From Single-Source to Best- of-Breed
Tue, 10/21/2008 - 5:18am
Submerged membrane bioreactor (MBR) technology continues to experience rapid growth due to the two main advantages over conventional biological wastewater treatment systems: significantly improved effluent quality and a substantially smaller footprint. Throughout the past decade, MBR systems have been installed in thousands of municipal and industrial wastewater treatment plants of all sizes, and for a wide variety of challenging wastewater applications in paper mills, breweries, food processors, chemical plants and textile manufacturers.
The history of membrane bioreactors follows a typical pattern of technology evolution. The idea of coupling an activated sludge bioreactor with an external membrane separation system dates back to the mid-1960s. The concept of submerging membranes in the bioreactor was first conceived in the late 1980s and early 1990s when independent teams in Japan and North America experimented with different membrane designs, notably hollow fibers and flat sheet panels. A number of proprietary MBR systems became commercially available, and each vendor has refined its respective designs over the years. As a result, the industry as a whole has benefited from the independent and competing innovations of the various MBR manufacturers.
Existing MBR users seek to benefit from these technological advances, especially as their first generation product reaches the end of its useful life. Most find that to implement the latest technology in their plant, the investment in changes to infrastructure, piping and utilities are miniscule when compared to the dramatic savings in lifecycle costs that are attainable with the newer designs.
Second Generation MBR
In recent years, MBR membranes have evolved to the point that they require less maintenance and, at the same time, have a longer useful life. An added benefit comes from the more advanced designs that consume significantly less energy and provide lower operating costs.
The new PURON™ module from Koch Membrane Systems, Inc. is a second generation MBR membrane module that employs hollow fibers. Like several other second generation membranes, the high-strength fibers that comprise the PURON modules also overcome the fiber breakage problems typical of first generation systems that utilize non-braided fibers. Unlike the “double header” design of other hollow fiber MBR membrane that pot the hollow fibers at both ends, the PURON module uses a single header with hollow fibers potted at the base, allowing the tips of the fibers to float at the top with a seaweed-like action. This design eliminates the build-up of fibrous debris by allowing material to pass through the module, rather than collecting at the top. This results in the single-header design placing less mechanical stress on the fibers.
Also new in the PURON design is the introduction of air scouring at the center of the fiber bundle. Low pressure compressed air creates coarse bubbles that shake the membranes and effectively scour the entire length of the membrane fiber, enabling the air to remove accumulated debris from the membrane fibers within the bundle. The ability to supply compressed air in a cyclical pattern avoids sludging and reduces energy consumption.
Unlike most flat sheet membranes that do not accommodate backflushing, the PURON membrane resists fouling and maintains flux by introducing a small portion of the filtrate back through the fiber pores from the inside-out at timed intervals. PURON hollow fibers also look to provide higher membrane surface area and higher filtration capacity within the same module footprint, as compared to flat sheet membrane designs.
For these reasons, a growing number of wastewater treatment plant operators have opted to replace their first generation MBR modules with PURON modules. When planning such a retrofit installation, several factors must be considered:
Moreover, membrane manufacturers use different air flow rates and cycle times for air scouring on those systems that are able to cycle the delivery of air. Therefore, retrofit installations may require adjustments to the air scouring sequence (typically a programming issue), and changes to the size of blowers and the position of automatic valves that move air.
Three Retrofit Examples: Municipal Wastewater
An example of a municipal MBR module retrofit is the wastewater treatment plant that serves Thélus, a community near Calais in the north of France. Generale-des-Eaux, a Veolia Water company, had been using a municipal MBR to treat the wastewater from this community since 2000. However, reliability and energy consumption of the existing double-header hollow fiber membrane modules never met the operator’s expectations. Therefore, in June 2006 the plant owner chose to retrofit the system with second generation PURON membrane modules.
The retrofitted MBR plant now has a capacity of 335 cubic meters/day (90,000 gal/day). Incoming wastewater passes through a 1 mm slot screen prior to entering the MBR. The blower for air scouring of the membrane modules was replaced by a unit that is half the size of its predecessor due to the reduced air scour demands of the PURON single header design that uses central aeration. The reduced air scour demand of the new membrane module reduced dissolved oxygen concentration in the recycle flow, thereby reducing the air entering the denitrification zone, improving the overall denitrification performance of the system. As a result, effluent values for total nitrogen have dropped considerably and effluent quality has improved significantly.
Another example of a retrofit is at the Ecopark De Wierde landfill for household and industrial waste in The Netherlands. Leachate from the landfill has been treated by means of a membrane bioreactor since February 2003, but in 2004 the 126,000 gal/day (477 cubic meters/day) capacity of the system was insufficient and consideration was given to expanding the MBR to a 222,000 gal/day (833 cubic meters/day) hydraulic capacity. In September 2005, two second generation PURON membrane modules were installed and operated in parallel with the existing MBR modules. This system was the first of its kind to employ submerged membrane modules from two different manufacturers. The stable performance of the new MBR modules was very encouraging, and after one year, the operator chose to retrofit the remaining two original double header modules. The retrofit was conducted in just one day.
Dekker Trucking Company
The Belgium transport company, Dekker, had been operating a wastewater treatment plant with submerged MBR membrane technology for a few years. The existing system was a first generation membrane design with non-reinforced hollow fibers fixed at both ends. The original membrane experienced various problems including fiber breakage and low membrane permeability as the existing membrane was not capable of a backwashing operation. After careful consideration the existing membrane modules were replaced by PURON modules that utilize a reinforced membrane fiber, which supports backwashing and contains a total membrane area of 2,500 square feet (232 square meters).
Retrofits With Second Generation Technology Make Sense
Second generation systems position the membrane bioreactor industry for even more rapid growth, as they work to offer reduced fouling and lower energy consumption, along with a more robust composition than first generation systems. Finally, they are usually designed with compatibility in mind, in order to minimize retrofitting costs. n