Multi medium filter for water treatment
Multi-media filters are the main force in water pretreatment and are widely used, especially in protecting subsequent precision treatment units. Their performance is highly dependent on reasonable filter media grading and thickness design, precise water distribution/collection system and optimized operation parameter control.
Operation parameters (determine actual filtration effect and efficiency)
Filtration flow rate:
Refers to the empty tank flow rate (Superficial Velocity), unit m/h.
Typical range: 8 – 12 m/h (most common), up to 15-20 m/h.
Too low flow rate: low equipment utilization, prone to surface blockage.
Too high flow rate: effluent water quality deteriorates (penetration), head loss increases rapidly, filter media is easy to caking, shortening the filtration cycle. It must be selected according to the raw water quality (turbidity, suspended solids content) and effluent requirements.
Influent water quality requirements (for the filter itself):
Turbidity: Generally, the influent turbidity is required to be < 20 NTU, and ideally < 10 NTU. Too high will lead to frequent backwashing.
Suspended solids: < 50 – 100 mg/L.
Oil and grease: should be as low as possible, which can easily cause the filter material to be calcified and difficult to clean.
Effluent water quality:
Target turbidity: usually 1 – 5 NTU (under good operating conditions, even lower to <1 NTU).
SDI15: When used as a reverse osmosis pretreatment, the core goal is to reduce the pollution index to SDI15 < 5 (preferably < 3) to meet the water inlet requirements of the RO membrane.
Operating pressure difference/head loss:
The initial pressure difference of the new filter material is very small (0.02-0.05MPa).
As the operation increases, the retained impurities increase and the pressure difference gradually increases.
Maximum allowable pressure difference/end pressure difference: usually set at 0.05 – 0.1 MPa (0.5 – 1.0 bar) or determined according to the system back pressure. Backwashing is required when this value is reached.
Filtration cycle:
The running time between two backwashes (hours).
Depends on the inlet water quality, filtration flow rate, filter layer thickness and dirt holding capacity. It can range from a few hours to dozens of hours. The design goal is to ensure a reasonable cycle (such as >8 hours or >24 hours) and avoid too frequent backwashing.
Backwashing parameters (the key to restore the filtration capacity of the filter media):
Backwashing method: Usually the combination of “water washing + air scrubbing” is the best. Water washing alone can also be used (with poor results).
Backwashing flow rate:
Water washing flow rate: 12 – 15 L/s·m² (equivalent to 43 – 54 m/h), ensuring that the expansion rate of the filter layer is 25% – 50%. Insufficient expansion rate will not be washed cleanly, and excessive expansion rate may cause material leakage.
Air scrubbing intensity: 50 – 60 m³/h·m² (or 10 – 20 L/s·m²), about 15-30 Nm³/h·m². Strong air flow agitation causes intense friction of filter particles, stripping off trapped impurities.
Backwash time:
Air scrubbing time: 3 – 5 minutes.
Water washing time: 5 – 15 minutes (air and water washing at the same time or water washing alone). Wait until the drainage is clear.
Backwash water source and water quality: Usually use filtered clean water (or raw water, but the effect is slightly worse), and the pressure must be sufficient (often a special backwash water pump is required).
Backwash sequence: Commonly: drainage water level reduction -> air scrubbing -> air and water washing at the same time -> water rinsing -> positive washing until qualified -> standby/put into operation.
Positive washing: After backwashing, rinse with inlet water according to the filtration direction until the outlet turbidity or SDI is qualified (usually a few minutes) before it can be put into normal operation.
