General aspects of a membrane bioreactor (MBR)

Technical questions

1. What is the typical pore size of an MBR membrane?

The size will depend on the composition of the water to be treated. Microfiltration (MF) membranes are used to separate suspended solids, large molecules, and bacteria, and are generally in the range of 0.1 to 0.4 μm.

When smaller suspended solids such as colloids, organic molecules, bacteria, and some viruses need to be filtered, ultrafiltration (UF) membranes are used, which have pore sizes of around 0.01 to 0.1 μm.

2. How does an MBR system remove contaminants from wastewater?

A membrane bioreactor combines two main processes: biological treatment, in which bacteria are responsible for removing organic matter, and membrane filtration, which separates excess sludge and recirculates active sludge to the biological reactor. This process produces high-quality treated water, which in many cases can be reused.

In summary, the biological process breaks down contaminants, and the membranes separate clean water from solids and microorganisms.

3. What are the operating pressures of MBR systems?

For submerged membranes, transmembrane pressure (TMP) between 0.1 and 0.5 bar is sufficient. This is achieved by suction using vacuum pumps.

In the case of external membranes, the pressure to be applied is 0.5 to 2 bar, and pumping is carried out to membranes housed in external modules.

4. How can membrane fouling be prevented in an MBR?

Preventing fouling of membranes in an MBR system is key to maintaining its efficiency and extending its service life. Several techniques are used, which can be combined with each other:

• Aeration, which is carried out by means of bubbling that generates turbulence, promoting continuous cleaning of the membranes.
• Periodic cleaning, which can be physical, using backwashing to reverse the flow and remove deposited dirt, or with specific chemical solutions such as hypochlorite or citric acid (CIP maintenance, or cleaning in place). Vigorous cleaning with other more aggressive reagents is also carried out to recover the membranes when there is significant fouling.

Chemical Cleaning of MF and UF Membranes in Water Treatment | PDF | Water | Aluminum

5. What is sludge retention time (SRT) in an MBR system?

SRT is the average time that sludge remains in the biological system before being removed.

SRT allows control of biomass composition, promoting the growth of slow-growing microorganisms such as nitrifiers, which leads to more efficient bacterial colonies, reduced sludge production, improved performance in the removal of complex pollutants, and greater process stability.

Typical SRT values in an MBR system are around 20 days for nitrification–denitrification and longer times to minimize residual sludge.

6. How are suspended solids leaks detected in MBR filtration systems?

The simplest system consists of observing whether there is any abnormal coloration or turbidity in the filtered water sample. However, the use of an online turbidimeter or laboratory analysis allows us to carry out safe and effective monitoring.

To detect which membrane or element is causing the leak, we use a method known as the integrity test, which consists of a pressure or air (bubble point) test. In this test, pressurized air or gas is applied and observed to see if there is any passage to the permeate side. If there is an increase in the gas passage rate, this indicates a leak in the membrane. If there are several membrane cartridges, they must be isolated to check which one is damaged.

Bubble Point Integrity Test: What Is It and What Is It Used For? – Brother Filtration

7. How can you tell if the membranes in a membrane bioreactor are dirty?

Fouling of membranes in an MBR system is usually detected by a decrease in filtration flow rate while maintaining the same operating pressure, or because it is necessary to increase the pressure to maintain the desired flow rate. In addition, it is common to observe an increase in the concentration of suspended solids in the permeate when fouling occurs.

These parameters are controlled by pressure measurement systems such as pressure transmitters, which trigger an alarm when a certain pressure differential between inflow and outflow is exceeded. The filter flow meter can also be equipped with an alarm if the flow rate drops. The filter turbidity meter indicates whether the normal turbidity value has been exceeded.

8. What concentration of suspended solids (MLSS) is used in an MBR process?

Normal MLSS concentration values in an MBR are between 8 and 12 g/l, and in certain systems they can operate at up to 15 g/l; whereas in activated sludge systems they operate at concentrations of between 2 and 4 g/l. This translates into a much smaller biological reactor volume and a considerably higher concentration of extracted sludge.

9 How are submerged membranes cleaned?

If the membranes are submerged in modules attached to the biological reactor, the filtration system is stopped, the modules are emptied, and chemical solutions are circulated in the module itself. If the membranes are inside the biological reactor, they must be removed and submerged in cleaning tanks.

Cleaning to remove scale and accumulated organic matter is carried out using hypochlorite solutions, citric acid, or specialized products.

10. What type of pretreatment is recommended before feeding an MBR reactor?

Before introducing the effluent into the membrane bioreactor, it is advisable to screen it to a level of 2-3 mm and, if necessary, remove sand. It is also important that there are no oils, fats, or other products that could clog the membranes.

With regard to salt concentration (_CaCO₃, CaSO₄, SiO₂, etc.), the saturation limits of these salts (LSI, PS) must be taken into account to prevent precipitation on the membranes. This can occur due to changes in pH, temperature, or evaporation of the liquid in the process.

Troubleshooting

1. Why is my MBR system experiencing membrane fouling?

Fouling of MBR membranes can be physical, chemical, or biological, and occurs when particles, microorganisms, or chemicals adhere to or deposit on the membranes, reducing their active surface area, which results in decreased permeability.

As the affected surface area increases, the rest must work under stressful conditions, which accelerates fouling. In turn, the dirt becomes deeply embedded in the deeper layers and is very difficult to remove, even with vigorous cleaning (irreversible fouling ).

2. How are odor problems in MBRs resolved?

To resolve or control odors, the following measures are taken:

• Operate under correct aerobic conditions and control the sludge load to prevent the formation of gases such as_SH2 or_NH3.
• Covering with domes or hoods, both for the biological reactor and for sludge treatment.
• Forced ventilation systems that conduct extracted air to gas treatment units such as biofilters, scrubbers,or beds with adsorbent fillers such as activated carbon.

3. What causes a decrease in flow in MBR membranes?

A decrease in flow in MBR membranes is usually related to several factors that affect their permeability. The most common causes are:

• Membrane fouling (biological, organic, inorganic, or colloidal fouling).
• Aging or damage to membranes.
• Insufficient aeration flow or turbulence.
• Accumulation of sludge.
• Inadequate or infrequent chemical cleaning leaves residues.
• Inadequate operating conditions (flow rate, pressure, temperature, etc.).

MBR membrane fouling: an in-depth analysis of causes, impacts, and solutions – SPERTA MEMBRANE

4. How are leaks detected in the membranes of an MBR reactor?

The most commonly used systems for detecting leaks in the membranes of an MBR system are:

• Integrity tests: Pressure or bubble test and vacuum test.
• Microbiological analysis and analysis of suspended solids in the permeate.
• An increase in the turbidity or SDI of the permeate may indicate fiber breakage or cracks.
• Visual inspection. If the membranes are submerged, they can be removed and checked for tears or cracks in the modules or their connections.

5. Why is the biological performance of my membrane bioreactor declining?

The main causes are as follows:

• When the effluent to be treated has a higher COD/BOD content than the design, the biomass in the biological reactor cannot process the organic load properly, which reduces the purification efficiency.
• The presence of inhibitory or toxic compounds can affect active biomass.
• Fouling of MBR membranes can reduce the filtration capacity and sludge recirculation of the system.
• Nutrient imbalance due to nitrogen or phosphorus deficiency causes biomass to develop improperly.
• Incorrect operating conditions (incorrect pH, pressure, temperature, oxygenation, HRT or SRT, etc.).

6. How are leaks in MBR membranes repaired?

Insulation:

• In hollow fiber membranes, once the broken fiber has been located, the damaged fiber is sealed with heat or a specific resin, or isolated with a special plug.
• On flat modules, a patch or special resin can be applied to the affected area.

Module replacement:

• When the leak is irreparable, it is recommended to replace the entire module.

CIP (cleaning in place):

• Sometimes, chemical cleaning can eliminate leaks caused by dirt.

7. How can sludge from a biological membrane reactor be concentrated?

Sludge from an MBR system can be concentrated using various techniques, depending on the treatment objectives and the type of sludge management required. Some of the most common methods are:

• Static sludge thickeners.
• Thickening using dissolved air flotation systems, which are more efficient than static thickeners.
• Mechanical sludge drying (filter presses, centrifugal decanters, belt filters, rotary screens, etc.).

Static and mechanical systems for sludge dewatering typically use coagulants and flocculants to improve their performance.

8 Can water from an MBR system be made drinkable?

Although a membrane bioreactor produces high-quality effluent, it does not completely eliminate pathogens, viruses, or traces of chemical contaminants (such as medicines, pesticides, etc.).

Water from an MBR system can be made drinkable, but it needs to undergo additional treatment stages, such as:

• Disinfection with_Cl₂,_O₃, UV, etc.
• Filtration by nanofiltration or reverse osmosis.
• Filtration/adsorption with activated carbon.
• pH adjustment + remineralization.

For high flow rates, it is also common to reinject water filtered by an MBR into an aquifer, where it mixes with natural water (indirect drinking water reuse).

9. What should be done with the effluent from cleaning the membranes of an MBR?

These discharges contain chemicals used to clean the membranes, such as acids, alkalis, chelating agents, and biocides. Therefore, they cannot be discharged directly into the environment without prior treatment.

The line to follow would be:

• Analytical characterization of discharge parameters (pH, conductivity, SS, metals, COD, etc.).
• Physicochemical treatment.
• Biological treatment in the same MBR reactor. Some plants can recirculate part of these pretreated discharges if it does not affect the biological process.
• External management when they cannot be recycled or reprocessed at the MBR plant (in the case of cleaning with highly aggressive or toxic agents).
• Use of mild cleaning agents and possible reuse of these agents for other cleaning tasks.

10. What happens if the pH of an MBR system becomes unbalanced?

Significant problems can occur that affect both the biological process and the functioning of the membranes:

• The biomass of the biological system can be damaged if the pH falls below 6 or rises above 9, as bulking, foaming, and bad odors can occur due to the inactivity of the affected bacteria. This leads to a reduction in treatment performance.
• Damage to the MBR membranes may also occur (chemical degradation, reduced permeability, fouling, which reduces the service life of the membranes).

https://europemembrane.com/bioreactores-de-membrana-mbr/criterios-de-diseno-de-un-mbr/

Comparative questions

1. How does an MBR reactor compare to activated sludge systems?

Both are biological purification processes, but they differ from each other:

• The MBR combines a biological reactor (aerobic or anaerobic) with a subsequent filtration system using ultra- or microfiltration membranes, while a biological activated sludge treatment system uses a secondary clarification system after the aerobic biological reactor.
• Compared to the activated sludge process, the MBR takes up less space, as it can operate at higher mass loads and does not require a secondary clarifier.
• Safer and more efficient filtration: While conventional activated sludge systems use settling processes or, in some cases, flotation (DAF), these methods can cause problems if the biological part does not function properly. This does not occur in an MBR system, as the membranes ensure that solids always remain within the system.
• Removal of contaminants and microorganisms: MBR systems use ultrafiltration membranes, which have a micron-level filtration capacity. This means they can remove virtually any contaminant.
• Greater safety: The main advantage of an MBR is that, if there is a problem with the biological part, the system will never allow solids to escape into the effluent.

2. What are the differences between an MBR and reverse osmosis (RO)?

Both are water treatment processes, but while an MBR reactor is designed to remove organic matter through biological processes and then filter the water with membranes to remove most of the suspended contaminants, the reverse osmosis process is designed to obtain high-purity water by separating dissolved salts from the water by overcoming the osmotic pressure of the solution.

3. Is MBR a suitable system for small-scale wastewater treatment plants?

The MBR is a process that allows for the construction of compact, modular equipment that houses the biological reactor and membrane assemblies.

This equipment is assembled and tested in the workshop and, in addition to the electromechanical components, includes a control and power panel to ensure the correct operation of the installation.

Decentralized plants for wastewater treatment

4. How does MBR work in combination with other technologies such as UV disinfection?

An MBR system can separate a significant proportion of COD, but toxic substances, viruses, and bacteria may still remain and need to be eliminated. If disinfection is desired, different systems are used, such as chlorination, ozonation, and UV radiation. The latter process is the cleanest, as it does not involve any reagents or oxidizing substances. Depending on the desired degree of microorganism elimination, high or medium UV radiation equipment with an effectiveness of over 99.9% is used.

5. What are the cost differences between a membrane bioreactor and conventional wastewater treatment?

Assuming that both processes consume similar amounts of_O2 for the biological oxidation of organic matter (approx. 1.5–2 kg_O2/kg BOD), the MBR system is more expensive to install and maintain due to the UF membranes, which have additional energy consumption due to aeration and pumping, and additional reagent consumption for chemical cleaning. However, an MBR system provides savings in excess sludge management and, above all, in the possible reuse of treated water due to its high quality.

Reuse of treated water using reactors (MBR).

6. How do MBR and MBBR processes differ?

While an MBR uses a biological reactor followed by a set of filter membranes, an MBBR process involves biological purification in one or more stages using a system of aerated reactors loaded with filler material (carriers). These carriers provide a large surface area for bacteria to colonize, which feed on the organic matter in the effluent to be treated.

An MBBR is more effective than an MBR for biological oxidation purposes, as the carriers are kept in constant motion, and limits excess sludge to the solids that detach from the carriers. Energy consumption, maintenance, and implementation are lower with MBBR, but the treated water is generally of better quality with MBR.

7. Why does an MBR system offer better performance than other biological processes?

A biological membrane reactor outperforms other biological processes because it works with biomass loads that are 3 to 4 times higher than the standard, but the big difference lies in the replacement of a secondary clarifier with the ultrafiltration membranes of the MBR, which provide a filtration light in the order of 0.1 µm, facilitating the separation of all suspended solids, most colloids and larger molecules, as well as most microorganisms, such as bacteria and viruses, which escape with the effluent resulting from the biological reactor.

8. How do we choose the most suitable type of filter membrane for an MBR process?

The membranes in an MBR reactor will depend on the degree of filtration required: microfiltration (MF) from 0.1 to 0.4 µm, or ultrafiltration (UF) from 0.01 to 0.1 µm.

On the other hand, depending on the application, submerged membranes can be used in the bioreactor, which consume less energy but also have a lower flow per^m2; or membranes external to the reactor can be used. This type is more resistant and allows for more efficient cleaning, but has a higher energy cost due to pumping.

Submerged membranes are typically used more in urban wastewater treatment, while external membranes are more common in industrial effluent treatment.

9 When is MBR used in combination with DAF flotation?

It is common to use dissolved air flotation as a primary treatment before membrane bioreactors, especially when there are floating materials or oil and grease residues that can clog UF membranes.

When required by the effluent, coagulation/flocculation will be necessary in order to improve the separation of colloidal material and suspended solids.

10. What are the differences between an MBR and an FBR?

In an MBR, secondary clarification is replaced by membrane filtration (MF/UF), while in an FBR, secondary clarification is carried out using a DAF system after the biological reactor. The MBR saves space, improves treatment performance and treated water quality, and with the FBR, sludge concentration is higher and installation is simpler for maintenance purposes.

Compact and modular membrane bioreactors

1. How does a compact MBR system work?

An MBR system combines two main processes:

1. Biological part. This is divided into two zones: an anoxic zone where no oxygen is introduced, allowing pollutants to be broken down without the presence of air, and an aerobic zone where oxygen is introduced via blowers, promoting bacterial activity that helps break down the organic pollutants present in the water.
2. Physical filtration using membranes: Submerged membranes are located in the aerobic part of the system, where they perform physical filtration of the water. This stage ensures high effluent quality by removing solids and other contaminants.

2. What are the main applications of compact MBR systems?

The main applications of compact MBR systems are highly varied, especially due to their ability to adapt to different needs and locations. Some key examples include:

1. Isolated areas: MBR systems are ideal for remote or hard-to-reach locations (rural or forested areas).
2. Agricultural work camps: In places where wastewater treatment systems are required due to the large number of workers or activity in the area.
3. Temporary or mobile installations: Thanks to their compact and modular design, MBR systems can be transported and installed quickly.

3. How much space does a compact, modular MBR system take up?

A compact and portable MBR system is similar in size to a shipping container, making it easy to install and move around. Plus, since it's a plug-and-play system, it doesn't need any extra assembly—just connect the water inlet and outlet. Its internal parts are pre-assembled.

4. Is a compact, modular MBR system expensive?

A compact MBR system is inexpensive to maintain. Its main advantage is that it significantly reduces sludge management costs. In conventional systems, large amounts of sludge are generated, requiring frequent and costly purges. In contrast, an MBR handles a higher concentration of biomass, producing less sludge. This reduces operating costs, as less sludge volume needs to be managed. Although the initial investment may be higher, the long-term savings make MBR more economical and efficient than conventional systems.

5. What is the expected service life of a compact MBR system?

A compact membrane bioreactor has a service life of between 15 and 25 years, depending on how it is used and maintained.

It is important to mention that membranes have a useful life of between 8 and 10 years, if they are properly maintained. Once this period has been reached, the membranes must be replaced.

6. Is pretreatment necessary before an MBR system?

Proper pretreatment of the water before it enters the MBR is also important, especially to remove solids larger than 1 mm, which ensures optimal system performance.