**2.2 Carrier filling degree**

A typical MBBR process can have a biocarrier filling ratio lower than 70%, where carriers are continuously mixed in the reactor and the whole reactor has homogenous conditions. Due to shear forces from mixing/aeration, biofilm growth and detachment processes are balanced to maintain a relatively constant biofilm thickness at steady-state condition. The limitation of filling degree is related to energy consumption and mixing effects for mass transfer purposes [26]. Higher filling degrees will result in higher energy requirement for sufficient mixing of the suspended carriers. It is especially challenging for aerobic systems where aeration energy consumption can account for more than 70% of the complete treatment energy demand [8, 13]. While different carrier filling degrees have been attempted, a different setup based on over 90% filling degree has been developed by biowater technology. The process is named continuously flow intermittent cleaning (CFIC®) which constitutes of two individual modes, a normal operation mode and a washing mode. In the normal operation, over 90% filling degree leads to an almost stagnant

**Figure 3.** *Biocarrier BWTX® (left) and BWT15® (right) (biowater technology AS, Norway) with biofilm growth [25].*

**7**

*Biofilm in Moving Bed Biofilm Process for Wastewater Treatment*

carrier bed. Oxygen field transfer efficiency (OTE) has been documented to be 1.5 times higher than in a normal MBBR with lower filling degree by applying coarse bubble aeration. Big bubbles are cut through the carrier bed with better utilization. Due to high filling degree, sludge will accumulate on carriers which also work as a filter bed media for wastewater treatment. High sludge accumulation will lead to effluent solid increase after certain times depending on load situations. A washing mode is therefore introduced by increasing the water level in the reactor which resembles a normal MBBR operation washed-out accumulated excess sludge (similar to backwashing of sand filters). Wastewater can be fed continuously to the reactor without stops during the washing cycle. This high filling degree process has been applied in full scale for petrochemical wastewater treatment [27] and municipal wastewater treatment both in China and Brazil for organic and ammonia removal, confirming high efficiency and compactness. Carrier filling degree around 30% is also applied for systems to remove dissolved oxygen before feeding to a

In a MBBR biofilm system, the process effectiveness depends on the active organism's concentration, mass transfer efficiency, and system setup, for example, feed distribution and mixing. Organisms' concentration is relatively constant in a stable process, depending on feed substrates and biofilm mass on carriers, which is on

for example, while the active organisms are mainly on the outer surface of the scaling mass. For processes like nitrification or anammox, the mass per area can be lower due to the slow growth rates. The organic loading rate in MBBR is generally based on

reduced removal efficiency is expected in such high load system where oxygen supply can be a limiting factor. Comparing to activated sludge system, a MBBR can sustain higher sludge concentration per reactor volume. With an on average 20 g/m2

MBBR process has also been developed for ammonia removal through both traditional nitrification and denitrification processes and anammox (**Figure 4**) [13]. In conventional nitrogen removal process, ammonium ion is oxidized to nitrate by complete nitrification, and subsequently nitrate is reduced to nitrogen gas by pre-/ post-denitrification. Such nitrogen removal is usually carried out in two different reactors. Inorganic carbon as alkalinity is normally supplied to perform ammonium oxidation. Denitrification requires easily degradable organic such as methanol as electro acceptor. Partial nitrification, called nitritation, and anaerobic ammonium oxidation can also be achieved to remove nitrogen from wastewater in one reactor by manipulating dissolved oxygen concentration into the biofilm. That means oxidation of nitrite to nitrate is suppressed, and denitrification can occur according

Ammonium removal by nitrite is performed by a group of autotropic bacteria, named anammox bacteria [28, 29]. The anammox process requires 40% less energy and generates 88% less CO2 emission comparing to traditional nitrogen removal process [10, 24]. Due to low growth rate (0.06 g VSS/g VSS d), a doubling time being ~10–14 days at relatively high temperature (30–35°C) [30], anammox requires long start-up period. The biofilm attached to the MBBR carrier, being protected from the environment, maintaining long sludge retention time,

on carrier surface and a filling degree of 70%, the sludge content is about 7 g/L for

reduces the operation complexity and equipment for sludge return is avoided.

. The carrier mass value can be higher in a system with scaling,

/d depending on the biofilm condition and loading history. A

carriers. This is achieved without sludge return and thus

/d. The organic loading rate can be as

biofilm

*DOI: http://dx.doi.org/10.5772/intechopen.88520*

denitrification system, for example.

the protected surface areas, such as gCOD/m2

/m3

**2.3 MBBR treatment process**

average below 20 g/m2

high as 100 gCOD/m2

a surface area 500 m2

to "shortcut" in **Figure 4** [13].

*Biofilm in Moving Bed Biofilm Process for Wastewater Treatment DOI: http://dx.doi.org/10.5772/intechopen.88520*

carrier bed. Oxygen field transfer efficiency (OTE) has been documented to be 1.5 times higher than in a normal MBBR with lower filling degree by applying coarse bubble aeration. Big bubbles are cut through the carrier bed with better utilization. Due to high filling degree, sludge will accumulate on carriers which also work as a filter bed media for wastewater treatment. High sludge accumulation will lead to effluent solid increase after certain times depending on load situations. A washing mode is therefore introduced by increasing the water level in the reactor which resembles a normal MBBR operation washed-out accumulated excess sludge (similar to backwashing of sand filters). Wastewater can be fed continuously to the reactor without stops during the washing cycle. This high filling degree process has been applied in full scale for petrochemical wastewater treatment [27] and municipal wastewater treatment both in China and Brazil for organic and ammonia removal, confirming high efficiency and compactness. Carrier filling degree around 30% is also applied for systems to remove dissolved oxygen before feeding to a denitrification system, for example.

### **2.3 MBBR treatment process**

*Bacterial Biofilms*

**2.1 Biocarriers**

1000 m2

/m3

828 (BWT15®) m2

requirements.

**2.2 Carrier filling degree**

Plastic carriers of different shapes and surface areas have been developed and applied in the MBBR systems as biofilm substratum. The carriers' shape, density, protected areas, and void volume are important factors that affect the performance of MBBR processes. Carriers can be made of different shapes such as squares, round, and sphere. The shape can affect the carrier's strength, shearing, and colliding conditions. The carrier density is normally lower than water at around 0.98 kg/L, so that it can be suspended in wastewater with biofilm attachment without introducing strong mixing. The carriers protected areas range from 300 to over

areas normally mean high complexity of the carrier structure and higher produc-

in full-scale wastewater treatment plants due to their costs and process benefits. **Figure 3** shows two different types of plastic carriers that are made of high-density polyethylene (HDPE) with respective protected surface area of 650 (BWTX®) and

and maintains active organisms in thin layers. A well-designed carrier enables stable biofilm in the MBBR process, so that the void is not easily blocked by wastewater particles or excessive biofilm accumulation. Effective mixing/aeration combining a good carrier design leads to good system performance and low-maintenance

A typical MBBR process can have a biocarrier filling ratio lower than 70%, where carriers are continuously mixed in the reactor and the whole reactor has homogenous conditions. Due to shear forces from mixing/aeration, biofilm growth and detachment processes are balanced to maintain a relatively constant biofilm thickness at steady-state condition. The limitation of filling degree is related to energy consumption and mixing effects for mass transfer purposes [26]. Higher filling degrees will result in higher energy requirement for sufficient mixing of the suspended carriers. It is especially challenging for aerobic systems where aeration energy consumption can account for more than 70% of the complete treatment energy demand [8, 13]. While different carrier filling degrees have been attempted, a different setup based on over 90% filling degree has been developed by biowater technology. The process is named continuously flow intermittent cleaning (CFIC®) which constitutes of two individual modes, a normal operation mode and a washing mode. In the normal operation, over 90% filling degree leads to an almost stagnant

*Biocarrier BWTX® (left) and BWT15® (right) (biowater technology AS, Norway) with biofilm growth [25].*

tion cost. Carriers of protected areas of 500–1000 m2

/m3

depending on the shapes and internal structure. Large carrier protected

/m3

. The biofilm on carriers develops as illustrated in **Figure 1**

are normally applied

**6**

**Figure 3.**

In a MBBR biofilm system, the process effectiveness depends on the active organism's concentration, mass transfer efficiency, and system setup, for example, feed distribution and mixing. Organisms' concentration is relatively constant in a stable process, depending on feed substrates and biofilm mass on carriers, which is on average below 20 g/m2 . The carrier mass value can be higher in a system with scaling, for example, while the active organisms are mainly on the outer surface of the scaling mass. For processes like nitrification or anammox, the mass per area can be lower due to the slow growth rates. The organic loading rate in MBBR is generally based on the protected surface areas, such as gCOD/m2 /d. The organic loading rate can be as high as 100 gCOD/m2 /d depending on the biofilm condition and loading history. A reduced removal efficiency is expected in such high load system where oxygen supply can be a limiting factor. Comparing to activated sludge system, a MBBR can sustain higher sludge concentration per reactor volume. With an on average 20 g/m2 biofilm on carrier surface and a filling degree of 70%, the sludge content is about 7 g/L for a surface area 500 m2 /m3 carriers. This is achieved without sludge return and thus reduces the operation complexity and equipment for sludge return is avoided.

MBBR process has also been developed for ammonia removal through both traditional nitrification and denitrification processes and anammox (**Figure 4**) [13]. In conventional nitrogen removal process, ammonium ion is oxidized to nitrate by complete nitrification, and subsequently nitrate is reduced to nitrogen gas by pre-/ post-denitrification. Such nitrogen removal is usually carried out in two different reactors. Inorganic carbon as alkalinity is normally supplied to perform ammonium oxidation. Denitrification requires easily degradable organic such as methanol as electro acceptor. Partial nitrification, called nitritation, and anaerobic ammonium oxidation can also be achieved to remove nitrogen from wastewater in one reactor by manipulating dissolved oxygen concentration into the biofilm. That means oxidation of nitrite to nitrate is suppressed, and denitrification can occur according to "shortcut" in **Figure 4** [13].

Ammonium removal by nitrite is performed by a group of autotropic bacteria, named anammox bacteria [28, 29]. The anammox process requires 40% less energy and generates 88% less CO2 emission comparing to traditional nitrogen removal process [10, 24]. Due to low growth rate (0.06 g VSS/g VSS d), a doubling time being ~10–14 days at relatively high temperature (30–35°C) [30], anammox requires long start-up period. The biofilm attached to the MBBR carrier, being protected from the environment, maintaining long sludge retention time,

### **Figure 4.**

*Nitrification and denitrification with shortcut mechanism illustrated [13].*

and thereby preventing the slow-growing organisms from being washed out of the system, is suitable for slow-growing anammox biomass. Limited research on anammox process in MBBR is documented, but it has been observed that removal rates of up to 1.2 kg N/m3 .d can be achieved using MBBR for side-stream reject wastewater treatment in municipal application [31]. Nitrite formation is a limiting step, and dissolved oxygen needs to be well controlled, so advanced process control is required for efficient MBBR anammox solutions.

MBBR has also been applied for biological phosphor removal in Norway by physically moving carriers with biofilm from anaerobic stage to aerobic stage and back to anaerobic stage so that the P-accumulating organisms undergo the same cycles as in activated sludge "Bio-P" processes [32].

### **2.4 Different MBBR reactor setups**

Due to MBBR's compact nature, high effectiveness, and reliability, the MBBR process is also integrated with other processes (summarized in **Table 1**), such as with activated sludge for enhancing ammonia removal, with anaerobic granular sludge process to form a hybrid system, such as HyVAB® [13, 27], for combined anaerobic and aerobic wastewater polishing, and with membrane bioreactor (MBR) for high strength and stricter wastewater treatment requirements [10].

Based on the MBBR technology, there are several commercially proven technologies available in the market [25], such as:


The typical layout for the above processes for organic removal is shown in **Figure 5**. **Table 1** briefly compares the abovementioned technologies and key benefits. Most of the technologies only focus on COD and nutrient removal from wastewater except HyVAB®. HyVAB® is the technology that focuses on both COD, nutrient removal together with energy recovery as biogas [13, 25]. Biogas production from the HyVAB® reactor makes the treatment process partially or fully energy self-sufficient.

### **2.5 MBBR operational issues**

Depending on MBBR process operational knowledge and full-scale design experience, several problems can be encountered for a full-scale MBBR process, such as

**9**

**Figure 5.**

**Table 1.**

*Biofilm in Moving Bed Biofilm Process for Wastewater Treatment*

**Technology Process Benefits**

attached biofilm removing both organic and inorganic nitrogen

process together with MBBR carriers, by introducing plastic carriers into the activated sludge process

90–99% that allows high substrate transfer efficiency. Operates in normal and washing modes with continuously wastewater feeding. Excess biomass removal is needed

anaerobic and aerobic high-rate processes. Anaerobic stage recovers energy (methane) from wastewater and the aerobic part with biocarrier removes the remaining organics and

*MBBR integrated technologies with other biological treatment process.*

• High effective surface area in carrier gives large protected growth area, hence less space

• Self-regulating biofilm on carriers requires less monitoring and ensures stable treatment

• Suitable for retrofitting existing activated sludge plant to enhance nitrification and BOD

• BOD, P, and N removal can be achieved together • Achieve low SVI, ensures efficient sludge removal

• Very compact and energy-efficient process (20% smaller footprint and 50% less energy

• Higher oxygen (1.5 times to MBBR) and substrate transfer efficiency

• Ultra-high rate and compact process • Suitable for wide range of application; reject water treatment and industrial wastewater

• Very low sludge production

• Very low SVI enables fast sludge settlement, 80% less effluent sludge than MBBR in normal

• High COD removal and generate high methane

demand than MBBR)

requirement

process

removal • Small foot print

mode

treatment

content biogas

*Typical layout of (1) MBBR, (2) CFAS, (3) CFIC, and (4) HyVAB for organic (BOD/COD) removal [25].*

*DOI: http://dx.doi.org/10.5772/intechopen.88520*

MBBR [12, 13, 25] Freely moving plastic carriers with

CFAS®/IFAS [33] Uses the existing activated sludge

CFIC® [34] High carrier filling degree of over

HyVAB® [27] Hybrid system integrates both

nutrients


*Biofilm in Moving Bed Biofilm Process for Wastewater Treatment DOI: http://dx.doi.org/10.5772/intechopen.88520*

### **Table 1.**

*Bacterial Biofilms*

**Figure 4.**

rates of up to 1.2 kg N/m3

is required for efficient MBBR anammox solutions.

*Nitrification and denitrification with shortcut mechanism illustrated [13].*

cycles as in activated sludge "Bio-P" processes [32].

**2.4 Different MBBR reactor setups**

gies available in the market [25], such as:

**2.5 MBBR operational issues**

• CFAS®—Combined fixed film activated sludge

• CFIC®—Continuous flow intermittent cleaning reactor

• HyVAB®—Hybrid vertical anaerobic biofilm reactor

and thereby preventing the slow-growing organisms from being washed out of the system, is suitable for slow-growing anammox biomass. Limited research on anammox process in MBBR is documented, but it has been observed that removal

wastewater treatment in municipal application [31]. Nitrite formation is a limiting step, and dissolved oxygen needs to be well controlled, so advanced process control

MBBR has also been applied for biological phosphor removal in Norway by physically moving carriers with biofilm from anaerobic stage to aerobic stage and back to anaerobic stage so that the P-accumulating organisms undergo the same

Due to MBBR's compact nature, high effectiveness, and reliability, the MBBR process is also integrated with other processes (summarized in **Table 1**), such as with activated sludge for enhancing ammonia removal, with anaerobic granular sludge process to form a hybrid system, such as HyVAB® [13, 27], for combined anaerobic and aerobic wastewater polishing, and with membrane bioreactor (MBR)

Based on the MBBR technology, there are several commercially proven technolo-

The typical layout for the above processes for organic removal is shown in **Figure 5**. **Table 1** briefly compares the abovementioned technologies and key benefits. Most of the technologies only focus on COD and nutrient removal from wastewater except HyVAB®. HyVAB® is the technology that focuses on both COD, nutrient removal together with energy recovery as biogas [13, 25]. Biogas production from the HyVAB® reactor makes the treatment process partially or fully energy self-sufficient.

Depending on MBBR process operational knowledge and full-scale design experience, several problems can be encountered for a full-scale MBBR process, such as

for high strength and stricter wastewater treatment requirements [10].

.d can be achieved using MBBR for side-stream reject

**8**

*MBBR integrated technologies with other biological treatment process.*

**Figure 5.**

*Typical layout of (1) MBBR, (2) CFAS, (3) CFIC, and (4) HyVAB for organic (BOD/COD) removal [25].*

feed pipe/effluent sieve blocking, nonhomogeneous mixing, carrier voids blocking, destroyed carriers, carrier accumulating at the effluent sieves, and carrier overflow out of reactor. These can be all prevented through skilled design, based on accumulated project knowledge and operation experience.

Depending on wastewater characteristics, problems such as chemical scaling on carries can happen, especially for wastewater that contains high calcium, ammonia, and other minerals, such as anaerobic digestion reject water and diary wastewater [35]. Mineral precipitation can occur when wastewater is supersaturated with relevant ion concentration [35]. The composition of mineral scaling varies and can contain struvite, hematite, hydroxyapatite, maghemite, etc. [8, 35]. Scaling on biofilm carriers creates negative effects on the reactor's performance by reducing effective surface area, hindering the mass transfer, and demanding more energy to keep the carriers in suspension. Carriers with excess scaling become heavier and settled down at the reactor bottom and need to be replaced [35]. The pH and concentration of the ions are the main factors influencing chemical precipitates on carriers. Minerals tend to precipitate more at higher pH; thus, pH control can alleviate scaling. Buffer dosing, reduced air stripping of CO2, and alkalinity removal could help to hinder scaling rates. Pure oxygen aeration is an option to avoid air stripping of CO2 to avoid pH increase. Pretreatment by chemical precipitation such as adding lime to remove calcium and magnesium could also be an option.

Feed wastewater composition changes can cause disturbances such as increased organic load in nitrification or anammox processes that will lead to a shift in competition between heterotrophic to autotrophic bacteria. In such cases, the heterotrophic bacteria that have higher growth rate can gradually dominate the MBBR biofilm, leading to unfavorable condition for ammonia removal.

Unwanted biofilm detachment caused by toxic chemicals or abrupt operational condition changes, such as sudden increase of aeration can lead to process problems and even failure in extreme cases, but inner layers of biofilms are protected by the outer layers, making biofilms quite resilient to such disturbances.
