Estimation of Water and Wastewater Flow

Before designing the proper waterworks project, it is essential to determine the quantity of water that is required daily. This involves the determination of the population and the rate of demand. The determination of the population is one of the most important factors in the planning of the project has to serve the community for a certain design period.

In order to determine the section of the sewer, it is essential to know the total quantity of wastewater or sewage that would flow through the sewer. The total flow is divided into two components and these are dry weather flow (DWF) and stormwater flow.

Content

1.1. Various types of Water Demand

1.2. Design Period

1.3. Population Forecasting

1.4. Variation in Rate of Demand

1.5. Estimation of Sewage Quantity

1.1. Various types of water demand

The total water requirement may be divided into the following categories.

• Residential or domestic use
• Institutional use
• Public use
• Industrial use
• Water system losses

1.1.1. Residential or domestic use

Residential or domestic use comprises about 50 % of the total water demand. It includes the water requirements for drinking, cooking, bathing, washing of clothes, utensils and house and flushing water closets. IS: 1172-1957 recommends a per capita water consumption of 135 litres per day. The breakup of domestic demand is given below.

Table 1: Water requirement for domestic purposes (IS1172-1957)

The Manual on Water Supply and Treatment (MWST), Ministry of Urban Development (MUD), New Delhi recommends the following rates for domestic demand.

Table 2: Domestic and non-domestic water demand (MUD)

1.1.2. Institutional demand

The Manual on Water Supply and Treatment, MUD recommends the values of water requirements for institutional need as given below.

Table 3: Water for institutional need

1.1.3. Public or civil use

Water required for public or civil uses may be for the following purposes.

• Road washing (5 lpcd)
• Sanitation – for cleaning public sanitary blocks, flushing sewer etc. (3-5 lpcd)
• Public parks (2-3 litre per sq. meter per day)
• Fire fighting
• Water required for fire fighting is usually known as fire demand.
• It is treated as a function of population and may be computed using the different formula given below.
• Kuichling’s formula

Where Q = quantity of water in lpm and P = population in thousand

• Buston’s formula
• Freeman’s formula
• National Board of Fire Underwriters formula

• The rate of consumption is very high.

1.1.4. Industrial uses

• The quantity of water required depends upon the type of industry.
• For a city with moderate factories, a provision of 20-25% of per capita consumption may be made for this purpose.
• The following table shows the water requirement for specific industry.

Table 4: Industrial water needs

1.1.5. Water system losses

• Losses from water distribution system consisting of
• Leakage and overflow from service reservoirs
• Leakage from main and service pipe connections
• Leakage and losses on consumer’s premises
• Under registration of supply meter
• Large leakage or wastages from public taps.
• Losses of water may be taken as 15-20% of the total consumption.

1.2. Design Period

• Water supply projects may be designed normally to meet the requirements over a thirty years period after completion.
• The time lag between design and completion of the project should also be taken into account and which should not exceed two to five years depending upon the project size.
• The thirty years period may be modified in regard to certain components of the project depending on their useful life.
• The following table shows the design periods for different components of the water supply scheme and the sewerage system.

Table 5: Design periods for components of water supply scheme

Table 6: Design periods for components of water supply scheme

1.3. Population Forecasting

Following are some of the important methods of population forecasts or population projections.

• Arithmetical mean method
• Incremental increase method
• Geometric mean method
• Decreasing rate method
• Graphical method

1.4. Variation in Rate of Demand

• The average daily per capita consumption is obtained by dividing the quantity of water supplied during the year by the number of days in the year and the number of persons served.
• This per capita demand varies not only from year to year and from season to season but more important from day to day and hour to hour.
• In most of the Indian cities, the peak demand occurs in the morning and evening. During the night hours, the consumption is below average.
• The variations are expresses as the percentage of the annual average daily consumption. Some common values are as under.
• Maximum seasonal consumption is 130% of the annual average daily rate of demand.
• Maximum monthly consumption is 140% of the annual average daily rate of demand.
• Maximum daily consumption is 180% of the annual average daily rate of demand.
• Maximum hourly consumption is 150% of the maximum daily consumption.
• Effect of variation in consumption on design
• A water supply system has several units and design of each unit should match with the hourly, daily and seasonal variations in the demand.
• The design principles taking into account the effect of variation in the consumption are given below.
• The filter units, as well as pumping units, are designed for 1.5 times the average daily demand.
• Distribution mains are designed for maximum hourly demand of the maximum day.
• Sedimentation tanks and water reservoirs are designed for the average daily rate of consumption.

1.5. Estimation of Sewage Quantity

• To find the design flow in the sanitary sewer the following steps are followed.

• Select the design period.
• Forecast the design population (P) of the area.
• Find the sewage flow per day by multiplying population with flow per day per capita of sewage. The sewage is taken as (70-80)% of average water supply.
• Select a peak factor (P.F.) to find the peak sewage flow (Note. Sewers are designed for peak flow).
• Calculate the allowance for industrial and commercial sewage.
• Calculate infiltration allowance.
• Find the design sewage flow by adding Peak Flow, industrial allowance and infiltration flow.

1.5.1. Design period

• A design period of 30 years (excluding planning and construction period) is recommended for all types of sewers.
• The period between design and construction should be taken as 3 to 6 years depending on the size of the project.

1.5.2. Tributary area

• The tributary area for any section under consideration has to be marked on the key plan and the area can be measured from the map.

1.5.3. Percapita sewage flow

• Dry weather flow (DWF) is defined as = population x per capita rate of sewage contributed per day
• (or) DWF = (Density of population x Area served by the sewer) x per capita rate of flow
• Per capita sanitary sewage = 80% of per capita water demand
• Since dry-weather-flow depends on the quantity of water used, and as there are fluctuations in the rate of water consumption, there will be fluctuations in dry weather flow hourly or seasonally.

1.5.4. Peak factor

• Sewer is designed for peak flow.
• The peak factor i.e. the ratio of maximum to average flow depends upon the contributory population.
• The minimum flow varies from 1/3 to ½ of the average flow.

1.5.5. Groundwater infiltration

• An estimate of sanitary sewage may include certain flow due to infiltration of groundwater through joints.
• Suggested estimates of groundwater infiltration for sewers laid below groundwater table is given.

1.5.6. Estimation of Storm Sewage

• A portion of rainfall that falls on the ground is lost as evaporation and percolation. The remaining flows over the ground surface as stormwater.
• The quantity of stormwater that goes to the sewers depends on
• Intensity and duration of rainfall
• Runoff coefficient
• Catchment area.