CLARIFIERS

Clarifiers are settling tanks built with mechanical means for continuous removal of solids being deposited by sedimentation. A clarifier is generally used to remove solid particulates or suspended solids from liquid for clarification and (or) thickening.
A clarification step is the first part of conventional treatment for waste and surfacewater treatment. It usually consist in: – Screening. – Physical chemical treatmentis a generic term for Coagulation-Flocculation. – Sedimentation or Flotation, upon particles properties, and water type.

Pretreatment

Before the water enters the clarifier, coagulation and flocculation reagents, such as polyelectrolytes and ferric sulfate, 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. Isolating the particle components first using these processes may reduce the volume of downstream water treatment processes like filtration.

Potable water treatment

Water is 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 at 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.

Waste water treatment

Sedimentation tanks have been used to treat wastewater for millennia. The primary treatment of sewage is the removal of floating and settleable solids through sedimentation. Primary clarifiers reduce the content of suspended solids and pollutants embedded in those suspended solids. 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 the treated water.
Sedimentation tanks are called secondary clarifiers to remove flocs of biological growth created in some methods of secondary treatment including activated sludge, trickling filters, and rotating biological contactors.

Mining

Methods used to treat suspended solids in mining wastewater include sedimentation and floc blanket clarification and filtration. 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 settle to the bottom.

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. 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. 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. Bafflesare 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.

Tube settlers

Tube settlers are commonly used in rectangular clarifiers to increase the settling capacity by reducing the vertical distance a suspended particle must travel. High efficiency tube settlers use a stack of parallel tubes, rectangles or flat pieces 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 These structures working 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.

Clarifiers employ gravity settling to remove suspended particles or solid particulates from wastewater. Sludge refers to concentrated contaminants, while scum refers to those that float to the liquid’s surface. Large settling tanks with built-in mechanical mechanisms, such as scraper blades, continuously transfer solids to the pipe or location where sludge and scum are collected. Furthermore, certain coagulants and poly-electrolytes are employed to improve the natural settling of solids by converting them into flocs.

Clarifiers are settling tanks designed to continuously remove solids. A clarifier is primarily used to remove solid particles or suspended solids from liquids for clarification and thickening. Sludge refers to concentrated pollutants released from the bottom of the tank, whereas scum refers to particles that float to the liquid’s surface.

How The Clarification Happens

Before the water enters the clarifier, coagulation and flocculation reagents such as polyelectrolytes and ferric are added to induce finely suspended particles to cluster together and create larger, denser particles known as flocs. These settle more rapidly and steadily, allowing the solids to separate in the clarifier more easily and efficiently.

Clarifier types:

In a wastewater treatment facility, clarifier designs vary based on the application and purpose. Clarifiers are classified as primary, secondary, circular, rectangular, lamella, and solid contact clarifiers.

Clarifiers are divided into two types: primary and secondary.

Primary clarifiers

Primary clarifiers separate settleable materials from raw incoming wastewater.This process involves sedimentation & is facilitated by Sedimentation tanks. These are located downstream from the plant. The primary clarifier’s main role is to remove all settleable and floating solid waste with a high oxygen demand – BOD. The entire process is known as primary clarification, sometimes known as sedimentation.

Secondary clarifiers

Secondary clarifiers, on the other hand, are positioned near biological treatment facilities, such as aeration basins or filters.These clarifiers are used to remove treated wastewater from the primary treatment stage. The major goal is to segregate treated wastewater from natural material. The main purpose is to return the activated sludge. The entire process is referred to as secondary clarification. The performance of a wastewater treatment plant is determined by how well primary and secondary clarifiers work.

Features of a Clarifier

1. Clarifiers can be installed by modifying existing infrastructure, such as a primary sedimentation tank or rainwater tank.
2. The treatment process employs the natural flow down method, allowing for consistent operation despite fluctuating flow rates.
3. Because of the availability of the non-flocculation method, the only costs for power are maintenance and management.
4. There is no need for an auxiliary tank because the washing period for filter media is only about 1 minute.
5. Operations management is more accessible. This is because neither a pre-treatment screen nor a bottom screen are required to impede the flow of filter media.