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Particles in Water Properties and Processes - Chpater 7 (end)

Trong chương này chúng tôi một thời gian ngắn tiến hành điều tra các phương pháp chính được sử dụng để loại bỏ các hạt từ nước. Chỉ có một phác thảo rộng sẽ được đưa ra, mà không đi sâu vào nhiều chi tiết kỹ thuật. Hầu hết các trọng tâm sẽ được vào các quy trình được sử dụng trong xử lý nước uống, mặc dù nhiều người trong số các khái niệm có liên quan đến các ứng dụng khác. Các phương pháp tách bao gồm như sau: • lắng • nổi (chủ yếu là hòa tan. | chapter seven Separation methods 7.1 Introduction In this chapter we shall briefly survey the main methods used to remove particles from water. Only a broad outline will be given without going into a lot of technical detail. Most emphasis will be on processes used in drinking water treatment although many of the concepts are relevant to other applications. The separation methods covered are as follows Sedimentation Flotation mainly dissolved air flotation Filtration including deep bed and membrane processes In all cases there is a strong influence of particle size and it is often found that increasing particle size by a coagulation flocculation procedure is a necessary preliminary step before one or more of the previously mentioned processes is used. Filtration can be an effective method but for various reasons it may be preceded by another separation process either sedimentation or flotation. This can greatly reduce the load on the subsequent filtration process and leads to longer filter runs. A typical sequence of steps in a solid-liquid separation procedure is shown schematically in Figure 7.1. Although the principles of coagulation and flocculation have been dealt with in some detail in the previous chapter some discussion of more practical aspects will be given here followed by sections on the three processes listed earlier. 7.2 Flocculation processes The main requirements for effective flocculation are as follows Rapid mixing of coagulants Opportunity for collisions of destabilized particles and hence flocculation 149 2006 by Taylor Francis Group LLC 150 Particles in Water Properties and Processes Coagulant Figure 7.1 Typical sequence of processes for particle separation in a water treatment plant. For the second step some form of fluid motion has to be generated which may be by mechanical stirring or flow or both . 7.2.1 Rapid mixing Essentially rapid mixing sometimes called flash mixing is necessary to distribute the coagulant species among the particles in