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Pre-treatment can optimize membrane filtration

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Pre-filter at plant
Installing a pre-filter helped extend the usable life of plant’s main membrane.

By Del Williams

Pre-filtration of water is critical to protect and extend the life of membrane systems, as well as to increase intervals between backwashing or necessary maintenance in municipal water treatment plants.

Pressure-driven membrane systems utilize thin, porous sheets of material to separate contaminants from water. This includes microfiltration, ultrafiltration, reverse osmosis filtration, and nanofiltration membranes. They are commonly used for potable water production and advanced wastewater treatment, including “purple pipe” reclaimed and recycled wastewater applications.

However, without adequate pre-filtering of the water entering the membranes, such systems can be susceptible to irreparable damage from large particles. In addition, water that is not adequately pre-filtered can cause excessive membrane fouling, leading to sub-par water production, as well as costly, premature replacement and unscheduled production downtime. As such, a growing number of municipalities are now utilizing low maintenance, multi-element, self-cleaning, pre-treatment filters.

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When one municipal water utility found that it needed to protect its membrane filters, a pre-filtering system was required. The plant’s well water had to be strained, to prevent any sand from getting through to the expensive membrane filters and damaging them.

In order to improve capacity, plant operators looked at a multi-element, self-cleaning pre-treatment filter from R. P. Adams. Its 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 pre-treatment 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 damaging the elements, caused by large solids becoming lodged between the media and the backwash arm.

The pre-filter has helped to prolong the usable life of the drinking water facility’s main membrane. With pre-filtering, the main membrane functioned well for about seven years before it needed to be replaced. Before pre-filtering, it had 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.

Del Williams is a technical writer for R.P. Adams. This article appears in ES&E Magazine’s December 2019 issue.

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