Are Media Filters Backwashed Too Frequently?
Principles of Media Filtration
Sand, dual media or multi-media filters are common stand-alone or pretreatment process steps – especially for high flow rate, membrane based systems. Often the first process step in an industrial water system, they are used to remove suspended solids such as corrosion byproducts, silt and colloidal material from the incoming feed water.
A simple mechanical separation process, most media filters act as a physical separator to remove particles – down to approximately the 10-20 micron range. Specialty filters are available for side stream cooling tower filtration, boiler make-up and point-of entry applications that reduce the nominal particles to even lower size ranges. Coagulating agents may also be added to the feed water to help improve the efficiency of the filtration process. For industrial water systems, removal of suspended solids will reduce the potential for plugging, fouling, or scaling downstream unit operations such as reverse osmosis (#RO) membranes.
For industrial water systems, removal of suspended solids will reduce the potential for plugging, fouling, or scaling downstream unit operations such as reverse osmosis (#RO) membranes.
Media filters are packed columns where water is introduced through the top of the vessel and flows down through the filtration media, exiting the vessel at the bottom or through a downtube collector. As the suspended solids accumulate in the media bed, the pathways for water flow become more tortuous and the pressure drop across the bed increases. To remove the entrapped particles, the direction of flow is reversed. For simplification of the process and controls, most filters are equipped with integral controllers which backwash the units based on time or water throughput. The flow rate required for backwash often exceeds the service flow rate to lift the media and achieve effective removal of the suspended solids.
The flow rate required for backwash often exceeds the service flow rate to lift the media and achieve effective removal of the suspended solids.
Backwash Frequency
As the feed water quality is unique for every system, there is no definitive calculation to predict the frequency of backwash that will be required. Commonly, media controllers are factory set to backwash on a certain day of the week or after an arbitrary volume of water has been treated. While the performance of a media filter can be monitored by sampling the inlet and outlet for suspended solids or turbidity, backwash should ideally initiated when the differential pressure across the filter exceeds a specified value. A 10 psig pressure drop is often used as a general design guideline.
By using differential pressure to trigger the backwash step, time between backwashes is often extended. We have observed media filters in the field that were pre-programmed to backwash weekly – and this has extended the time between backwashes to months apart. For larger systems, the water savings can be extraordinary.
By using differential pressure to trigger the backwash step, time between backwashes is often extended.
Other Considerations
As suspended material is entrapped in the bed, the filtration efficiency may actually improve. This can be a result of less void volume throughout the bed resulting in the filtration of smaller particles, or interparticle attraction between the free flowing and trapped colloidal material. This would lead to less maintenance and cleaning of downstream components.
Another reason to backwash media filters as infrequently as possible is that the backwash step may decrease the lifetime of the media. When a backwash occurs, media may be projected into the upper bed distributors or the upper head of the column. This can cause the media to fracture into smaller, lighter particles that could be flushed out through the backwash line, or consequently increase the pressure drop across the bed.
Recommendations
The additional instrumentation controls – or effort to provide for media backwash initiation based on differential pressures – can lead to significant water savings. Simultaneously, the filtration efficiency of your industrial water treatment system can be improved. Systems can easily be retrofitted to take advantage of this operational improvement. This is especially true for those fed from a potable water source which is likely quite low in suspended solids. Some cities, towns, or municipalities may even have qualifying credits or incentive programs for water conservation efforts like these.