Membrane bioreactor

A reinforced immersed hollow fiber membrane cassette[11]

In the immersed Membrane Bioreactor (iMBR) configuration, the filtration element is installed in either the main bioreactor vessel or in a separate tank. The modules are positioned above the aeration system, fulfilling two functions, the supply of oxygen and the cleaning of the membranes. The membranes can be a flat sheet or tubular or a combination of both and can incorporate an online backwash system which reduces membrane surface fouling by pumping membrane permeate back through the membrane. The backwash system can be optimized using IPC membranes, as developed by Blue Foot Membranes. In systems where the membranes are in a separate tank from the bioreactor, individual trains of membranes can be isolated to undertake cleaning regimes incorporating membrane soaks, however, the biomass must be continuously pumped back to the main reactor to limit MLSS concentration increase. Additional aeration is also required to provide air scouring to reduce fouling. Where the membranes are installed in the main reactor, membrane modules are removed from the vessel and transferred to an offline cleaning tank.[12] Usually, the internal/submerged configuration is used for larger-scale lower strength applications.[13] To optimize the reactor volume and minimize the production of sludge, submerged MBR systems typically operate with MLSS concentrations comprised between 12000 mg/L and 20000 mg/L, hence they offer good flexibility in the selection of the design Sludge retention time. It is mandatory to take into account that an excessively high content of Mixed liquor suspended solids may render the aeration system to be not so effective and the useful flux of water treated by the membrane would decrease; the classical solution to this optimization problem is to ensure a concentration of mixed liquor suspended solids which approaches 10.000 mg/L, to guarantee a good mass transfer of oxygen with a good permeate flux. This type of solution is widely accepted in larger-scale units, where the internal/submerged configuration is typically used, because of the higher relative cost of the membrane, compared to the additional volume required into the tank.[14]

Immersed MBR has been the preferred configuration due to its low energy consumption level, high biodegradation efficiency, and low fouling rate compared to sidestream membrane bioreactors. In addition, iMBR systems can handle higher SSLM concentrations, while traditional systems work with an MLSS concentration between 2.5-3.5, iMBR can handle a concentration between 4-12 g/L, an increase of 300%. This type of configuration is adopted in industrial sectors, including textile, food & beverages, oil & gas, mining, power generation, pulp & paper in light of its benefits.[15]

In sMBR technology, the filtration modules are outside the aerobic tank, hence the name side-stream configuration. Like the iMBR configuration, the aeration system is also used to clean and supply oxygen to the bacteria that degrade the organic compounds. The biomass is either pumped directly through several membrane modules in series and back to the bioreactor, or the biomass is pumped to a bank of modules, from which a second pump circulates the biomass through the modules in series. Cleaning and soaking of the membranes can be undertaken in situ with the use of an installed cleaning tank, pump, and pipework. The quality of the final product is such that it can be reused in process applications due to the filtration capacity of the micro- and ultrafiltration membranes.

Usually, the external/side stream configuration is used for small-scale higher strength applications; the main advantage that the external/side stream configuration shows is the possibility to design and size the tank and the membrane separately, with practical advantages for the operation and the maintenance of the unit. As in other membrane processes, a shear over the membrane surface is needed to prevent or limit fouling; the external/side stream configuration provides this shear using a pumping system, while the internal/submerged configuration provides the shear through the aeration in the bioreactor, and since there is an energy requirement to promote the shear, this configuration shows this additional cost. Moreover, the MBR module fouling is more consistent, due to the higher fluxes involved in this configuration.[16]

Finally, in order to have a comparison of the characteristics and capabilities of the two configurations, a few points are given below where they are compared: - iMBR offers a lower cleaning frequency, and lower energy consumption, but, otherwise, sMBR can handle higher concentrations than iMBR could MLSS (Mixed liquor suspended solids). For this reason, it is easier to carry out maintenance operations, module replacements and cleanings, since the system is more compact.

The MBR filtration performance inevitably decreases with filtration time. This is due to the deposition of soluble and particulate materials onto and into the membrane, attributed to the interactions between activated sludge components and the membrane. This major drawback and process limitation has been under investigation since the earlier MBRs, and remains one of the most challenging issues facing further MBR development.[17][18]

Fouling is the process by which the particles (colloidal particles, solute macromolecules) are deposited or adsorbed onto the membrane surface or pores by physical and chemical interactions or mechanical action. This produces a reduction in size or blockage of membrane pores.

Membrane fouling can cause severe flux drops and affects the quality of the water produced. Severe fouling may require intense chemical cleaning or membrane replacement.[19] This increases the operating costs of a treatment plant. Membrane fouling has traditionally been thought to occur through four mechanisms: 1) complete pore blocking, 2) standard blocking, 3) intermediate blocking, and 4) cake layer formation.[2] There are various types of foulants: biological (bacteria, fungi), colloidal (clays, flocs), scaling (mineral precipitates), and organic (oils, polyelectrolytes, (humics).

Membrane fouling can be recognized either by the reduction in permeation flux while holding Transmembrane Pressure (TMP) constant or an increase in Transmembrane Pressure in constant flux mode. Nevertheless, most of the wastewater treatment plants are operated in constant flux mode. Hence, fouling phenomena generally are perceived by observing the variation of TMP with time. In recent reviews covering membrane applications to bioreactors, it has been shown that, as with other membrane separation processes, membrane fouling is the most serious problem affecting system performance. Fouling leads to a significant increase in hydraulic resistance, manifested as permeate flux declines or transmembrane pressure (TMP) increases when the process is operated under constant-TMP or constant-flux conditions respectively.[20] In systems where flux is maintained by increasing TMP, the energy required to achieve filtration increases. Alternatively, frequent membrane cleaning is therefore required, increasing, significantly, the operating costs as a result of cleaning agents and production downtime. More frequent membrane replacement is also expected.

Membrane fouling results from the interaction between the membrane material and the components of the activated sludge liquor, which include biological flocs formed by a large range of living or dead microorganisms along with soluble and colloidal compounds. The suspended biomass has no fixed composition and varies with feed water composition and MBR operating conditions. Thus, though many investigations of membrane fouling have been published, the diverse range of operating conditions and feedwater matrices employed, the different analytical methods used, and the limited information reported in most studies on the suspended biomass composition, have made it difficult to establish any generic behavior pertaining to membrane fouling in MBRs specifically.

Factors influencing fouling (interactions in red)

The air-induced cross flow obtained in submerged MBR can efficiently remove or at least reduce the fouling layer on the membrane surface. A recent review reports the latest findings on applications of aeration in submerged membrane configuration and describes the enhancement of performances offered by gas bubbling.[18] As an optimal air flow rate has been identified behind which further increases in aeration have no effect on fouling removal, the choice of aeration rate is a key parameter in MBR design.

Many other antifouling strategies can be applied to MBR applications. They comprise, for example:

• Intermittent permeation or relaxation, where the filtration is stopped at regular time intervals before being resumed. Particles deposited on the membrane surface tend to diffuse back to the reactor; this phenomenon being increased by the continuous aeration applied during this resting period.
• Membrane backwashing, where permeate water is pumped back to the membrane, and flows through the pores to the feed channel, dislodging internal and external foulants.
• Air backwashing, where pressurized air in the membrane's permeate side builds up and releases a significant pressure within a very short period of time. Membrane modules, therefore, need to be in a pressurized vessel coupled to a vent system. Air usually does not go through the membrane. If it did, the air would dry the membrane and a re-wet step would be necessary, by pressurizing the feed side of the membrane.
• Proprietary antifouling products, such as Nalco's Membrane Performance Enhancer Technology.[21]

In addition, different types/intensities of chemical cleaning may also be recommended:

• Chemically enhanced backwash (daily);
• Maintenance cleaning with higher chemical concentration (weekly);
• Intensive chemical cleaning (once or twice a year).

Intensive cleaning is also carried out when further filtration cannot be sustained because of an elevated transmembrane pressure (TMP). Each of the four main MBR suppliers (Kubota, Evoqua, Mitsubishi and GE Water) have their own chemical cleaning recipes, which differ mainly in terms of concentration and methods (see Table 1). Under normal conditions, the prevalent cleaning agents remain NaOCl (sodium hypochlorite) and citric acid. It is common for MBR suppliers to adapt specific protocols for chemical cleanings (i.e. chemical concentrations and cleaning frequencies) for individual facilities.[10]

Intensive chemical cleaning protocols for four MBR suppliers (the exact protocol for chemical cleaning can vary from a plant to another)

Simply due to the high number of microorganisms in MBRs, the pollutants uptake rate can be increased. This leads to better degradation in a given time span or to smaller required reactor volumes. In comparison to the conventional activated sludge process (ASP) which typically achieves 95 percent, COD removal can be increased to 96 to 99 percent in MBRs (see table,[22]). COD and BOD5 removal is found to increase with MLSS concentration. Above 15 g/L COD removal becomes almost independent of biomass concentration at >96 percent.[23] Arbitrary high MLSS concentrations are not employed, however, as oxygen transfer is impeded due to higher and non-Newtonian fluid viscosity. Kinetics may also differ due to easier substrate access. In ASP, flocs may reach several 100 μm in size. This means that the substrate can reach the active sites only by diffusion which causes an additional resistance and limits the overall reaction rate (diffusion-controlled). Hydrodynamic stress in MBRs reduces floc size (to 3.5 μm in side stream MBRs) and thereby increases the apparent reaction rate. Like in the conventional ASP, sludge yield is decreased at higher SRT or biomass concentration. Little or no sludge is produced at sludge loading rates of 0.01 kgCOD/(kgMLSS d).[24] Because of the imposed biomass concentration limit, such low loading rates would result in enormous tank sizes or long HRTs in conventional ASP.

Nutrient removal is one of the main concerns in modern wastewater treatment, especially, in areas that are sensitive to eutrophication. Nitrogen (N) is a pollutant present in wastewater that must be eliminated for multiple reasons: it reduces dissolved oxygen in surface waters, is toxic to the aquatic ecosystem, poses a risk to public health, and together with phosphorus (P), are responsible for the excessive growth of photosynthetic organisms like algae. All these factors make its reduction focus on wastewater treatment. In wastewater, nitrogen can be present in multiple forms. Like in the conventional ASP, currently, the most widely applied technology for N-removal from municipal wastewater is nitrification combined with denitrification, carried out by bacteria nitrifying and the involvement of facultative organisms. Besides phosphorus precipitation, enhanced biological phosphorus removal (EBPR) can be implemented which requires an additional anaerobic process step. Some characteristics of MBR technology render EBPR in combination with post-denitrification an attractive alternative that achieves very low nutrient effluent concentrations.[23] For this, a MBR benefits the retention of solids, which provides a better biotreatment, supporting the development of slower-growing microorganisms, especially nitrifying ones, so that it makes them especially effective in the elimination of N (nitrification).

Nutrients removal in MBRs for municipal wastewater treatment[22]

Anaerobic MBRs (sometimes abbreviated AnMBR) were introduced in the 1980s in South Africa and currently see a renaissance in research. However, anaerobic processes are normally used when a low-cost treatment is required that enables energy recovery but does not achieve advanced treatment (low carbon removal, no nutrients removal). In contrast, membrane-based technologies enable advanced treatment (disinfection), but at a high energy cost. Therefore, the combination of both can only be economically viable if a compact process for energy recovery is desired, or when disinfection is required after anaerobic treatment (cases of water reuse with nutrients). If maximal energy recovery is desired, a single anaerobic process will always be superior to a combination with a membrane process.

Recently, anaerobic MBRs have seen successful full-scale application to the treatment of some types of industrial wastewaters—typically high-strength wastes. Example applications include the treatment of alcohol stillage wastewater in Japan[25] and the treatment of salad dressing/barbecue sauce wastewater in the United States.[26]

Like in any other reactors, the hydrodynamics (or mixing) within an MBR plays an important role in determining the pollutant removal and fouling control within an MBR. It has a substantial effect on the energy usage and size requirements of an MBR, therefore the whole life cost of an MBR is high.

The removal of pollutants is greatly influenced by the length of time fluid elements spend in the MBR (i.e. the residence time distribution or RTD). The residence time distribution is a description of the hydrodynamics/mixing in the system and is determined by the design of the MBR (e.g. MBR size, inlet/recycle flowrates, wall/baffle/mixer/aerator positioning, mixing energy input). An example of the effect of mixing is that a continuous stirred-tank reactor will not have as high pollutant conversion per unit volume of reactor as a plug flow reactor.

The control of fouling, as previously mentioned, is primarily undertaken using coarse bubble aeration. The distribution of bubbles around the membranes, the shear at the membrane surface for cake removal and the size of the bubble are greatly influenced by the mixing/hydrodynamics of the system. The mixing within the system can also influence the production of possible foulants. For example, vessels not completely mixed (i.e. plug flow reactors) are more susceptible to the effects of shock loads which may cause cell lysis and release of soluble microbial products.

Example of computational fluid dynamic (CFD) modelling results (streamlines) for a full-scale MBR (Adapted from the Project AMEDEUS – Australian Node Newsletter August 2007[27]).

Many factors affect the hydrodynamics of wastewater processes and hence MBRs. These range from physical properties (e.g. mixture rheology and gas/liquid/solid density etc.) to the fluid boundary conditions (e.g. inlet/outlet/recycle flow rates, baffle/mixer position etc.). However, many factors are peculiar to MBRs, and these cover the filtration tank design (e.g. membrane type, multiple outlets attributed to membranes, membrane packing density, membrane orientation, etc.) and its operation (e.g. membrane relaxation, membrane backflush, etc.).

The mixing modeling and design techniques applied to MBRs are very similar to those used for conventional activated sludge systems. They include the relatively quick and easy compartmental modelling technique which will only derive the RTD of a process (e.g. the MBR) or the process unit (e.g. membrane filtration vessel) and relies on broad assumptions of the mixing properties of each sub-unit. Computational fluid dynamics modelling (CFD), on the other hand, does not rely on broad assumptions of the mixing characteristics and attempts to predict the hydrodynamics from a fundamental level. It is applicable to all scales of fluid flow and can reveal much information about the mixing in a process, ranging from the RTD to the shear profile on a membrane surface. Visualization of MBR CFD modeling results is shown in the image.

Investigations of MBR hydrodynamics have occurred at many different scales, ranging from examination of shear stress at the membrane surface to RTD analysis of the whole MBR. Cui et al. (2003)[18] investigated the movement of Taylor bubbles[28][29][30][31] through tubular membranes. Khosravi, M. (2007)[32] examined the entire membrane filtration vessel using CFD and velocity measurements, while Brannock et al. (2007)[33] examined the entire MBR system using tracer study experiments and RTD analysis.

The market of MBR is segmented based on end-user, which includes municipal and industrial, and geography, which comprises Europe, Middle East and Africa (EMEA), Asia-Pacific (APAC) and the Americas.[34]

In this line, in 2016, some studies and reports showed that APAC region took the leadership position in terms of market share, owning 41.90% of it. On the other hand, EMEA's market share is approximately 31.34% and eventually the Americas own 26.67% of it.[34]

APAC has the largest membrane bioreactors market. Developing economies, such as India, China, Indonesia, and the Philippines are majorly contributing to the growth. APAC is considered one of the most disaster-prone regions in the world. In 2013, thousands of people died from water-related disasters in the region, accounting for nine-tenth of the water-related deaths, globally. In addition to this, the public water supply system in the region is not as developed when compared to other countries, such as the US, Canada, the countries in Europe , etc.[34]

The membrane bioreactors market in EMEA has witnessed stable growth. Countries as Saudi Arabia, the UAE, Kuwait, Algeria, Turkey, and Spain are majorly contributing to that growth rate. The scarcity of clean and fresh water is the key driver for the increasing demand for efficient water treatment technologies. In this regard, the increased awareness about water treatment and safe drinking water is also driving the growth.[34]

Ultimately, the Americas has been witnessing major demand from countries, such as the US, Canada, Antigua, Argentina, Brazil, and Chile. MBR market has grown on account of stringent regulatory enforcement towards the safe discharge of wastewater. The claim of using this emerging technology comes mainly from the pharmaceuticals, food & beverages, automotive, and chemicals industries.[34]