Moving Bed Biofilm Reactor (MBBR) vs. Bead Filters for Wastewater Treatment

The MBBR and the Floating Bead Filter are both plastic floating media biofilters that can eliminate soluble Biological Oxygen Demand (BOD) from the system.  These filters are widely used in aerobic treatment and can be redesigned to function in an anaerobic format for denitrification. The term “C/N” ratio refers to the ratio of organic carbon to ammonia in a nitrifying filter.  This defines the relative abundance of heterotrophic and nitrifying that can be expected in a biofiltration operation.  Often estimated by the BOD5/ammonia ratio, a high C/N indicates excessive heterotroph growth, which can suppress the development of a healthy nitrifying biofilm.

MBBR Filter

AST Filter: Ammonia Treatment 1.2 MGD, Georgia

MBBR systems are fixed film bioreactors that use a moving bed of support material, typically plastic biocarriers, to grow microorganisms. In aerobic treatment, the biocarriers are constantly mixed by air injection, which keeps the water aerated and provides abrasion to remove excess biofilm.  The discharge from the MBBR is screened to contain the biocarriers. Biocarriers tend to be large 10-20 mm to avoid screen clogging.  Originally developed for aquaculture applications, they are capable of good rates of conversion of dissolved organics (BOD) and ammonia.  They have also successfully applied to domestic and industrial waters.  They mimic suspended growth treatment, such as activated sludge.  MBBRs are not as sensitive as activated sludge to flow variations and compete best under low-concentration regimes where floc development issues can plague an activated sludge unit. The specific surface areas for most MBBR biocarriers are typically reduced by as much as 50% by over-abrasion of externally exposed surface area, but hull volume to medial volumes is low (about 2).  Nitrification rates for low ammonia (<1 mg-N/L) concentration regimes of 300-500 gm/m3 media-day are common and conversions of 1 kg-N/m3 media-day are achievable with low C/N and higher ammonia concentrations.  Internally protected surface area is poorly abraded under heavy organic loading so the MBBR should always be protected by aggressive solids removal to lower organic loading (lower C/N ratios).  MBBRs constantly shed solids and are normally followed by a solids capture device.

Floating Bead Filters are granular filters that use a static bed low-density plastic (specific gravity <0.92) media bed to filter up flowing water.  The media (typically 2-3 mm) is retained by an overlying screen.  When used as a clarifier, the media captures suspended solids by the same mechanisms as a sand filter (micro-scale settling, interception, or surface adsorption).  The units also effectively operate as fixed film bioreactors, particularly when charged with plastic biocarriers that have been shaped to improve porosity and provide biofilm protection.  Floating bead filters that are designed to function as a biofilter inherently capture suspended solids and are referred to as bioclarifiers since they perform a dual function.  Floating bead filters have about twice the effective surface area (all external) as an MBBR, reflecting the smaller media size, but the hull-media ratio is moderate at about 3-4.  Nitrification rates for low-concentration regimes (<1 mg/L) are typically 800-1000 g-N/m3 media-day, and triple that rate is achievable with higher concentration and good backwash management.  Modern floating bead filters are washed pneumatically (with bubbles) that have been shown to “gently” wash the bead surface to minimize biofilm damage.  Backwashing is timed (1-6 times daily) to match the loading regime while maintaining optimum levels of oxygen delivery, ammonia transport, and turbulence in the bed.  Backflushing scours excessive bacteria off the outer layers of the biofilm, creating ideal conditions for diffusion of critical nutrients to nitrifying populations that normally reside close to the biocarrier surface. Backwashing effectively mitigates the negative impact of organics (particulate and dissolved), so floating bead bioclarifiers are not sensitive to C/N ratios since they selectively harvest heterotrophic growths during backwashing. These bioclarifiers internally remove excess biofilm and do not need to be followed by a particulate removal device.  Aerobic units tend to support higher flowrates to transport oxygen.  Anoxic HPPG (or Propellor-washed) units are characterized by low flows as the bed is often used to deoxygenate the waste flow to create the ideal redox conditions for denitrifying.

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There are three different Floating Bead Filter models that dominate commercial-scale operation.  The Propeller-washed (PBF) format cleans beads by the action of a set of embedded propellors.  This is the original AST format.  The newer High Profile PolyGeyser (HPPG) employs pneumatic backwashing, which simplifies operations and moderates’ abrasion during backwashing.  For all practical purposes, the PBF and HPPG are interchangeable and both lack the internal aeration to make them effective as aerobic bioclarifiers.  They are single-pass units designed primarily as clarifiers.  They can be linked to an external aerated tank to become effective in a bioclarifier mode. For example, they are very effective bioclarifiers when linked to an aerated fish tank in a recirculating mode.  The Recirculating PolyGeyser (RCPG) links an internal head tank with an airlift to facilitate the treatment of organic or ammonia-rich waste waters that require very high oxygen demand.  The water is rapidly re-aerated and passed through the bed multiple times to achieve the treatment objectives.   The RCPG can replace an MBBR and its associated clarifiers. The HPPG and PBF are normally used to support an MBBR, replacing one or both of the associated MBBR clarifiers.

The vast majority of MBBR and Floating Bead Filter units are operated aerobically. The principal difference between the MBBR and the family of Floating Bead Filters is the operational mode of the bed.  The MBBR operates continually mixed, and the Floating Bead Filter beds do not move during filtration.  Thus, the Floating Bead Filters inherently capture suspended solids while the MBBR generates them. MBBR abrasion is continuous and intermittent in the Floating Bead Filter.  Both use a plastic biocarrier.  MBBR media floats or is neutrally buoyant.  Floating bead filters always use a floating plastic, typically low-density polyethylene, with a specific gravity <0.92.  The MBBR, HPPG, and MBBR are washed pneumatically.   However, the MBBR is washed all day (86,000 seconds/day) and the Floating Bead Filter (6-16 seconds daily/day), so they operate in totally difference biofilm abrasion regimes.  Changing the backflush frequency in an RCPG or HPPG adjusts the biofilm abrasion rate. The MBBR operates at an aggressive and fixed abrasion rate.   The PBF is aggressively washed by the propellor at 1750 rpm for 15-30 seconds (1-2 backwashes daily) per day when used as a bioclarifier.  Increasing the duration or frequency of PBF backwashing usually over-abrades the biofilm.

 

MBBR (Moving Bed Biofilm Reactor):

  • Filtration Mechanism: MBBR systems have no solids removal mechanism and generate solids that must be removed by a clarifier.  They use plastic media with a moderate specific surface area to support the growth of biofilm, which helps break down dissolved organic matter.
  • Flow capacity: MBBR systems can be fabricated on-site at any flow scale required.
  • Pressure loss: There is normally little to no pressure loss associated with a well-designed MBBR.   The units are easily operated as gravity-fed units.
  • Biological Treatment: An MBBR is a fixed-film bioreactor. In the normal aerated configuration, they can be used to oxidize organics or nitrify for a variety of applications.  When configured with mechanical mixing, the MBBR can be used for denitrification.
  • Nitrification: THE MBBR is effective at nitrification when the organic loading is controlled, provided the pH is basic (>7.0), and carbonate alkalinity is high (>100 mg-CaCO3).  They are normally protected by clarifiers to remove particulate organics since nitrification capacity is sensitive to increases in C/N ratios.
  • Oxygen Supply: An MBBR is internally aerated by the air injection that provides mixing.  No external source is normally required.
  • Carbon Dioxide Stripping: The large amount of air injected into an MBBR has the secondary benefit of enhancing carbon dioxide stripping.   This feature can be of secondary benefit in low pH systems (<6.5) as it can locally raise the pH into a zone more favorable to high-rate notification.
  • Organic Loading: MBBR nitrification is sensitive to both particulate and dissolved organic loading.  It is most effective as a nitrification unit at the end of the treatment train after the organics have been addressed.  MBBR is used to control BOD for a variety of applications.
  • Footprint: MBBR systems are compact and have similar space requirements to floating bead filters for similar treatment capacities. Two solid removals are required for most MBBR applications and require additional area.
  • Maintenance: MBBR systems require little maintenance.  Periodic inspection of outlet screens may be required in some applications.  Routine maintenance of blowers is required. Most MBBR reactors operate without internal moving parts. Periodic cleaning of injectors may be required in hard waters.  Replacement of media every decade may be required for some biocarrier designs.

Floating Bead Filters:

  • Filtration Mechanism: AST Bead filters operate through biological and physical filtration. The use of small plastic beads as a static filter media allows them to remove suspended solids and creates a large surface area to support the growth of biofilm. The bacteria in the biofilm break down dissolved organic matter in wastewater.  The biofilm can also be effective at nitrification. The static bead bed simultaneously removes suspended solids, dissolved organics, and ammonia from wastewater with periodic backwashing.
  • Flow Capacity: The current lines of fiberglass HPPG single pass units have flow capacities of up to about 1 mgd.  Custom single pass (HPPG) stainless steel has unit flow capacities over 4 mgd.   Ultimately, flow capacity is limited by the internal aeration capacity of the unit, not hydraulic design.   On-site construction can accommodate larger clarification flows.  RCPG treatment capacity is limited by organic and ammonia loadings.  Typical treatment flows for an RCPG 500 in a polishing application are in the range of 0.5-1 mgd.   For the typical high concentration (BOD5>1,000 mg/L)  surcharge application, this drops to 0.1-0.5 mgd.   Larger flows are accommodated by multiple parallel units.
  • Pressure loss: There is little pressure loss passing through a properly washed floating bead bioclarifier packed with Enhanced Nitrification media.   Moderately loaded units with 2-3 foot beds operate with internal head losses of 6-9” principally attributed to piping and diffusers.  Heavily loaded RCPG 500 surcharge units are designed to operate with internal recirculation drops of up to 18 inches.  The flow through the units, however, requires only a few inches and is compatible with gravity-fed operations.  HPPG and PBF units are designed for pressure operation and can be configured to operate at low pressure (<1 psi) but generally operate under peak pump pressures of about 10 psi.  In specialized situations, hull pressure of 60 psi can be accommodated, but drops across the unit are usually <5 psi.
  • Oxygen Supply: All AST floating bead filters can be configured for bioclarification by adding the “Enhanced Nitrification” biocarrier.  HPPG models do not have an internal aeration mechanism and are normally used with an attached external aeration tank.  In aquaculture, this is the aerated fish tank.  RCPG units are fully designed as bioclarifiers and have internal aeration in the form of recirculating airlifts.
  • Biological Treatment: Non-AST bead filters can support some biological activity on the surface of the beads; their primary function is mechanical filtration. Unlike other bead filters, AST filters are as efficient as MBBR in terms of biological treatment. They are excellent at reducing organic contaminants and nitrogen compounds (like ammonia) through nitrification.  The HPPG is used for denitrification.  When a carbon source is dosed, the stratified bed is operated from the bottom up to first deoxygenate the incoming water and then, as the redox falls, to denitrify. 
  • Nitrification: Floating Bead filters are effective at high rate nitrification across a broad spectrum of organic and ammonia loadings.  They are insensitive to the C/N ratio provided the backflush frequency is appropriately adjusted.  They do not need to be protected by an upstream clarifier or followed by a downstream clarifier in most applications.  High-rate nitrification in a floating bead filter depends on satisfactory pH and alkalinity levels.
  • Footprint: Bead filters require similar space compared to MBBR systems for equivalent treatment capacity. AST bead filters offer the ability to consolidate the footprint of the overall treatment train with its solid’s removal capabilities.
  • Maintenance: RCPG and HPPG require little maintenance as there are no internal moving parts.  Weekly inspection of beads through the viewing window and adjustment of the backwash air delivery rate as required is typically recommended for bioclarification  Some units require periodic manual sludge removal, or it can be automated. Larger RCPG units are equipped with pneumatic sludge discharge that require no manual labor, automated valves, or pumps. Monthly cleaning of algae off RCPG outlet screens may be required in some outdoor applications.  The manufacturer may specify quarterly maintenance of blowers and the backflushing compressor.  Annual acid treatment RCPG airlift injectors may be required in hard waters to control scaling.  Biocarriers generally never have to be replaced.  Screens in PBF and HPPG used in marine (or corrosive ) should be annually inspected for screen corrosion.  PBF propellor shaft seals should be inspected annually for leakage, and routine replacement of seals every decade should be considered.

Interfacing with the MBBR

Figure 1: The core of most domestic treatment follows a classical treatment sequence.

The classical treatment sequence for treatment of domestic waste is a four-step process (Figure 1).  First, suspended solids are removed, reducing the organic loading by about 40%.  Then, a biofilter, in this illustration an MBBR, converts dissolved organics to bacteria biomass. Then, a second clarifier, the secondary clarifier, removes the particulate biomass.  Finally, for many modern facilities, the wastewater stream is then nitrified before it passes on to the disinfection system.  It is a very effective approach that forms a baseline for many variants.

Figure 2: The HPPG floating bead filter can be used to supplement or replace, the secondary clarifier, the primary clarifier, or both.

For example, Figure 2 illustrates how an HPPG can be used to replace either the primary clarifier, the secondary clarifier, or both.   The replacement of the secondary clarifier is a straightforward substitution motivated by the need to further polish the suspended solids in the effluent or to conserve space.  An HPPG or PBF clarifier requires 1/40th of the area as a settling basin.  The replacement of the primary settling system may be motivated by the desire to take advantage of the fine solids capture ability of the Floating Bead Filter to lower the C/N ratio entering the MBBR to improve the nitrification rate.   However, this substitution would result in some loss of flow equalization and may complicate sludge handling for the facility.

Figure 3: Alternately an RCPG bioclarifier can be used to polish the MBBR effluent or provide a pretreatment to the MBBR allowing it to be reduced in size.

As Figure 3 illustrates, the treatment power of the RCPG bioclarifier can be used to polish the MBBR effluent or to provide pretreatment for the MBBR by removing organic solids, oxidizing soluble organics, removing the biomass produced, and nitrifying.  The MBBR biofiltration contribution can facilitate a significant reduction in MBBR sizing, while the substantial reduction in the C/N ratio realized will contribute to the MBBR’s ability to nitrify.

Figure 4: The RCPG bioclarifier can be used aa a total solution replacing all the core components and with the addition of a coarse screening pretreatment and disinfection provide a treatment solution for an application.

Ultimately, the RCPG bioclarifier can be used to replace the entire traditional domestic wastewater treatment train core.  A single unit sized to treat a small industrial or rural domestic discharge.  This approach can reduce demands for staffing,  treatment area and  capital investment.

 

Summary:

 

A single pass HPPG/PBF and MBBR are often paired.  Here, the HPPG/PBF receives all its oxygen as dissolved oxygen in the influent water stream.  Since oxygen saturation typically limits the amount of oxygen delivered, barely 1 mg-N/L of ammonia can be oxidized before the oxygen is depleted and nitrifying stops, reducing the HPPG to a clarifier only format.   Only when the HPPG is utilized in recirculating format does it display significant (aerobic)  biological capability.  Either the HPPG or PBF can effectively reduce the suspended solids load, and thus the C/N ratio for the MBBR allowing it to operate with a thinner, more efficient biofilm thickness.

Although the MBBR and the RCPG floating bead filter may appear to be comparable, they are distinctive and potentially complementary technologies.  They both use plastic biocarriers, but the MBBR operates with continuous mixing and abrasion.  The Floating Bead Filter operates with a floating plastic media bed that is static for the vast majority of the time and is only mixed a few times a day when solids removal and biofilm abrasion is required. The Floating Bead Filter always acts as a clarifier, removing suspended solids. The RCPG is configured by media selection and internal airlift recirculation also to be an excellent fixed film biofilter, the zone of overlap for the two technologies.  An MBBR is just a biofilter; the RCPG integrates the solids capture and removal solids mechanisms into a format that can be reduced to an excellent biofilter, but is capable of replacing the entire treatment train whenever costs treatment objectives, or secondary constraints. They can also be used in a complementary manner to enhance MBBR performance by polishing the effluent for BOD, ammonia, and solids or as a pretreatment to reduce organic loading.

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