Excessive backwashing time for multi-media filters can negatively impact water quality through three pathways: media loss, structural damage, and secondary contamination. Specifically:
Lightweight filter media loss
Prolonged, high-intensity backwashing (>15 minutes) will wash away the top layer of lightweight filter media such as anthracite, resulting in insufficient filter layer thickness, destruction of the "coarse on top and fine on bottom" gradient structure, short-circuiting of water flow, and an increase in effluent turbidity to 2-5 NTU, or even the penetration of suspended solids.
After the filter media is lost, it needs to be replenished with the same specification of filter media. However, mixing new and old filter media will further reduce the filtration stability and cause irreversible performance degradation.
Filter layer structure damage
Excessive rinsing can disrupt the stratification of the filter media, allowing large particles of impurities to enter the lower fine filter media zone, clogging the pores and causing fluctuations in the quality of the effluent (such as turbidity rising from 1 NTU to 3-5 NTU).
II. Indirect water quality decline: equipment performance degradation
Damage to water distribution system
Prolonged high-pressure backwashing (>0.2MPa) can cause excessive impact on the water distributor (filter cap, screen plate), leading to filter cap breakage, screen plate displacement, or cracks in the perforated pipe, uneven water flow distribution, and localized loss of filter media due to excessive scouring, resulting in deterioration of the effluent water quality.
Filter media caking and secondary pollution
If the backwashing time is too long but the intensity is insufficient (e.g., <10L/(m²・s)), impurities will embed deep into the filter media and form clumps. Even if the time is shortened later, it will be difficult to recover, and some of the aged filter media will need to be replaced (cost increases by 30%-50%).
