Compared to conventional biological wastewater treatment systems, submerged membrane bioreactors (MBR) produce significantly improved effluent quality and have a substantially smaller footprint. Throughout the past decade, MBR systems have been installed in thousands of municipal and industrial wastewater treatment plants for challenging wastewater applications for plants including paper mills, breweries, food processors, chemical plants and textile manufacturers.
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 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. The investment in changes to infrastructure, piping and utilities are miniscule when compared to the dramatic savings in life cycle costs attainable with the newer designs.
Second Generation MBR
In recent years, MBR membranes have evolved to require less maintenance but have a longer useful life. More advanced designs consume significantly less energy and lower operating costs.
One second generation MBR membrane module available employs hollow fibers. Like several other second generation membranes, the high-strength fibers that comprise these 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 new module uses a single header with hollow fibers potted at the base, allowing the tips of the fibers to float freely at the top with a seaweed-like action. This design eliminates the buildup of hair and other fibrous debris by allowing this material to pass through the module rather than collecting at the top as is typical of dual header designs. Compared to double header designs, the single header design also places less mechanical stress on the fibers.
Another advantage of this 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, this second generation 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. The hollow fibers provide significantly higher membrane surface area and higher filtration capacity within the same module footprint compared to flat sheet membrane designs.
Retrofit Considerations
For these reasons, a growing number of wastewater treatment plant operators have opted to replace their first generation MBR modules with second generation modules. When planning such a retrofit installation, several factors must be considered:
Performance of the MBR System
The most important consideration in the retrofit of an existing MBR is the performance of the biological system. In many cases, poor performance in the bioreactor results in problems with membrane performance. In these situations, changes to the biological treatment system may be needed to improve the system; simply changing the membrane modules may not be sufficient.
Physical Module Dimensions
The new modules must have a flexible design to accommodate the existing tank size and module layout. The frame of the new modules must have the necessary exterior dimensions (height, width, length). The membrane area must also closely match the system design to support the required operating flux.
For example, one new 1,500-m2 module is designed specifically for large-scale MBR retrofit applications. Features such as optimized permeate extraction manifold and air supply lines reduce the number of piping connections during installation.
Module Operation
Each MBR manufacturer has optimized performance in different ways depending on the membrane module and membrane configuration. Careful consideration is required to evaluate the existing equipment that supports backwashing, air scouring and chemical cleaning sequences. This equipment may need modification or an upgrade along with a membrane retrofit.
For example, some systems backwash the membranes with permeate while others do not. The choice depends upon the robustness of the membrane when pressure is applied from the opposite direction. Typically, flat sheet membranes and some unsupported hollow fibers cannot be backwashed, so the necessary piping and pumping capacity for backwashing may not be in place.
Moreover, membrane manufacturers use different air flow rates and cycle times for air scouring on those systems capable of cycling air delivery. 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. In June 2006, the plant owner chose to retrofit the system with second generation membrane modules.
The retrofitted MBR plant now has a capacity of 335-m3/day (90,000-gpd). 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 half the size of its predecessor due to the reduced air scour demands of the 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 and 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. See the figure below.
Landfill Leachate
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-gpd (477-m3/day) capacity of the system was insufficient and consideration was given to expanding the MBR to 222,000-gpd (833-m3/day) hydraulic capacity.
In September 2005, two second generation 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 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.
Trucking Company
Dekker, a Belgian transport company, 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 proved incapable of backwashing operation. After careful consideration, the existing membrane modules were replaced by modules with a reinforced membrane fiber that supports backwashing and contains a total membrane area of 2,500-ft2 (232-m2).
Retrofitting with Second Generation Technology
Second generation systems position the membrane bioreactor industry for even more rapid growth. They offer reduced fouling and lower energy consumption. They are also more robust than first generation systems and are usually designed with compatibility in mind to minimize retrofitting costs.