Comparison of Membrane Technology

There are different types of membranes available for use in MBR's: Tubular, Hollow Fiber and Flat Sheet. Below is a comparison of the technologies.

Hollow Fiber, Flat Plate, and Tubular Membranes

 

Tubular Membranes

Tubular membranes were available for laboratory use as early as the 1920's and were first used in industrial applications in the 1960's.  Tubular membrane modules have one or more tubes of varying diameter.  The tubes themselves are constructed of a microporous substrate material which provides mechanical strength and the membrane is cast on the inside of the tube as a finely porous surface layer.  Wastewater is pumped through the membrane tubes and permeate flows through the engineered pores to produce treated water.

Tubular membranes are known for their sturdy construction, long membrane life and high flux rates.  Of all membrane types, they are more robust and can be subjected to high pressures in demanding applications.  Tubular membranes are backwashed in some applications.  With Backwash pressures of up to 15 psi, the large flow in the reverse direction efficiently removes solids from the membrane surface.

Membrane Damage May occur from:

  • Chemical Oxidation

 

Tubular Membranes are configured in several ways for Membrane Bioreactors (MBR's):

HiRate™ Membrane Bioreactor (MBR)

A HiRate™ Membrane Bioreactor (MBR) is a configuration where mixed liquor is pumped at high velocity through the tubular membrane module. The high velocity continuously scours the membrane surface and prevents solids from building up.  This process produces high flux rates and simple operation. No backwash is required. HiRate™ Membrane Bioreactor's (MBR’s) are the lowest capital cost option because of the high flux rates and simple operation. Energy requirements are significantly higher than other types of Membrane Bioreactor (MBR) systems.

Advantages 

  • Low Cost
  • Simple Operation
  • No special building or tank requirements
  • Entire system may be placed indoors
  • Membrane tank not required

Disadvantages 

  • High Energy

 

Dynalift™ Membrane Bioreactor (MBR)

Dynalift™ Membrane Bioreactor (MBR) systems use tubular membranes mounted vertically. A recirculation pump feeds mixed liquor from the bioreactor to the bottom of each module where air is injected.  This, in effect, acts as an airlift pump increasing the velocity that aids scouring inside the membranes. The scouring mixture discharges from the top of each module and is returned to the bioreactor (or the anoxic zone, if de-nitrification is taking place). Back-flushing with permeate water is initiated on a timed cycle to each bank of modules. This removes any cake formed on the inside of the membrane tubes, thus maintaining flux rates.  This membrane configuration allows for low energy operation.

Advantages 

  • Low Energy
  • No special building or tank requirements
  • Entire system may be placed indoors
  • Membrane tank not required
  • Very cost effective for small installations

Disadvantages 

  • Not Cost Effective for large plants

 

Hollow Fiber Membranes

Hollow Fiber Membranes were developed in the 1960's.  They are constructed of a microporous structure having a dense selective layer on the outside surface.  Many fibers must be packed into bundles and potted into tubes to form a membrane module.  Modules with a surface area of even a few square feet may require many miles of fibers.  Because a module must contain no defective or broken fibers, production requires strict quality control.

Hollow fiber membranes can withstand very high pressures from the outside, but are limited on pressure exerted from the inside of the fiber, therefore backwash rates are limited to around twice the normal permeate rate.  The feed fluid is applied on the outside of the fibers and the permeate is removed down the fiber bore.

Hollow Fiber membranes are applied in clean water applications and are used in MBR, but are limited to lower concentrations of solids than other membrane types. 

Membrane damage may occur from:

  • Chemical oxidation,
  • Foreign bodies introduced into the system
  • Mechanical Stresses of over aeration or manual cleaning

Advantages

  • Low Energy
  • Membrane Space efficient for large installations >5 MGD
  • Cost effective for large installations

Disadvantages

  • Not Cost Effective for small plants
  • Membranes damaged easily
  • System is somewhat complicated
  • Can not operate with high MLSS

 

Flat Plate Membranes

Flat Plate Membranes have been used in Membrane Bioreactor (MBR) systems for the last 20 years.  They are constructed of a paper like backing material with a membrane cast on the surface of the paper.  The membrane sheet is ultrasonically welded to both sides of a plastic plate.  These plates are assemled into a submerged membrane unit and spaced about 1/4" apart.  The permeate passes through the membrane unit from the outside in.  Course bubble air is sparged between the plates to prevent solids from building up on the plates and dewatering the membranes. 

Flat plate membranes used in Membrane Bioreactor (MBR) applications require a biofilm to be attached to the membrane.  The membane is a support structure to the biofilm which actually provides the filtration.  Care must be taken to maintain the biofilm on the membranes to prevent failure.

Flat plate membranes can be damaged by:

  • Chemical oxidation
  • Foreign bodies introduced into the system
  • Excessive movment due to air scouring
  • Backpressure on the membranes from unintended operation or chemical cleanings
  • Irregular aeration patterns though the membrane caused by improper installation or diffuser fouling
  • Operating membranes with low solids

Advantages

  • Can be Low Energy
  • Cost effective for medium installations 0.5-5 MGD

Disadvantages

  • Not Cost Effective for small plants
  • Membranes damaged easily
  • Membrane can not be backwashed
  • Can not operate with low MLSS 

 

Operating Parameters

ParameterTubular HiRate™Tubular Dynalift™Hollow FiberFlat Plate

Estimated Life

10+ years 10+ years 5-7 years 10+ years
MLSS Range 1000 - 20,000 mg/L 1000 - 20,000 mg/L 1,000 - 8,000 mg/L 8,000 - 18,000 mg/L
Flux 50-100 gfd 20-30 gfd 5-8 gfd 8-20 gfd
Backwash Not Required 10Q 2Q

Membrane Damaged if backwashed

Configuration Skid Mounted Skid Mounted Membrane Tank Membrane Tank
System Energy 4-6 KWhrs /M3 0.4 Kwhrs / M3 0.4 Kwhrs / M3 0.4 Kwhrs / M3
TMP 10-50 psig 1-3 psig 7-10 psig 1-3 psig

 

Conclusion

There are several membrane configurations available for MBR applications.   Requirements of the project should drive the decision for membrane supplier.  Tubular membranes offer cost effective, reliable systems for small plants because the requirements for a tubular system require no special tankage or installation considerations.  Because flat plate and hollow fiber membranes must be placed into a tank and serviced with cranes the installed costs of these systems are much higher than tubular systems for small plants.  Additionally tubular system installations are typically completely installed in a building without concrete tanks.  For larger applications, submerged systems offer economies of scale that make them viable options in large plants.

Information Sheets