BACKWASH METHOD DESCRIPTION
When using a traditional “sand” filter media, a backwash rate of 42m3/hr/m2 is a common and fundamental requirement. Whilst different medias may have different backwash rates, it is essential that the required bed expansion be provided. Failure to do so can result in a cumulative build-up of solids that will detract from the filter’s long-term performance. This loss of performance can include unnecessary service problems, replacement of the filter media and possible damage to the filter’s underdrain.
The choice of backwash method is largely determined and dictated by the required filter rate and the number of filter vessels that are selected for the specific project.
For single tank systems (with filter areas of up to 3.5m2) single lever backwash valves are available as an optional item.
These valves are currently available in two standard sizes (dn100 and dn150). Single lever valves can also be automated with a pneumatic actuator and associated controls. The CGF single lever valve is a simple mechanism that is designed to obviate the need to operate a number of commercially available valves in a very a specific Four Tank MHS1500 filters with 100mm Multiport Valves and controlled sequence.
CGF single lever valves have special relevance when the services of a trained plant room operator are not necessary available, eg., as is often the case with a school pool.
For projects that require a two-tank system, a standard “tandem” installation is strongly recommended. In this instance, the filters are arranged in parallel to operated at a conservative rate of 21m3/hr/m2. This filter rate is established on the basis of being half of the required backwash rate (ie., 42m3/hr/m2.) Backwashing a “tandem system” is achieved by using the full duty of the pump through one individual filter module. When backwashing is undertaken, the other unit is isolated and is temporarily taken “off-line”.
Whilst deviations of the above “tandem system” include pump selections that are based on using the maximum filter rate of 42m3/hr/m2 this method requires very careful planning. Furthermore, it does not provide the same dirt holding capacity that is available with the “tandem” filter system that operates at a conservative filter rate of 21m3/hr/m2.
“Shunt Backwashing” has been developed to permit the use of conservative filter rates without compromising the need to obtain the specified/mandatory backwash rate. In many high load applications (such as indoor heated pools and many industrial processes), filter rates as low as 15 to 25 m3/hr/m2 are often technically required. Since CGF first introduced shunt backwashing to the commercial swimming pool market (in 1992) this backwash method has been widely used and specified by many leading Consultants.
Shunt back washing is applicable to installations that use three or more filter modules that are arranged in a filter bank. In practical terms, shunt backwashing should not be applied to any more than six filter modules in any one filter bank.
The use of shunt backwashing simplifies the application engineering that is often required when selecting a pump’s duty point to best suit the “filter” & the “backwash” requirements. Shunt backwashing also has the benefit that it uses clean filtered water for backwash purposes.
Compared to all other conventional methods of “reverse flow” backwashing, extensive field-testing has established that shunt backwashing is capable of maintaining filter beds in a far more permeable condition.
Maintaining a permeable filter bed is particularly relevant in high load applications where body oils and fats are an issue. It is also relevant where a high level of solids is expected; such as encountered with equine pools and aquaculture. In these applications, reverse flow backwashing is not recommended on the basis that underdrain systems can be very easily blocked with solids. Blockages of this type can result in premature failure of the filter’s underdrain, poor filter performance, and
high service/maintenance problems.
Typical Shunt Backwash with Stainless Steel Face Plumbing
Apart from simplifying the commissioning process, shunt backwashing also provides the opportunity to increase the backwash rate to resolve filter bed maintenance problems. With the shunt backwashing each filter is backwashed by operating three simple butterfly valves. Shunt backwashing can also be readily upgraded to include various levels of automation.
Apart from reducing the chance of Operator error, shunt backwashing provides a constant albeit reduced process flow during the backwash cycle. As minor as what this benefit may seem, it has a very considerable impact on the extent of automatic control loops that are required.
When using a shunt backwash system, the shunt backwash valve must be modified so that it cannot be
fully closed. Once the required backwash flow has been established (by volumetric analysis or other means), the notch plate setting on the shunt valve should be marked and recorded as being the setting required for normal backwash.
The maximum recirculation rate for a shunt backwash system should be considered to be no more than the total number of filter vessels, minus one, times the filter area of each filter module, times the maximum filter rate of 42m3/hr/m2. This requirement ensures that when one filter module is being backwashed the filter rate through the remaining “on-line” filters does not exceed the filter’s maximum filter rate of 42m3/hr/m2. The minimum recirculation rate for a shunt backwashing system must not be less than the required backwash flow for one filter (in m3/hr) times 1.2. This requirement obviates the need for the shunt backwashing valve to ever be fully closed.