FOR MUNICIPAL WATER authorities, prefiltration of water is critical to protect and extend the life of membrane systems, as well as increase intervals between backwashing or necessary maintenance.
Pressure-driven membrane systems utilize thin, porous sheets of material to separate contaminants from water. The technology, in the form of microfiltration, ultrafiltration, reverse osmosis filtration, and nanofiltration membranes, provides superior water quality with less waste than technologies like sand filtration. Because of this, it is commonly used for potable water production and advanced wastewater treatment, including “purple pipe” reclaimed and recycled wastewater applications.
However, without adequate prefiltering of the water entering the membranes, such systems can be susceptible to irreparable damage from large particles. In addition, water that is not adequately prefiltered can cause excessive membrane fouling, leading to sub-par water production as well as costly, premature replacement and unscheduled production downtime.
Fortunately, a growing number of municipalities are ensuring superior water quality, as well as membrane longevity and reliable production, utilizing low maintenance, multi-element, self-cleaning, pretreatment filters.
OPTIMIZING MEMBRANE LONGEVITY AND PRODUCTION
When a municipal water utility plant in the southeastern United States needed to protect its membrane filters, a new prefiltering system was required.
According to an instrumentation technician who installs, programs, configures, and troubleshoots monitoring devices in the drinking water facility, as soon as the water is pulled from the wells it goes through a strainer. From there, it goes to other parts of the plant for further treatment and cleaning, with membrane filtration utilized at the end of the process.
“The well water has to be strained,” the technician said. “If the sand got through, it would destroy the membrane. The membrane is sensitive to large particles and expensive [to replace].”
Although the facility has utilized a sand strainer, the instrumentation technician felt it was too small for the volume of the plant, which produces millions of gallons of drinking water per day.
In response, the plant turned to a multi-element, self-cleaning pretreatment filter from R. P. Adams. The company first introduced and patented the technology in the 1960s and has over 10,000 installations worldwide today.
The design provides an alternative to sand filters, centrifugal separators, and basket-type strainers. Unlike those designs, which have limitations in particle size filtration and also require frequent maintenance, the multi-element, self-cleaning pretreatment filter can provide continuous removal of suspended solids. When utilized as the first line of defense for pre-membrane water filtration, it typically filters out sand, silt, and other suspended solids from 500 to 75 microns.
Another significant feature of the multi-element design is in the engineering of the backwash mechanism. With traditional strainers, the backwash mechanism comes into direct contact with the straining media. This can be problematic, as large, suspended solids often encountered with raw water can become lodged between the straining media and the backwash arm. The result is straining media damage and/or rupture that can compromise membranes and other downstream equipment, hindering plant operation.
Instead, the multi-element design utilizes a tube sheet to separate the straining media from the backwash mechanism. This prevents the backwash mechanism from coming into contact with the media and causing damage to the membrane elements.
According to the instrumentation technician, the pre-filter has helped to prolong the usable life of the drinking water facility’s main membrane.
“With pre-filtering, our main membrane functions well for about seven years before it needs to be replaced,” he said. “Without pre-filtering, it would need to be replaced much sooner.”
Municipal water plants also need to consider how to best reduce membrane fouling and required maintenance. Traditional strainers, however, due to limitations in straining area, can become clogged quickly. When that occurs, cleaning, media replacement, or backwashing is necessary, which adversely affects productivity as well as maintenance costs.
In this regard, the multi-element design provides three to four times the surface area of traditional strainers and pre-filters. This translates directly into less frequent backwashing so less water goes to waste, less power is consumed, and less maintenance is required.
“We have had no real issues since we bought the pre-filter,” said the technician.
When considering pre-filter technology for municipal potable water or wastewater systems, automatic multi-element, self-cleaning, pretreatment filters are an increasingly popular choice and a reliable, cost-effective solution.
