Water Distribution Systems – Part 1

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Presented by:Agha Safdar

Contents Methods of Distributing Water Capacity Requirements

System Losses Population Forecast Water Demand Fire Demand Pressure Design Period Work Examples

Storage Reservoirs Elevated Reservoirs Work Example

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Water Distribution Systems (WDS) The objective of WDS is to deliver water to individual

consumers with appropriate quality, quantity and pressure.

The distribution system describes collectively the facilities used to supply water from its source to the point of usage.

This may include extensive system of pipes, storage reservoirs, pumps and related appurtenances.

The proper functioning of a water distribution system is critical to providing sufficient drinking water to consumers as well as providing sufficient water for fire protection

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Definitions

Average day demand. The total annual quantity of water production for an agency or municipality divided by 365.

Maximum day demand. The highest water demand of the year during any 24-h period.

Peak hour demand. The highest water demand of the year during any 1-h period.

Peaking factors. The increase above average annual demand, experienced during a specified time period. Peaking factors are customarily used as multipliers of average day demand to express maximum day and peak hour demands.

Distribution pipeline or main. A smaller diameter water distribution pipeline that serves a relatively small area. Water services to individual consumers are normally placed on distribution pipelines. Distribution system pipelines are normally between 150 and 400 mm (6-16 in.).

Transmission pipeline or main. A larger-diameter pipeline, designed to transport larger quantities of water during peak demand-periods. Water services for small individual consumers are normally not placed on transmission pipelines. Transmission mains are normally pipelines larger than 400 mm (16 in.).

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Methods of Distributing WaterDepending on the topography between the source and the

consumer the following may be used to transport water to consumers with adequate pressure:

Gravity – when the source is at a sufficient elevation above the consumer to produce the desired pressure. Highly economical

Pumping – Pumps are used to develop the necessary head (pressure) to distribute to consumers and storage reservoirs

Pump-Storage System- storage reservoirs are used to maintain adequate pressure during periods of high demand and emergency (fires & power failures). During low consumption, water is pumped and stored in the storage reservoir.

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Capacity RequirementsWhen designing a water –supply

system, a major consideration is the population to be served, the fire flows needed and the proximity to the source.

Categories of water demand are given in the table and can vary from city to city or country to country.

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Category

Avg Use(liters/day)/person % of Total

Residential 260 40Commercial 90 14Industrial 190 29Public 70 10Loss 50 7

660 100

Category MCM/Yr (2005) % of TotalDomestic 142 40Major Industrial 66 18Minor Industrial 11 3Irrigation 10 3Loss 128 36

357 100

Consuming Water Demand for Trinidad and Tobago (WRA)

Typical Distribution of Water Demand (Chin 2000)

Residential – Typical for third world- 135 lpcd

Bath – 55 LWashing Cloths – 20 LWC – 30 LWashing House – 10 LWashing Utensils – 10 LCooking – 5 LDrinking – 5 L

Demand vary throughout Demand vary throughout the 24hr period and can the 24hr period and can rangerange - from 25-40% of the avg - from 25-40% of the avg daily demand between daily demand between 12.00 to 6.00am 12.00 to 6.00am - to as high as 150 to - to as high as 150 to 175% during morning 175% during morning and evening peakand evening peak

Demand also vary from Demand also vary from year to year; season to year to year; season to season; day to day and season; day to day and house to househouse to house

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Capacity Requirements - Water DemandCapacity Requirements - Water Demand

The range of demand conditions expected within a distribution are specified by demand factors or peaking factors. See Table

NOTE:Max daily demand – demand on the day of the year that uses

the most water = 1.8 x Avg daily demandMax hourly demand – the demand during the hour that uses the

most water = 3.25 x avg daily demand

8Capacity Requirements - Water DemandCapacity Requirements - Water Demand

System Losses Leaking and overflow from reservoir Leaking from main and service pipelines Leaking and losses on premises Leaking from public taps

In a well maintained system losses are about 20%. Partially maintained systems are about 50%

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Capacity Requirements –

In water-supply projects the water demand at the end of the design life of the project forms the basis for design.

Design flow rate =population (at the end of service life) x per capita water demand

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Design Periods Design period: Time periods for which the system

or components is to be adequate (useful life) The design periods of water & wastewater

facilities depends on several factors including : Point of diminishing returns:

(Repair and maintenance vs cost of new facility) Ease of replacement and expansion Likelihood of obsolescence by technological

advances Future needs (expected growth) Cost and interest rate

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Design Period-Drinking Water Systems13

(1)Allow for expansion

Design Period Wastewater Systems14

(1)Allow for expansion

Population Projection

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Population Projection 16

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Work Example – Population Forecast18

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Work Example –water DemandA community have a population of 225,000

persons with the average daily demand 600L/capita/day. 1- Calculate the water demand; giving the following data Maximum daily demand factor is 1.8 Max. peak hour demand factor is 2.7 Fire demand should be calc for 10 hrs2- Based on your calculation, Should we

include a storage reservoir in our design and if yes, estimate the size of the reservoir

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Work Examples – Water DemandWork Examples – Water Demand

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Work Examples – Water Demand26

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