Concept of sedimentation and Softening in Industrial Effluent Treatment

 

Concept of sedimentation What Is Sedimentation?

Sedimentation is the process of separating small particles and sediments in water. This process happens naturally when water is still because gravity will pull the heavier sediments down to form a sludge layer. However, this action can be artificially stimulated in the water treatment process. This mechanical assistance is called thickening.

Why Is Sedimentation Used?

 

The sedimentation process is used to reduce particle concentration in the water. The advantage of sedimentation is that it minimizes the need for coagulation and flocculation. Typically, chemicals are needed for coagulation and flocculation, but improved sedimentation controls the need for additional chemicals. Additionally, sedimentation can be used after coagulation to increase the effectiveness of ongoing filtration in the process.

 

Types of Sedimentation process

 

Sedimentation is also called clarification. Sedimentation removes settleable solids by gravity. The process takes place in sedimentation basins that are either rectangular, square or circular. By increasing the volume of the flow channel, the flow velocity is slowed to the point where there is no turbulence and the suspended solids are allowed to settle out. This process follows the coagulation/flocculation process and is right before filtration and can effectively remove over 90% of the raw water turbidity if the coagulation/flocculation process are both functioning properly.

As you have studied, what is sedimentation and why you need sedimentation in your water treatment process? Next, you can study what are the types of sedimentation process.

There are two types of sedimentation process, the first one is plain sedimentation and the second one is sedimentation with coagulation( clarification).

1.Plain Sedimentation

When the suspended solids (impurities) are settled out by the action of natural forces alone, by gravitation is called plain sedimentation.

 

2.Sedimentation with coagulation( Clarification)

When chemicals or other substances are used to hasten and settling of finely divided suspended matter or colloids, this operation or types of sedimentation is called as clarification or sedimentation with coagulation.

 

Types of Sediments (Settlings)

There are various types of sediments in runoff or suspended water. These sediments will settle in various layers of water in the sedimentation tank. The settling of suspended particles takes place according to their specific gravity.

That is denser particle will settle in the deeper area of the tank than the surface of the water. Types of settlings are divided into four.

 

1.Discrete settling

 

Discrete settling refers to when suspended particles having low concentration settles at the top layer of the water as shown in the above figure. If you don’t know the meaning of discrete, they are individually separate and distinct particles.

Discrete settling is also called as free settling because they have the little chances for flocculating. Flocculate means they these small particles ( discrete particles) have a tendency to form or unite and lead to the formation of small masses.

 

2.Hindered Settling

 

Hindered settling particles which are seen below the discrete settling particles as shown in the above figure.

They are formed by flocculation during the sedimentation process. The hindered setlling particles will be larger than discrete settling particles.

The flocculation process can increase the mass of the sediments and can promote the rate of sedimentation at a faster rate.

 

3.Zone settling

 

The zone settling comes under the third category. They have intermediate concentration. The interparticle forces are high and they can unite each other and can form large groups.

 

4.Compression settling

Compression settling particles have a high concentration and they are wool like structures. They are merged and growth into one body.

Sedimentation Zones

All sedimentation basins have four distinct zones - the inlet (influent) zone, the settling zone, the sludge zone, and the outlet (effluent) 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 rectangular sedimentation basins, such as the one shown below:

However, those same zones are in a circular clarifier as well. In a circular 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 effluent zone is on the outside of the settling zone, while the effluent weir goes around the entire circumference of the clarifier.

 

 

 

 

 

 

 

 

 

 

 

 

 

Inlet Zone

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. In addition to preventing short-circuiting, inlets control the velocity of the incoming flow. 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. 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.

 

 

 

 

 

 

Settling Zone

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. These are discussed in more detail below.

Tube settlers and lamella plates-

Water flows up through slanted tubes or along slanted plates. Flow 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.

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.

 

 

 

 

 

Outlet Zone

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 primary component of the effluent structure is the effluent launder, a trough which collects the water flowing out of the sedimentation basin and directs it to the effluent piping. The sides of a launder typically have weirs attached. Weirs are walls preventing water from flowing uncontrolled into the launder. The weirs serve to skim the water evenly off the tank.

 

A weir usually has notches, holes or slits along its length. These holes allow water to flow into the weir. The most common type of hole is the V-shaped notch shown on the picture above, which allows only the top inch or so of water to flow out of the sedimentation basin.

Sludge Zone

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. Thank 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 basin often enough, the effective (usable) 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 cause 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.

 

Modified Clarification Processes

High rate basins are designed for a better treatment with high load and less detention time. They are compact units. Detention time is generally 1 to 2 hours, as compared to 4 to 6 hours in the conventional basins. These basins consist of tube settler basins, plate settler basins, and solid contact basins.

 

Plate and Tube Settlers

Plate and tube settlers are either one or the other. Plates are spaced about 2 inches apart and are between 3 and 6 feet in length. Tubes have similar dimensions.

Based on the principles that with increased surface area for particles to settle onto, higher flow rates can be achieved in the same settling volume. This allows existing surface water treatment plants to increase their flow capacity without increasing the plant footprint.

High-rate tube and plate settlers are installed to increase the flow rate through the conventional sedimentation basin or in package plant applications. Water moves upward through the plates or tubes while the settled sludge slides downward, settling at the bottom of the clarifier. This is called counter current settling. Plates or tubes become self-cleaning when they are installed at a 50-60° angle.

Lamella clarifier

·        Lamella clarifier is a compact, inclined plate type of clarifier. It is used for clarification of water, waste water and liquid having suspended and colloidal particles. Principle of lamella clarifier is based on settling under gravity, providing number of inclined plates to give large projected surface area.

 

·       WORKING PRINCIPLE of lamella clarifier

·        The lamella clarifier provides a means of water clarification at a large saving of plant surface area. The clarifier consists of a series of inclined overlapping plates, which are arranged to form a separate sedimentation chamber or the cells between each pair of adjacent plates. The overlapping additive projected area of several plates is a factor of increased surface settling area proportioned to the number of plates used.

·        Before entering to lamella clarifier, water is first fed to flash mixer and flocculation tank (FMFT). Chemicals like alum, ferric chloride, lime are added in flash mixer in which high-speed agitator is provided for proper mixing of chemicals in water. Water from flash mixer enters in flocculation chamber in which paddle type agitator is provided for gentle mixing. Polymer is added for flocculation of coagulated particles. Sufficient residence time is provided in this chamber for particles to become heavy before entering into lamella clarifier.

·        Static mixer can replace the flash mixer. In such case, chemicals were added prior to static mixer. The zigzag vanes are provided in static mixer to do proper mixing of chemicals.

·        The pre-treated feed stream enters the lamella and transverses through feed ducts longitudinally, along each side of the lamella plates, through a bottomless distribution duct. The liquid/ solid feed stream then enters each plate chamber near the bottom section of the plates and flows upward between them. As the feed stream moves upwards, solids settle downward on the plates descending a short distance onto the surface area provided by the plates. Solids continue to slide down the plate surfaces to a collection hopper.

·        Near the top of each plate, water leaves each cell through a pair of circular openings in the adjustable weir plate located along each side of the clarifier. The weir plate should be set horizontally and in level so as to provide proper distribution of liquid through each circular opening. It should also be set at a height to provide a design water level below top of the tank.

·       Sludge is periodically removed by opening the drain valve provided in the hopper bottom of the lamella clarifier.

 

 

APPLICATIONS

• Ash/ Scrubber waste treatment
• Brine clarification
• Clarification of water
• Coal and other mineral separation
• Filter backwash water recovery
• Food and dairy processing and wastewater
• Iron removal
• Methyl hydroxide separation
• Mill scale separation
• Municipal water and wastewater treatment
• Phosphorous acid waste treatment
• Semi-conductor process waste treatment

 

 

 

 

 

 

 

 

 

 

 

 

 

Review of chapter

• There are 4 zones in all conventional sedimentation basins: influent, settling, effluent and sludge zones.
• Mechanical sludge removal equipment in sedimentation basins should be drained and inspected once a year.
• The turbidity of settled water before it is applied to the filters (post sedimentation process) should be kept below 1-2 ntu.
• The sedimentation/settling zone is particularly affected by the other zones in a sedimentation basin.
• The material that accumulates on the surface of a DAF unit is called float.
• The solids concentration in solids-contact basins with fairly constant water quality parameters should be determined at least twice per day.
• The effluent launder collects the settled water as it leaves the sedimentation basin.
• Dissolved air flotation is an alternative clarification process that is particularly good for removing light-weight solids, such as algae.
• The maximum design weir loading rate for a solids-contact basin is normally 10 gpm/ft.
• The major cause of short circuiting in a sedimentation basin is poor inlet bafflind.
• The normal detention time in mechanical mixers is typically 15-45 seconds.
• When ferric coagulants react with the natural alkalinity in the source water, they form ferric hydroxide.
• The normal maximum design weir overflow rate for a conventional sedimentation basin is 20,000 gpd/ft.
• Short-circuiting is the most likely result of high winds on a sedimentation basin.
• Aluminum hydroxide is formed when the alum reacts with the natural alkalinity in water.
• Within the influent zone, baffles reduce flow velocity, evenly distributes flow across the basin, prevents short-circuiting and prepares water for the settling zone.
• The transition between the settling zone and the effluent weir can see currents or eddies that can cause sludge to carry over through the effluent weir.
• The scum trough is the collection point for any surface scum that the scum skimmer or flights collect.
• The scum baffle prevents any floatable surface scum from passing and leaving the sedimentation basin.
• Water leaving the sedimentation basin (clarifier) flows over adjustable weirs, which are attached to the effluent launder. The launder conveys the water to the next step, filtration
• The sludge pipe draws off the collected sludge and sends it to the solids handling facility (sludge lagoon, drying beds or mechanical dewatering equipment).
• The flights are attached to a chain that rotates through the basin and move the sludge to the influent end of the basin. At the same time, they also move surface scum towards the scum trough.
• The drive unit moves the sludge rake and scum scraper in a circular motion, providing the power to move settled sludge to the center of the clarifier for collection and removal.

 

 

 

Softening

·        Softening of water is the removal of bivalent calcium and magnesium ions (Ca⁺², Mg⁺²). These ions come from the dissolved compounds of calcium and magnesium. Their presence is known as hardness of water.

·        Hardness

·        Hard water is usually defined as water which contains a high concentration of calcium and magnesium ions.  Measurements of hardness are given in terms of the calcium carbonate equivalent (CaCO₃), which is an expression of the concentration of hardness ions in water in terms of their equivalent value of calcium carbonate.  Water is considered to be hard if it has a hardness of 100 mg/L or more as calcium carbonate. 

·        Softening is the removal of hardness from water.  This is not a required part of the water treatment process since hard water does not have any health consequences.  However, hard water is problematic for a variety of reasons.  Hard water makes soap precipitate out of water and form a scum, such as the ring which forms around bathtubs.  In addition to being unsightly, the reaction of hard water with soap results in excessive use of soaps and detergents.  Hard water may also cause taste problems in drinking water and may shorten the life of fabrics washed in hard water.  Finally, hard water harms many industrial processes, so industries often require much softer water than is usually required by the general public.

·        Excessively hard water will nearly always have to be softened in order to protect the water treatment plant equipment and piping systems.  At a hardness of greater than 300 mg/L as calcium carbonate, scale will form on pipes as calcium carbonate precipitates out of the water.  The scaling can damage equipment and should be avoided.  Water that is naturally low in total hardness (soft water) may be corrosive. Note the water softening industry measures hardness in grains per gallon. (1 grain/gallon = 17.1 mg/L CaCO₃) Total hardness is a test of overall water quality; there are no health concerns related to total hardness. Values near 150 mg/L are generally ideal from an aesthetic viewpoint. Water less than 150 mg/L are considered soft water while values greater than 200 mg/L are considered hard water. Sources of hardness are primarily dissolved carbonate minerals from soil and rock materials. When carbonate minerals dissolve they increase the amount of calcium and magnesium ions in water.

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·        Water hardness classifications
Classification	CaCO3 equivalent (mg/L)
Soft	< 75
Moderately hard	75 - 150
Hard	150 - 300
Very hard	> 300

The two primary constituents of water that determine the hardness of water are calcium and magnesium. If the concentration of these elements in the water is known, the total hardness of the water can be calculated. To make this calculation, the equivalent weights of calcium, magnesium, and calcium carbonate must be known. The equivalent weights are given below:

 

Equivalent Weights
Calcium (Ca)	20.04
Magnesium (Mg)	12.15
Calcium carbonate (CaCO3)	50.045

 

·        Types of Hardness

As mentioned above, hardness in water is caused by a variety of divalent cations, primarily calcium and magnesium.  These cations have a tendency to combine with anions (negatively charged ions) in the water to form stable compounds known as salts.  The type of anion found in these salts distinguishes between the two types of hardness - carbonate and noncarbonate hardness. 

Carbonate hardness compounds	Noncarbonate hardness compounds
Calcium carbonate (CaCO3) Magnesium carbonate (MgCO3) Calcium bicarbonate (Ca(HCO3)2) Magnesium bicarbonate (Mg(HCO3)2) Calcium hydroxide (Ca(OH)2) Magnesium hydroxide (Mg(OH)2)	Calcium sulfate (CaSO4) Magnesium sulfate (MgSO4) Calcium chloride (CaCl2) Magnesium chloride (MgCl2

 

As you can see in the table above, carbonate hardness is caused by metals combined with a form of alkalinity.  As you may remember, alkalinity is the capacity of water to neutralize acids and is caused by compounds such as carbonate, bicarbonate, hydroxide, and sometimes borate, silicate, and phosphate.  In contrast, noncarbonate hardness forms when metals combine with anything other than alkalinity.

Carbonate hardness is sometimes called temporary hardness because it can be removed by boiling water.  Noncarbonate hardness cannot be broken down by boiling the water, so it is also known as permanent hardness.  In general, it is important to distinguish between the two types of hardness because the removal method differs for the two. 

When measuring hardness, we typically consider total hardness which is the sum of all hardness compounds in water, expressed as a calcium carbonate equivalent.  Total hardness includes both temporary and permanent hardness caused by calcium and magnesium compounds.

Problems Caused by Hardness

Hardness is undesirable for several reasons. For example, hardness is

• A nuisance in laundering, due to wastage of soap and collection of dirty precipitate on fibers.
• A nuisance in bathing.
• A source of a dirty ring in the tubs and sinks.
• Responsible for a residue on washed objects like cars and utensils.
• Responsible for deposits on faucets and shower heads.
• Responsible for forming a carbonate scale inside the steam boilers.

 

 

 Softening Processes

The reduction of hardness, or softening, is a process commonly practiced in water treatment. Chemical precipitation and ion exchange are the two softening processes most commonly used. Softening of hard water is desired (for domestic users) to reduce the amount of soap used, increase the life of water heaters, and reduce encrustation of pipes.

In each of the treatment processes, the goal is the same.  Softened water should have a hardness of about 80 to 90 mg/L as calcium carbonate.  If the water is softened further (as in the ion exchange process) then the hard water must be mixed with the softened water to achieve the desired hardness.  Excessively soft water can be nearly as problematic as excessively hard water since it causes corrosion of pipes. 

1.     Chemical Precipitation

·        In chemical precipitation, it is necessary to adjust pH. To precipitate the two ions most commonly associated with hardness in water, calcium and magnesium, the pH must be raised to about 9.4 for calcium and about 10.6 for magnesium. To raise the pH to the required levels, lime is added. Chemical precipitation is accomplished by converting calcium hardness to calcium carbonate and magnesium hardness to magnesium hydroxide. This is normally accomplished by using the lime-soda ash or caustic soda processes.

·        * The lime-soda ash process reduces the total mineral content of the water, removes suspended solids, removes iron and manganese, and reduces color and bacterial numbers. The process, however, has a few disadvantages. For example, the process produces large quantities of sludge, requires careful operation, and if the pH is not properly adjusted, may create operational problems downstream of the process.

·        Softening is the process of removing calcium and magnesium from water, which can cause hard water deposits known as "scale." Following the chemical pre-treatment, the water enters the softener. The softener creates a reaction zone for the hardness, which causes ions to precipitate. The lime and (occasionally) soda ash are added just before the feed enters the clarifier in the reaction zone. To remove calcium and magnesium hardness, lime is added to raise the pH.

 

·         If the raw water contains enough alkalinity to remove the calcium hardness, soda ash can be added to remove it. Solids that have settled react with lime and soda ash to form larger, faster settling particles. The clarified water is routed through weirs, and the solids (sludge) are scraped to the centre for removal and dewatering. Typically, the overflow contains less than 10 mg/L of suspended solids. The amount of hardness remaining will depend on the water chemistry and proper chemical addition.

·        

 

·        In the caustic soda process, the caustic soda reacts with the alkalinity (the capacity of water to neutralize acids) to produce carbonate ions for reduction with calcium. The process works to precipitate calcium carbonate in a fluidized bed of sand grains, steel grit, marble chips, or some other similar dense material. As particles grow in size by deposition of CaCO₃, they migrate to the bottom of the fluidized bed from which they are removed. This process has the advantages of requiring short detention times (about 8 seconds) and producing no sludge.

2.     Ion Exchange Softening

·         Hardness can be removed by ion exchange. In water softening, ion exchange replaces calcium and magnesium with a non-hardness cation, usually sodium. Calcium and magnesium in solution are removed by interchange with sodium within a solids interface (matrix) through which the flow is passed. Similar to the filter, the ion exchanger contains a bed of granular material, a flow distributor, and an effluent vessel that collects the product. The exchange media include greensand (a sand or sediment given a dark greenish color by grains of glauconite), aluminum silicates, synthetic siliceous gels, bentonite clay, sulfonated coal, and synthetic organic resins and are generally in particle form, usually ranging up to a diameter of 0.5 mm. Modern applications more often employ artificial organic resins. These clear, BB-sized resins are spherical and have the advantage of providing a greater number of exchange sites.  

·          Each of these resin spheres contains sodium ions, which are released into the water in exchange for calcium and magnesium. As long as exchange sites are available, the reaction is virtually instantaneous and complete.

 

Softening in the Treatment Process

As mentioned previously, lime softening uses the equipment already found in most treatment plants for turbidity removal.  An overview of the lime treatment process is shown below.

·                  Softening Reaction Chemistry

·                  Clark’s process: Temporary hardness can be removed, by the addition of a calculated amount of lime, where magnesium carbonate or calcium carbonate is precipitated.

 

         Ca(HCO3​)2​+Ca(OH)2​→2CaCO3​+2H2​O
         Mg(HCO3​)2​+Ca(OH)2​→CaCO3​+MgCO3​+2H2​O

·                  Permanent hardness: With sodium carbonate. On treatment with washing soda, Ca2+ and Mg2+ in hard water are precipitated. The precipitate of the insoluble carbonates thus formed is removed by filtration.

 

Ca2++CO32−​⟶WhitepptCaCO3​​

Mg2++CO32−​⟶MgCO3​

 

The anion may remain in solution but in this form, it is not dangerous.

·                  Ion-exchange method. The common substances used for this process are zeolite which is hydrated sodium aluminum silicate, NaAl(SiO2​)2​. The exchange occurs when on passing over the zeolite bed, sodium ions from zeolite are replaced by calcium and magnesium ions. Thus.

2NaZe+Ca2+⟶(Ze)2​Ca+2Na+;2NaZe+Mg2​+⟶(Ze)2​Mg+2Na+

When all the sodium ions of zeolite have been replaced, the zeolite is said to be exhausted. It can be regenerated by treatment with a strong solution of sodium chloride.

 

2Na+(Ze)2​Ca→2ZeNa+Ca

 

 

 

 

 

 

 

 

 

 

           

 

Summary

·        Hardness in water is caused by the presence of certain positively charged metallic ions in solution in the water. The most common of these hardness-causing ions are calcium and magnesium; others include iron, strontium, and barium. Softening of water is the removal of these ions, which come from the dissolved compounds of calcium and magnesium.

·        Hardness can cause soap scum, or scale inside of pipes, so softening the water is sometimes necessary to aid in household maintenance. The two main methods for softening drinking water is chemical precipitation, through the addition of lime-soda ash or caustic soda, and ion exchange. Both lime softening and ion exchange softening create waste disposal problems. Recarbonation (stabilization) is the adjustment of the ionic condition of water so it will neither corrode pipes nor deposit calcium carbonate, which can produce an encrusting film.

·        During or after the lime-soda ash softening process, this recarbonation is accomplished through the reintroduction of carbon dioxide into the water.