給水排水專業(yè)英語(yǔ)課件(李田)10

1、1-12-12-22-32-43-13-24-14-24-3Filter OperationnFig.27 gives an illustration of the behavior of the filter during a filter run.n It shows the initial breakthrough, a transitory jump in effluent turbidity, and a terminal breakthrough. 5-1nSince initial breakthrough usually lasts no more than 30 min, a

2、nd therefore has no great effect on the general plant average quality, in most plants no corrective action is taken. nIn some plants, water is filtered to waste for 15 to 30 min at the start of each filter run.n British waterworks practice usually includes a 30 min slow start for each filter run as

3、a measure to prevent the initial breakthrough.6-17-17-2n The initial and maintenance costs of the fairly complex rate control system are high;n The filtered water quality is not as good through gravity granular media filters as through declining-rate filters in potable water filtration;n The rate or

4、 level control systems frequently do not function properly, causing sudden changes in rate, with the effect described previously on water quality. In many existing works, the control systems are nonfunctional.7-3Filter BackwashingBoth sand and mixed-media filters are normally cleaned by reversal of

5、the flow through the bed, or backwashing.nBackwashing expands the mediums depth, allowing particulates to be released into the water which are collected above the media in wash troughs. nThis washing process may be improved by jet of water, air injection, or mechanical agitation at the surface. 8-1n

6、Cleasby et al., cite a method of backwashing that may be used in various situations: (1) High-rate backwash with full bed expansion and fluidization (bed expansion 20 to 50 percent) preceded by a lower-rate backwash. nThis method may be used for single-medium or multimedia filters. n(2) Low-rate bac

7、kwash without fluidization for expansion but with auxiliary scour. This method is used for single-medium filtration only. 8-2n(3) Water surface wash.n(4) Air and water surface wash has several options, including: (a) air with low-rate water backwash for a single medium only; (b) air with high-rate w

8、ater backwash for a single media and multimedia; (c) simultaneous air and low-rate water backwash for a single medium or multimedia. 8-3nBackwash efficiency is affected by medium size, shape, density, water quality, and coagulants used previous to or during the filtration process. nBackwash water is

9、 either discharged to the sewer or settled with the supernatant returned to the front of the plant and the sludge treated in some acceptable manner.8-4nThe best criteria for backwash effectiveness are the quality of the filtered water and long-term absences of dirty filters and mudball formation. nAmirtharaja has indicated that few or no contacts occur between fluidized particles.nHence, particulate fluidization with water alone is an intrinsically weak cleaning process. Air scour, which causes abrasions between particle