Membrane bioreactor

It is possible to operate membrane bioreactor processes at higher mixed liquor suspended solids concentrations compared to conventional settlement separation systems, thus reducing the reactor volume to achieve the same loading rate.

[1] Membrane bioreactors have become an attractive option for the treatment and reuse of industrial and municipal wastewater, as evidenced by their consistently rising numbers and capacity.

[5] The global membrane bioreactor market is expected to grow in the near future due to various driving forces, for instance increasing scarcity of water worldwide which makes wastewater reclamation more profitable; this will likely be further aggravated by continuing climate change.

[6] Growing environmental concerns over industrial wastewater disposal along with declining freshwater resources across developing economies also account for increasing demand for membrane bioreactor technology.

[5] Membrane bioreactors can be used to reduce the footprint of an activated sludge sewage treatment system by removing some of the liquid components of the mixed liquor.

Because of the poor economics of the first-generation devices, they only found applications in niche areas with special needs such as isolated trailer parks or ski resorts.

Aeration maintains solids in suspension, scours the membrane surface, and provides oxygen to the biomass, leading to better biodegradability and cell synthesis.

The next key steps in membrane bioreactor development were the acceptance of modest fluxes (25 percent or less of those in the first generation) and the idea to use two-phase (bubbly) flow to control fouling.

[9] Despite the more favorable energy usage of submerged membranes, there continued to be a market for the side stream configuration, particularly in smaller flow industrial applications.

For ease of maintenance, side stream configurations can be installed on a lower level in a plant building, and thus membrane replacement can be undertaken without specialized lifting equipment.

It is mandatory to take into account that an excessively high content of mixed liquor suspended solids may render the aeration system less effective; 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 permeation 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 tank volume required.

This type of configuration is adopted in industrial sectors including textile, food & beverage, oil & gas, mining, power generation, pulp & paper.

This major drawback and process limitation has been under investigation since the earliest membrane bioreactors and remains one of the most challenging issues facing further development.

[2] There are various types of foulants: biological (bacteria, fungi), colloidal (clays, flocs), scaling (mineral precipitates), and organic (oils, polyelectrolytes, (humics).

Most wastewater treatment plants are operated in constant flux mode, and hence fouling phenomena are generally tracked via the variation of transmembrane pressure with time.

A recent review reports the latest findings on applications of aeration in submerged membrane configuration and describes the performance benefits of gas bubbling.

Each of the four membrane bioreactor suppliers Kubota, Evoqua, Mitsubishi and GE Water have their own chemical cleaning recipes; these differ mainly in terms of concentration and methods (see Table 1).

Hydrodynamic stress in membrane bioreactors reduces floc size (to 3.5 μm in side stream configurations) and thereby increases the effective reaction rate.

[23] Because of the imposed biomass concentration limit, such low loading rates would result in enormous tank sizes or long hydrodynamic residence times in conventional activated sludge processes.

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.

Like in the conventional activated sludge process, 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.

Some characteristics for membrane bioreactor technology render enhanced biological phosphorus removal in combination with post-denitrification an attractive alternative that achieves very low nutrient effluent concentrations.

[22] For this, a membrane bioreactor improves 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).

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).

[25] Like in any other reactors, the hydrodynamics (or mixing) within a membrane bioreactor plays an important role in determining the pollutant removal and fouling control within the system.

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.

Computational fluid dynamics modeling, on the other hand, does not rely on broad assumptions about the mixing characteristics and instead 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 residence time distribution to the shear profile on a membrane surface.

[34] Ultimately, the Americas region has been witnessing major demand from countries including the US, Canada, Antigua, Argentina, Brazil, and Chile.