To determine whether the filter media in a shallow sand filter needs replacement, the core indicators are filtration efficiency, the condition of the filter media itself, and operating resistance . A comprehensive judgment should be made by combining visual observations and data. Specific standards are as follows:
1. Decreased filtration efficiency (the most direct indicator)
The turbidity of the effluent continues to fail to meet the standard: The normal turbidity of the effluent should be ≤5 NTU. If the turbidity of the effluent is consistently >5 NTU after monitoring for 3-5 consecutive days, and there is still no improvement after ruling out factors such as sudden changes in the quality of the influent and incomplete backwashing.
The removal rate of suspended solids has been significantly reduced: the removal rate has dropped from the usual 80%-90% to below 70%, resulting in a significant increase in the frequency of clogging of subsequent water-using equipment (such as heat exchangers and drippers).
Deterioration in water clarity: If the water is visibly cloudy and contains suspended solids, or if impurities are deposited in the water used by the user, it indicates that the filter media has failed to retain water.
The filter media was severely broken and pulverized: After opening the manhole and taking out some filter media, it was found that the proportion of broken and pulverized particles exceeded 30% after screening. The excessive fine sand caused the filter layer pores to be blocked, and backwashing could not restore it.
Insufficient filter layer thickness: The normal filter layer thickness is 100-300mm. If the thickness is less than 100mm after measurement, and new filter media of the same specification is added, filter media loss still occurs frequently (a large amount of sand particles are discharged from the drain outlet).
Filter media caking or sticking: The filter media clumps together to form hard lumps, which cannot be fluidized and dispersed during backwashing, or a large amount of sticky organic matter and scale adheres to the surface, and cannot be restored to a loose state after chemical cleaning.
) III. Operating Resistance and Backwashing Anomalies (Data Support for Judgment)
The pressure difference cannot be reset after backwashing: The pressure difference between the inlet and outlet is 0.03-0.05MPa before backwashing, and the pressure difference is still higher than 0.02MPa after backwashing, indicating that the filter media is severely blocked and cannot remove impurities through backwashing.
The backwash cycle has been significantly shortened: the original backwash cycle was 8-24 hours, but it has been gradually shortened to less than 6 hours, and the backwash water consumption has continued to rise (exceeding 3% of the treated water volume), but it still cannot alleviate the blockage.
Poor backwashing and sewage discharge effect: The sewage outlet water is turbid for a long time during backwashing. Even if the backwashing time is extended and the backwashing intensity is increased, it is difficult to discharge impurities, indicating that the adsorption and retention capacity of the filter media has been saturated.
IV. Auxiliary Judgment in Special Scenarios
Water containing oil or chemicals: Oil film appears on the surface of the filter media, discoloration (such as turning black or yellow), accompanied by an odor, which cannot be removed even after chemical cleaning, affecting the quality of the effluent.
Long-term overload operation: The filter media has reached the service life of 3-5 years (the upper limit of normal lifespan). Even if the current filtration effect is still acceptable, it is recommended to replace it gradually to avoid sudden failure affecting the water supply.
