Clarifiers/thickeners solids recycle

Another type of disc centrifuge has nozzles or orifices at the periphery where thickened solids are discharged continuously, and sometimes a fraction of this discharge is recycled to ensure that the nozzles are able to prevent breakthrough of the clarified supernatant liquid. Sometimes these nozzles are internally fitted to the bowl. Others are opened and closed electrically or hydrostatically from a build up of the sludge within the bowl. 

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. 

Water Clarification. Process water that needs to be clarified comes from several different sources in the recycling mill rejects from screens and mechanical cleaners rejects from washers, thickeners, and flotation cells water that drains from the pulp as it is converted into paper on the paper machine (white water) and water from felt washers. These waters contain different dissolved chemicals and suspended solids and are usually processed separately. 

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. 

Overflow from 1 thickener contains 150 to 200 ppm of solids. This is reduced to 50 to 75 ppm by addition of flocculant and settling in a 60-ft (18-m) diameter by 20-ft (6-m) deep reactor-clarifier. Fine slimes from the clarifier are recycled to leach tank 1. 

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