Biological filtration of poor quality brackish water reducing Reverse Osmosis membrane fouling

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Abstract

A primary limitation in applying Reverse Osmosis (RO) is loss of performance caused by membrane fouling resulting from compounds that bacteria can use as energy or nutrient sources. When these compounds enter RO membranes, they are capable of sustaining extensive bacterial growths on the membrane sheets as well as within feed channel spacers. This leads to the entrapment of particles and incorporation of dissolved substances into biological matrices exacerbating the biological fouling. Treating the water with oxidizing chemicals such as chlorine, ozone and potassium permanganate, increase the quantities of compounds that bacteria can use thereby increasing the membrane fouling potential. Also, traces of oxidants reaching the membranes can cause membrane damage decreasing the capability to reject both dissolved and particulate material (including viruses). It is also disconcerting that some organic compounds with relatively high molecular weights are able to pass through even tight RO membranes causing microbial growth in distribution systems. Therefore, even when using RO membranes, quality distributed water cannot be assured. If no corrective action is taken microbes can grow in the RO membranes as well as in the distribution lines.

The solution to this dilemma is to grow microbes in biological filters ahead of the RO membranes. If a high quality filtration material, such as Filtralite expanded clay, is used for microbial attachment, it is possible to effectively remove both microbial energy and nutrient compounds even at low temperatures (6°C). Pilot and full-scale plant experiences from the Canadian prairies using biological filtration have advanced these treatment processes from experimental to proven technologies and are currently being evaluated as potentially becoming “best available technology” in the treatment of extremely poor quality brackish groundwater. The first Integrated Biological and RO Treatment Plant was commissioned in December 2003, and after two years of full-scale testing, two more plants were commissioned in December 2005. At one of these plants, conventional manganese greensand treatment was followed by RO treatment resulting in frequent chemical RO cleanings as well as membrane replacements every eight months. Removing the manganese greensand in the existing filters and replacing them with Filtralite® material resulted in a rapid improvement of treated water quality and a literal stop to frequent RO cleanings. The biological filters need to be backwashed 36 times less than the manganese greensand filters (100 filter backwashes per year vs. 3,600). Backwash water use decreased to 0.4 million L from 23 million L and backwash labor decreased to 40 hours from 1,440 hours per year. Combining these savings with decreased RO cleanings, no need for frequent membrane replacements, and decreased chemical costs, it has been estimated that this water treatment plant serving 1,200 people will save more than $100,000 per year.

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