Water Demand Choice Of Sources Engineering Essay

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The optimal choice of the source for potable water supply must be adequate and reliable on forecast yields, the quality of water supply must clean and economic analysis of capital and operation costs is low. Sources of the water may come from the direct runoff from rivers, supply from rivers after the development of reservoirs or dam, and groundwater. Most designers preferred to construct a scheme which directly abstracts direct runoff from a natural river because it is the cheapest among all the solutions. Constructing a dam for water supply source will create great social and environment impacts. The natural habitats of wild life will be destroyed and need to resettle the community.

Water Quality Criteria:

The water quality of water supply source is very important. Usually, raw water requires conventional treatment in order to turn raw water safe for human consumption. However, certain elements such as heavy metal and magnesium cannot be removed by the conventional treatment process. Hence, the selection of water supply source must consider carefully.

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Water Treatment Processes

The main objective of water treatment process is to ensure that publics have the safe potable water to use. Basically, water treatment process includes coagulation, sedimentation, filtration and disinfection.

Process

Objective

Coagulation

To remove dirt and suspended particles

Sedimentation

To remove settleable particles, bacteria and viruses

Filtration

To remove finely divided particles

Disinfection

Water is placed in storage tank in order to kill micro-organisms

http://www.epa.gov/ogwdw/kids/watertreatmentplant/images/image_watertreatmentcycle.gif

http://water.epa.gov/learn/kids/drinkingwater/watertreatmentplant_index.cfm

Water demand:

Before begin any design and construction for potable water for certain area served, water demand must be determine first. Usually, water demand is based on population served, per capita consumption, service factor, industrial and other special demand. Various type of directly and indirectly factors needed to be taking into account when estimating water demand. These factors include population growth rapidly, unsatisfied demand due to improvement in living standards and impact of climate change. According to The Malaysia Water Association, "The study of water demand for any urban water supply scheme should cover a period of at least 20 years with projections made at 5 year intervals." Consultant engineers always find difficulty on estimating water demand because it is impossible to fully account in all the directly and indirectly factors. Potable water supply is totally different from electricity supply. For electrical supply, the amount of output (electrical produced) by the electrical generator is equal to amount of usage (electrical energy used). However, the amount of potable water carries from treatment plant to demand area is not equal. According to R. McKenzie and C. Seago, there is a leakage during the transmission of potable water. Most of the consultant designers will use waterCAD to modeling and management the water distribution system.

Water distribution system

The water distribution system consists of mainly 4 sections.

Transmission - treated water is transfer from a treatment plant to a reservoir

Distribution pipelines - pipelines that distribute treated water to the reticulation pipelines from reservoir or treatment plant

Reticulation pipelines - pipelines that use to distribute treated water to water demand areas.

Balancing and service reservoirs - store treated potable water

The main objective of water distribution system is to ensure that the supply potable water has appropriate pressure and quantity to the users.

Basically, water distribution system is categorized into 3 types:

Gravity system

Direct pumped system

Gravity and pumped combination system

The selection of type of distribution system depends on the topography, location and extent of the distribution area, elevation and site conditions (The Malaysia Water Association 1994). The most preferred type of distribution will be the gravity system. It is more reliable and low operation cost. However, if the gravity system type of distribution is not adequate, then the gravity and pumped combination system shall be considered. Most consultant engineer least preferred using direct pumped system because of the problems associated with operation and maintenance of pumping system. The direct pumped system only used for certain circumstances.

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There are a few general considerations need to be consider by the consultant engineer during the planning of distribution and reticulation layouts. The distance between service reservoir and water demand area must be as close as possible in order to provide least capital and operating costs. Besides, the layout of the reticulation pipes should in a network form system and discourage dead ends. Pipelines usually lay adjacent to roads.

Choice of Construction Materials

The 2 common factors that affect the selection of construction materials will be the costs and suitability. For a scheme potable water project, the main construction material is the pipe materials. Table XXX shows a few types of water supply pipe materials with its advantages and disadvantages.

Pipe material

Advantages

Disadvantages

Asbestos Cement (A.C)

Low manufacture cost

Strength and rigid

Corrosion resistant

Ease of installation

Difficult to maintain

May affect healthy on potable lines

Ductile Iron (D.I)

Ease of jointing

High strength and toughness

Ease of installation

Heavy

If internal and external protection damage, corrosion will occur

Polyethylene (HDPE)

Low cost

Corrosion resistant

Flexible

Ease of installation

Ease of jointing (Fusion joint)

Skill and equipment needed for fusion joint

Steel

High tensile and compressive strength

Flexible

Ease of jointing (welded)

Skill and equipment needed for welded joint

High cost

http://www.puretechltd.com/types_of_pipe/

The designer must always consider the major factors when selecting the type of pipe to be use. These factors include:

Working and test pressure

Strength of pipe to withstand designed internal and external loads

Durability of the pipe

Suitability and workability for laying and operating requirements

Capital, operation and maintenance costs and

Extent of possible leakage

Most designers preferred using HDPE pipe for potable water pipeline. As compare with other pipes, HDPE pipe is much cheaper. The internal surface of HDPE pipe is very smooth, hence the friction head loss produce by water flow will also very small. The entire external and internal surfaces of D.I pipe are protected by bitumen and cement mortal respectively. Once the protections get damaged, the D.I pipe will start to rust. If there is external load acting on the HDPE pipe, it will break easily. Therefore, the pipelines should bury 1200mm below the ground level. Whenever the pipe has to cross culverts, D.I pipe should be considered first due its high strength and toughness. Besides, it also able to expose under different weather and condition.

Storage tank or reservoir

The main functions of a reservoir have shown below:

To minimize the interruption of water supply whenever there is a breakdown.

To provide water for fire fighting

To fulfill the fluctuating water demand

There are many types of reservoirs in water distributing system:

Reinforced concrete (R.C.)

Prestressed concrete

Gralvanised pressed steel

Fiberglass reinforced polyester

Glass fused to steel panels

Various factors needed to be consider when selecting the types of reservoirs for distribution system. These factors include:

Capital cost

Cost of maintenance

Topography

Life expectancy

Reinforced concrete reservoirs are always preferred by designer as it easy to maintain. Apart from that, the life expectancy is much longer as compared with other type of reservoirs. For most cases, the R.C. reservoir will build on the group level, only if there is a pressure requirement, then the R.C. reservoir will build on R.C. supporting towers. The ground reservoir can either be circular or rectangular shape. Circular ground reservoir is much more economy due to its structural design and geometry. Circular ground reservoir can provide least amount of wall for a given volume and depth when compare with rectangular ground reservoir. However, rectangular ground reservoir will be selected when there is constraint on space. Besides, the construction cost of rectangular ground reservoir is relatively lower as compare with circular ground reservoir. When there is not high ground, elevated tanks will be selected to meet the water supply requirement. Most designer will avoid design elevated tanks as the construction cost is much higher than ground reservoir. The reservoir must be structurally safe and water tight.

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