Clarifier

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Three wastewater/sewage clarifiers at the `Aikahi wastewater treatment plant in Hawaii. They appear to have a floating cover
to reduce the odor because the plant is very close to a residential area. Wastewater Clarifiers.jpg
Three wastewater/sewage clarifiers at the ʻAikahi wastewater treatment plant in Hawaii. They appear to have a floating cover to reduce the odor because the plant is very close to a residential area.
Circular clarifier with surface skimmer visible in the lower right. As the skimmer slowly rotates around the clarifier, skimmed floating material is pushed into the trap visible above the fenced enclosure at the lower left. Bundesarchiv Bild 183-1984-1002-002, Gustrow, Zuckerwerk, Klarwerk.jpg
Circular clarifier with surface skimmer visible in the lower right. As the skimmer slowly rotates around the clarifier, skimmed floating material is pushed into the trap visible above the fenced enclosure at the lower left.

Clarifiers are settling tanks built with mechanical means for continuous removal of solids being deposited by sedimentation. [1] A clarifier is generally used to remove solid particulates or suspended solids from liquid for clarification and/or thickening. Inside the clarifier, solid contaminants will settle down to the bottom of the tank where it is collected by a scraper mechanism. [2] Concentrated impurities, discharged from the bottom of the tank, are known as sludge, while the particles that float to the surface of the liquid are called scum. [3]

Contents

Applications

Pretreatment

Before the water enters the clarifier, coagulation and flocculation reagents, such as polyelectrolytes and ferric sulfate, [4] can be added. These reagents cause finely suspended particles to clump together and form larger and denser particles, called flocs, that settle more quickly and stably. This allows the separation of the solids in the clarifier to occur more efficiently and easily, aiding in the conservation of energy. [4] Isolating the particle components first using these processes may reduce the volume of downstream water treatment processes like filtration.

Potable water treatment

Drinking water, water being purified for human consumption, is treated with flocculation reagents, then sent to the clarifier where removal of the flocculated coagulate occurs producing clarified water. The clarifier works by permitting the heavier and larger particles to settle to the bottom of the clarifier. The particles then form a bottom layer of sludge requiring regular removal and disposal. Clarified water then proceeds through several more steps before being sent for storage and use. [4]

Wastewater treatment

Sedimentation tanks have been used to treat wastewater for millennia. [5]

Primary treatment of sewage is removal of floating and settleable solids through sedimentation. [6] Primary clarifiers reduce the content of suspended solids and pollutants embedded in those suspended solids. [7] :5–9 Because of the large amount of reagent necessary to treat domestic wastewater, preliminary chemical coagulation and flocculation are generally not used, remaining suspended solids being reduced by following stages of the system. However, coagulation and flocculation can be used for building a compact treatment plant (also called a "package treatment plant"), or for further polishing of the treated water. [8]

Sedimentation tanks called 'secondary clarifiers' remove flocs of biological growth created in some methods of secondary treatment including activated sludge, trickling filters and rotating biological contactors. [7] :13

Mining

Methods used to treat suspended solids in mining wastewater include sedimentation and floc blanket clarification and filtration. [9] Sedimentation is used by Rio Tinto Minerals to refine raw ore into refined borates. After dissolving the ore, the saturated borate solution is pumped into a large settling tank. Borates float on top of the liquor while rock and clay settles to the bottom. [10]

Technology

Rectangular sedimentation tanks with effluent weir structure visible above the fluid surface. Decantacao.jpg
Rectangular sedimentation tanks with effluent weir structure visible above the fluid surface.
Drained circular sedimentation tank showing central inlet baffles on the right with solids scraper and skimmer arms visible under the rotating bridge. Sedimentation tank.jpg
Drained circular sedimentation tank showing central inlet baffles on the right with solids scraper and skimmer arms visible under the rotating bridge.

Although sedimentation might occur in tanks of other shapes, removal of accumulated solids is easiest with conveyor belts in rectangular tanks or with scrapers rotating around the central axis of circular tanks. [3] Mechanical solids removal devices move as slowly as practical to minimize resuspension of settled solids. Tanks are sized to give water an optimal residence time within the tank. Economy favors using small tanks; but if flow rate through the tank is too high, most particles will not have sufficient time to settle, and will be carried with the treated water. Considerable attention is focused on reducing water inlet and outlet velocities to minimize turbulence and promote effective settling throughout available tank volume. Baffles are used to prevent fluid velocities at the tank entrance from extending into the tank; and overflow weirs are used to uniformly distribute flow from liquid leaving the tank over a wide area of the surface to minimize resuspension of settling particles. [11]

Tube settlers

Tube settler installation in clarifier Tube settler installation in clarifier tank.jpg
Tube settler installation in clarifier

Tube or plate settlers are commonly used in rectangular clarifiers to increase the settling capacity by reducing the vertical distance a suspended particle must travel. Tube settlers are available in many different designs such as parallel plates, chevron shaped, diamond, octagon or triangle shape, and circular shape. [12] High efficiency tube settlers use a stack of parallel tubes, rectangles or flat corrugated plates separated by a few inches (several centimeters) and sloping upwards in the direction of flow. This structure creates a large number of narrow parallel flow pathways encouraging uniform laminar flow as modeled by Stokes' law. [13] These structures work in two ways:

  1. They provide a very large surface area onto which particles may fall and become stabilized.
  2. Because flow is temporarily accelerated between the plates and then immediately slows down, this helps to aggregate very fine particles that can settle as the flow exits the plates.

Structures inclined between 45°  and 60°  may allow gravity drainage of accumulated solids, but shallower angles of inclination typically require periodic draining and cleaning. Tube settlers may allow the use of a smaller clarifier and may enable finer particles to be separated with residence times less than 10 minutes. [13] Typically such structures are used for difficult-to-treat waters, especially those containing colloidal materials.

Tube settlers capture the fine particles allowing the larger particles to travel to the bottom of the clarifier in a more uniform way. The fine particles then build up into a larger mass which then slides down the tube channels. The reduction in solids present in the outflow allows a reduction in the clarifier footprint when designing. Tubes made of PVC plastic are a minor cost in clarifier design improvements and may lead to an increase of operating rate of 2 to 4 times. [14] [15]

Operation

In order to maintain and promote the proper processing of a clarifier, it is important to remove any corrosive, reactive and polymerisable components first, or any material that may foul the outlet stream of water to avoid any unwanted side reactions, changes in the product or damage to any of the water treatment equipment. This is done through routine inspections in order to ascertain the extent of sediment build up, as well as frequent cleaning of the quiescent zones, the inlet and outlet areas of the clarifier to remove any scouring, litter, weeds or debris that may have accumulated over time. [16]

Water being introduced into the clarifier should be controlled to reduce the velocity of the inlet flow. Reducing the velocity maximizes the hydraulic retention time inside the clarifier for sedimentation and helps to avoid excessive turbulence and mixing; thereby promoting the effective settling of the suspended particles. To further discourage the overt mixing within the clarifier and increase the retention time allowed for the particles to settle, the inlet flow should also be distributed evenly across the entire cross section of the settling zone inside the clarifier, where the volume is maintained at 37.7 percent capacity.[ citation needed ]

The sludge formed from the settled particles at the bottom of each clarifier, if left for an extended period of time, may become gluey and viscous, causing difficulties in its removal. This formation of sludge promotes anaerobic conditions and a healthy environment for the growth of bacteria. This can cause the resuspension of particles by gases and the release of dissolved nutrients throughout the water fluid, reducing the effectiveness of the clarifier. Major health issues and problems can also occur further down the track of the water purification system, or the health of the fish found downstream of the clarifier may be hindered.[ citation needed ]

New development

Improvements and modifications have been made to enhance clarifier performance depending on the characteristics of the substance undergoing the separation.

Addition of flocculants is common to aid separation in clarifiers, but density difference of flocculant concentrate may cause treated water to have an excessive flocculant concentration. Uniform flocculent concentration can be improved and flocculant dosage reduced by installation of an intermediate diffused wall perpendicular to the flow in the clarifier. [17]

The two dominant forces acting upon the solid particles in clarifiers are gravity and particle interactions. Disproportional flow can lead to turbulent and hydraulic instability and potential flow short-circuiting. Installation of perforated baffle walls in modern clarifiers promotes uniform flow across the basin. Rectangular clarifiers are commonly used for high efficiency and low running cost. Improvements of these clarifiers were made to stabilize flow by elongation and narrowing of the tank.

See also

Related Research Articles

<span class="mw-page-title-main">Water purification</span> Process of removing impurities from water

Water purification is the process of removing undesirable chemicals, biological contaminants, suspended solids, and gases from water. The goal is to produce water that is fit for specific purposes. Most water is purified and disinfected for human consumption, but water purification may also be carried out for a variety of other purposes, including medical, pharmacological, chemical, and industrial applications. The history of water purification includes a wide variety of methods. The methods used include physical processes such as filtration, sedimentation, and distillation; biological processes such as slow sand filters or biologically active carbon; chemical processes such as flocculation and chlorination; and the use of electromagnetic radiation such as ultraviolet light.

<span class="mw-page-title-main">Water treatment</span> Process that improves the quality of water

Water treatment is any process that improves the quality of water to make it appropriate for a specific end-use. The end use may be drinking, industrial water supply, irrigation, river flow maintenance, water recreation or many other uses, including being safely returned to the environment. Water treatment removes contaminants and undesirable components, or reduces their concentration so that the water becomes fit for its desired end-use. This treatment is crucial to human health and allows humans to benefit from both drinking and irrigation use.

<span class="mw-page-title-main">Settling basin</span>

A settling basin, settling pond or decant pond is an earthen or concrete structure using sedimentation to remove settleable matter and turbidity from wastewater. The basins are used to control water pollution in diverse industries such as agriculture, aquaculture, and mining. Turbidity is an optical property of water caused by scattering of light by material suspended in that water. Although turbidity often varies directly with weight or volumetric measurements of settleable matter, correlation is complicated by variations in size, shape, refractive index, and specific gravity of suspended matter. Settling ponds may be ineffective at reducing turbidity caused by small particles with specific gravity low enough to be suspended by Brownian motion.

<span class="mw-page-title-main">Sedimentation</span> Tendency for particles in suspension to settle down

Sedimentation is the deposition of sediments. It takes place when particles in suspension settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration, or electromagnetism. Settling is the falling of suspended particles through the liquid, whereas sedimentation is the final result of the settling process.

<span class="mw-page-title-main">Flocculation</span> Process by which colloidal particles come out of suspension to precipitate as floc or flake

In colloidal chemistry, flocculation is a process by which colloidal particles come out of suspension to sediment in the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended, under the form of a stable dispersion and are not truly dissolved in solution.

<span class="mw-page-title-main">Activated sludge</span> Wastewater treatment process using aeration and a biological floc

The activated sludgeprocess is a type of biological wastewater treatment process for treating sewage or industrial wastewaters using aeration and a biological floc composed of bacteria and protozoa. It uses air and microorganisms to biologically oxidize organic pollutants, producing a waste sludge containing the oxidized material.

<span class="mw-page-title-main">Industrial wastewater treatment</span> Processes used for treating wastewater that is produced by industries as an undesirable by-product

Industrial wastewater treatment describes the processes used for treating wastewater that is produced by industries as an undesirable by-product. After treatment, the treated industrial wastewater may be reused or released to a sanitary sewer or to a surface water in the environment. Some industrial facilities generate wastewater that can be treated in sewage treatment plants. Most industrial processes, such as petroleum refineries, chemical and petrochemical plants have their own specialized facilities to treat their wastewaters so that the pollutant concentrations in the treated wastewater comply with the regulations regarding disposal of wastewaters into sewers or into rivers, lakes or oceans. This applies to industries that generate wastewater with high concentrations of organic matter, toxic pollutants or nutrients such as ammonia. Some industries install a pre-treatment system to remove some pollutants, and then discharge the partially treated wastewater to the municipal sewer system.

<span class="mw-page-title-main">Sand filter</span> Water filtration device

Sand filters are used as a step in the water treatment process of water purification.

<span class="mw-page-title-main">Upflow anaerobic sludge blanket digestion</span>

Upflow anaerobic sludge blanket (UASB) technology, normally referred to as UASB reactor, is a form of anaerobic digester that is used for wastewater treatment.

Electrocoagulation (EC) is a technique used for wastewater treatment, wash water treatment, industrially processed water, and medical treatment. Electrocoagulation has become a rapidly growing area of wastewater treatment due to its ability to remove contaminants that are generally more difficult to remove by filtration or chemical treatment systems, such as emulsified oil, total petroleum hydrocarbons, refractory organics, suspended solids, and heavy metals. There are many brands of electrocoagulation devices available and they can range in complexity from a simple anode and cathode to much more complex devices with control over electrode potentials, passivation, anode consumption, cell REDOX potentials as well as the introduction of ultrasonic sound, ultraviolet light and a range of gases and reactants to achieve so-called Advanced Oxidation Processes for refractory or recalcitrant organic substances.

Dissolved air flotation (DAF) is a water treatment process that clarifies wastewaters by the removal of suspended matter such as oil or solids. The removal is achieved by dissolving air in the water or wastewater under pressure and then releasing the air at atmospheric pressure in a flotation tank basin. The released air forms tiny bubbles which adhere to the suspended matter causing the suspended matter to float to the surface of the water where it may then be removed by a skimming device.

The physical process of sedimentation has applications in water treatment, whereby gravity acts to remove suspended solids from water. Solid particles entrained by the turbulence of moving water may be removed naturally by sedimentation in the still water of lakes and oceans. Settling basins are ponds constructed for the purpose of removing entrained solids by sedimentation. Clarifiers are tanks built with mechanical means for continuous removal of solids being deposited by sedimentation; however, clarification does not remove dissolved solids.

<span class="mw-page-title-main">Secondary treatment</span> Biological treatment process for wastewater or sewage

Secondary treatment is the removal of biodegradable organic matter from sewage or similar kinds of wastewater. The aim is to achieve a certain degree of effluent quality in a sewage treatment plant suitable for the intended disposal or reuse option. A "primary treatment" step often precedes secondary treatment, whereby physical phase separation is used to remove settleable solids. During secondary treatment, biological processes are used to remove dissolved and suspended organic matter measured as biochemical oxygen demand (BOD). These processes are performed by microorganisms in a managed aerobic or anaerobic process depending on the treatment technology. Bacteria and protozoa consume biodegradable soluble organic contaminants while reproducing to form cells of biological solids. Secondary treatment is widely used in sewage treatment and is also applicable to many agricultural and industrial wastewaters.

<span class="mw-page-title-main">Aerobic granular reactor</span>

Aerobic granular reactors (AGR) or Aerobic granular sludge (AGS) are a community of microbial organisms, typically around 0.5-3mm in diameter, that remove carbon, nitrogen, phosphorus and other pollutants in a single sludge system. It can also be used for wastewater treatments. Aerobic granular sludge is composed of bacteria, protozoa and fungi,which allows oxygen to follow in and biologically oxidize organic pollutants. AGS is a type of wastewater treatment process for sewages and/or industrial waste treatment. AGR was first discovered by UK engineers, Edward Ardern and W.T. Lockett who were researching better ways for sewage disposal. Another scientist by the name of Dr. Gilbert Fowler, who was at the University of Manchester working on an experiment based on aeration of sewage in a bottle coated with algae. Eventually, all three scientists were able to collaborate with one another to discover AGR/AGS.

A lamella clarifier or inclined plate settler (IPS) is a type of clarifier designed to remove particulates from liquids.

<span class="mw-page-title-main">Aerobic granulation</span>

The biological treatment of wastewater in the sewage treatment plant is often accomplished using conventional activated sludge systems. These systems generally require large surface areas for treatment and biomass separation units due to the generally poor settling properties of the sludge. Aerobic granules are a type of sludge that can self-immobilize flocs and microorganisms into spherical and strong compact structures. The advantages of aerobic granular sludge are excellent settleability, high biomass retention, simultaneous nutrient removal and tolerance to toxicity. Recent studies show that aerobic granular sludge treatment could be a potentially good method to treat high strength wastewaters with nutrients, toxic substances.

Clarifying agents are used to remove suspended solids from liquids by inducing flocculation, causing the solids to form larger aggregates that can be easily removed after they either float to the surface or sink to the bottom of the containment vessel.

Mixed liquor suspended solids (MLSS) is the concentration of suspended solids, in an aeration tank during the activated sludge process, which occurs during the treatment of waste water. The units MLSS is primarily measured in milligram per litre (mg/L), but for activated sludge its mostly measured in gram per litre [g/L] which is equal to kilogram per cubic metre [kg/m3]. Mixed liquor is a combination of raw or unsettled wastewater or pre-settled wastewater and activated sludge within an aeration tank. MLSS consists mostly of microorganisms and non-biodegradable suspended matter. MLSS is an important part of the activated sludge process to ensure that there is a sufficient quantity of active biomass available to consume the applied quantity of organic pollutant at any time. This is known as the food to microorganism ratio, more commonly notated as the F/M ratio. By maintaining this ratio at the appropriate level the biomass will consume high percentages of the food. This minimizes the loss of residual food in the treated effluent. In simple terms, the more the biomass consumes the lower the biochemical oxygen demand (BOD) will be in the discharge. It is important that MLSS removes COD and BOD in order to purify water for clean surface waters, and subsequently clean drinking water and hygiene. Raw sewage enters in the water treatment process with a concentration of sometimes several hundred mg/L of BOD. Upon being treated by screening, pre-settling, activated sludge processes or other methods of treatment, the concentration of BOD in water can be lowered to less than 2 mg/L, which is considered to be clean, safe to discharge to surface waters or to reuse water.

<span class="mw-page-title-main">Coagulation (water treatment)</span> In water treatment, the addition of compounds that promote clumping

In water treatment, coagulation and flocculation involve the addition of compounds that promote the clumping of fine floc into larger floc so that they can be more easily separated from the water. Coagulation is a chemical process that involves neutralization of charge whereas flocculation is a physical process and does not involve neutralization of charge. The coagulation-flocculation process can be used as a preliminary or intermediary step between other water or wastewater treatment processes like filtration and sedimentation. Iron and aluminium salts are the most widely used coagulants but salts of other metals such as titanium and zirconium have been found to be highly effective as well.

References

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  10. Rio Tinto Minerals. "Mining & Refining Borates." Archived 10 November 2013 at the Wayback Machine Accessed 13 October 2013.
  11. Weber, pp. 128–131.
  12. Smith, Aaron (March 8, 2020). "Advantages of different lamella clarifier designs". wordpress.com.
  13. 1 2 Weber, p. 130.
  14. SBS Enviro Concepts (2008). sbsenviro.com; "Tube Settlers" Archived 2013-10-29 at the Wayback Machine Accessed 14 October 2013
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Bibliography