MEMBRANE BIOREACTOR: APPLICATION OF MEMBRANE TECHNOLOGY IN WASTEWATER TREATMENT

Why do industrial plants treat their waste water? There are few important reasons:

• Conservation of natural resources


• Compliance to environmental laws

• Financial benefits

Before we go the the main topic, let us first look at the stages of a conventional waste water treatment facility:

Preliminary Treatment

It may be a physical or mechanical process which aims to remove large or coarse particles

Examples: Screens; grit chambers


Primary Treatment

A physical or mechanical process which removes Settleable Solids, Oil and Grease and about 40% of TSS and BOD

Examples: Sedimentation (primary clarifier); oil/water separator; aeration


Secondary Treatment

Physical, Biological and/or Chemical process; process to convert Dissolved Solids and Suspended Solids into a form that can be removed by physical means; removes up to 85% of TSS and BOD and a small percentage of TDS.

Examples: Sequencing Batch Reactor; Activated Sludge; Rotating Biological Contactor; Trickling Filter; chemical precipitation; flocculation; coagulation


Tertiary/Advanced Treatment

Any level of treatment, may be physical or chemical, beyond the secondary treatment focusing on the removal of nutrients and disease-causing microorganisms

Examples: Filtration; Ammonia Stripping; pH adjustment; disinfection using chemical, UV light or ozone

To understand the revolutionary potential of Membrane Bioreactor (MBR) Technology it is helpful to first consider how a conventional wastewater treatment plant operates. Each conventional plant consists of three basic parts:

This treatment process is 85-95 percent effective in removing TSS, BOD and COD but is often ineffective in terms of removing microorganisms.

Typically the discharge from a conventional plant will contain 10,000 to 100,000 microbes per milliliter.

Membrane Bioreactors, simply called MBRs offer an optimum solution:

• Membrane modules are submerged in the activated sludge to combine the biological step and the solid-liquid separation step into a single process.

• Essentially, membrane bioreactors replace the solids separation function of secondary clarifiers and sand filters in a conventional activated sludge system


• Produces effluent with much better quality than that produced by a conventional plant

Since the membrane acts as a barrier to microorganisms, the effluent quality is much better than that produced by a conventional plant. Also, the membrane barrier eliminates the need for secondary clarifier and allows the activated sludge to be more highly concentrated. This reduces the capacity needed for biological tanks, saving space and money.

MBR Plant can be configured either as internal/submerged, or external/sidestream.

• Internal/Submerged is the type where the membranes are immersed in and integral to the biological reactor


• External/Sidestream is where membranes are a separate unit process requiring an intermediate pumping step

Advantages of MBR Technology versus conventional process:

• Improved Water Quality - it meets stringent effluent requirements and filters out nearly all solids


• Allows Wastewater Reuse - as part of a treatment scheme, provides water for potable reuse; reduces wastewater discharge fees; provides water for non-potable applications where fresh water is in short supply


• Lowers Capital Costs - that is, Clarifier is not needed and biological step can be scaled down since bacteria concentration is higher


• Reduces Plant Space Requirements - Footprint is up to 50% smaller than conventional plant. It allows for easy expansion in terms of capacity within existing buildings


• Fewer Operational Problems - Bulking and floating sludge problems are minimized


• Easily Retrofittable to Existing MBRs and conventional WWTF - the system has few module connections and there is little need to modify infrastructure
  
  

Membrane Technology: Emerging Technology in Water Purification

Membrane Technology a generic term for a number of different, very characteristic separation processes. These processes are of the same kind, because all of these use semi-permeable membrane. It works without addition of chemicals and with low energy use. Separation process based on the presence of a semi-permeable membrane.

Membrane acts as a very specific filter that will let water flow through, while it catches suspended solids and other substances.

Benefits of Membrane Technologies over Existing Water Purification Methods

• Process can take place while temperatures are low


• Uses smaller space requirements (footprint); membrane equipment requires 90 to 95% less space than conventional plants


• Most of the energy that is required is used to pump liquids through the membrane, therefore, there is lower energy costs


• The process can easily be upgraded or expanded

MEMBRANE TECHNOLOGIES

Reverse Osmosis

A high-pressure, cross flow process, which separates dissolved solids from water through the use of a semi-permeable membrane.

In Osmosis, water flows from a column with low dissolved solids content to a column with a high dissolved solids content to attain equilibrium. Osmotic pressure is the pressure that is used to stop the water from flowing through the membrane, in order to create balance. By pursuing pressure that exceeds the osmotic pressure, the water flow will be reversed; water flows from the column with a high dissolved solids content to the column with a low dissolved solids content; thus the term Reverse Osmosis.

Nanofiltration

Nanofiltration is a pressure-related process, however, feed pressure is generally lower compared to RO Systems.

Process is the same as reverse osmosis in terms of concept and operation. However, while Reverse Osmosis removes the monovalent ions at 98-99% level at 200 psi, Nanofiltration membranes’ removal of monovalent ions varies between 50% to 90% depending on the material and manufacture of the membrane.

Microfiltration

MICROFILTERS membranes with pore size ranging from 0.1 to 10 microns. Microfiltration is a low pressure (10-100 psig) process for separating larger size solutes from aqueous solutions by means of a semi-permeable membrane. It removes bacteria and partially filters out viruses.

Ultrafiltration

Utilize filter membranes with pore size ranging from 0.005 to 0.1 micron. It is a low pressure (5-150 psig) process for separating larger size solutes from aqueous solutions by means of a semi-permeable membrane. Removes suspended solids, colloidal particles, bacteria and viruses and retains oils, particulate matter, bacteria and suspended solids, macromolecules and proteins.