BACKWASH METHODSINTRODUCTION

Apart from providing the required amount of filter area, the system designer (& the installing contractor) has an obligation to ensure that the proposed filters are capable of being effectively backwashed. Regardless of which backwash method you may choose, it is important to get it right. Whilst system flaws may not be readily apparent upon the initial start-up, failure to achieve the correct backwash rates will affect the long-term performance of your filter system.

CGF’s technical literature nominates the required backwash flow for each filter Model. This “litre per second” flow is based on a required backwash rate of 42 m3/hr/m2. It is also extended to reflect the amount of washwater (in cubic metres) that is required for a standard four-minute backwash. This washwater consumption helps to establish the size and the capacity of the required detention tank, sludge lagoon, and or washwater recovery system.

The above backwash requirement is based upon the use of “silica filter sand” that has been produced in accordance with AWWA B100-89 Standard. CGF’s recommend grade of filter sand provides an effective grain size of 0.8mm to 2.0mm with a specific gravity of 2.65. Whilst other filter Medias may be used, backwash rates (& the required bed expansion) may vary according to the media’s specific gravity.

METHOD NO 1 – SHUNT BACKWASHING

The shunt backwash method uses clean filtered water for backwashing purposes. It is applicable to at least three filters arranged in parallel. Individual filters can provide up to 7.25m2 of effective filter area. This backwash method enables the use of conservative filter rates, without compromising the need for aggressive wash rates.

This backwash process is particularly relevant to applications that may have a high level of suspended solids. The process will help to maintain the permeability of your filter bed. Precise backwash rates can be readily obtained by regulating the shunt valve located in the filtered water outlet.

Since CGF first introduced hunt backwashing to the commercial pool market in 1992, this method has been widely used and specified by many of Australia’s leading pool professionals. 

METHOD NO 2 – MULTIPORT VALVE

For filters with up to 3.5m2 of filter area.
Available in two valve sizes (either 100mm or 150mm). Applicable to single filters with dedicated pump sets or filters within a filter bank. Single Lever valves; reduce the chance of operator error. The orientation of the valve ports may be manufactured to suit your specific installation requirements.

METHOD NO 3 – VALVE MATRIX

(Using Commercially Available Valves)

Applicable to all filter models, either as single filters or filters within a filter bank. The number of operating cycles will vary depending upon the use of either a four or five-valve matrix. Most applicable to systems that require air scouring

 

METHOD NO 4 – TANDEM SYSTEMS

Tandem systems refer to two filters arranged in parallel to “filter” at a rate of 21m3/hr/m2. With this arrangement, each filter is individually backwashed with the full pump duty to obtain a rate of 42m3/hr/m2.

Tandem systems can apply to all filters with filter areas up to 7.25m2. As shown in this case, the tandem arrangement is equipped with single lever backwash valves. Tandem arrangements can also use a valve matrix.

Tandem filter systems provide all of the benefits associated with a conservative filter rate ie., high dirt holding capacity & improved process efficiency.