In recent years, the use of membrane bioreactor (MBR) technology for wastewater treatment has grown dramatically. An MBR is a hybrid process in which a biological wastewater treatment process is combined directly with a membrane process. The main task of the membrane within this process is to separate the treated effluent from the biomass, retaining suspended solids and bacteria. Membranes available today fulfill this requirement well.
The second task of the membranes is to always maintain an economic permeate flux.
Investment costs for MBR plants are often lower than those of conventional wastewater treatment plants.
However, MBR operating costs are still higher than most conventional plants. In the future, these costs will be reduced by optimizing the entire system, and MBR technology will provide three main advantages compared with conventional technology:
- Better effluent quality;
- Approximately half the foot- print; and
- Lower overall lifecycle costs. With this outlook, MBR technology will be the key technology for future wastewater treatment.
Economics of MBR
The lifecycle costs of MBR technology are based on three main factors: energy consumption, membrane lifecycle costs and filtration rate.
The energy consumption in MBR plants is always higher than in conventional plants, as the operation requires additional energy, mainly for air to scour the membranes. The module design affects the energy efficiency of aeration. Membrane lifecycle cost is governed by the initial membrane cost for a new plant, the membrane replacement cost and the membrane life. Membrane life is affected by the operating conditions of the total process.
For example, poor performance in the biological step has a direct impact on the fouling behavior of the membranes, which leads to a higher chemical demand for membrane cleaning and therefore, to accelerated membrane aging. The consequences are shorter membrane life and higher lifecycle costs.
Over the last few years, the price of the modules has been declining and membrane life has increased. As a result, the overall lifecycle costs have declined, but they still have the potential to fall further.
Higher filtration rates lead to a reduction in the required membrane area for an MBR process, thus reducing membrane costs. Furthermore, the peripheral costs related to the membrane modules, such as valves, pipes and process control, generally go down with reduced membrane area.
Influencing factors
Development work in MBR technology is focused on maximizing the filtration rate of the membranes and maintaining steady operation at a high filtration rate. The average permeate fluxes of currently available membrane modules in MBR applications are usually between 10 and 15 gfd (gal per sq ft per day). This limitation is less influenced by the hydraulic capability of the membranes, and more by the operating conditions of the process. Permeability, which is the specific permeate flux (filtration rate divided by membrane area) divided by the transmembrane pressure, indicates the stability of membrane performance.
There are two main groups of factors influencing the permeability. Membrane-based factors include membrane material, membrane fouling and membrane aging. Module-based factors include module design, module operation and module clogging. Additionally, the permeability of the membranes is affected by the design and the performance of the biology.
Membrane fouling is caused by deposits on or in the membrane. Fouling leads to an increase in the hydraulic resistance of the membrane and therefore, to a decrease in the permeability. Fouling arises as a result of deposits on the membrane surface, or substances that are able to penetrate the membrane pores and adsorb to the membrane structure. Factors affecting fouling include the material composition of the membrane polymer, the hydrophilicity of the material, the membrane porosity, the pore size distribution, the surface charges, and the adsorption behavior of the different substances.
Fouling always occurs to some extent. Chemical cleaning of the membrane modules is used to reverse membrane fouling that is not removed by air scouring or physical backwash steps. Because many cleaning chemicals have damaging effects on organic polymer membranes, a compromise must be found between the effectiveness of the cleaning and membrane aging. Aging is a change over time in the mechanical and/or chemical structure of the membranes and, as a result, their filtration properties. The cleaning strategy depends not only on the selected membrane cleaning chemical but also on the module and MBR system design. The requirements for an optimized membrane include material, morphological and economic requirements.
Material requirements:
- Hydrophilicity;
- Low fouling tendency;
- Chemical & thermal stability; and
- Mechanical stability. Morphological requirements:
- Narrow pore distribution;
- Minimized number of membrane pinholes;
- High porosity; and
- Low hydraulic resistance. Economic requirements:
- Cost-effective materials; and
- Cost-effective assembly.
Besides fouling and aging, the permeability is also influenced by module design. During filtration, local dewatering of the sludge takes place at the membrane surface. Good operation requires retained particles and solids be removed easily and reliably from the module.
Module clogging and sludging can be a problem with some membrane modules. Clogging is caused by hair and fibrous materials that are in the biological tank and become wrapped around the membranes. The rising air bubbles carry them upwards. If hollow fiber membranes are fixed at both ends between two headers, the hair and fibrous materials cannot be removed from the module, and these materials clog the modules starting from the top.
Additionally, sludging occurs at the bottom of the modules. The reason for sludging is insufficient aeration in the potting area of the membrane fibers. This leads to local concentration of sludge and blockage of that respective module area. Both clogging and sludging reduce the active membrane area of the module and affect the filtration rate of the system.
The sensitivity of the MBR to hair and fibrous material in the sludge depends on the design of the individual module system. To reduce the risk of clogging, some module suppliers recommend using very fine pre-screens, but these increase the system capital and operating costs.
The requirements for membrane module design are listed below. Design requirements:
- Good solids management;
- Simple backwash system;
- Equal hydraulic load;
- High mechanical stability; and
- Effective air injection. Economic requirements:
- High packing density;
- Cost-effective assembly;
- Cost-effective membrane replacement;
- Long membrane life; and
- Low-cost materials.
Submerged membrane modules
A product that optimizes both membrane and module design is the Puron™ submerged hollow fiber UF module from Koch Membrane Systems. The patented module is designed to avoid clogging and sludging. The Puron module features hollow fiber membranes with a pore size of approximately 0.05 micron. The lower ends of the membrane fibers are fixed in a header while the upper ends are individually sealed and are free to move laterally. All solids and particulates remain on the outside of the fibers while permeate is sucked out of the inside of the fibers by means of a vacuum.
Membrane bundle
The fibers are arranged in bundles and are submerged vertically into the activated sludge. To maintain the filtration rate of the membrane modules, air scouring is carried out at regular intervals. An air nozzle is integrated into the center of bundles of fibers to apply the air for scouring purposes. The central arrangement of the air nozzles inside the membrane bundles reduces energy consumption because air is injected at the place where the risk of sludging is highest. The module design ensures that even hair and fibrous compounds will be removed reliably from the system, so that a coarser pre-screen can be used, thus improving capital and operating costs.
A special feature of Puron membranes is their enormous mechanical strength, which is provided by a braid inside the membrane material.
The individual fiber bundles are connected in rows. Several of the rows are mounted into a frame made of stainless steel to form a module. The free-moving fibers combined with central aeration ensure stable filtration during plant operation, long membrane life, and low operating costs by reducing the need for energy, cleaning and maintenance.
MBR systems using the Puron module are in operation treating both municipal and industrial wastewater. The Puron module is the next generation of MBR technology and can help reduce the overall lifecycle costs for MBRs, matching the costs of conventional wastewater treatment plants.
