Backwashing is the reversal of water flow through the media bed to cleanse the media. It is a critical maintenance task, to ensure the smooth operation of your filtration vessels and to maintain the quality of the filtered water being discharged from the filter.
During the filtration cycle, the filter media bed forms thousands of water canals which collect impurities and solid residues present within the water stream. As these canals fill up a pressure differential will occur. When the appropriate pressure differential point (Recommended 60 kpa/0.6 bar) has been reached between the inlet and outlet of your vessel, it means that the filter bed needs to be cleaned. In order to clean the filter bed, it is necessary to carry out a periodic backwash phase to return the filter to optimum working conditions.
In addition to pressure differential, there are a number of other parameters that determine when to backwash your media filters. These include:
- The flow rate through the filter decreases until it is insufficient to meet the demand.
- The removal efficiency of the filter bed decreases to the point where filter water quality deteriorates or results in dirt or debris passing through the system.
- When the inlet pressure is between 50 to 70 kpa (1 to 10 psi or 0.5 to 0.7 bar) higher than the start up pressure.
- If the differential pressure required to trigger a backwash is not achieved, a backwash should be performed at regular time intervals to prevent media solidifying or channelling.
- If the filter is connected to mains water, pressure rise is not an accurate indicator as mains pressure tends to fluctuate. It is best to rely on the actual flow rate or a predetermined time.
- For filters with Nozzle Plate, it is advised to backwash before the pressure difference between inlet and outlet connection is greater than 0.6 Bar and to avoid exceeding a pressure differential of 0.8 Bar. If a pressure differential of greater than 0.8 bar is passed between the two chambers this could result in the rupture of the nozzle plate.
During the backwash cycle, the water flow is reversed and increased so that the water enters the vessel through the lower outlet pipe, going through the lower distribution system at a high velocity. The reversal of flow causes the filter media bed to expand and fluidise, thus releasing trapped particles. These particles are then discharged to the top of the vessel, exiting out of the upper diffuser system along with the backwash water.
Filters that are configured with a nozzle plate floor or slotted lateral system for air scour, can be air scoured prior to backwash. During the air scour process, air is introduced into the filter in the opposite direction of the normal flow of water. The injected air agitates the filter media, helping reduce the bulk density of the media causing it to expand and break up any accumulated solids or debris. This agitation helps dislodge the trapped particles from the filter media, allowing them to be carried away by the backwash water. This saves water during backwashing as it reduces the length of time required to flush with water.
A rinse cycle is recommended immediately after the backwash phase. The objective of this process is to expel the remains of residues that could have penetrated the collectors during the backwash cycle and remain inside the vessel. This is performed by discharging the filtrate to waste directly after the backwash cycle and can only be performed on filters with valves that have connection between the filtered water and the drain.
The importance of backwashing cannot be overstated. Dense filter media can become “packed” without proper and frequent backwashing. Debris will remain trapped and create channelling within the filter bed. This will result in the filter bed exhausting early. If debris is not flushed from the media grains, the filter bed will become blocked and the filter operation will fail.