What is the difference between sedimentation and flotation

In addition, 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. The rest of this lesson will be concerned with sedimentation following coagulation and flocculation.

We will consider types of sedimentation basins and parts of a typical sedimentation basin, as well as the disposal of sludge. Types of Basins. A fourth type of sedimentation basin is more complex. Solids-contact clarifiers , also known as upflow solids-contact clarifiers or upflow sludge-blanket clarifiers combine coagulation, flocculation, and sedimentation within a single basin.

Solids-contact clarifiers are often found in packaged plants and in cold climates where sedimentation must occur indoors. This type of clarifier is also often used in softening operations. The sedimentation and flotation units commonly found are primary clarifiers, secondary clarifiers, combined sedimentation-digestion units, flotation units, and Imhoff tanks. The most important function of the primary clarifier is to remove as much settleable and floatable material as possible.

Removal of organic settleable solids is very important because they cause a high demand for oxygen BOD in receiving waters or subsequent biological treatment units in the treatment plant. If the horizontal velocity of water is slowed to a rate of 1. Specific gravity of water is 1. There are many factors that will influence settling characteristics in a particular clarifier. A few of the more common ones are temperature, short circuits, detention time, weir overflow rate, surface loading rate, and solids loading.

In general, as water temperature increases, the settling rate of particles increases; as temperature decreases, so does the settling rate. Molecules of water react to temperature changes. They are closer together when liquid temperature is lower; thus density increases and water becomes heavier per given volume because there is more of it in the same space. As water becomes more dense, the density difference between water and solid particles becomes less; there the particles settle more slowly.

All sedimentation basins have four zones - the inlet zone, the settling zone, the sludge zone, and the outlet zone. Each zone should provide a smooth transition between the zone before and the zone after. In addition, each zone has its own unique purpose. Zones can be seen most easily in a rectangular sedimentation basin, such as the one shown below:.

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:. The two primary 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.

In addition, inlet devices act to prevent turbulence of the water. The incoming flow in a sedimentation basin 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. We will discuss short-circuiting in the next lesson.

In addition to preventing short-circuiting, inlets control the velocity of the incoming flow. If the water velocity is greater than 0. Breakup of floc in the sedimentation basin will make settling much less efficient. Two types of inlets are shown below. 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. 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. After passing through the inlet zone, water enters the settling zone where water velocity is greatly reduced.

This is where 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 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, in packaged plants, or to increase the capacity of shallow basins. The outlet zone controls the water flowing out of the sedimentation basin - both the amount of water leaving the basin and the location in the basin from which the outflowing water is drawn.

Like the inlet zone, the outlet zone is designed to prevent short-circuiting of water in the basin. In addition, a good outlet will ensure that only well-settled water leaves the basin and enters the filter. The outlet can also be used to control the water level in the basin.

Outlets are designed to ensure that the water flowing out of the sedimentation basin has the minimum amount of floc suspended in it. The best quality water is usually found at the very top of the sedimentation basin, so outlets are usually designed to skim this water off the sedimentation basin.

A typical outlet zone begins with a baffle in front of the effluent. This baffle prevents floating material from escaping the sedimentation basin and clogging the filters. After the baffle comes the effluent structure, which usually consists of a launder, weirs, and effluent piping. A typical effluent structure is shown below:. The sludge zone is found across the bottom of the sedimentation basin where the sludge collects temporarily. Velocity in this zone should be very slow to prevent resuspension of sludge.

A drain at the bottom of the basin allows the sludge to be easily removed from the tank. The tank bottom should slope toward the drains to further facilitate sludge removal.

In some plants, sludge removal is achieved continuously using automated equipment. In other plants, sludge must be removed manually. If removed manually, the basin should be cleaned at least twice per year, or more often if excessive sludge buildup occurs.

It is best to clean the sedimentation basin when water demand is low, usually in April and October. Many plants have at least two sedimentation basins so that water can continue to be treated while one basin is being cleaned, maintained, and inspected.

If sludge is not removed from the sedimentation basin often enough, the effective useable volume of the tank will decrease, reducing the efficiency of sedimentation. In addition, the sludge built up on the bottom of the tank may become septic , meaning that it has begun to decay anaerobically. Septic sludge may result in taste and odor problems or may float to the top of the water and become scum.

Sludge may also become resuspended in the water and be carried over to the filters. After primary sedimentation, wastewater always contains some suspended solids that neither settle nor float to the surface and therefore remain in the liquid as it passes through the clarifier.

Dissolved solids will, of course, travel through the clarifiers because they are unaffected by these units. Colloids and emulsions are two other forms of solids that are very difficult to remove. Colloids are very small, finely divided solids particulates that do not dissolve that remain dispersed in liquid for a long time due to their small size and electrical charge.

Colloids are usually less than millimicrons in size, and generally will not settle readily. Organic colloids exert a high oxygen demand, so their removal is desirable. Sludge Disposal Sludge Composition. The sludge which is found in the bottom of a sedimentation tank is primarily composed of water. The solids in the sludge are mainly excess coagulant, such as alum. The greater solids concentration of manually removed sludge is due to a small amount of gravity thickening.

Many options exist for disposal of sedimentation sludge. Here we will discuss disposal in streams, sanitary sewers, lagoons, and landfills. Disposal in Streams and Sewers. In the past, sludge and backwash water was typically released into streams and other bodies of water. However, this practice is becoming much less common and is now well regulated.

Backwash water and sludge can only be released into streams if a discharge permit has been granted by the NPDES. The NPDES further requires extensive daily monitoring of the water quality when the sludge is being discharged. Alternatively, sludge may be piped directly to the sanitary sewer.

However, this disposal option also has its disadvantages. Sludge can cause sewer blockages. In addition, fees charged by the wastewater treatment plant can be expensive. Releasing sludge into the sewage lines in large batches makes treatment of the wastewater problematic, so sludge is typically released slowly over a long time period, which requires a large holding tank at the water treatment plant.

Thickening Sludge. Most of the other alternatives require transporting sludge away from the treatment plant. Sludge is typically dried before it is trucked away since the greater volume of wet sludge makes it much more expensive to transport. Formed specimens less likely to contain trophozoites can be kept for up to one day, with overnight refrigeration if needed, prior to examination.

Specimens preserved in formalin can be tested directly wet mount, immunoassay, chromotrope stain, UV fluorescence or can be concentrated prior to further testing.

Concentration procedure separate parasites from fecal debris and increase the chances of detecting parasitic organisms when these are in small numbers. They are divided into flotation techniques and sedimentation techniques. The main advantage of this technique is to produce a cleaner material than the sedimentation technique. The disadvantages of most flotation techniques are that the walls of eggs and cysts will often collapse, thus hindering identification.

Also, some parasite eggs do not float. Sedimentation techniques use solutions of lower specific gravity than the parasitic organisms, thus concentrating the latter in the sediment. Sedimentation techniques are recommended for general diagnostic laboratories because they are easier to perform and less prone to technical errors. The sedimentation technique used at CDC is the formalin-ethyl acetate technique, a diphasic sedimentation technique that avoids the problems of flammability of ether, and which can be used with specimens preserved in formalin, MIF or SAF.

Specimens preserved in PVA are mostly used for permanent staining with trichrome. Prior to staining, they are processed as follows:. Slides may be trichrome stained or kept for several months in a protective slide tray or box for future staining. For additional information on stool processing, call the Division of Parasitic Diseases at DPDx is an educational resource designed for health professionals and laboratory scientists. For an overview including prevention, control, and treatment visit www.

Section Navigation. Facebook Twitter LinkedIn Syndicate. Stool specimens can be examined fresh or preserved. Strain 5ml of the fecal suspension more or less depending on its consistency through wetted cheesecloth-type gauze placed over a disposable paper funnel into a 15 ml conical centrifuge tube.

Conical paper cups with the tips cut off are sufficient.