AERATION, MICROSTRAINING AND FLOW MEASUREMENTS

 

AERATION, MICROSTRAINING AND FLOW MEASUREMENTS

Aeration

          Is a pretreatment process which involves bringing water and air in close contact in order to remove dissolved gases (such as carbon dioxide) and oxidizes dissolved metals such as iron, hydrogen sulfide, and volatile organic chemicals (VOCs). Aeration is often the first major process at the treatment plant.

Processes

•      Turbulence of aeration physically removes these gases from solution allowing them to escape into surrounding air.

•      The second removal process is through oxidation. Oxidation helps to remove certain gases and minerals.

•      Oxidation is a chemical combination of oxygen from the air with certain undesirable metal in the water eg. iron and Mn. Once oxidized these material become very fine suspended material in the water which can be removed by filtration.

Removal of dissolved gases (degasification)

CO₂

          High solubility of CO2 reduces the pH of water which causes excessive consumption of lime or other neutralizing agents in coagulation and softening process. The corrosiveness of water is also higher at lower pH values. Exposure of water droplets to air for 2sec will lower the CO₂ by 70-90%.

          CO2 > 10mg/L  aeration is recommended

          Otherwise lime addition should be used to neutralize the CO.

 

METHANE CH4

          Is a colorless gas that is highly flammable and explosive and can be found in groundwater. It may be formed by the decomposition of organic matter. It can be found in water from aquifers that are near natural-gas deposits. Methane tends to make the water taste like garlic. The gas is only slightly soluble in water, has a low boiling point, vaporizes easily, and therefore is easily removed by the aeration of the water.

 

TASTE &ODOR

          Aeration is also very good in removing taste and odours that caused by volatile materials (materials that turn to vapour easily and have low boiling point) or taste and odour caused by materials that can be readily oxidized.

          Methane and Hydrogen sulphides are two common dissolved gases that can be removed by aeration.

          Aeration is not an efficient method for removing the taste & odor compounds produced by algae because algal oils cause taste and odor not volatile

Sources of taste and odour:

Ø Dissolved gases

Ø Organic matter from algae and other microorganisms

Ø Presence of phenol

Ø Chlorine

          Aeration is effective in removing taste and odour which are caused by volatile materials.

 

HYDROGEN SULFIDE

          H2S (highly soluble in water). Occurs mainly in groundwater supplies, and may be caused by the action of iron or sulphur reducing bacteria in the well. The rotten-egg odor often noticed in well waters is caused by hydrogen sulfide. Hydrogen sulfide in a water supply will disagreeably alter the taste of coffee, tea, and ice. Occasional disinfection of the well can reduce the bacteria producing the hydrogen sulfide.

Ø H2S poisoning is one of the lauding causes of accidents in the field. Hazardous levels are:

•      5ppm: Moderate odor.

•      10ppm: Eye irritation begins

•      Hazardous levels 30ppm: Strong , unpleasant odor of rotten egg

•      100ppm: Loss of smell

•      >300ppm: Unconsciousness, death

Ø oxygenation of water (gasification, absorption)

Ø Removal of iron and manganese (oxidation) (gasification, absorption)

 

IRON AND MANGANESE

Ø Water containing more than 0.3 mg/l of iron will cause yellow to reddish-brown stains of plumbing fixtures or almost anything that it contacts andcan deposit on pipe walls, wells and valves

Ø  If the concentration exceeds 1 mg/l, the taste of the water will be metallic and the water may be turbid.

Ø Manganese in water, even at levels as low as 0.1 mg/l, will cause blackish staining of fixtures and anything else it contacts. Manganese concentration levels that can cause problems are 0.1 mg/l and above.

Ø If the water contains both iron and manganese, staining could vary from dark brown to black.

Ø Typical consumer complaints will be that laundry is stained and that the water is red or dirty. Water containing both iron and manganese should not be aerated unless filtration is provided

Ø .

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.

WATER-INTO-AIR AERATORS

Ø Cascade Aerators

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

Ø Cone Aerators

          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.

Ø 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.

Ø 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.

Ø Spray Aerators

          Spray aerators have one or more spray nozzles connected to a pipe manifold. Water moves through the pipe under pressure, water is distributed into air in the form of small droplets by means of orifices and nozzles mounted on a stationary pipe system. Orifices and nozzles may be constructed to discharge water vertically and at an angle, in upward or downward direction.

Ø Spray aeration is successful in oxidizing iron and manganese and increases the dissolved oxygen in the water.

Air-Into-Water Aerators

Ø Diffused – Air Aeration Systems

          air is introduced into liquid being aerated in the form of bubbles which typically rise through the liquid

Ø common device for ;

•      transferring oxygen in aerobic biological treatment systems

•      air stripping of volatile organics

Ø the size of bubbles varies from coarse to fine-bubble diffusers and coarse bubble diffusers

Ø 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.

Ø Centrifugal Aerators

          Centrifugal aerators create enhanced conditions for dissolving gas into liquid phase, including bubble size, and bubble size distribution and duration of interaction with liquid. Centrifugal aerators combine several elements:

Ø High turbulence swirling flow of liquid;

Ø Orthogonal flow of liquid and gas;

Ø Constant pressure inside the vessel;

Ø Optimum flow velocity generating centrifugal forces thereby extending diffusion rate within the vessel;

Ø Very small pores, through which gas permeates into the liquid and is sheared off into liquid phase, thereby forming small bubbles.

Combination aerators

•      Mechanical aerators

          Operate by producing a large air-water interface the transfer of oxygen from atmosphere is enhanced. The aerator can be a vertical shaft or horizontal shaft.

Ø Very small pores, through which gas permeates into the liquid and is sheared off into liquid phase, thereby forming small bubbles.

Micro straining

          Is a very fine screen used to remove algae, other aquatic organisms and small debris that can clog treatment plant filters.

•      The microstrainer unit consists of a rotating drum that is lined with finely woven materials such as stainless wire fabric. The drum rotates slowly, usually 4 to 7 revolutions per minute (rpm) as water enters the inside of the drum and flows outward through the fabric.

•      Algae and other aquatic organisms deposited on the inside of the fabric form a mat of debris, which adheres to the fabric and rotates up to the backwash hood area.

•      At the top of rotating drum, a high pressure jet spray the back side of the fabric causing the matted debris to break away. The debris and backwash water fall by gravity into a debris trough inside the drum and flow either direct to a disposal point or to a pond or tank that separates the debris from water.

•      Microstrainers removes from 50-90 % of the filter clogging materials from water depending on type of algae present.

•      Microstrainers can not remove all algae and they do not remove bacteria, viruses or most suspended matters that contributes to turbidity. Even eggs of tiny aquatic organism can pass through the fabric. Microstrainers have no effect on the removal of dissolved substances such as organic and inorganic chemicals.

•      Microstrainers should not be used as a substitute for coagulation / flocculation, filtration.

Ø Flow measurements

•      Water flow is measured to assess how much water is available for a supply and to check the quantity of water flowing through a system or treatment plant.

•      Flow control is essential in water plants and for effective disinfection of distribution networks. Most water treatment processes required a controlled constant flow of water in order to operate effectively and efficiently.

•      Water flow must be controlled continuously and accurately during treatment. This is important because in most treatment processes, if the flow of water is not constant (within set limits), the treatment may be inefficient and the water supplied of poor quality.

•      Flow measurements give an indication of the efficiency of a process and indicate if filter bed needs cleaning. Measurement of water flows through treatment processes must be done regularly. Types of measuring devices for water flow in treatment plant are as follows:

•      Ultrasonic meters

•      Magnetic flow meters.

•      Pressure differential meters

•      Velocity meters

 

 

Written by James Theodory …/ jamesteodory@gmail.com