Lecture (11): Water Distribution Systems

Lecture (11): Water Distribution Systems Lecture (11): Lecture objectives: By the end of this lecture students should be able to: To know the methods of distributing water. To know the water distribution system requirements To know how to estimation water consumption and population forecast.

Methods of Distributing Water Depending 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.

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

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.

Definitions 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.).

Water Distribution System Requirements Capacity Requirements System Losses Population Forecast Water Demand Fire Demand Pressure Design Period Storage Reservoirs Elevated Reservoirs

Capacity Requirements When 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. Residential – Typical for third world- Bath – 55 L Washing Cloths – 20 L WC – 30 L Washing House – 10 L Washing Utensils – 10 L Cooking – 5 L Drinking – 5 L

Capacity 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 demand Max hourly demand – the demand during the hour that uses the most water = 3.25 x avg daily 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%.

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

Design Periods Point of diminishing returns: 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

Design Period-Drinking Water Systems

Population Projection

Work Example – Population Forecast