DUAL MEDIA FILTER OPERATION
Models VHT & VDF are particularly suitable for dual media filtration, using a capping of Anthracite (or filter coal) on top of a conventional silica sand bed. Whilst the depth of each filter media may vary according to the application and the specific media details, Table 5 in DIN19643 nominates that both anthracite and the filter sand beds should be 600mm deep in order to provide a total bed depth of 1200mm. In cases where deeper filter beds are required, CGF’s taller range of Vertical Column Filters (VCF) are available for this specific purpose.

The potential benefits of dual media filtration include extended filter cycles and improved water quality, particularly when high-suspended solids may be encountered. Other benefits include a higher retention of solids and generally lower pressure losses.

WHY DUAL MEDIA?

In a mono-sand filter, the reverse flow of the backwash process can inevitably cause the smallest particles to be lifted to the top of the filter bed. This condition is known as “inverse stratification” and it is largely responsible for problems that are associated with the formation of mud balls and channeling. In order to minimize this effect best practice suggests very careful selection of the filter media and generally good filter management by the plant Operator.

Ideally, the ultimate stratification for a granular filter would place the coarsest, largest granules being placed at the top of the filter bed. This would provide a technically “coarse to fine” filter with larger matter being detained by the filter’s top layers. This arrangement would also tend to prevent the bottom (fine) layer from being prematurely loaded and blocked.

In essence, dual media filtration permits much finer materials to be used in the bottom of the filter bed than can be otherwise tolerated in a conventional mono-sand filter. In a mono-sand filter, only a comparatively small part of the total voids are used to store particulates. When high-suspended solids are encountered, the head loss increases quite rapidly. Under certain circumstances, this may result in unacceptably short filter cycles.

WHY COAL OR ANTHRACITE?

Anthracite is the ideal top layer in a dual media filter. Anthracite has a greater surface area and a greater dirt holding capacity than sand. More importantly, when correctly applied and operated, the Anthracite will remain at the top of the filter bed after backwashing has subsided.

During its service life, sand tends to lose its entrapment capacity as it is progressively eroded from sharp-edged and irregular granules to roundly smooth, uniformly sized, smaller granules. The hard-carbon nature of anthracite resists erosion and therefore is not subject to a loss of efficiency due to the upward migration of the smaller particles that can be produced in a mono-sand filter.

With dual media filtration, the upper layer of the media provides in depth filtration and large dirt storage capacity.
The finer sand in the bottom of the filter (with its low uniformity co-efficient) remains largely unpolluted and it is available to provide very effective filtration. By remaining relatively clean, the sand’s layer’s dependence upon backwashing is also reduced. Similarly, the chance of the bottom sand layer becoming biologically corrupt is greatly reduced.

CURRENT & FUTURE TRENDS

Despite the fact that the dual media concept was established in the early 1060’s, the use of a mono-sand filter remains the most reliable and convenient mainstay of the commercial swimming pool Industry. The use of dual media filtration is more commonly and correctly applied to the industrial water treatment Industry, where the nature of the dirt load is usually more fragile and sticky. Equally many processes in general water treatment require single pass performance, whereas the swimming pool Industry is largely founded upon the Law of Consecutive Dilution with filtration being provided by multiple passes.

Dual media filtration does require application engineering and proper selection of appropriate filter medias to suit specific applications. Backwashing rates, bed expansion, and freeboard height needs to be carefully established in relation to the media’s effective size and its uniformity of co-efficient. Operational care is also required to prevent hydraulic surges that could cause bed disturbance.

The effectiveness of dual media filtration (particularly for the water treatment industries) was clearly the catalyst for the further development of the true “multi or mixed” media filtration. In this case, a layer of Garnet is provided at the bottom of the filter bed.

Recent industrial trends indicate the use of coarser media sizes and lower uniformity co-efficient in deeper beds operating with higher hydraulic loading rates. This approach has been developed to increase the amount of storage volume within the filter bed (per square meter of filter area) for generally better plant economics and performance.

FILTER MEDIA COMPARISIONS

MEDIA CLASSIFICATION – DEFINITIONS

“Effective Size” (es)

The size opening that will pass 10% (by weight) of a representative sample.
“Uniformity Coefficient” (UC) The ratio of the size opening that will just pass 60% (by weight) divided by the size opening that will just pass 10% (by weight) of the same sample

TYPICAL BED COMPOSITION

This Table demonstrates the “range” of typical dual media filters.