9.1 General Provisions
9.1.1 The selection of water treatment process flow and the composition of main structures shall be determined through investigation and research, tests of different process combinations, or reference to the operation experience of existing water plants under similar conditions, combined with local operation and management conditions, and comprehensive technical and economic comparison, based on raw water quality, designed production capacity, and treated water quality requirements.
9.1.2 The designed production capacity of water treatment structures shall be determined based on the maximum daily water supply plus the water consumption for the plant's own use, and shall also include the supplementary fire water supply if necessary.
The self-consumed water of urban water plants shall be determined through calculation based on factors such as raw water quality, adopted treatment processes and types of structures. The self-consumed water rate of urban water plants is generally 5% to 10% of the designed water volume.
Note: When the backwash water of filters is reused, the self-consumed water rate may be appropriately reduced.
9.1.3 The design parameters of water treatment structures shall be checked according to the maximum water supply required under the most adverse conditions of raw water quality (such as sand peaks, low temperature, low turbidity, etc.) if necessary.
9.1.4 When designing an urban water plant, consideration shall be given to meeting the water supply requirements at that time when any structure or equipment is out of service for maintenance or cleaning.
9.1.5 Water purification structures shall be equipped with sludge discharge pipes, emptying pipes, overflow pipes or pressure flushing facilities as required.
9.1.6 Urban water plants shall properly treat and dispose of sludge discharge water in accordance with local environmental protection requirements.
9.1.7 When the backwash water of filters is reused, the accumulation of harmful substances, pathogenic microorganisms, etc. shall be avoided, and it may be reused after appropriate treatment if necessary.
9.2 Pretreatment
9.2.1 Pretreatment may be added before conventional treatment when the raw water has high contents of sediment concentration, color, organic matter, mutagen precursors, obvious odor and taste, or to improve the coagulation effect.
9.2.2 When the raw water has high sediment concentration, pre-sedimentation measures shall be adopted. When natural terrain is available, water storage measures may also be adopted for use during sand peak periods.
9.2.3 The selection of pre-sedimentation methods shall adopt sand settling, natural sedimentation or coagulation sedimentation according to factors such as raw water sediment concentration and its composition, duration of sand peaks, sludge discharge requirements, treated water volume and water quality requirements, combined with terrain conditions.
9.2.4 The design data of pre-sedimentation tanks shall be determined through raw water sedimentation tests or by reference to the operation experience of similar water plants.
9.2.5 Pre-sedimentation tanks are generally designed according to the daily average sediment concentration of raw water during the duration of sand peaks. When the raw water sediment concentration exceeds the designed value, the possibility of adjusting coagulant dosage or adopting other measures shall be considered.
9.2.6 Mechanical sludge discharge shall be adopted for pre-sedimentation tanks.
9.2.7 Biological pretreatment may be adopted when the raw water for domestic drinking water has high contents of ammonia nitrogen, odor threshold, organic micro-pollutants and algae. The design of biological pretreatment tanks shall be based on raw water test data. The raw water for biological pretreatment shall have good biodegradability, contain no harmful substances inhibiting biological treatment, and the water temperature shall be higher than 5℃.
9.2.8 Aeration devices shall be installed in artificial filler biological pretreatment tanks.
9.2.9 The hydraulic retention time of artificial filler biological contact oxidation tanks is preferably 1~2h, and the aeration gas-water ratio is preferably 1:1~2:1.
9.2.10 Granular filler biological filters can be down-flow or up-flow. The filler particle size is preferably 2~5mm, the filler thickness is preferably 2m, the filtration rate is preferably 4~7m/h, and the aeration gas-water ratio is preferably 1:1. The air-water backwash intensity of down-flow tanks is preferably: water 10~15L/s·m², air 10~20L/s·m².
9.2.11 When adopting the chlorine pre-oxidation process, the chlorine dosing point and dosage shall be determined through tests to minimize the generation of disinfection by-products.
9.2.12 The adoption of ozone pre-oxidation shall comply with the relevant provisions of Section 9.9 of this code.
9.2.13 The adoption of potassium permanganate pre-oxidation shall comply with the following provisions:
1 Potassium permanganate is generally added at the water intake of the water plant; if dosed in the water treatment process, the dosing time before other water treatment agents shall not be less than 3min. The water after potassium permanganate pre-oxidation must be filtered through a filter;
2 The dosage of potassium permanganate for pre-oxidation shall be determined through tests and accurately controlled. The dosage of potassium permanganate for removing organic micro-pollutants, algae and controlling odor and taste is generally 0.5~2.5mg/L;
3 Dry dosing may be adopted when the dosage of potassium permanganate exceeds 15kg/d. The concentration of wet dosing solution is preferably 4%.
9.2.14 Powdered activated carbon adsorption may be adopted when the raw water contains a high concentration of soluble organic matter and has abnormal odor and taste in a short time. The adoption of powdered activated carbon adsorption shall comply with the following provisions:
1 The dosing point of powdered activated carbon shall be comprehensively determined according to the water treatment process. It is generally added to the raw water, and coagulant or chlorine is added after mixing and contacting with water for 10~15min;
2 The dosage of powdered activated carbon is determined according to tests, generally 5~30mg/L;
3 The concentration of wet-dosed powdered activated carbon slurry can be 5~10% (by weight);
4 Dust-proof, dust-collecting and fire-proof facilities shall be provided in the storage, transportation and dosing workshops of powdered activated carbon.
9.3 Dosing of Coagulants and Coagulant Aids
9.3.1 Coagulants or coagulant aids for domestic drinking water must meet the requirements of the *Code for Hygienic Safety Evaluation of Chemical Treatment Agents for Domestic Drinking Water* issued by the Ministry of Health.
9.3.2 The selection and dosage of coagulants and coagulant aids shall be comprehensively determined based on the results of raw water coagulation and sedimentation tests or reference to the operation experience of water plants under similar conditions.
9.3.3 The dosing method of coagulants can be wet dosing or dry dosing.
For wet dosing, the dissolution and dilution of coagulants shall adopt hydraulic, mechanical or compressed air mixing methods according to the dosage and properties of coagulants.
9.3.4 When dosing coagulants by wet method, the number of dissolutions shall be determined according to factors such as coagulant dosage and preparation conditions, generally not more than 3 times a day.
When the coagulant dosage is large, a belt conveyor shall be installed or the solid dissolution tank shall be set underground. When the coagulant dosage is small, the dissolution tank can also serve as the dosing tank. A standby dosing tank shall be provided.
9.3.5 The solution concentration for coagulant dosing can be 5%—20% (calculated by solid weight).
9.3.6 Lime shall be made into lime milk for dosing.
9.3.7 Metering equipment shall be designed for coagulant dosing and measures shall be taken to stabilize the dosing amount, generally using metering pumps for dosing.
9.3.8 Automatic control dosing is preferred for coagulants or coagulant aids.
9.3.9 The inner walls of tanks, equipment, pipelines and floors in contact with coagulants shall take corresponding anti-corrosion measures according to the properties of coagulants.
9.3.10 The dosing room shall be located in a well-ventilated area as far as possible. Ventilation equipment and labor protection measures to ensure the hygienic safety of staff must be installed indoors.
9.3.11 The dosing room shall be close to the dosing point.
9.3.12 The floor of the dosing room shall have a drainage slope.
9.3.13 Measuring tools and handling equipment shall be installed in the reagent warehouse and dosing room according to specific conditions.
9.3.14 The fixed reserve of coagulants shall be determined according to local supply and transportation conditions, generally calculated as 15 days of the maximum dosage. The turnover reserve shall be determined according to local specific conditions.
9.3.15 When calculating the area of storage warehouses for solid coagulants and lime, the stacking height can be 1.5~2.0m for coagulants and 1.5m for lime.
The stacking height can be appropriately increased when mechanical handling equipment is adopted.
9.4 Coagulation, Sedimentation and Clarification
(1) General Provisions
9.4.1 Sedimentation and clarification referred to in this section mean coagulation sedimentation and clarification after adding coagulants.
9.4.2 When selecting the type of sedimentation tank or clarifier, it shall be determined through technical and economic comparison according to raw water quality, designed production capacity, treated water quality requirements, considering factors such as changes in raw water temperature, uniformity of water production and continuous operation, combined with local conditions.
9.4.3 The number of sedimentation tanks and clarifiers or the number of separate emptying compartments shall not be less than two.
9.4.4 The turbidity of water after sedimentation or clarification treatment before entering the filter shall generally not exceed 5 NTU, and shall not exceed 10 NTU in case of high-turbidity raw water or low-temperature and low-turbidity raw water.
9.4.5 Uniform water distribution and collection shall be considered in the design of sedimentation tanks and clarifiers.
9.4.6 The volume of the sludge accumulation area of sedimentation tanks and the sludge concentration chamber (hopper) of clarifiers shall be determined through calculation according to factors such as suspended matter content of inlet and outlet water, treated water volume, sludge discharge cycle and concentration.
9.4.7 Mechanical or automatic sludge discharge devices shall be adopted when sedimentation tanks and clarifiers are large in scale or have frequent sludge discharge.
9.4.8 Sampling devices shall be installed in clarifiers.
(2) Mixing
9.4.9 The design of mixing equipment shall ensure rapid and sufficient mixing of reagents and water according to the type of coagulant adopted.
9.4.10 Mixing methods generally include mechanical mixing and hydraulic mixing.
(3) Flocculation
9.4.11 Flocculation tanks shall be built jointly with sedimentation tanks.
9.4.12 The selection of flocculation tank type and the adoption of flocculation time shall be determined according to raw water quality, operation experience under similar conditions or through tests.
9.4.13 The design of baffle flocculation tanks shall comply with the following requirements:
1 The flocculation time is generally 20~30min;
2 The flow velocity of the flocculation tank corridor shall be designed as a gradual change from high to low, the initial velocity is generally 0.5~0.6m/s, and the final velocity is generally 0.2~0.3m/s;
3 The net distance between baffles is generally greater than 0.5m.
9.4.14 The design of mechanical flocculation tanks shall comply with the following requirements:
1 The flocculation time is generally 15~20min;
2 3~4 sets of mixers are generally installed in the tank;
3 The speed of the mixer shall be calculated according to the linear velocity at the edge of the paddle, which shall gradually decrease from 0.5m/s in the first stage to 0.2m/s in the last stage;
4 Facilities to prevent short-circuit of water flow shall be installed in the tank.
9.4.15 The design of folded plate flocculation tanks shall comply with the following requirements:
1 The flocculation time is generally 10~20min;
2 The velocity in the flocculation process shall be reduced section by section, the number of sections shall generally not be less than three, and the velocity of each section can be:
First section: 0.25~0.35 m/s;
Second section: 0.15~0.25 m/s;
Third section: 0.10~0.15 m/s.
3 The included angle of folded plates is 90°~120°.
9.4.16 The design of grid (mesh) flocculation tanks shall comply with the following requirements:
1 The flocculation tank shall be designed as a multi-cell vertical flow type;
2 The flocculation time is generally 10~20min;
3 The vertical shaft velocity, grid (mesh) velocity and hole velocity of the flocculation tank shall decrease section by section, generally divided into three sections, and the velocities can be:
Average vertical shaft velocity: 0.14~0.12m/s in the first and middle sections, 0.14~0.10m/s in the last section;
Grid (mesh) velocity: 0.30~0.25m/s in the first section, 0.25~0.22m/s in the middle section;
Hole velocity between vertical shafts: 0.30~0.20m/s in the first section, 0.20~0.15m/s in the middle section, 0.14~0.10m/s in the last section.
4 The flocculation tank is generally arranged in 2 or more parallel groups.
5 Sludge discharge facilities shall be installed in the flocculation tank.
Note: The flocculation time can be appropriately extended for low-temperature and low-turbidity water.
(4) Horizontal Flow Sedimentation Tank
9.4.17 The sedimentation time of horizontal flow sedimentation tanks shall be determined according to raw water quality, water temperature, etc., with reference to operation experience under similar conditions, generally 1.5~3.0h.
9.4.18 The horizontal flow velocity of horizontal flow sedimentation tanks can be 10~25 mm/s, and excessive turning of water flow shall be avoided.
9.4.19 The effective water depth of horizontal flow sedimentation tanks is generally 3.0~3.5m. The width of each cell (or spacing of guide walls) of the sedimentation tank is generally 3~8m, maximum not exceeding 15m, the ratio of length to width shall not be less than 4; the ratio of length to depth shall not be less than 10.
9.4.20 Perforated wall water distribution and overflow weir water collection are preferred for horizontal flow sedimentation tanks, and the overflow rate is generally not more than 500m³/m·d.
(5) Tube (Plate) Sedimentation Tank with Different Flow Directions
9.4.21 Tube (plate) sedimentation tanks with different flow directions are suitable for raw water with turbidity lower than 1000 NTU for a long time.
9.4.22 The surface load of the tube (plate) sedimentation zone shall be determined according to operation experience under similar conditions, generally 5.0~9.0m³/m²·h.
9.4.23 The following data are generally adopted for tube (plate) design: tube diameter 30~40mm; plate spacing 80~100mm; tube length 1.0m; inclination angle 60°.
9.4.24 The protection height of the clear water zone of tube (plate) sedimentation tanks shall generally not be less than 1.0m; the height of the bottom water distribution zone shall not be less than 1.5m.
(6) Lateral Flow Plate Sedimentation Tank
9.4.25 The design of lateral flow plate sedimentation tanks shall comply with the following requirements:
1 The designed particle settling velocity and surface load of the plate sedimentation zone shall be determined through tests or reference to the operation experience of similar water plants. Generally, the designed particle settling velocity can be 0.16~0.3mm/s, and the surface load can be 6.0~12m³/m²·h. The lower limit shall be adopted for low-temperature and low-turbidity water;
2 The plate spacing of inclined plates is generally 80~100mm;
3 The inclination angle of inclined plates is preferably 60°;
4 The length of a single-layer inclined plate shall not be greater than 1.0m.
(7) Mechanical Agitation Clarifier
9.4.26 Mechanical agitation clarifiers are suitable for raw water with turbidity lower than 5000 NTU for a long time.
9.4.27 The upward flow velocity of the clear water zone of mechanical agitation clarifiers shall be determined according to operation experience under similar conditions, generally 0.8~1.0mm/s.
9.4.28 The total residence time of water in mechanical agitation clarifiers can be 1.2~1.5h.
9.4.29 The lifting flow of the mixing impeller can be 3~5 times the influent flow, and the impeller diameter can be 70%~80% of the inner diameter of the second flocculation chamber, and a device for adjusting the impeller speed and opening degree shall be provided.
9.4.30 Whether a mechanical sludge scraper is installed in a mechanical agitation clarifier shall be determined according to factors such as tank diameter, bottom slope, influent suspended matter content and particle composition.
(8) Hydraulic Circulation Clarifier
9.4.31 Hydraulic circulation clarifiers are suitable for raw water with turbidity lower than 2000 NTU for a long time, and the production capacity of a single tank is generally not more than 7500m³/d.
9.4.32 The upward flow velocity of the clear water zone of hydraulic circulation clarifiers shall be determined according to operation experience under similar conditions, generally 0.7~0.9mm/s.
9.4.33 The effective height of the draft tube (second flocculation chamber) of hydraulic circulation clarifiers is generally 3~4m.
9.4.34 The reflux water volume of hydraulic circulation clarifiers can be 2~4 times the influent flow.
9.4.35 The included angle between the inclined wall of the hydraulic circulation clarifier and the horizontal plane shall not be less than 45°
(9) Pulse Clarifier
9.4.36 Pulse clarifiers are suitable for raw water with turbidity lower than 3000 NTU for a long time.
9.4.37 The upward flow velocity of the clear water zone of pulse clarifiers shall be determined according to operation experience under similar conditions, generally 0.7~0.9mm/s.
9.4.38 The pulse period can be 30~40s, and the filling-discharging time ratio is 3:1~4:1.
9.4.39 The height of the suspension layer and the clear water zone of the pulse clarifier can be 1.5~2.0m respectively.
9.4.40 Perforated pipe water distribution shall be adopted for pulse clarifiers, and herringbone flow stabilizing plates shall be installed on the top.
9.4.41 An exhaust device shall be installed on the main water distribution pipe of the siphon pulse clarifier.
(10) Flotation Tank
9.4.42 Flotation tanks are generally suitable for raw water with turbidity less than 100 NTU and low-density suspended substances such as algae.
9.4.43 The upward flow velocity of the contact chamber is generally 10~20 mm/s, and the downward flow velocity of the separation chamber is generally 1.5~2.0 mm/s.
9.4.44 The single-cell width of the flotation tank shall not exceed 10m; the tank length shall not exceed 15m; the effective water depth is generally 2.0~2.5m.
9.4.45 The pressure and reflux ratio of the dissolved air tank shall be determined according to the raw water flotation test or reference to the operation experience under similar conditions. The dissolved air pressure is generally 0.2~0.4MPa; the reflux ratio is generally 5%~10%.
The model and number of dissolved air releasers shall be determined according to the outlet flow and action range of a single releaser under the selected pressure.
9.4.46 The total height of the pressure dissolved air tank is generally 3.0m, the tank shall be filled with filler with a height of 1.0~1.5m, and the cross-sectional hydraulic load of the tank can be 100~150m³/h·m².
9.4.47 A slag scraper shall be adopted for sludge discharge in flotation tanks. The traveling speed of the slag scraper shall generally not be greater than 5m/min.
9.5 Filtration
(1) General Provisions
9.5.1 Filter materials shall have sufficient mechanical strength and corrosion resistance, generally quartz sand, anthracite, heavy ore, etc. can be used.
9.5.2 The selection of filter type shall be determined through technical and economic comparison according to factors such as designed production capacity, operation and management requirements, effluent quality and elevation layout of water purification structures, combined with plant site terrain conditions.
9.5.3 The number of filter compartments shall be determined through technical and economic comparison according to filter type, production scale, operation, maintenance and overhaul conditions, generally not less than 4 compartments except for valveless filters and siphon filters.
9.5.4 The single-cell area of the filter shall be determined through technical and economic comparison according to filter type, production scale, operation, uniformity of filtered water collection and flushing water distribution.
9.5.5 The ratio of filter material thickness (L) to effective particle size (d10) (L/d10 value): greater than 1000 for fine sand and double-layer filter material filtration; greater than 1250 for coarse sand and three-layer filter material filtration.
9.5.6 Filters shall be equipped with initial filtered water discharge facilities except for filters where initial filtered water discharge facilities cannot be installed due to filter structure and operation.
(2) Filtration Rate and Filter Material Composition
9.5.7 Filters shall be designed according to the filtration rate under normal conditions and checked according to the forced filtration rate under maintenance conditions.
Note: Normal condition means all filters of the water plant are in operation; maintenance condition means one or two compartments of all filters are out of service for maintenance, flushing or sand turning.
9.5.8 The selection of filter rate and filter material composition shall be determined through tests or reference to the operation experience of existing filters under similar conditions according to factors such as influent water quality, filtered water quality requirements and filter structure. Generally, it shall be adopted in accordance with Table 9.5.8.
Table 9.5.8 Filter Rate and Filter Material Composition of Filters
Filter Material Type Filter Material Composition Normal Filtration Rate (m/h) Forced Filtration Rate (m/h)
Particle Size
(mm) Uniformity Coefficient (k80) Thickness
(mm)
Single-layer Fine Sand
Filter Material Quartz Sand
d10＝0.55 <2.0 700 7～9 9～12
Double-layer
Filter Material Anthracite
d10＝0.85 <2.0 300～400 9～12 12～16
Quartz Sand
d10＝0.55 <2.0 400
Three-layer
Filter Material Anthracite
d10＝0.90 <1.7 450 16～18 20～24
Quartz Sand
d10＝0.5 <1.5 250
Heavy Ore
d10＝0.25 <1.7 70
Uniform Graded
Coarse Sand Filter Material Quartz Sand
d10=0.9～1.2 <1.4 1200～
1500 8～10 10～12
Note: The relative density of filter materials is: quartz sand 2.6～2.65; anthracite 1.4～1.6; heavy ore 4.7～5.0.
9.5.9 When a large-resistance water distribution system is adopted for the filter, the supporting layer shall be adopted in accordance with Table 9.5.9.
Table 9.5.9 Supporting Layer Material, Particle Size and Thickness of Fine Sand Graded Filter Material Filter
Layer (From Top to Bottom) Material Particle Size (mm) Thickness (mm)
1 Gravel 2～4 100
2 Gravel 4～8 100
3 Gravel 8～16 100
4 Gravel 16～32 The top of this layer shall be 100mm higher than the holes of the water distribution system
9.5.10 The supporting layer of three-layer filter material filter shall be adopted in accordance with Table 9.5.10.
Table 9.5.10 Supporting Layer Material, Particle Size and Thickness of Three-layer Filter Material Filter
Layer (From Top to Bottom) Material Particle Size (mm) Thickness (mm)
1 Heavy Ore 0.5～1 50
2 Heavy Ore 1～2 50
3 Heavy Ore 2～4 50
4 Heavy Ore 4～8 50
5 Gravel 8～16 100
6 Gravel 16～32 The top of this layer shall be 100mm higher than the holes of the water distribution system
Note: Layer 6 can be omitted if the water distribution system uses filter bricks with a pore size ≤ 4mm.
9.5.11 When a long-handled head water (air) distribution system is adopted, the supporting layer can be coarse sand with a particle size of 2～4mm and a thickness of 50mm～100mm.
(3) Water and Air Distribution System
9.5.12 The water and air distribution system of the filter shall be selected according to factors such as filter type, flushing method, single-cell area, and uniformity of air and water distribution. For single-water flushing, water distribution systems such as perforated pipes, filter bricks and filter heads can be selected; for air-water flushing, water and air distribution systems such as long-handled filter heads, plastic filter bricks and perforated pipes can be selected.
9.5.13 The ratio of the total hole area of the large-resistance perforated pipe water distribution system to the filter area is 0.20%～0.28%; the ratio of the total hole area of the medium-resistance filter brick water distribution system to the filter area is 0.6%～0.8%; the ratio of the total gap area of the small-resistance filter head water distribution system to the filter area is 1.25%～2.00%.
9.5.14 The large-resistance water distribution system shall be designed according to the flushing flow rate and determined through calculation based on the following data:
1 The flow velocity at the inlet of the water distribution main pipe (channel) is 1.0～1.5m/s;
2 The flow velocity at the inlet of the water distribution branch pipe is 1.5～2.0m/s;
3 The outlet flow velocity of the holes of the water distribution branch pipe is 5～6m/s.
An exhaust pipe shall be installed on the top of the main pipe (channel), and the exhaust outlet shall be above the water surface of the filter.
9.5.15 The long-handled filter head air and water distribution system shall be designed according to the flushing air volume and water volume, and shall be determined through calculation based on the following data:
1 The flow velocity at the inlet end of the air distribution main pipe is 10～15m/s;
2 The outlet flow velocity of the air distribution holes is about 10m/s;
3 The flow velocity at the inlet end of the water distribution main pipe is about 1.5m/s.
4 The outlet flow velocity of the water distribution holes is 1～1.5m/s.
An exhaust pipe shall be installed on the top of the main pipe (channel), and the exhaust outlet shall be above the water level of the filter.
(4) Flushing
9.5.16 The selection of filter flushing method shall be determined through tests or reference to the experience of existing filters under similar conditions according to the filter material layer composition and type of air and water distribution system. Generally, it shall be selected in accordance with Table 9.5.16.
Table 9.5.16 Flushing Method and Procedure
Filter Material Composition Flushing Method and Procedure
Single-layer Fine Sand Graded Filter Material (1) Water Flushing
(2) Air Flushing－Water Flushing
Single-layer Coarse Sand Uniform Graded Filter Material Air Flushing－Air-water Simultaneous Flushing—Water Flushing
Double-layer Anthracite-sand Graded Filter Material (1) Water Flushing
(2) Air Flushing－Water Flushing
Three-layer Anthracite-sand-heavy Ore Graded Filter Material Water Flushing
9.5.17 The flushing intensity and flushing time of single-water flushing filters shall be adopted in accordance with Table 9.5.17.
When a surface flushing device is added, the surface flushing intensity is preferably 2～3L/m²·s (fixed type) or 0.50～0.75L/m²·s (rotary type), and the flushing time is 4～6min.
Table 9.5.17 Water Flushing Intensity and Flushing Time (Water Temperature 20℃)
Filter Material Type Flushing Intensity (l/m²·s) Expansion Rate (%) Flushing Time (min)
Single-layer Fine Sand Graded Filter Material 12～15 45 7～5
Double-layer Anthracite-sand Graded Filter Material 13～16 50 8～6
Three-layer Anthracite-sand-heavy Ore Graded Filter Material 16～17 55 7～5
Note:
