Water Quality Control and Treatment Water Treatment

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Water Quality Control and Treatment Water Treatment

Transcript of Water Quality Control and Treatment Water Treatment

  • Water Quality Control and Treatment Water Treatment By Dr. Khamis AL-Mahallawi
  • Water treatment Overview of the Water Treatment Process Preliminary Treatment Presedimentation Aeration Primary Sedimentation Sedimentation and flotation zones Aeration Adsorption Ion Exchange Coagulation and Flocculation Filtration Membrane Processes / Electro-dialysis Nanofiltration and reverse osmosis Softening Treatment Groundwater types and treatment Surface water treatment Disinfection
  • Water Treatment Process abstraction - treatment transport storage - distribution
  • Water treatment processes Preliminary Treatment Preliminary treatment is any physical, chemical or mechanical process used on water before it undergoes the main treatment process. The purpose of preliminary treatment processes is to remove any materials which will interfere with further treatment. Pretreatment may include screening, presedimentation, chemical addition, flow measurement, and aeration.
  • Preliminary Treatment / Screens The screens are used to remove rocks, sticks, leaves, and other debris. Very small screens can be used to screen out algae in the water. All objects are removed by physical size separation Screens on the outside of intakes are often cleaned by flushing water from the treatment plant backwards There are two primary types of screens - bar screens and wire-mesh screens. A bar screen is used to remove large debris. The spaces between the bars are two to four inches wide. A wire-mesh screen is used to remove smaller debris. The gaps are about half an inch wide. Water must be flowing slowly in order to pass through a wire-mesh screen - velocities should be no greater than 3.5 inches per second. A wire-mesh screen A bar screen
  • Preliminary Treatment / Presedimentation - Aeration Presedimentation is to settle out sand, grit, and gravel which will settle rapidly out of the water without the addition of chemicals at the beginning of the treatment process. Presedimentation depends on gravity and includes no coagulation and flocculation. Presedimentation will reduce the load on the coagulation/flocculation basin and on the sedimentation chamber, as well as reducing the volume of coagulant chemicals required to treat the water. Presedimentation basins are useful because raw water entering the plant from a reservoir is usually more uniform in quality than water entering the plant without such a holding basin Here in pretreatment, the purpose of sedimentation is to make the chemical treatment phase of the water treatment process more efficient by removing sediment from the raw water. In presedimentation basin, activated carbon may be added to the basin for taste, odor, and color problems, and some chemicals to control the growth of algae. Aeration removes carbon dioxide and hydrogen sulfide from the water. It also oxidizes the iron and manganese.
  • Preliminary Treatment / Monitoring Flow Measurement : to adjust chemical feed rates, calculate detention times, and monitor the amount of water being treated. It is also monitored for a variety of characteristics including pH, turbidity, total alkalinity, temperature, and coliform bacteria. The pH and total alkalinity of the water will influence the amount of alkali to be added and can also influence the flocculation conditions The level of turbidity will influence the amount of polymer (coagulant) added to the water. Temperature is also measured since cold water does not floc as well as warm water and requires the addition of more polymer
  • Primary Sedimentation Notes: sedimentation may not be necessary in low turbidity water of less than 10 NTU In this case, coagulation and flocculation are used to produce pinpoint (very small) floc which is removed from the water in the filters Sedimentation is a treatment process in which the velocity of the water is lowered below the suspension velocity and the suspended particles settle out of the water due to gravity. The process is also known as settling or clarification Settled solids are removed as sludge, and floating solids are removed as scum The efficiency or performance of the process is controlled by: detention time, temperature, tank design, and condition of the equipment.
  • Primary Sedimentation / Location in the Treatment Process The most common form of sedimentation follows coagulation and flocculation and precedes filtration. This type of sedimentation requires chemical addition (in the coagulation/flocculation step) and removes the resulting floc from the water. sedimentation following coagulation/flocculation is meant to remove most of the suspended particles in the water before the water reaches the filters, Sedimentation at this stage in the treatment process should remove 90% of the suspended particles from the water, including bacteria. The purpose of sedimentation here is to decrease the concentration of suspended particles in the water, reducing the load on the filters. Sedimentation can also occur as part of the pretreatment process, where it is known as presedimentation.
  • Types of sedimentation basins Rectangular basins: have a variety of advantages - predictability, cost-effectiveness, and low maintenance. In addition, rectangular basins are the least likely to short-circuit, especially if the length is at least twice the width. A disadvantage of rectangular basins is the large amount of land area required. Double-deck rectangular basins: This type of basin conserves land area - has higher operation and maintenance costs. Square or circular sedimentation basins with horizontal flow are known as clarifiers. This type of basin is likely to have short-circuiting problems. Solids-contact clarifiers , also known as upflow solids-contact clarifiers or upflow sludge- blanket clarifiers combine coagulation, flocculation, and sedimentation within a single basin. found in packaged plants and in cold climates where sedimentation must occur indoors
  • Sedimentation and flotation zones All sedimentation basins have four zones - the inlet zone, the settling zone, the sludge zone, and the outlet zone. In a clarifier, water typically enters the basin from the center rather than from one end and flows out to outlets located around the edges of the basin. But the four zones can still be found within the clarifier A rectangular sedimentation basin
  • Sedimentation and flotation zones/Inlet Zone Purposes of the inlet zone of a sedimentation basin are to distribute the water and to control the water's velocity as it enters the basin. inlet devices act to prevent turbulence of the water. The incoming flow must be evenly distributed across the width of the basin to prevent short-circuiting. Short-circuiting is a problematic circumstance in which water bypasses the normal flow path through the basin and reaches the outlet in less than the normal detention time. If the water velocity is greater than 0.5 ft/sec, then floc in the water will break up due to agitation ) of the water.
  • Sedimentation and flotation zones/Inlet Zone Two types of inlets. 1. The stilling ( ) wall, also known as a perforated baffle wall , spans the entire basin from top to bottom and from side to side. Water leaves the inlet and enters the settling zone of the sedimentation basin by flowing through the holes evenly spaced across the stilling wall. 2. The second type of inlet allows water to enter the basin by first flowing through the holes evenly spaced across the bottom of the channel and then by flowing under the baffle in front of the channel. The combination of channel and baffle serves to evenly distribute the incoming water
  • Sedimentation and flotation / Settling Zone water enters the settling zone where water velocity is greatly reduced. the bulk of floc settling occurs and this zone will make up the largest volume of the sedimentation basin. For optimal performance, the settling zone requires a slow, even flow of water. The settling zone may be simply a large expanse of open water. But in some cases, tube settlers and lamella plates, such as those shown below, are included in the settling zone. Tube settlers and lamella plates Water flows up through slanted tubes or along slanted plates. Floc settles out in the tubes or plates and drifts back down into the lower portions of the sedimentation basin. Clarified water passes through the tubes or between the plates and then flows out of the basin. Actual Area vs. Effective Area
  • Sedimentation and flotation / Settling Zone Why Tube settlers and lamella plates: To increase the settling efficiency and speed in sedimentation basins. Each tube or plate functions as a miniature sedimentation basin, greatly increasing the settling area. Tube settlers and lamella plates are very useful in plants where site area is limited, or to increase the capacity of shallow basins. Adding inclined settling surface technology to an existing clarifier can increase water treatment flow by as much as 75%.