Clarifiers/thickeners sludge removal

Liquid waste streams with a high-suspended solids content can be cleaned up by solids removal in clarifiers, thickeners, and liquid cyclones and by accelerated settling by inclined Chevron settlers or the like [73]. For waste streams with very finely divided solids in suspension (i.e., less than about 100 pm) dissolved air flotation techniques have been shown to be more efficient than methods employing sedimentation. Final dewatering of the sludges obtained may be carried out on a continuous filter or a centrifuge. The clarified water product can be accepted for more potential options of reuse or final disposal options than untreated water, and the separated solids may be burned or discarded to landfill, as appropriate [74]. 

Lamella separators can be used as liquid/liquid separators (as in oil-water separation - discussed at some length in Section 5C), or as clarifiers, for the removal of small amounts of suspended solids - an application of increasing use in water treatment. Liquid/liquid separation and solid removal can also be achieved in the one separator. The device can, in addition, be used as a thickener, in which case there will be a sludge collection space below the plate assembly, which must be deep enough to allow the formation of a sludge compression zone. 

Clarifiers typically are used in chemical precipitation and biological treatment processes to remove precipitated metal soHds and suspended biological soHds. To prevent the sludge blanket from becoming too thick or heavy, part of the sludge blanket is removed continuously or intermittently from the system and thickened prior to disposal. 

Dissolved air flotation (DAF) is a process commonly used in refineries to enhance oil and suspended solids from gravity-separator effluent. In some refineries it is used as a secondary clarifier for activated sludge systems and as a sludge thickener. The process involves pressurizing the influent or recycled wastewater at 3-5 atm (40-70 psig) then releasing the pressure, which creates minute bubbles that float the suspended and oily particulates to the surface. The float solids are removed by a mechanical surface collector. 

This stream cools the vapors and scrubs the remaining catalyst out of the cracked products. Most of the slurry of catalyst in heavy oil withdrawn from the bottom of the tower is recirculated to the top of the disc-and-donut section, while a small portion is withdrawn for recovery of catalyst. The latter stream (slurry return) usually amounts to 3 to 10% of the volume of fresh feed to the reactor. Catalyst concentration in the slurry can be decreased by increasing the rate of withdrawal, and is usually maintained below 0.5 lb./gallon to avoid erosion of slurry pumps and valves. The slurry-return stream may be pumped to a separate settler (e.g., a Dorr thickener or a simple cone-bottom tank) or the settler may be incorporated in the bottom of the fractionating tower (25). About 70% of the heavy oil is removed from the settler as a clarified oil containing less than 0.01 lb. catalyst/gallon. The sludge is diluted with fresh feed and pumped to the reactor to return the catalyst to the system. 

The WRS includes two clarifiers, two thickeners, two filter presses, and auxiliary equipment. The ICB effluent is transferred to the WRS clarifiers, where a polymer will be injected to provide chemical coagulant for enhancing removal of suspended solids. The clarified effluent will be transferred to the BRS. The clarifier sludge will be pumped to the WRS thickeners, where a polymer may be added to enhance thickening. Thickener overflow is recycled to the clarifiers, and underflow is pumped to the dewatering filter presses. The Alter press separates the solids from the liquid stream. The liquid is recirculated to the clarifiers, and the filter cake, containing 20-25 percent dry weight sohds, is a secondary waste. 

Another technique is to remove sludge continuously at a reasonable rate by pumping it to an elevated tank. This provides some clarification, with the brine overflowing to the clarifier. A conical bottom on the tank enhances thickening of the sludge, which can go to a receptacle or to further processing. 

Alternative Clarification Processes. The discussion so far has assumed the use of a single conventional clarifier, with a cylindrical body and conical bottom with a thickening rake, in continuous operation. In small plants, the rake may be eliminated the use of a conical bottom for easy removal of the sludge is then very important. In very small plants, clarification may be a batch process, often in the same tanks used for chemical treatment. Very large clarifiers, on the other hand, are sometimes rectangular... 

Carbonation. In this process, the juice is heated to 80 to 90°C, limed, and then sent to a carbonator for gassing with carbon dioxide. The resulting mixture, containing insoluble lime salts, chiefly calcium carbonate, is pumped to subsiders (thickeners, clarifiers) to remove the insolubles by settling. From the subsider, the partially clarified filtrate is recarbonated, and the residual lime is precipitated. This treatment is followed by press filtration. The sludge from the subsiders is filtered on rotary-drum filters, and the sugar-laden filtrate is returned to the first carbonation step. The carbonation process can be either continuous or batch. 

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