1 INTRODUCTION Intake structures are used at the beginning of a water supply system. They act as the medium by which water enters the system. These structures must be taken into account broad differences in water depth, flow, quality and temperature. Sources of pollution and the direction that current of flow are also debating factors. This report will cover the basic forms of intake structures that exist, with primary focus on the structures that are used in Guyana for surface water. The climate and usage of these structures will determine their choice in a supply system. The report begins by listing the various types of intake structures, the mechanisms by which they work, and their specific application use. It is required that after the water passes the intake, it must been screened, and as such, screening and its systems are briefly covered in the later part of the report. Screening, as you will discover later, is essential in removing unwanted objects from the water before it enters the treatment system. This screening prevents any form of damage to the system and acts as a pre-filtering of the water to be treated.
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INTRODUCTION
Intake structures are used at the beginning of a water supply system. They act as the
medium by which water enters the system. These structures must be taken into account
broad differences in water depth, flow, quality and temperature. Sources of pollution and
the direction that current of flow are also debating factors.
This report will cover the basic forms of intake structures that exist, with primary
focus on the structures that are used in Guyana for surface water. The climate and usage of
these structures will determine their choice in a supply system. The report begins by listing
the various types of intake structures, the mechanisms by which they work, and their
specific application use.
It is required that after the water passes the intake, it must been screened, and as
such, screening and its systems are briefly covered in the later part of the report. Screening,
as you will discover later, is essential in removing unwanted objects from the water before
it enters the treatment system. This screening prevents any form of damage to the system
and acts as a pre-filtering of the water to be treated.
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1.0 Intake
Water intake involves brining water from the source to the treatment facility. Surface water from rivers, lakes or reservoirs flows into the transmission system of the plant, through an intake structure. Ground water flow in an intake pipe from the aquifer or source, via pumping mechanism through a transmission conduit to the treatment or distribution system. As such, intake structures are set up to channel the water into a transmission system.
FIG 1.0 - The essentialness of intake and screening in the water supply process (drawn by J. Takchandra)
In the case of ground water structures, they safely bring up the water from the source with no interception from soil, debris or other infestations. Once the water enters the plant, treatment is proceeded.
1.1 Roles of intake structures
Intake structures are placed for two intrinsic purposes: i. to supply water of the best possible quality from the source, and
ii. to protect the downstream equipment and piping from damage or clogging from debris, flooding or wave action.
1.2 Types of intake structures
Types Intake structures may be classifies in two categories as seen in the table below: Category Design type Notes 1. Exposed Tower integral with dam Applicable to large systems Tower in lake Navigation impact
Shore inlet Designed for floating debris
Floating or moveable Good access O&M Siphon well Applicable to small systems
2. Submerged Screened inlet crib No navigational impact
Gravel-packed wells No navigational impact Horizontal collection systems
(infiltration bed) No navigational impact, must have
favourable geology
Table 1.1 - Types of intake structures (McKenzie. L. Davis 2003, pg. 3-5)
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Mechanisms Intake structures can utilize one of two mechanisms:
i. Pumping A pump is a device that converts mechanical energy into hydraulic energy. It lifts water from a lower to higher level and delivers it at a high pressure, or along the same potential level. Pumps are employed to retrieve the water from the source and are used for the following purposes:
To lift water from low areas
To deliver water at a desired pressure for transport
To supply pressure for fire hydrants
ii. Conveyance The conveyance system channels the water from the source into a transmission main. This then travels to the treatment plant. The main purpose of these structures is to provide calm and still water for the water supply scheme. An example of this would be a sluice.
Applications Here are the common forms of intake systems that are applicable to water supply domains:
1.2.1 River intake Both submerged and inlet structures are used in rivers and include small sluices or inlets, and submerged pumping structures. Large rivers that are equipped with dams, the choice of intake is less critical since the water is not so unregulated. Shore-based systems however, provide best access for operation and maintenance.
1.2.2 Reservoir and lake intakes Exposed structures are used in large bodies of water (e.g. Canal No. One conservancy, W.B.D.). These structures are widely used in warm climates since the climate permits year round access (by a boat or bridge) for maintenance. A classic tower may be used with many sections (or cells for intake) to provide redundancy. Submerged structures avoid many problems like that of exposed systems but are difficult to maintain because of inherent lack of access (e.g. water mains from Wales Sugar Estate to Patentia housing scheme; passes through and under back-dam trenches, uses water for industrial processes).
FIG 1.1 – Submerged intake for lakes and reservoirs (Joanne E. Drinan – pg. 62)
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1.2.3 Canal intake These see the form of sluices that can control water levels in the canal especially if the source river or reservoir is tidal. They provide water supply that may be used for simple screening for livestock farms or irrigation - see FIG 1.3 below.
1.2.4 Conduits intake1 The intake conduit connects the inlet works with the pump station. A tunnel or pipeline may be used. An example of this type of structure would be city sewage systems that recycle waste water. Large concrete or masonry conduits take waste water to the treatment plant, and are situated under roads and buildings.
1 It is important to note that these structures are less common in Guyana since lakes (or creeks) are not within the location context to set up water supply system
FIG 1.2 – Intake tower structure ((Joanne E. Drinan
– pg. 60)
FIG 1.3 – Intake sluice on river (drawn by J.
Takchandra)
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2.0 Screening
A screen, in context of water supply systems, is a device with openings that removes bigger suspended or floating matter in sewage which would otherwise damage equipment or interfere with satisfactory operation of treatment units. Screening may occur once or a number of times as the water flows from intake to treatment plant. Consequently, screening follows intake. They remove rocks, sticks, leaves dirt lumps, and other debris. Very small screens can be used to screen out algae in the water. Screens are used in conjunction with intake structures to achieve a certain level of water standard, which enters the treatment plant. The choice of screening apparatus widely depends of the type of treatment required, the quality of raw water, and the source of the water.
Screens maybe classified as either: Coarse screens
Large openings: 70 mm – 150 mm also called racks (main bar screens, see below) and composed of vertical
inclined bars Medium screens
Clear openings: 20 mm – 50 mm Made up of bars
Fine screens Perforated plates: less than 20mm openings Cleaned devices with woven wire or cloth Not suitable for sewage
2.1 Types of screens
2.1.1 Trash screens Trash racks or rakes collect and trap large debris from the incoming water. These are commonly used as the primary screening device (coarse screen) to remove the largest debris before the influent water enters a smaller screen. They are constructed of steel or durable high density polymers. To achieve greater effectiveness, multiple trash screens are stacked along the screening rack. These can be installed on dam walls or on the sides of buildings depending on the intake requirements and set up.
2.1.2 Travelling water screens These consist of a continuous series of wire mesh panels that are attached to a continuous closed loop chain that are assembled on two rollers. The setup is then placed in a channel of flowing water to remove floating or suspended debris. The
raw water passes through the revolving baskets and trapped debris is carried out of the water by the roller action. High pressure sprays are then used to blast the debris off the basket to restart the cycle. These are best suited for slow flowing river installations.
2.1.3 Bar screens Bar screens are designed to handle large debris. The actual screen consists of a rack of straight steel bars welded at both ends to horizontal steel members. Powered rakes move up and down the bar rack face to remove debris, as they move in and out of flow. They are installed at an angle to the water surface for effective debris removal.
2.1.4 Drum screens The screen is mounted on a periphery cylinder that resembles an open ended drum. The screen turns slowly on its horizontal axis picking up debris as the water flows through it.
FIG 2.2 – Drum screen (drawn by J. Takchandra)
2.1.5 Passive screens These are also called stationary screening cylinders because they have no moving parts. Cylindrical meshes are mounted on a horizontal axis and are oriented parallel to the current in a water body. It takes advantage of natural current in the water to trap the debris in the basket-like assembly.
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3.0 References
Drinan, Joanne E. 2001. Water & Waste Water Treatment . CRC.
