Backwashing is an indispensable part of the operation of all granular media filters, consisting of fluidisation of the bed (the percentage at which the bed must be expanded depends on each granular medium). Incorrect execution or omission of backwashing is the most frequent cause of problems in granular media filters. Problems that can occur from improper backwashing are as follows:
- Sludge formation on the surface causing a reduction in the area available for filtration. This causes an increase in hydraulic load (surface velocity) and pressure drop.
- Leakage of particles carried by the effluent.
- As stated above, if the bed cannot be expanded during backwashing, or if it is not backwashed often enough (the granular medium can be cemented).
Figure 1. Johnson type element design
To achieve an even distribution of the backwash flow, the following is recommended:
- The opening of the holes must be small enough to have a controlled pressure drop. The pressure drop at the distributors during backwash varies between 0.1 and 3 m water column. This depends on the design of the valves and the speed of the backwash.
- The flow velocity in the distribution system pipe should be uniform throughout the entire cross-sectional area of the filter.
Figure 2. (a) Johnson type stainless steel nozzle, (b) Johnson type ABS hydrosphere, (c) Bajonet Pentair diffuser
Separation between each nozzle or hydrosphere:
By using individual nozzles or distributors, an even distribution of the liquid to be treated is generated inside the filter. This ensures that the filter media is not concentrated on one side or in the center of the tank, and that the bed remains as horizontal as possible.
A good arrangement and distribution of the nozzles is as follows:
Placement distance between centers of two nozzles: Optimal 6 inches and maximum 12 inches between each.
- (2016). ANSI/AWWA B100-16 Granular Filter Material. American Water Works Association.
- (1999). Water Quality & Treatment a Handbook of Community Water Supplies. McGraw-Hill.
- (2011). Operational control of coagulation and filtration processes. American Water Works Association.
- (2017). Pretreatment for reverse osmosis desalination. Voutchkov Nikolay.