are harmless.
c) Aerobic bacteria: It helps the decomposition of sewage in oxidation ponds, lagoons, etc.
d) Anaerobic bacteria: It helps the decomposition of sewage in a septic tank, cesspool, etc.
e) Facultative bacteria: It has no function in sewage treatment.
The microorganism like algae, fungi, and protozoa help the process of decomposition of sewage by photosynthesis or by breaking the organic compounds.