Sedimentation basins

Clarifier Settling tank, sedimentation basin, A tank or basin in which wastewater is held for a period of time, during which the heavier solids settle to the bottom and the lighter material will float to the water surface. 

Sedimentation basins can be used to collect and store surface water flow and to settle suspended solid particles. Seepage basins and ditches can be used to discharge uncontaminated or treated water downgradient of the site. It is important to separate clean surface runoff from contaminated water and store and treat them separately. Table 16.4 summarizes the surface water control methods. 

Chutes and downpipes Seepage basins and ditches Sedimentation basins and X ... 

To provide this finished water quality, a system incorporating flocculation, settling, and mixed-media filtration is used. Fig. 6 schematically illustrates the flow of the system used. After flocculation, the coagulated water is processed through a shallow-depth sedimentation basin using tube settlers. Tube settlers provide efficient sedimentation in greatly reduced detention times. The tube settlers are essentially a honeycomb of 1-in. trays operating in parallel. The... 

Alternatively, plain sedimentation basins with mechanical equipment for continuous collection of the fresh sludge may be used. The fresh sludge, so collected, is discharged into separate sludge digestion tanks which operate on the principle of the lower story of the Imhoff tank except that by means of higher and better temperature control the digestion cycle is much more rapid and efficient than for the Imhoff tank. 

Blackwell, B. A. B. In Tracking Environmental Change Using Lake Sediments Basin Analysis, Coring and Chronological Techniques , Last, W. M. Smol, J. P., Eds. Kleuwer Dordecht, 2001 Vol. 1, pp 283-368. 

Figure 5.4 shows a portion of a circular primary sedimentation basin used at the Back River Sewage Treatment Plant in Baltimore City, MD. In this type of clarifier, the raw sewage is introduced at the center of the tank and the solids settled as the wastewater flows from the center to the rim of the clarifier. The schematic elevational section in Figure 5.5 would represent the elevational section of this clarifier at the... 

Primary Settling and Water-Treatment Sedimentation Basins... 

The settling tank has an inherent capacity to damp out fluctuation in rate of inflow. In the present example, the flow is being damped out or sustained in a period of 2.5 h. Because this flow has taken effect during the detention time, it must be the flow corresponding to this detention time and, hence, must be the one adopted for design purposes. To size sedimentation basins, the flow to be used should therefore be the sustained flow corresponding to the detention time chosen. This detention time must, in turn, be a value that limits septicity. The method of calculating sustained flows was discussed in Chapter 1. 

What sustained flow is equivalent to an average flow of 0.15 m /s There are, actually, two sustained flows that must be determined the sustained peak flow and the sustained low flow gms- The sustained peak flow is used to size the tank the sustained low is used to determine the amount of recirculation desired so as to deter septicity from occurring. The occurrence of the sustained low flow will produce a much, much longer detention time. As mentioned before, detention times greater than 2.5 h (or some other value depending upon the type of waste) can cause septicity and thus inefficiency in sedimentation basins. 

One other method of design is to use equalization basins ahead of the sedimentation basins. In this case, the inflow would be the equalized flow. There should be no need for recirculation, because only one flow rate is introduced into the sedimentation basin. Design of equalization basins was discussed in Chapter 3. 

Example 5.9 To meet effluent limits, it has been determined that the design of a primary sedimentation basin must remove 65% of the influent suspended solids. The average influent flow is 6,000 mVd the peak daily flow is 13,000 mVd and the minimum daily flow is two-thirds of the average flow. It was previously determined that 65% removal corresponds to an overflow rate of 28 m/d. Design a circular basin to meet the effluent requirement at any cost. 

Inlet zone—In a sedimentation basin, the transition into the settling zone aimed at properly introducing the inflow into the tank. 

Outlet zone—In a sedimentation basin, the part where the settled water is taken off into the effluent launder. 

Settling zone—In a sedimentation basin, this is the part where the suspended solids load of the inflow is removed to be deposited into the sludge zone below. 

Sludge zone—In a sedimentation basin, this is the part where solids are deposited. 

Consider ihe contents inside an activated sludge reactor laden with dissolved oxygen and pollutants subjected to aeration. Also, consider the sedimentation basin that follows the reactor and all the associated pipings. For the purpose of performing the material balance, let the boundaries of these items encompass the control volume. According to the Reynolds transport theorem, the total rate of increase of the concentration of dissolved oxygen is equal to its partial (or local) rate of increase plus its convective rate of increase. 

Ce Concentration of dissolved oxygen in the effluent of the secondary sedimentation basin... 

In practice, two types of plants are generally used for chemical precipitation hardness removal One type uses a sludge blanket contact mechanism to facilitate the precipitation reaction. The second type consists of a flash mix, a flocculation basin, and a sedimentation basin. The former is called a solids-contact clarifier. The latter arrangement of flash mix, flocculation, and sedimentation were discussed in previous chapters on unit operations. 

If the concentration of iron and manganese is less than about 10 mg/L, it may not be necessary to use a sedimentation basin before filtration. Iron(II) and manganese(II) are also removed by lime/soda-ash softening, described in another section. 

The total amount of heavy metals stored in mud sediments of the Arkona Basin is probably much greater than those metals bound by iron-manganese concretions of a relatively small area. However, this assumption needs to be proved by mass balance calculations and may not be true for other areas. It would be interesting to investigate whether and how changing redox conditions in bottom water layers of sedimentation basins could favor the formation of Fe-Mn microconcretions and the accumulation of heavy metals therein. 

With hydraulic residence times ranging from months to years, lakes constitute efficient sedimentation basins for particles. Lacustrine sediments are sinks for nutrients that support biological growth and for pollutants such as those toxic metals and synthetic organic compounds that associate with particulate matter. Natural aggregation increases particle sizes and thus particle settling velocities, accelerates particle removal to bottom sediments, and decreases the concentrations of particles and particle-reactive pollutants in the water column of a lake. 

Valioulis, I. A., and E. J. List (1984a), Numerical Simulation of a Sedimentation Basin. 1. Model Development, Environ. Sci. Technol. 18, 242-247. 

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