Unit - 1

Water Demand

1.1 Population Forecast and Design Period:

• Along with other economic developments, the grow rate of population of that urban settlement is very important factor which is used to fix the design period i.e. what would the total population of that urban centre after five or ten years.
• The population increase is confined to two reasons e.g. (i) The difference between the crude births and the crude deaths in a given period of time is called as Net Increase of population. Generally the birth rates are higher than the death rates, so the net increase is always positive. (ii) In large city has another variable called as Migration of population i.e. some people leave the city for many reasons like health problems, better jobs, marriage, etc.
• It is called as outgoing migrations. Some people come from other places for better jobs or for health problems. It is called as "Incoming migrations". In an urban area where the incoming migrants are greater in numbers than the outgoing migrants, the total population increases.
• The methods which used to calculate the increase in the population are called as population forecasting methods or population projection methods. Some of the important methods of population forecasting are as below,
• The important methods of population forecasting are as follows:

1. Arithmetical Increase Method
2. Geometrical Increase Method
3. Incremental Increase Method
4. Logestic Method
5. Declining Growth [Negative Growth] Model

1. Arithmetical Increase Method:

• In this method it is assumed that for all the decades, the birth rates, death rates, are constant. The following equation is used for the Arithmetical method.

Where in,

Pn = Population after nth decade.

Po = Present Population

n = Number of decades

i = Growth rate of population per decade.

• This provides accurate growth of any region but the assumption about the birth rates and death rate may vary from the reality because for a longer period of 25 to 50 years the birth or the death rates cannot remain constant.

2. Geometrical Increase Method

• In this method, it is assumed that the rate of growth is proportional to the increase in population. The following equation is used to calculate the growth of population.

Where in,

Pn= Population after nth decade

Po = Present population

n= Number of decades

i = Growth rate per decade

• For the regions having unlimited growth this method gives accurate results.

3. Incremental Increase Method

• In this method both the arithmetical and geometrical increase of population is taken into account. This method gives almost accurate results because for the future decades the addition is made at an average rate for two consecutive decades. The equation used for the calculation is as below:

Where in,

Pn= Population after nth decade

Po =Present population

n = Number of decades

r = Average increase for two consecutive decades

4. Logistic Method

• If the mathematical calculations are plotted on a logistic paper of the population data, we get the logistic curve. In fact if the growth rate remains constant for all the decade, it appears to be straight line.
• If the straight line is not obtained then the hypothesis of the population growth is not valid.

5.Declining Growth [Negative Growth] Model

• In this method it is assumed that the population growth rate is declining after reaching the point of saturation. In this method it is assumed that the growth rate is proportional to the deficit population.

Ratio Method:

• It is very general and crude method to calculate the population of a given region.
• By comparing two cities having almost similar size and geographical conditions, will help to project the population of another city i.e. if the growth rate of 'A' cities has been calculated it can be applied to 'B' city to project its population in future.

Curvilinear Method:

• If the population growth of a region is plotted on a graph it can be extended with the same rate to project the future decade population.
• In all the methods the general trend of population can be shown and not the accurate figure because the variables are more than the constant. i.e. The physical environment is constant but the economic, social, cultural, political environment etc. are the variables.

Examples on Forecasting of Population:

Q.1. Forecast the future population of a town for the year 2021 by incremental increase method, with help of follow censers data.

Soln.:

Step 1: By using the following formula we can calculate the probable population for the given census year.

Where in, Pn = Probable population

P = Present population

Ia = Average arithmetical increase

Ic= Average incremental increase

Step 2: Let us apply this formula to the data supplied

Step 3: So the probable population at the end of next decade would be as given below:

The probable population in 2021 would 91,378.

Q.2.Following is the census data of a term. Calculate prospective population in the year 2040 by incremental increase method.

Soln:

Step 1: By using the following formula we can calculate the probable population for the given census year.

Where in, Pn = Probable population

P = Present population

Ia = Average arithmetical increase

Ic= Average incremental increase

Step 2: Let us apply this formula to the data supplied

Step 3: So the probable population at the end of next decade would be as given below:

So the probable population of the town in 2040 would be 1,01,665.

Q.3.The population of a town obtained from census report is as given below.

Estimate the population in the year 2031 by incremental method.

Sol:

Step 1: By using the following formula we can calculate the probable population for the given census year.

Where in, Pn = Probable population

P = Present population

Ia = Average arithmetical increase

Ic= Average incremental increase

Step 2: Let us apply this formula to the data supplied

Step 3: So the probable population at the end of next decade would be as given below:

So the probable population of the town in 2031 would be 97896.

Q.4.Forcast the future population of a town for the year 2021 by the incremental increase method with the help of following census data.

Soln.:

Step 1: By using the following formula we can calculate the probable population for the given census year.

Where in, Pn = Probable population

P = Present population

Ia = Average arithmetical increase

Ic= Average incremental increase

Step 2: Let us apply this formula to the data supplied

Step 3: So the probable population at the end of next decade would be as given below:

So the probable population of the town in 2011 would be 85378.

1.2 Factors affecting population growth:

• Population boom is primarily based totally on 3 essential elements: beginning charge, dying charge, immigration and emigration.
• Population boom charge= (beginning charge + immigration) – (dying charge + emigration)

1. Birth Rate:

• The populace logically escalates if the range of births is greater than the range of deaths at any precise time or if the dying charge is much less or slower in appreciate to the beginning charge.
• The components for calculating crude beginning charge is:

Crude Birth Rate (CBR) = (Number of births inside a 12 months/Total mid-12 months populace) × 1000

2. Mortality Rate/Death Rate:

• A vital purpose affecting the boom of the populace is the dying, or mortality charge.
• As the beginning of latest people escalates the populace length, deaths decline it. The troubles that have an effect on the mortality charge include the accessibility and affordability of nice fitness care and every day lifestyles practices.
• The components for calculating Crude Death Rate is:

Crude Death Rate (CDR) = (Number of deaths inside a 12 months/Total mid-12 months populace)× 1000

3. Immigration and Emigration:

• Migration is the moving of human beings from one place/u . s . to a unique place/u . s.
• There are  kinds of migration; Immigration and Emigration.
• Immigration (transferring Into the place/u . s .) is the motion of people right into a populace from different areas.
• Emigration (Exiting from a place/u . s .) is the motion of people out of a populace.
• Both Immigration and Emigration influences the populace length of preferred nations.
• A range of elements, which includes strolling farfar from war, looking and locating desirable schooling, pursuing a career, looking new jobs or union with own circle of relatives members, outcomes in emigration.
• When an man or woman emigrates from a nation, its populace declines gradually.
• When someone travels to a rustic from every other place, it's miles called immigration (into the u . s .)
• There are pull and push elements of migration.
• Pull elements appeal to human beings toward a sure place/u . s .. Example of pull elements include; higher task opportunities, social securities, higher fitness centers and advanced schooling structures etc.
• Push elements pressure human beings to go away the place/u . s .. Examples of push elements include; loss of social concord and peace, accelerated conflict, war, negative schooling structures, negative task opportunities, loss of good enough sources etc.

1.3 Water Demand:

• Among all the important needs, water is one of the most important needs of the human beings. So, when any new township is planned along with other amenities, the required water supply for that community, is thought about.
• Water is required for various purposes, drinking water stands first among the all. So the team of town planners have to determine the water demand of that township or the urban settlement.
• Once the demand of water is calculated the next step is to find out the sources of water, which can meet the expected water demand.
• In case of large cities such as Ahmadabad, or Baroda or the industrial complex like Ankaleshwar, one source of lewater supply or one water-works is not enough.
• So in cases of such large populated cities, various water sources are thought about, and more than one water-works for treatment and supply of water, are designed.

Classification of Water Demands:

• We need water for various purposes. So on the basis of the purpose, we can classify the water demands as follows.
• Classification of Water Demands are as follows:

1. Domestic Water Demand
2. Commercial and Industrial Water Demand
3. Water Demand for Fire Fighting
4. Demand of Water for Public Use
5. Demand of Water for Domestic Animals
6. Per Capita Water Demand

1. Domestic Water Demand:

• The domestic needs of water include, for drinking, cooking, washing, bathing etc. The domestic water needs generally depend upon the social status, habits, customs of the people.
• The needs also change as the climatic conditions change e.g. In the tropical hot and dry climates more water is consumed than in a settlement, located in the cool climates.
• Under the normal climatic conditions, in India, the per day, per person consumption of water is about 135 litres, while in case of developed, western countries this water consumption is more than 350 litres, per day, per person.
• This higher consumption rate of domestic water is the higher standard of living using air coolers, dishwashers, house laundries etc.
• Out the average total consumption of water, per day, per person, maximum of 55 litres is used for bathing and 20 litres for washing of clothes. Another 30 litres are being used for flushing of latrines. 5 litres each are used for drinking and cooling purposes.

2. Commercial and Industrial Water Demand

• Among the long list of commercial users of water, maximum demand comes from the commercial constructions, Hospitals, Hotels, Cinema houses, Schools Hostels, Sport-Clubs etc.
• The need of the water by any industry depends upon, the process the raw material, seasonality of the industry etc. In case of industrial demand of water also, is measured in terms of per capita, per day demand, so the water needed by any industry also depends upon the number of workers also.
• Sugar mills, paper mills, cotton textile mills, breweries, chemical factories etc. have a very huge demand of water. Generally these industries have their own water supply arrangements.

3. Water Demand for Fire Fighting

• The fire in the urban settlement is generally caused by short circuits, explosion, or by the domestic causes or the industrial processes. So, all the large populated cities have to have the fire-fighting arrangements.
• On an average, the need of water for fire-fighting and depends upon the total population of the urban area and on the types of buildings, and also on the basis of the density of population per sq. Unit of land. Generally, for the fire fighting the water requirement can be calculated as:

1. For a city having low buildings-2200 Lit/min.
2. For a city having high building-4500 Lit/min.
3. For market areas, public places 7,650-13,500 Lit/min.
4. For high density of population and for multi-storeyed buildings - 27000 Lit/min.

• The need of water for fire-fighting varies seasonally also i.e. the fire in summer needs more water.

4. Demand of Water for Public Use

• The quantity of water needed for the public use such as washing the roads, watering the public parks, cleaning of the sewers etc. covers about 5% of the total consumption of the water by the urban settlement.
• On an average the water required by the public parks is about 16,850 Lits per hec per day.

5. Demand of Water for Domestic Animals

• For milk, meat, skin, fur etc. the live stock farming is carried around the urban settlements. Each domestic animal has a different need of water for consumption and for cleaning. e.g. Cows need about 68.25 Lits, of water per day per animal.
• While in case of houses it is about 45.50 lits per day per animal. The need of water for chickens is minimum i.e. 0.09 lits. Per day, per bird.

6. Per Capita Water Demand

• As we have already seen that in any community, water is used for various purposes by the common man. The demand is measured as litres, per person, per day. On this basis the total water demand of any settlement with a given population can be calculated as below

Per capita water demand=

Where, Q = Total quantity of water required by town per year (in lits.)

P = Total population of the town.

1.4 Factors affecting rate of demand:

• When the demand for water by any urban settlement is calculated, average, yearly, per capita demand is calculated average yearly, per capita demand is calculated and it is multiplied by the total population.
• In the reality the per capita consumption of water varies from person to person and also from season to season, even the consumption rate varies from hour to hour in a day.
• Let us study the important factors which affect the rate of consumption of water and so the demand of water.

1. Seasons
2. Size of the Community
3. General Standard of Living
4. Economic Activities
5. Cost of Water
6. Sanitation System

Seasons:

• The quality of water required is directly connected to the climatic conditions i.e. during hot summers, the need of Air coolers, Air conditioners, the sprinklers is increased which use more water than the average yearly need. More water is used for bathing, cleaning the rooms also.
• On the other hands, during winters, as the climate is cool, the use of Air conditions, Air coolers is reduced the watering of plants also needs less quantity of water.
• Generally the water consumption is more in the tropical countries due to climatic conditions while in the temperate countries having cooler conditions, need less water, but during very cool winter, they also use more water for keeping the house warms.
• During the winter when the temperature is about 0°C, they keep the house taps open, which does not allow the freezing of water in the taps.

Size of the Community:

• As the community size goes on expanding more roads,more public places, more public latrines, more public vehicles, large size of sewers are required. This increases the water demand of the community and so the per capita water demand also increases.
• In a small community with reasonable population, the total demand and so the per capita demand remain less.

General Standard of Living:

• As the general standard of living of the persons in any community increases, the need of water also increases e.g. The people who are economically better can altered to make use of Air conditioners, dish washers, washing machines, personal vehicles (which need more water for keeping them clean).
• So, higher the standard of living, higher would be the consumption of water per person, per day.

Economic Activities:

• Each economic activity needs water but the demand varies i.e. the turning needs huge amount of water but the rain takes care of their demand, while in case of industry, the consumptions of water is huge and and sure water supply is required.
• So for an industrial town, the total water consumption and so the per capita water consumption is huge. For the commercial activities also huge water is required.

Cost of Water:

• In the urban areas people have to the water tax to the public authorities. This proportion of tax has a variation among the cities within a state also. So if the water taxes are high, people prefer to use less water than the people paying less tax for the water.
• In case of the cities giving fix tax per family, they do not bother about the use and over use of water, but when one has to pay per litre, the consumption is kept in limits and the wastage of water is required.

Sanitation System:

• In an urban settlement if a proper sanitation system has been setup, the per capita demand of water would be more than the settlement having non-functioning or no drainage system.
• With all the above factors, the size of the population is one of the most important factors which increases the demand of water e.g.
• On an average the a small township having a population upto 20,000 will have the water demand of 110 litres per day, per person, while a very large city. Having more than a million population, would have the average water demand between 240 and 270 litres per day, per person.

1.5 Variation in rate of demand:

• It is not easy to estimate the quantity of water required. i.e. the water demand because the consumptive and non consumptive use of water and so the demand of water varies from region to region, season to season and from-culture to culture.
• The main factors, as already seen are the rate of demand and total /population on which the water demand can be estimated.
• For that the future population i.e. population forecasting is carried out (the methods we have discussed in the chapter only) to design the water supply schemes for a given region having and given population.
• Let us see the average water demand for various purposes on which we can estimate the required quantity of water.

Consumptive Use:

This demand of water can be sub-divided as following

Drinking purpose:

• The quantity of water that a person requires for drinking also varies on the bases of age, type of work, season etc.
• On an average, under normal climatic conditions; a person needs about 2 litres of water/per day. So the water demand for drinking water can be estimated on the basis of the total population of the settlement x 2 litres x 365 days.
• The estimated amount of water may look small, but it is not be quantity of water which is important but the quality of water to be supplied i.e. it must be free from any type of impurities which lead to epidemic. So the water must be protected, potable and palatable.

Cooking:

• The water required for cooking also has a variation mainly it depends upon the type of food to be cooked and the methods of cooking. On an average, for cooking, about 5 liters of water/ per person/ per day, is estimated.

Bathing:

• The water needed for bathing also varies according to the climatic conditions and also, according to the social habits of the people in a given community. On an average, the water needed for bathing is about 50 to 80 liters/ per person/ per day.

Sanitation:

• For personal sanitation, about 50 to 60 liters of water, per person, per day is required. This water demand also has a variation from region to region.

Domestic animal's water demand

• Each one of the domestic animal like cow, dog, horse has different requirement of water that also has seasonal variation. On an average the water required by these domestic animals per head / per day is given in the following Table

Table: Water estimation for domestic animals

Non-Consumptive Use:

The quantity of water required for the public utilisation can be sub-divided as below:

Road washing:

• To keep the roads clean, i.e. dust-proof, about 5 liters of water/ per head / per day is required.

Public sanitation:

• For keeping the public sanitation blocks and also for carrying domestic waste water i.e. sewage about 2 to 3 liters of water/per head/ per day is estimated.

References:

1. G.B Masters, Introduction to Environmental Engineering and Science, Pearson Education,2013
2. M.Chandrasekhar, Environmental science, Hi Tech Publishers,2009
3. Gerard Kiely, Environmental Engineering, McGraw Hill Education Pvt.Ltd, Special Indian Edition, 2007