Liquid Filtration

It�s a common practice to use a filter of sand or other loose media to clean particulate from water in industrial applications down to residential pools.� Application usage can be varied including: drinking water, mine water, food and beverage, boilers, irrigation, wastewater, and so on.� These types of filters are good at removing suspended solids and particulates in the water.

Filters may be just sand filters with one layer of sand or multimedia filters with multiple layers of filter media.� Larger filters are open and the water flows through the media by gravity.� Process filters are pressurized vessels containing the filter media.� Multimedia filters usually contain at least three media layers: anthracite coal, sand, and gravel or garnet.� There may be several layers of gravel or sand with different densities.� ���

All filters will eventually become full of the particulate that they are removing from the process to the point that they are inefficient or not working properly.� Filters are either cleaned or replaced when this happens.� With sand and multimedia filters the units are cleaned when they become inefficient. If the units are not cleaned regularly the filter may become so clogged that it will stop the flow through the system.� If the process is consistent in filtration then a regularly scheduled cleaning can be performed based on filter run time.� Filter cleaning can be initiated manually such as with swimming pool filters.�

It is more common to perform an on-demand automatic cleaning of the filter based on actual filter status.� Dirty status of the filter can be determined by measuring the turbidity (cloudiness) of the out flowing water or by measuring the differential pressure across the filter.� Using filter differential pressure is more economical to turbidity.� As the filter becomes full of particulate the differential pressure across the filter will increase.� Cleaning of the filter is then initiated based on a set differential pressure according to the filter manufacturer.�

These types of filters are cleaned by backwashing the filter media.� Clean water is pumped in the reverse direction through the filter.� This cleaning cycle lifts the filter media, fluidizing it, making it expand allowing the filter media to loosen and release the particulate that the filter trapped.� The particulate is lighter than the filter media allowing it to float off and flow out of the filter.� The filter media is not lost out of the filter during the backwash process.� Clean water is run through the system until the filter is clean based on pressure drop, timing, or turbidity. �Efficient backwashing results in cleaner media and shorter backwash cycles using less clean water.� ������

Dwyer Instruments, Inc. offers a wet/wet differential pressure switch, the DX Series, that is ideal for differential pressure monitoring on sand and media filters.� The DX Series is connected to the filter to measure the differential pressure across the filter media.� An adjustable set point is set to the value at which the manufacturer feels is best to initiate the backwash cycle.� Weatherproof rated, NEMA 4X, the DX can be used for outdoor filter locations.� The DX is a robust and very economical switch for controlling automatic filter backwash.� �