top of page

MEMBRANE BIO-REACTOR (MBR)

MBR – MBR PACKAGE AND SMALL SCALE WWTP’S
 

A Membrane Bio-Reactor (MBR) system consists of the combination of a membrane process with an activated sludge bioreactor. The inclusion of a membrane separator enhances the performance of a biological system by many times and hence is able to treat high concentration waste water economically. Additionally, the operation & maintenance of the system is simplified.

 

An MBR system is able to produce treated effluent of high quality enough to be discharged to surface waterways or to be reclaimed for non-potable use with disinfection. Other advantages of an MBR system over conventional packaged sewage treatment processes include having a small footprint as well as being easy to retrofit and/or upgrade old wastewater treatment plants.  An MBR system does not need a conventional clarifier and the aeration basin is of significantly reduced size. Figures 1 depicts the membrane bioreactor process respectively. It should be noted that in an MBR system, the settler/clarifier and sand filters are not needed. They are replaced by the membrane separation system. With the MBR configuration, the discharged effluent is free of any bio-solids.

 

COMPARISON BETWEEN  MBR AND SBR WASTEWATER TREATMENT (WWT)

Basic Similarities

 

There are two main similarities between MBR and Sequential Batch Reactor (SBR) wastewater treatment (WWT) technologies in general.

 

They both are forms of the activated sludge process and they both can operate with only a single tank to achieve biological treatment and activated sludge separation from the treated effluent water.

 

Basic Differences

 

There are two key differences between these technologies. One difference is in their method of separating the mixed liquor from the treated wastewater. SBR technology relies on gravity settling, while MBR technology uses the membrane filtration as the physical method for separation. The second key difference is that an MBR is a continuously operating process while an SBR operates as a batch process sequentially. Also, most MBR’s do not require sludge digesters whereas SBR’s do.

 

Features & Advantages of MBR Over SBR Systems

The use of a membrane physical barrier for separation and its being continuous operating process provides the MBR WWT technology numerous advantages of SBR systems in terms of performance, operating ease & capital cost.

 

  • Superior Effluent Quality

The membrane physical barrier of an MBR WWT system provides an effective opening of less than 0.1μm. The enables superior permeate quality with suspended solids less than 2 mg/l and in most cases non-detectable. Additionally, the membrane achieves 6-log removal of bacteria and 3-log removal of virus without disinfection. In addition, effluent turbidity is less than 0.2 NTU in the discharge of an MBR.  SBR technology, even when followed by tertiary filtration, does not achieve values this low as is summarized in a comparison provided in Table 1 below. The typical opening for most tertiary filters used in SBR’s is on the order of 10μm, 100x larger than the membrane. In this regard, MBR technology reduces the burden on the final disinfection system (typically UV or Chlorine).

 

 

Table 1.  Comparison of Effluent Qualities For  Activated Sludge Processes.

 

  • Reduced Footprint

One of the benefits of MBR technology over other activated sludge processes is its ability to operate at high biomass concentrations (MLSS). A typical MBR design will operate at an MLSS of 12,000 mg/l, while a typical SBR design will have an MLSS in the range of 3,000 mg/l. This difference in biomass concentration leads to much smaller process basins for MBR technology, and results in the MBR system having an overall plant footprint 50 – 70% smaller than an SBR system. Further, because it relies on phase separation, the SBR cannot operate at elevated biomass concentrations, as the sludge loses its ability to settle into distinct layers once the MLSS gets above 6,000 – 8,000 mg/l. Settling characteristics are not relevant with MBR technology because it utilizes a physical barrier for separation.

 

  • Reduced Sludge Production

An MBR system will produce less waste sludge than an SBR system. The reason for this is the MBR’s ability to operate at much longer sludge retention times (SRT). Again, the reason an SBR can’t operate at longer SRTs is the negative impact on the settling of sludge.

 

  • Ease of Operation

Ease of operation is an important feature of MBR technology. Eliminating gravity phase separation (sludge settling) from the process greatly reduces the operator oversight required to keep the system running efficiently. Most operators of activated sludge facilities will tell you they spend the majority of their time focused on the settling characteristics of the sludge at their plant. There are many factors that impact settling characteristics and these can change from one day to the next. Not only does this require time spent in the lab testing of sludge samples, but also subsequent adjustments to the plant (i.e. adjusting process cycles in an SBR) to maintain good settling characteristics. If the sludge doesn’t settle into a distinct layer, the plant runs the risk of compromised effluent quality. This lab and plant adjustment work is greatly reduced with an MBR system since settling of sludge is not an issue.

To achieve reuse quality water, the SBR system will need to be followed by a tertiary filtration system including polymer dosing prior to filtration.  By comparison, MBR technology does not require tertiary filtration, polymer addition, or any further treatment processes to meet reuse standards for suspended solids and turbidity. This reduction in the number of unit processes further improves system reliability and reduces process oversight by the operator.

 

  • Equipment Sizing & Quantity

SBR operates cyclically in batch mode and hence one operation is idle while another operation is active. The equipment sizing will have to be larger than the sizing required for average design flow to cover the idle time. This will increase the capital cost. In addition, the longer detention times of the solids, associated with higher MLSS concentrations sustainable in an MBR system, results in a more thoroughly digested sludge which typically permits elimination of the requirement for a sludge thickener or, at minimum, a much smaller thickener and the additional power usage.

 

  • Specific Comparisons Between CES MBR WWT & SBR’s

 

Contact CES TO assist in computing CAPEX & OPEX comparisons for your application

bottom of page