Short-Term Gain, Long-Term Pain

November 13, 2012

Considerations for properly configuring a GAC filter system

gac filter system_residential water treatment

Granular activated carbon (GAC) is the great equalizer, providing superb filtration for a wide range of end-user concerns. It is a versatile, high-capacity powerhouse medium that can reduce and capture a multitude of contaminants. Its massive surface area and physical structure allow for contaminant reduction, primarily via adsorption, catalytic reduction and physical filtration.

GAC will in some way, shape or form improve just about any potable municipal or well water supply. It works so well in so many applications that it takes relatively little effort to produce a stellar performance. While the information herein applies to many types of filter media, it will focus on the ever-popular and successful GAC.  Following are some situations and configurations to consider.

Backwashing

The proper rate, frequency and duration of backwash are determined based on many related factors. The fully wetted density of the GAC, desired bed expansion, available freeboard (typically 50%, which means the empty side shell space is 50% of the media volume), influent water temperature and application-specific operating parameters must be considered.

The proper length of the backwash cycle generally, but not exclusively, is 10 to 15 minutes. Be sure to use a gravel base in conjunction with a good distributor screen or a special lower distributor plate, which will provide the best backwash performance.

The primary functions of a point-of-entry (POE) filter backwash cycle are to minimize particulate and precipitate accumulation at or near the top of the media bed; to provide consistent mixing and subsequent redistribution of the media; and to dispose of media fines that would eventually clog the filter. Proper backwashing helps maintain original flow rate capabilities, optimize contaminant reduction and facilitate higher capacity potential.
 
System Sizing

POE filters need to be sized based on site- and application-specific operating conditions. For GAC, it is all about contact time, or, technically, empty bed contact time. Water needs to maintain intimate contact with GAC for a specific amount of time to provide the desired results. This contact time can vary greatly depending on influent conditions, required effluent results and the contaminants being targeted.

For residential applications, the service flow rate typically is sized as 1 to 6 gal per minute (gpm) peak per cubic foot of GAC. It is better to err on the side of more contact time, as decreasing the contact time exacerbates capacity loss at a higher rate than the actual water use would suggest. Thus, if you know your contaminants, size accordingly and backwash appropriately, it will be difficult to fail.

Contact Time

It is popular to install POE GAC filters, generally under the guise of saving water, with a non-backwashing inlet-outlet head operating in an upflow or downflow service mode. This setup will reduce the initial system cost significantly—providing short-term gain—by substituting a simple distributor head in lieu of a backwashing controller. The water normally used to backwash the filter is “saved,” as the system is not designed for, nor capable of, an automatic backwash cycle.

While there may be a few applications in which a non-backwashing configuration can be effective, it is not a productive design for most common filter applications. Downflow systems that are consistently manually backwashed, portable exchange filters, anomalous applications and specially designed systems are potential exceptions.  

Upflow service non-backwashing filters present an array of challenges. In theory, the continuous upward flow through the media tank will keep the media constantly “backwashed” and clean. This can work, provided the upward flow rate through the media is high enough and long enough to mimic a standard backwash cycle.

A 10-in.-diameter GAC filter would require about 5 gpm upward flow for about 10 minutes to effectuate a solid backwash. For example, consider a 10-by-54-in. mineral tank with 1.4 cu ft of GAC in an upflow, non-backwashing configuration. Even if the service flow adequately satisfies the backwashing needs, there are several other potential problems, because the performance of GAC is highly dependent on contact time.

Water passing upflow through an expanding, partially fluidized GAC bed will inherently have less contact time, as the water will partially pass around the carbon. This upflow service will tend to lift the smallest and lightest GAC particles to the top of the bed and out to the service flow. A typical top screen will not prevent this, and specialty felt-style top screens may prevent the fines from exiting, but will be susceptible to clogging. You could try installing an oversized mineral tank with at least 100% freeboard, an effluent flow controller to limit service flow and an outlet sediment filter to capture exiting fines, but none of these can fix the problem of the lost contact time in the partially fluidized media bed.  

Because upflow does not provide the best results, you might consider a downflow, non-backwashing system. The downflow service contact time as the water passes through the well-compacted media bed is far better than the upflow version; however, there are many obstacles to consider. Water will follow the path of least resistance inside the mineral tank. If the GAC media is never mixed or redistributed, the water will find the preferred route, or “channel,” and will continue to follow the same flow paths through the media bed.

Channeling negatively impacts the contact time and greatly reduces the overall capacity. The downflow water direction will tend to deposit particulates on the top of the GAC bed. As these particles accumulate, the water path will be further restricted, and channeling will be accelerated and severe. Maintaining a pre-sediment filter may slow the channeling process, but will not prevent the poor flow characteristics that ultimately will lead to less-than-optimal filtration performance and reduced capacity.

Pay close attention to the key filtration factors surrounding the backwash performance, system sizing and contact time. Like running through a sprinkler leaves you mostly dry but swimming through a lake soaks you to the bone, blasting water through GAC results in unintended remaining contaminants, but proper contact time will deliver the expected quality. Shun the short-term gain of lower-cost equipment, and instead take the route that leads to peace of mind, satisfied customers and long-term gain.

Jerry Horner, CWS-VI, CI, is regional sales manager for Watts Water Quality. Horner can be reached at hornerjg@watts.com or 800.659.8400.

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