Lecture 9 Settling chamber design

Transcription

1 Lecture 9 Settling chamber design

2 INDERED (ZONE) SETTLING During settling of highh concentration slurries, both hindered or zone settling and compression settling usually occur in addition to discrete (free) and floculent settling. Various zones that get formed during settling of high concentration slurries are shown in following figure: Figure Phases of settling during hindered (zone) settling in a settling column, SZ: hindered settling zone; TSZ: transtion settling zone; CSZ: compressive settling zone. During this type of settling, the liquid tends to move up through the interstices of the contacting particles and therefore, the contacting particles tend to settle as a zone maintaing the same relative position with respect to each other. The phenomenon is known as hindered settling (SZ). The rate of settling in the hindered settling regionn is a function of the concentration of the solids and their characteristics. As the particles settle, a relatively clear layer of water is produced above the particles in the settling region. Remaining light particles usually settle as discrete or flocculant particles. An interface usually develops between the upper region and the hinderedd settling region. As settling continues, a compressed layer of particles begins to form on the bottom of the cylinder. In this compression setting region, particles remain in close physical contact and form a structure. A transition region of settling between the hindered and compression settling zones

3 gets formed. As the time progresses, first hindered and then transtion settling zones get removed and finally only clear water zone and compressed settling layer are only obtained [1]. These methods are, however, seldom used in the design of treatment plants because of less concentration of slurries. Area requirement based on single-batch test results. For purposes of design, the final overflow rate selected should be based on a consideration of the following factors: 1) The area needed for clarification 2) The area needed for thickening, and 3) The rate of sludge withdrawal. Column settling tests can be used to determine the area needed for the settling region directly. owever, because the area required for thickening is usually greater than the area required for the settling, the rate of free settling rarely is the controlling factor. In the case of the activated-sludge process, where light fluffy floc particles are present, the free flocculant settling velocity of these particles could control the design. AREA REQUIRED FOR TICKENING [1] Assume that a column of height o is filed with a suspension of solids of uniform concentration (C o ). The rate at which the interface subsided is then equal to the slope of the curve at that point in time. According to the procedure, the area required for thickening is given by [2]: Qt u A (3.9.1) o Where, A is area required for sludge thickening (m) 2, Q is the flow rate into tank (m 3 /s), o is the initial height of interface in column (m), t u is the time to each desired underflow concentration (s). The critical concentration controlling the solids handling capability of the tank occurs at a height 2 where the concentration is C 2. This point is determined by extending the tangents to the hindered settling and compression regions of the subsidence curve to the point of intersection and bisecting the angle thus formed. The time t u can be determined as follows:

4 (a) Construct a horizontal line at the depth solids are at the desired underflow concentration using the following expression [2]: C u that corresponding to the depth at which the C u. The value of u is determined o o u (3.9.2) Cu (b) Construct a tangent to the settling curve at the point indicated by C 2. (c) Construct a vertical line from the point of intersection of the two lines drawn in steps 1 and 2 to the time axis to determine the value of t u. With this value t u, the area required for the thickening is computed using equation earlier. The area required for clarification is then determined. The larger of the two areas is the controlling value. Problem 3.9.1: The settling curve shown in the following diagram was obtained for an activated sludge with an initial solid concentration C o of 3000 mg/l. The initial height of the interface in the settling column is 1 m. Determine the area required to yield a thickened solids concentration (C u ) of mg/l with a total flow of 3 m 3 /min. Also determine the solids loading (kg/m 2.d) and the overflow rate (m 3 /m 2.d). Solution: [A] Determination of the area required for thickening Co u C u o m A horizontal line is constructed at u =0.2 m which meets the settling curve at point C 1. Tangents are drawn to the curve at point C 1 and as well as at time t=0. Line bisecting the angle formed between the tangents meet the settling curve at point C 2. A tangent is further drawn to the settling curve at C 2 (the mid-point of the region between hindered and compression settling). The intersection of the tangent at C 2 and the line u =0.2 m determines t u =255 min. Thus for t u =255 min, and the required area is Qt u A 765m 2 o

5 [B] Determination of the clarification areaa a. Determinatio n of the interface subsidence velocity : determined by computing the slope of the tangent drawn from the initial portion of the interface settling curve. The computed velocity represents the unhindered settling rate of the sludge m/min 38 b. Determinatio n of the clarification rate: Since, the clarification rate is proportional to the liquid volume above the critical sludge zone, it may be computed as: Q c m 3 /min 1 clarification rate by the settling velocity. The subsidence velocity is c. Determinatio n of the clarification area: The required area is obtained by dividing the

6 Q c A 285 m 2 The controlling area is the thickening area (765 m 2 ) because it exceeds the area required for clarification (285 m 2 ). [C] Determination of the solids loading: The solids loading is computed as follows: Solids, kg/d = kg / d 3 10 Solids loading = kg/m2.d [D] Determination of the hydraulic loading rate: ydraulic loading rate = m 3 /m 2.d REFERENCES [1] Metcalf & Eddy, Tchobanoglous, G., Burton, F. L., Stensel,. D. Wastewater engineering: treatment and reuse/metcalf & Eddy, Inc., Tata McGraw-ill, [2] Wang, L. K., ung, Y.-T. Shammas, N. K. Biosolids Treatment Processes, andbook of Environmental Engineering, umana Press Inc., Volume 6, 2007.