4.3 Selection of Equipment
4.3.1 Design of clarification（sedimentation）equipment shall comply with the following provisions:
1 The output of clarification（sedimentation）equipment shall be checked based on the equipment type and design water temperature，and when necessary，it can be determined by test or with reference to operation experience of similar plants；
2 The clarification equipment shall be provided with sludge reflux system for seawater of low turbidity or inconsistent quality.
4.3.2 Design of media filtration equipment shall comply with the following provisions:
1 The backwash times of media filtration equipment shall be determined according to such factors as inlet and outlet water quality and the dirt intercepting capability of the filtering materials.There should be no more than 2 backwashes every 24 hours；
2 The backwash method of media filtration equipment shall be determined according to equipment types；
3 The media filtration equipment shall be provided with air scrubbing facilities，and the gas used should be supplied by a dedicated Roots blower；
4 The design filtration velocity of the media filtration equipment should comply with the provisions in Table 4.3.2-1；
Table 4.3.2-1 Filtration velocity of media filtration equipment
5 The filtering materials grading and backwash intensity of the media filtration equipment should comply with the provisions in Table 4.3.2-2；
Table 4.3.2-2 Filtering materials grading and backwash intensity of the media filtration equipment
Notes:1 Data in the table is based on backwash water temperature of 20℃，and the backwash intensity shall be increased or decreased accordingly by 1% when the water temperature increases or decreases by 1℃.
2 Filtering material backwash expansion ratio:45% with quartz sand in single layer and 50% in double layer.
3 The low value of backwash intensity can be taken when surface flushing facilities are provided.
4 The backwash intensity can be adjusted as appropriate according to factors of water temperature and water quality variation over the year.
6 Provisions shall be made to restrict flow for backwash，forward wash inflow water or discharge water for all types of filtration equipment.
4.3.3 Design of ultra（micro）filtration unit shall comply with the following provisions:
1 It shall be possible to automatically adjust the inflow water of pressurized ultra（micro）filtration unit，and frequency conversion controlled feed water pump or regulator valves should be used.The product water pump of submerged ultra（micro）filtration unit shall have frequency conversion function；
2 The ultra（micro）filtration membrane assembly shall be provided with chemical cleaning device and dedicated backwash water pump.Product water from ultra（micro）filtration unit should be used as backwash water，and chemical cleaning water should be fresh water；
3 The ultra（micro）filtration membrane device shall have integrity detection function for membrane element；
4 The type and quantity of ultra（micro）filtration membrane elements shall be determined according to membrane flux attenuation rate and water temperature.
4.3.4 Design of water tanks（pools）and water pumps shall comply with the following provisions:
1 The total effective volume of all types of water tanks（pools）of the pretreatment system shall be determined according to the water consumption of the system itself，the equipment capacity configuration of upstream and downstream systems and the system operation requirements，and should be taken as the water consumption by the subsequent system for 0.5h-1h；
2 There shall be no less than 2 sets of water tanks（pools）for each type，and when 1 set of water tank（pool）is under maintenance，the remaining equipment shall meet the normal water supply demand of the system；
3 When the water pump is located above the lowest water level of the water pool，each pump shall have a separate water suction pipe；
4 When water pumps of different applications are connected to the same water tank，they should be provided with separate water suction pipes；
5 Water pumps connected in parallel shall be provided with standby pumps.