During the last decade,microfiltration and ultrafiltration (MF/UF) membranes have become widely acceptedas a viable alternative to conventional drinking water treatment technologiessuch as coagulation, settling, and sand filtration. Over the past few years,continuous advances in cost and reliability, coupled with an increasing concernabout pathogens (such as Cryptosporidium), have made MF/UF membranes thepreferred alternative to conventional technology for new plants. In 1991, afterreviewing a number of approaches, ZENON Environmental Inc. of Oakville, Ontariodetermined that immersed membranes will ultimately be the most cost effective andreliable technology for drinking water treatment. Immersed membranes differ frompressurized membranes in that pressure vessels are not used. Instead, shell-lesshollow fibers are immersed into a tank open to the atmosphere and a gentlesuction is applied to draw clean water through the fibers. Through the use oftanks open to the atmosphere, ZENON's immersed membrane technology conform to thenormal manner that water plants are operated while saving the cost of pressurevessels. Furthermore, through the use of low trans-membrane pressures (suction)ZENON was able to minimize fouling. As a result, by the late 1990s, ZENON'simmersed membranes had become the leading membrane technology in North America.Ever since 1991, ZENON has continually evolved new generations of backwards-retrofitable immersed membranes. The first four generations are referred to asZeeWeed(R) 150 and ZeeWeed(R) 500 a, b, and c. These membranes have been designedto handle water of almost any level of turbidity as well as very high solidsapplications such as direct coagulant addition or sand filter backwash treatment.ZENON is now introducing its fifth generation immersed membrane technology, theZeeWeed(R) 1000 that has been optimized for lower solids applications. Ourexperience led us to develop a membrane that is suited for upgrading existingplants for microbial safety, improved treated water quality, and increasedcapacity. This paper will present a general assessment of factors that drive thecost of ownership of a membrane plant, key design and operational characteristicsof ZeeWeed(R) 1000 membrane system, and its particular applicability to retrofitexisting filter beds. Includes tables, figures.
