Pavement Surface Characteristics – Friction, Unevenness, Light Reflection & Drainage

The features of the cross-section of the pavement influences the life of the pavement as well as the riding comfort and safety. Of these, pavement surface characteristics affect both of these. Camber, kerbs and geometry of various cross-sectional elements are important aspects to be considered in this regard. For safe and comfortable driving four aspects of the pavement surface are important; the friction between the wheels and the pavement surface, smoothness of the road surface, light reflection characteristics of the top of pavement surface and drainage to water.

1) Friction

Friction between the wheel and the pavement surface is a crucial factor in the design of horizontal curves and thus the safe operating speed. Further, it also affects the acceleration and deceleration ability of vehicles. Lack of adequate friction can cause skidding or slipping of vehicles. Skidding happens when the path travelled along the road surface is more than the circumferential movement of the wheels due to friction. Slip occurs when the wheel revolves more than the corresponding longitudinal movement along the road. Various factors that affect friction are given below.

• Type of the pavement (like bituminous, concrete or gravel)
• Condition of the pavement (dry or wet, hot or cold etc.)
• Condition of the tyre (new or old)
• Speed and load of the vehicle

The frictional force that develops between the wheel and the pavement is the load acting multiplied by a factor called the coefficient of friction and denoted as “f”. The choice of the value of this is a very complicated issue since it depends on many variables. IRC suggests the coefficient of longitudinal friction as 0.35 - 0.4 depending on the speed and coefficient of lateral friction as 0.15. The former is useful in sight distance calculation and the latter in horizontal curve design.

2) Unevenness

It is always desirable to have an even surface, but it is seldom possible to have such a one. Even if a road is constructed with high quality pavers, it is possible to develop unevenness due to pavement failures. Unevenness affect the vehicle operating cost, speed, riding comfort, safety, fuel consumption and wear and tear of tyres.

Unevenness index

It is a measure of unevenness which is the cumulative measure of vertical undulations of the pavement surface recorded per unit horizontal length of the road. An unevenness index value less than 1500 mm/km is considered as good, a value less than 2500 mm/km is satisfactory up to speed of 100 kmph and values greater than 3200 mm/km is considered as uncomfortable even for 55 kmph.

3) Light Reflection

White roads have good visibility at night, but caused glare during day time. Black roads have no glare during day, but has poor visibility at night. Concrete roads has better visibility and less glare. It is necessary that the road surface should be visible at night.

4) Drainage

The pavement surface should be absolutely impermeable to prevent seepage of water into the pavement layers. Further, both the geometry and texture of pavement surface should help in draining out the water from the surface in less time.

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Aeration Process

Aeration is the process by which the area of contact between water and air is increased either by natural methods or by mechanical devices. In other words, it is the method of increasing the oxygen saturation of the water. Aeration is usually effective against several pollutants like carbon dioxide, some taste and odour producing compounds like methane, hydrogen sulphide, volatile organic compounds like industrial solvents etc. Principle of treatment underlines on the fact that volatile gases in water escape into atmosphere from the air-water interface and atmospheric oxygen takes their place in water, provided the water body can expose itself over a vast surface to the atmosphere. This process continues until an equilibrium is reached depending on the partial pressure of each specific gas in the atmosphere.

Aeration brings water and air in close contact in order to remove dissolved gases (such as carbon dioxide) and oxidizes dissolved metals such as iron, hydrogen sulphide and volatile organic chemicals (VOCs). Aeration is often the first major process at the treatment plant. During aeration, constituents are removed or modified before they can interfere with the treatment processes.

Aeration brings water and air in close contact by exposing drops or thin sheets of water to the air or by introducing small bubbles of air (the smaller the bubble, the better) and letting them rise through the water. The scrubbing process caused by the turbulence of aeration physically removes dissolved gases from solution and allows them to escape into the surrounding air. Aeration also helps to remove dissolved metals through oxidation, the chemical combination of oxygen from the air with certain undesirable metals in the water. Once oxidized, these chemicals fall out of solution and become particles in the water and can be removed by filtration or flotation.

Oxygen is added to water through aeration and can increase the palpability of water by removing the flat taste. The amount of oxygen in which the water can hold depends primarily on the temperature of the water. (The colder the water, more oxygen the water can hold). Water that contains excessive amounts of oxygen can become very corrosive. Excessive oxygen can also cause problems in the treatment plant i.e. air binding of filters.

Efficiency

The efficiency of aeration depends on the amount of surface contact between air and water, which is controlled primarily by the size of the water drop or air bubble. This contact is controlled primarily by the size of the water droplet or air bubble. The goal of an aerator is to increase the surface area of water coming in contact with air so that more air can react with the water. As air or water is broken up into smaller drops/bubbles or into thin sheets, the same volume of either substance has a larger surface area.

Aeration Process

Aeration removes or modifies the constituents of water using two methods - scrubbing action and oxidation. Scrubbing action is caused by turbulence which results when the water and air mix together. The scrubbing action physically removes gases from solution in the water, allowing them to escape into the surrounding air. Scrubbing action will remove tastes and odours from water if the problem is caused by relatively volatile gases and organic compounds. Oxidation is the other process through which aeration purifies water. Oxidation is the addition of oxygen, the removal of hydrogen or the removal of electrons from an element or compound. When air is mixed with water, some impurities in the water, such as iron and manganese, become oxidized. Once oxidized, these chemicals fall out of solution and become suspended in the water. The suspended material can then be removed later in the treatment process through filtration.

Problems with Aeration

Aeration typically raises the dissolved oxygen content of the raw water. In most cases, this is beneficial since a greater concentration of dissolved oxygen in the water can remove a flat taste. However, too much oxygen in the water can cause a variety of problems resulting from the water becoming supersaturated. Supersaturated water can cause corrosion (the gradual decomposition of metal surfaces) and sedimentation problems. In addition, air binding occurs when excess oxygen comes out of solution in the filter, resulting in air bubbles which harm both the filtration and backwash process. Aeration can also cause other problems unrelated to the supersaturated water. Aeration can be a very energy-intensive treatment method which can result in over use of energy. In addition, aeration of water can promote algae growth in the water and can clog filters.

Types of Aerators

Aerators fall into two categories. They either introduce air to water or water to air. The water-in-air method is designed to produce small drops of water that fall through the air. The air-in-water method creates small bubbles of air that are injected into the water stream. All aerators are designed to create a greater amount of contact between air and water to enhance the transfer of gases and increase oxidation. Pumping water through air is much more energy efficient than pumping air through water.

I) Water-Into-Air Aerators

1) Cascade Aerators

A cascade aerator consists of a series of steps that the water flows over similar to a flowing stream. Cascade aerators allow water to flow in a thin layer down steps. In all cascade aerators, aeration is accomplished in the splash zones. Splash zones are created by placing blocks across the incline. They are the oldest and most common type of aerators. Cascade aerators can be used to oxidize iron and to partially reduce dissolved gases.

Fig. 1 Cascade Aerator

2) Cone Aerators/Cone Tray Aerator

Cone aerators are used primarily to oxidize iron and manganese from the ferrous state to the ferric state prior to filtration. The design of the aerator is similar to the cascade type, with the water being pumped to the top of the cones and then being allowed to cascade down through the aerator. The cone tray aerator consists of several cones in which water flows through the cone and over the rim of the cone.

Fig. 2 Cone Tray Aerator

3) Slat and Coke Aerators

Slat and coke trays are similar to the cascade and cone aerators. They usually consist of three-to-five stacked trays, which have spaced wooden slats in them. The trays are then filled with fist-sized pieces of coke, rock, ceramic balls, limestone or other materials. The primary purpose of the materials is providing additional surface contact area between the air and water. Coke tray aerators also pass water through air in small streams. A coke tray aerator is comprised of a series of activated carbon trays, one above another, with a distributing pan above the top tray and a collecting pan below the bottom tray. The distributing pan breaks the water up into small streams or drops. The holes in the trays should be designed to develop some head loss to provide for equal distribution to the lower tray.

Fig. 3 Coke Tray Aerator

4) Draft Aerators

Draft aerators are similar to other water-into-air aerators, except that the air is induced by a blower. There are two basic type of draft aerators. One has external blowers mounted at the bottom of the tower to induce air from the bottom of the tower. Water is pumped to the top and allowed to cascade down through the rising air. The other, an induced-draft aerator, has a top-mounted blower forcing air from bottom vents up through the unit to the top. Both types are effective in oxidizing iron and manganese before filtration. This type of aerator is most effective in the reduction of hydrogen sulphide and carbon dioxide.

Fig. 4 Forced Draft Aerator

5) Spray Aerators

Spray aerators have one or more spray nozzles connected to a pipe manifold. Water moves through the pipe under pressure and leaves each nozzle in a fine spray and falls through the surrounding air, creating a fountain affect. Spray aeration is successful in oxidizing iron and manganese and increases the dissolved oxygen in the water.

Fig. 5 Spray Aerator

II) Air-Into-Water Aerators

1) Pressure Aerators

There are two basic types of pressure aerators. One uses a pressure vessel; where water to be treated is sprayed into high-pressure air, allowing the water to quickly pick up dissolved oxygen. The other is a pressure aerator commonly used in pressure filtration. Air is injected into the raw water piping and allowed to stream into the water as a fine bubble, causing the iron to be readily oxidized. The higher the pressure, the more readily the transfer of the oxygen to the water. The more oxygen that is available, the more readily the oxidation of the iron or manganese.

2) Centrifugal Aerators

Centrifugal aerators create enhanced conditions for dissolving gas into liquid phase, including bubble size, bubble size distribution and duration of interaction with liquid. Centrifugal aerators combine several elements like high turbulence swirling flow of liquid, orthogonal flow of liquid and gas, constant pressure inside the vessel, optimum flow velocity generating centrifugal forces thereby extending diffusion rate within the vessel and very small pores, through which gas permeates into the liquid and is sheared off into liquid phase, thereby forming small bubbles.

3) Air Diffusion Aerator

Air diffusion is a type of aerator in which air is blown through a trough of water. As water runs through the trough, compressed air is blown upward through porous plates on the bottom. This method is not very efficient due to limited air transfer.

Fig. 6 Air Diffusion Aerator

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Screening

Screening is the first unit operation used at wastewater treatment plants. A screen is a device with openings for removing bigger suspended or floating matter in sewage which would otherwise damage equipment or interfere with satisfactory operation of treatment units. Screening removes objects such as rags, paper, plastics and metals to prevent damage and clogging of downstream equipment, piping and appurtenances. Some modern wastewater treatment plants use both coarse screens and fine screens.

Fig. 1 Screen

The primary treatment incorporates unit operations for removal of floating and suspended solids from the wastewater. They are also referred as the physical unit operations. Screen is used to remove larger particles of floating and suspended matter by coarse screening. This is accomplished by a set of inclined parallel bars, fixed at certain distance apart in a channel. The screen can be of circular or rectangular opening. Industrial wastewater treatment plant may or may not need the screens. When packing of the product and cleaning of packing bottles/ containers is carried out, it is necessary to provide screens even for industrial wastewater treatment plant to separate labels, stopper, cardboard and other packing materials. The cross section of the screen chamber is always greater (about 200 to 300 %) than the incoming sewer. The length of this channel should be sufficiently long to prevent eddies around the screen.

Fig. 2 Fixed Bar Screen (Coarse or Medium)

Advantages

Manually cleaned screens require little or no equipment maintenance and provide a good alternative for smaller plants with few screenings. Mechanically cleaned screens tend to have lower labour costs than manually cleaned screens and offer the advantages of improved flow conditions and screening capture over manually cleaned screens.

Disadvantages

Manually cleaned screens require frequent raking to avoid clogging and high backwater levels that cause build-up of a solids mat on the screen. The increased raking frequency increases labour costs. Removal of this mat during cleaning may also cause flow surges that can reduce the solids-capture efficiency of downstream units. Mechanically cleaned screens are not subject to this problem, but they have high equipment maintenance costs.

Types of Screens

Screens can be broadly classified depending upon the opening size provided as coarse screen (bar screens) and fine screens. Based on the cleaning operation they are classified as manually cleaned screens or mechanically cleaned screens. Due to need of more and more compact treatment facilities many advancements in the screen design are coming up.

1) Coarse Screens

It is used primarily as a protective device and hence used as first treatment unit. Coarse screens also called racks, are usually bar screens, composed of vertical or inclined bars spaced at equal intervals across a channel through which sewage flows. Common type of these screens are bar racks (or bar screen), coarse woven-wire screens and comminutors. Bar screens are used ahead of the pumps and grit removal facility. Bar screens with relatively large openings of 75 to 150 mm are provided ahead of pumps, while those ahead of sedimentation tanks have smaller openings of 50 mm. Types of coarse screens include mechanically and manually cleaned bar screens, including trash racks.

Bar screens are usually hand cleaned and sometimes provided with mechanical devices. These cleaning devices are rakes which periodically sweep the entire screen removing the solids for further processing or disposal. Hand cleaned racks are set usually at an angle of 45° to the horizontal to increase the effective cleaning surface and also facilitate the raking operations. Mechanical cleaned racks are generally erected almost vertically. Such bar screens have openings 25% in excess of the cross section of the sewage channel.

Grinder or Comminutor

It is used in conjunction with coarse screens to grind or cut the screenings. They utilize cutting teeth (or shredding device) on a rotating or oscillating drum that passes through stationary combs (or disks). Object of large size are shredded when it will pass through the thin opening of size 0.6 to 1.0 cm. Provision of bye pass to this device should always be made.

2) Medium Screens

Medium screens have clear openings of 20 to 50 mm. Bar are usually 10 mm thick on the upstream side and taper slightly to the downstream side. The bars used for screens are rectangular in cross section usually about 10 x 50 mm, placed with larger dimension parallel to the flow.

3) Fine Screens

Fine screens are typically used to remove material that may create operation and maintenance problems in downstream processes, particularly in systems that lack primary treatment. Fine screens are mechanically cleaned devices using perforated plates, woven wire cloth or very closely spaced bars with clear openings of less than 20 mm. Typical opening sizes for fine screens are 1.5 to 6 mm (0.06 to 0.25 in). Very fine screens with openings of 0.2 to 1.5 mm (0.01 to 0.06 in) placed after coarse or fine screens can reduce suspended solids to levels near those achieved by primary clarification. Fine screens are not normally suitable for sewage because of clogging possibilities.

Fine screens are also used to remove solids from primary effluent to reduce clogging problem of trickling filters. Various types of micro screens have been developed that are used to upgrade effluent quality from secondary treatment plant. Fine screen can be fixed or static wedge-wire type, drum type, step type and centrifugal screens. Fixed or static screens are permanently set in vertical, inclined or horizontal position and must be cleaned by rakes, teeth or brushes. Movable screens are cleaned continuously while in operation. Centrifugal screens utilize the rotating screens that separate effluent and solids are concentrated.

Velocity

The velocity of flow ahead of and through the screen varies and affects its operation. The lower the velocity through the screen, the greater is the amount of screenings that would be removed from sewage. However, the lower the velocity, the greater would be the amount of solids deposited in the channel. Hence, the design velocity should be such as to permit 100% removal of material of certain size without undue depositions. Velocities of 0.6 to 1.2 m/s through the open area for the peak flows have been used satisfactorily. Further, the velocity at low flows in the approach channel should not be less than 0.3 m/s to avoid deposition of solids.

Head loss

Head loss varies with the quantity and nature of screenings allowed to accumulate between cleanings. The head loss created by a clean screen may be calculated by considering the flow and the effective areas of screen openings, the latter being the sum of the vertical projections of the openings. The head loss through clean flat bar screens is calculated from the following formula.

where, 

         h = head loss in m

        V = velocity through the screen in m/s

        v = velocity before the screen in m/s

Another formula often used to determine the head loss through a bar rack is Kirschmer's equation.

where

         h = head loss, m

        K = bar shape factor (2.42 for sharp edge rectangular bar, 1.83 for rectangular bar with semicircle upstream, 1.79 for circular bar and 1.67 for rectangular bar with both upstream and downstream face as semi-circular).

        W = maximum width of bar upstream of flow, m

        b = minimum clear spacing between bars, m

       hv = velocity head of flow approaching rack, m = v²/2g

        θ = angle of inclination of rack with horizontal

The head loss through fine screen is given by

where, 

       h = head loss, m

      Q = discharge, m³/s

      C = coefficient of discharge (typical value 0.6)

      A = effective submerged open area, m2

The quantity of screenings depends on the nature of the wastewater and the screen openings.

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