给水排水专业英语课件(李田)8.ppt

1、Unit 8,Mixing and Flocculation,New words and expressions,coagulate v. 凝结 Blood coagulates when it meets air. adj. 凝结的 settling n. 沉淀 沉淀作用 沉淀物 disperse v. (使)分散, (使)散开, 疏散 The police dispersed the crowd. instantaneous adj. 瞬间的 即刻的 即时的 aluminium n. 铝 hydroxide n. 氢氧化物 calcium n. 钙 carbonate n. 碳酸盐 vt.

2、 使变成碳酸盐, 使充满二氧化碳,New words and expressions,agglomerate vt. 使成团 使成块 使凝聚 gradient n. 坡度 梯度 adj. 倾斜的 pilot plant 试验工厂 中试装置 collision n. 碰撞 冲突 constraint n. 约束, 强制, 局促 geometry constraint n. 几何约束 dynamic n. 动力的 动力学的 动态的 viscosity n. 粘度 粘滞性,New words and expressions,shaft n. 轴, 杆状物 the shaft of an arrow

3、static mixer n. 静态混合器 Parshall flume n. 巴歇尔计量槽 rule of thumb n. 经验法则 经验方法 empirical rule collide v. 碰撞 turbine n. 涡轮 impeller n. 推进器 叶轮 baffle n. 挡板 导流片 v. 阻碍 vortex n. 旋涡, 旋风, 涡流 taper n. 锥形, 锥度 v. 逐渐变细, 逐渐减少,Clearly, if the chemical reactions in coagulating and softening a water are going to take

4、place, the chemical must be mixed with the water. In this section we will begin to look at the physical methods necessary to accomplish the chemical processes of coagulation and softening. The basis of what will be presented applies to any operation of mixing, flocculating, settling, or filtering.,1

5、-1,Mixing, or rapid mixing as it is called, is the process whereby the chemicals are quickly and uniformly dispersed in the water. Ideally, the chemicals would be instantaneously dispersed throughout the water. During coagulation and softening the chemical reactions that take place in rapid mixing f

6、orm precipitates.,2-1,Either aluminum hydroxide or iron hydroxide form during the case of coagulation , while calcium carbonate and magnesium hydroxide form during the case of softening. After forming the precipitates, it is necessary to bring them into contact with one another so that they can aggl

7、omerate and form larger particles, called flocs. This contacting process is called flocculation and is accomplished by slow, gentle mixing.,2-2,In the treatment of water and wastewater the degree of mixing is measured by the velocity gradient, G. The velocity gradient is best thought of as the amoun

8、t of mixing taking place; that is, the higher the G value the more violent the mixing. The velocity gradient is a function of the power input into a unit volume of water.,3-1,where Gvelocity gradient (s-1); Ppower input (W); Vvolume of water (m3); dynamic viscosity (Pa.s).,3-2,It is given by,From li

9、terature, experience, laboratory, or pilot plant work it is possible to select an optimal G value for a particular application. The total number of particle collisions is proportional to Gt0, where t0 is the detention time in the basin. Typical design values for G and Gt0 will be discussed elsewhere

10、.,4-1,Rapid mixing is probably the most important physical operation affecting coagulant dose efficiency. The chemical reaction in coagulation is completed in less than 0.1s; therefore, it is imperative that mixing be as instantaneous and complete as possible.,5-1,Rapid Mix,Rapid mixing can be accom

11、plished within a tank utilizing a vertical shaft mixer, within a pipe using an in-line mixer, Or in a pipe using a static mixer. Less efficient methods are sometimes used, Such as Parshall flumes, hydraulic jumps, baffled channels, or air mixing.,5-2,vertical shaft mixer,static mixer,in-line mixer,R

12、apid mix tanks usually have detention times of 10 to 30 s with velocity gradients of 600 to 1000 s-1. The volume of the rapid mix tank seldom exceeds 8 m3 because of mixing equipment and geometry constraints. The mixing equipment consists of an electric motor, gear-type speed reducer, and either a t

13、urbine or axial flow impeller as shown in Fig. 2-4.,6-1,6-2,The turbine impeller provides more turbulence and is preferred for rapid mixing. The tanks should be horizontally baffled into at least two and preferably three compartments in order to provide sufficient residence time. They are also baffl

14、ed vertically to minimize vortexing.,6-3,Chemicals should be added below the impeller, the point of the most mixing. Some common rules-of-thumb are that the design liquid depth is 0.5 to 1.1 times the basin diameter or width; that the impeller diameter be between 0.30 and 0.50 times the tank diamete

15、r or width; and that the vertical baffles extend into the tank about 10 percent of the tank width or diameter.,6-4,Although they may be larger, impeller diameters normally do not exceed 1 m in diameter. The liquid depth may be increased to between 1.1 and 1.6 times the tank diameter if dual impeller

16、s on the shaft are employed.,6-5,When dual impellers are employed on gear driven mixers, they are spaced approximately two impeller diameters apart. Two straight blade turbines impart about 1.9 times as much power to the water as one turbine alone for the same motor power.,6-6,Flocculation While rap

17、id mix is the most important physical factor affecting coagulant efficiency, flocculation is the most important factor affecting particle-removal efficiency. The objective of flocculation is to bring the particles into contact so that they will collide, stick together, and grow to a size that will r

18、eadily settle.,7-1,Enough mixing must be provided to bring the floc into contact and to keep the floc from settling in the flocculation basin. Too much mixing will shear the floc particles so that the floc is small and finely dispersed. Therefore, the velocity gradient must be controlled within a re

19、latively narrow range.,7-2,Flexibility should also be built into the flocculator so that the plant operator can vary the G value by a factor of two to three. The heavier the floc and the higher the suspended solids concentration, the more mixing is required to keep the floc in suspension.,7-3,Softening floc is heavier than coagulation floc and therefore requires a higher G value. An increase in the floc concentration (as measured by the suspended solids concentration) also increases the req