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. 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.

Technology

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. 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.

Incline Plate Clarifiers

A incline plate clarifier is designed to remove particulates from liquids. They are often employed in primary water treatment in place of conventional settling tanks. They are used in industrial water treatment. Unlike conventional clarifiers they use a series of inclined plates. These inclined plates provide a large effective settling area for a small footprint . The inlet stream is stilled upon entry into the clarifier. Solid particles begin to settle on the plates and begin to accumulate in collection hoppers at the bottom of the clarifier unit. The sludge is drawn off at the bottom of these hoppers and the clarified liquid exits the unit at the top by weir.

Incline plate clarifiers can be used in a range of industries including mining and metal finishing, as well as used to treat groundwater, industrial process water and backwash from sand filters. Incline plate clarifiers are ideal for applications where the solids loading is variable and solids sizing is fine and are more common than conventional clarifiers at many industrial sites due to their smaller footprint. One specific application is as pre-treatment stage for effluent entering membrane filters. Incline plate clarifiers are considered to be one of the best options for pre-treatment ahead of membrane filters. Their all steel design means there is a low chance that part of the inclined plate will chip off and be carried over into membrane, especially when compared to tube settlers which are constructed of plastic. Further incline plate clarifiers are able to maintain the required water quality to the membrane without the use of chemicals. This is a huge cost saving measure both in purchasing chemicals and limiting damage to the membrane, as membranes do not work well with the large particles contained in flocculants and coagulants. Incline plate clarifiers are also used in the municipal wastewater treatment processes. The most common wastewater application for incline plate clarifiers is as part of the tertiary treatment stage. Incline plate clarifiers can be integrated into the treatment process or stand-alone units can be used to increase the flow through existing water treatment plants. One option for integrating incline plate clarifiers into existing plants is for conventional or sludge blanket clarifiers to be upgraded by attaching a bundle of inclined plates or tubes before the overflow in the so-called “clear water zone”. This can increase the settling area by two-fold resulting in a decrease in the solids loading in the overflow.

Applications

Potable water treatment

By conducting coagulation and flocculation processes to cause finely suspended particles to clump together and form larger and denser particles that settle more quickly and stably, prior to the sedimentation process in the clarifier, allows the separation of particles 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. Sedimentation in potable water treatment generally follows a step of chemical coagulation and flocculation, which allows grouping particles together into flocs of a bigger size. This increases the settling speed of suspended solids and allows settling colloids. Polyelectrolytes and ferric sulfate cause fine and suspended particles in the water to clump together. Water may be treated with these coagulant chemicals prior to entering the clarifier. After coagulation, flocculation by large mechanical paddles allow the coagulate to form denser particles that settle more easily. The water is then sent to the clarifier where separation of clarified water from the flocculated coagulate occurs by permitting the heavier and larger particles to settle to the bottom of the clarifier. The particles then form a layer of sludge requiring regular removal and disposal. Clarified water may then be passed through granular filters to remove any residual particles remaining in the water. Filtered water then flows to a pumping station for storage and use.

Waste water treatment

Sedimentation tanks have been used to treat wastewater for millennia. Primary treatment of sewage is 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 of the treated water. 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.

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 settles to the bottom.


This article uses material from the Wikipedia article "Clarifier", which is released under the Creative Commons Attribution-Share-Alike License 3.0.