Thickening effluent

Thickening and clarification are sedimentation processes, and the equipment used for the two techniques are similar. The primary purpose of thickening is to increase the concentration of a relatively large quantity of suspended solids, whereas that of clarifying, as the name implies, is to remove a small quantity of fine solids to produce a clear liquid effluent. Thickening and clarification are relatively cheap processes when used for the treatment of large volumes of liquid. 

Sometimes it is necessary to reslurry the cake discharged from the decanter. This is generally for washing out contaminants from the cake, as in lactose production. It is sometimes done in effluent thickening, and when a greater dryness than is required is produced by the decanter for easy control. Then the cake is back mixed with a feed by-pass which reduces overall flocculant consumption, because the by-pass does not need flocculant. The by-passing material is, of course, 100% recovery. 

Dissolved Air Flotation. Dissolved air flotation (DAF) is used to separate suspended soflds and oil and grease from aqueous streams and to concentrate or thicken sludges. Air bubbles carry or float these materials to the surface where they can be removed. The air bubbles are formed by pressurizing either the influent wastewater or a portion of the effluent in the presence of air. When the pressurized stream enters the flotation tank which is at atmospheric pressure, the dissolved air comes out of solution as tiny, microscopic bubbles. Dissolved air flotation is used in many wastewater treatment systems, but in the United States it is perhaps best known with respect to hazardous waste because it is associated with the Hsted waste, K048, DAF flotation soflds from petroleum refining wastewaters. Of course, the process itself is not what is hazardous, but the materials it helps to remove from refining wastewaters. 

Drainage from active mining areas is considerably less than from inactive mines because of the disposal methods currently employed. Prior to discharge, Hquid effluents are limed and settled to precipitate metals as hydroxides. Elocculants are used to reduce the total suspended soHds and, in some instances, filtration of thickener overflow is practiced. 

The bubble size in these cells tends to be the smallest (10 to 50 Im) as compared to the dissolved-air and dispersed-air flotation systems. Also, very httle turbulence is created by the bubble formation. Accordingly, this method is attractive for the separation of small particles and fragile floes. To date, electroflotation has been applied to effluent treatment and sludge thickening. However, because of their bubble generation capacity, these units are found to be economically attractive for small installations in the flow-rate range of 10 to 20 mVh. Electroflotation is not expected to be suitable for potable water treatment because of the possible heavy metal contamination that can arise due to the dissolution of the electrodes. 

Water treatment Clarification of potable water, industrial effluents, municipal waste water thickening and dewatering of sludge filtration of primary sludge, digested sludge food processing... 

When a pilot-scale fermenter is run in continuous mode with a fresh feed flowrate of 65 1/h, the effluent from the fermenter contains 12 mg/1 of the original substrate. The same fermenter is then connected to a settler-thickener which has the ability to concentrate the biomass in the effluent from the tank by a factor of 3.2, and from this a recycle stream of concentrated biomass is set up. The flowrate of this stream is 40 1/h and the fresh feed flowrate is at the same time increased to 100 1/h. Assuming that the microbial system follows Monod kinetics, calculate the concentration of the final clarified liquid effluent from the system. /x, = 0.15 h-1 and Ks = 95 mg/1. 

The above-cited studies demonstrate the performance of a particular unit system for the treatment of specific type of waste stream. A particular unit system alone may not be able to treat the wastewater to a level of effluent standard prescribed for its safe disposal. Hence a number of pretreatments, such as screening, sedimentation, equalization, and neutralization, and post-treatment units such as secondary sedimentation, sludge thickening, digestion and disposal, disinfection, and so on, are extremely important for complete treatment. The effluent treatment and disposal facilities adopted by various types of pharmaceutical industries are described in the following sections. 

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. 

Cold-Water Process. The cold-water bitumen separation process has been developed to the point of small-scale continuous pilot plants. The process uses a combination of cold water and solvent. The first step usually involves disintegration of the tar sand charge, which is mixed with water, diluent, and reagents. The diluent may be a petroleum distillate fraction such as kerosene and is added in a ca 1 1 weight ratio to the bitumen in the feed. The pH is maintained at 9-9.5 by addition of wetting agents and ca 0.77 kg of soda ash per ton of tar sand. The effluent is mixed with more water, and in a raked classifier the sand is settled from the bulk of the remaining mixture. The water and oil overflow the classifier and are passed to thickeners, where the oil is concentrated. Clay in the tar sand feed forms emulsions that are hard to break and are wasted with the underflow from the thickeners. 

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