Posted by William Vander Plaats on Thu, May 27, 2010 @ 01:59 PM
Selecting the best tool and technique for a job is often a simple task. No one wants to waste time or be forced to "fix" something once it's been fixed already. As carpenters like to say "Measure twice, cut once." We always want to DO IT RIGHT THE FIRST TIME.
However, when it comes to filtering solids from water or other liquids, the wisest and most efficient method is sometimes a 2-step approach in which:
- Large particles or debris are removed in the "first pass" by one method or tool (the pre-filtration step)
- Smaller particles are removed in a "2nd pass"
For small particle removal (Step 2), some type of barrier filter that catches tiny particles is often the tool of choice, such as bag filters and 
cartridge filters. But barrier filters get clogged and fill quickly when too many large particles are present -- thus the need for a pre-filter.
The pre-filter will:
(a) extend the life of the barrier filter, reducing usage and overall costs of purchasing filters and
(b) minimize labor time spent changing bags and cartridges
This is true in a wide variety of situations and applications, from turf and agriculture, to industry and more.
Example: Source Water From A River
Take, for example, a situation in which river water or sea water is used as the main source for an industrial water process. Quite often, debris of many shapes and sizes are present in the water, and no single type of filtration would remove large AND small solids very efficiently. Using a two or three-step process by taking out the larger debris and solids first with a low-maintenance solution is often used. This Step 1 can be some sort of large strainer or screen to keep large debris from entering the system, followed in Step 2 by a centrifugal separator to remove the other solids. If additional particle removal is required, some type of barrier filter could be used.
Agricultural and Irrigation Uses
There are similar applications in agricultural settings. Centrifugal separators can be used as pre-filtration to a drip irrigation system. In this situation the primary filtration concern is to keep the drip emitters from getting plugged, and sand media tank filters are most commonly used. However, a pre-filter can be used before the source water reaches the media tanks, reducing the number of backwash cycles the tanks must go through to keep operating efficiently. This backwash reduction system saves energy, water, and equipment.
Getting Tired Changing Barrier Filters?
Sometimes the original design of a filtration system consists solely of barrier filters, but maintenance staff grow tired of the constant bag and cartridge changes. When barrier filters are not changed as needed, pressure drops and other negative effects occur, and the system no longer operates within design parameters. Adding a centrifugal separator upstream of the bag filters is a perfect solution.
Want To Read More Details? Download Our Pre-Filtration Case Studies: Read a couple of detailed Case Studies that explain pre-filtration in practice. Read how pre-filtration in a sugar mill reduced water waste and improved process efficiencies. And read how pre-filtration of river water prevents clogging of barrier filters at an oil production plant.
Posted by William Vander Plaats on Thu, Nov 05, 2009 @ 11:00 AM
Sand media filters may look the same, but they don't all operate the same. Some claim to use a "magical media" sand of one sort or another that has some mystical power that lends itself to finer filtration. But under a microscope, those particles appear no different than many others.
Independent testing by 3rd party agencies, however, has determined there is ONE measurable trait of a sand media filter which actually DOES make one filter more efficient than the next. That trait is "available surface area" of the underdrain. That is, the total area in square inches and mm that are available to take the flush of the filtered liquid as it passes through the sand/media/gravel pack. 
It makes sense when you stop and think about it. Water passes through the media tank, carrying debris which are caught in the media/sand as the water migrates towards the holes in the underdrain and out of the tank. What if there are only a FEW holes, spaced closely together? The debris "bumps" into other debris, as does the water, and the debris starts to pile up in certain places. The media sand does too. This leads to the "channeling effect" as the once level and flat bed of sand/media is turned into one that is uneven due to uneven flow through the underdrain. Watch this animation showing exactly how channeling takes place and how water flows through different styles of underdrains.
Now think of the opposite situation. Water passing through a sand filter carrying debris and leaving through a LARGE number of holes in an expansive underdrain, where the holes are further apart. The water is more evenly dispersed as it passes through the sand/media and underdrain, and no channeling takes place. The bed of sand remains relatively level and uniform because the water has more holes on the greater surface area of the underdrain to pass through the entire media filter.
Why Should You Care?
The reasons you should care about the efficiency of the sand media filter are:
- Greater efficiency and no "channeling" means fewer backwash cycles are required, which reduces wear and tear on ALL components of the system. This translates to longer equipment life.
- Water Savings.
- Lower pressure loss through the sand filter and reduced energy usage.
Don't forget to read the Independent 3rd Party testing results comparing the underdrains of some of the largest media tank filters manufacturers. See link below:
Comparison Performance Testing Results of Various Sand Media Filters