Environmental Engg.

Lesson-19 Domestic waste water – quantity and characteristics

Quantity of domestic waste water

In order to find out a suitable section for the sewer, it is necessary to determine the quantity of sewage that will flow through the sewer.  The sewage consists of the following two categories:

1. Dry weather flow

2. Storm water

 DRY WEATHER FLOW

This is sometimes written as D.W.F. and it consists of two types of sewage:

1. Domestic or sanitary sewage

2. Industrial sewage

The quantity of D.W.F. is determined by considering the following four factors:

1. Infiltration and exfiltration

2. Nature of industries

3. Population

4. Rate of water supply

Infiltration and exfiltration

The term infiltration is used to indicate the leakage of water from the ground surrounding the sewer and the term exfiltration is used to indicate the leakage of sewage from the sewer into the ground surrounding the sewer.

The infiltration and exfiltration are both undesirable.  The infiltration unnecessarily increases the quantity of sewage.  The exfiltration pollutes the underground sources of water, if any.  The infiltration, however, is preferred to exfiltration.

The infiltration and exfiltration can be prevented to some extent by constructing watertight joint of sewers.

The quantity of water through infiltration depends on the following four factors:

• Head of subsoil water level

• Length of sewer

• Nature and type of soil through which sewer is laid and

• Size of sewer

Nature of industries

The quantity of industrial sewge will depend upon the nature of industries.  A careful study is, therefore, made of the industries contributing to the flow of sewage and the quantity of industrial sewage is then accordingly worked out.

Population

Just as in case of water supply projects, the future population after two or three decades is determined by applying any suitable method of population forecast.  The design period of different parts of the sewerage system is different and the following factors are taken into account while fixing the probable life of a particular part of the sewerage system

• Chances of improvement in the equipment

• Cost of adding an extra unit of the equipment

• Labour conditions having important influence on the cost of equipment

• Maintenance cost and overhead charges

• Operation methods and

• Original cost of the part

Usually, the design periods of the following parts of sewerage system are:

• Laterals: These are designed for 50 years or so

• Branches and submains:  These are also designed to last for 50 years or more

• Main sewers and trunk sewers: These are designed for 30 to 50 years

• Pumping plant:  As additional pump can be installed with short notice, the design period of pumping plant is about 5 to 10 years.

• Treatment units:  These are designed for 10 to 30 years.

Rate of water supply

Usually the quantity of water entering the sewer will be slightly less than the quantity of water supplied. For practical purposes, it is assumed that the quantity of water which does not enter sewer is very nearly equal to the extra quantity of water which enter sewer.  In other words, the rate of sewage is assumed as equal to the rate of water supply.  The rate of sewage may, however, be assumed lower than the rate of water supply to the extent of about 60 to 70 per cent where there are sufficient reasons to justify such assumption.

Following two factors should be carefully considered while comparing rate of sewage with rate of water supply.

• Intensity of pressure:  If water is supplied at high pressure, there is more consumption of water and more wastage of water from leakage in pipes, valves etc.

• Use of water:  The use for which water is consumed should be carefully studied.  It is quite likely that water which is supplied may not appear as sewage after its consumption.  For instance, water that is supplied to fill up the tanks of railway locomotives will not appear as sewage.  Similarly, the quantity of water supplied and the quantity of sewage formed by various prominent industries such as cotton mills, milk plants, etc. should be studied and accordingly the rate of sewage should be decided.

STORM WATER

Wherever possible, the storm water is to be collected and conveyed in sewers at proper places for the following reasons:

• Damp conditions are created which are unhygienic as they provide flourishing ground for micro organisms

• Existence of waterpools affects the foundations of structures

• Initial washings of streets by storm water contain organic matter and hence such water requires to be collected and to be taken to the treatment plant

• Low lying areas get flooded and transport system is paralysed. It leads to loss of revenue.

• Stagnant waterpools serve as breeding places for mosquitoes.

The quantity of storm water, which is known as wet weather flow and mentioned as W.W.F., that will enter sewer is to be carefully determined.  It involves various factors such as intensity of rainfall, characteristics of catchment area, duration of storm, etc.  Following two methods are generally employed for calculating the quantity of storm water for the purpose of designing sewers:

• Rational method
• Empirical method

 Rational method

In this method, the following three factors are combined in the form of an equation:

Q= K I A / 360

Where,

Q = peak runoff in m³ per second

K = Impermeability factor

I = Intensity of rainfall, mm per hour

A = Area in hectares

Catchment area

The catchment area to be served by a storm water sewer is measured directly form the map of the locality

Impermeability factor

Some quantity of rain water that falls on the ground is absorbed by soil and the percentage of rain water that enters the sewer is known as impermeability factor. 

Intensity of rainfall

The intensity of rainfall can be worked out from the rainfall records of the area under consideration.  Where rainfall records are not available, the intensity of rainfall is obtained by applying suitable empirical formula. 

The general empirical formula adopted to calculate intensity of rainfall is:

           \[R=\frac{{25.4\cdot a}}{{t + b}}\]

            Where, R = Intensity of rainfall in mm per hour

                        t = Duration of storm in minutes

                        a and b are constants

The values of a and b are as follows:

a = 30 and b = 10 when duration of storm is 5 to 20 minutes

a = 40 and b = 20 when duration of storm is 20 to 100 minutes

Characteristics of domestic waste water

In order to determine the line of treatment, constituents of which sewage is composed are to be properly determined.  Quality of sewage plays an important role in the design and construction of various treatment units.  The treatment given to sewage should be such that it can be easily disposed off in natural stream or river. 

Properties of sewage

The properties of sewage will be studied in the following three groups:

1. Physical properties

2. Chemical properties

3. Biological properties

Physical properties

Specific gravity of sewage is very nearly equal to that of water and as such, no modification of hydraulic formulae is necessary.

Colour of fresh sewage is earthy or grey and it has soapy or oily smell.  It starts to give objectionable odour after few hours of its production.

Normal sewage is usually turbid and it contains some matter which can be easily identified when the sewage is fresh.  Such matter includes faecal matter or night soil, pieces of paper, cigarette ends, grease, fruit skins, soap, match sticks, vegetable debris, etc.

Sewage contains a very small amount of solid in relation to large amount of water.  Liquid content of normal sewage is about 99.90 per cent and the total amount of solid matter present either in suspension state or dissolved state is only about 0.10 per cent.  For normal sewage, it is estimated that two tones or 2000 kg of sewage will hardly contain 1 kg of solids.  The amount of 1 kg of solids will normally include 0.50 kg in solution stage, 0.25 in settleable state and 0.25 kg in suspension state.

Sewage contains organic and inorganic matter.  It will be interesting to note the proportions of these matters in a normal sewage. For illustration, one lakh parts of sewage will normally contain only 100 parts of solids.  Distribution of 100 parts of solids in sewage will roughly be as follows:

As a general rule, presence of inorganic solids in sewage is not harmful.  It requires only mechanical appliances for its removal in the treatment plants. Suspended and dissolved organic solids require treatment and they are as such responsible for creating troubles in sewage disposal.  As seen above, the amount of such solids in one lakh parts of sewage is about (20 + 25) = 45, say 50 parts only, i.e., they form only 1/2000^th part of the entire quantity of sewage and such a small part requires heavy treatment for its purification.

Inorganic solids in sewge include mineral matter such as gravel, grit, debris, dissolved salts, sand, chlorides, sulphates, etc.  Organic solids in sewage can be grouped in the following three categories:

• Carbohydrates such as cellulose, cotton filter, starch, sugar, etc.

• Fats and oils received from kitchens, laundries, garages, shops, etc.

• Nitrogenous compounds which are nothing but proteins and their decomposed products and they include wastes from animals, urea, fatty acids, hydrocarbons, etc.

Chemical properties

Nature of fresh sewage and treated or purified sewage is alkaline.  Nature of stale sewage is acidic.

In addition to solids and liquids, sewage also contains various gases such as hydrogen sulphide, methane, ammonia and carbon dioxide.  These gases are obtained either from atmosphere or formed by the decomposition of organic matter present in sewage.

It may be noted that the sewage containing industrial wastes may possess unusual chemical properties.

Biological properties

Sewage contains bacteria and other living micro-organisms such as algae, fungi, protozoa, etc.

Bacteria are present in sewage in large number and depending upon their nature, they may be classified as pathogenic bacteria and non-pathogenic bacteria.  Pathogenic bacteria are harmful and they are responsible for causing diseases.  Sewage obtains such bacteria from the discharges of persons and animals suffering from various diseases.  Non-pathogenic bacteria are harmless.

The major part of bacteria in sewage is engaged in carrying out the process of breaking the complex organic compounds into simple and stable compounds which may be organic or inorganic