MF(Microfiltration)

 ▶ MF is loosely defined as a membrane separation process using membranes witha pore size of approximately 0.03 to 10 microns,

     a MWCO of greater than 100,000 daltons, and a relatively low feedwater operating pressure of approximately 100 to 400 kPa (15

     to 60 psi).

     Representative materials removed by MF include sand, silt,clays,Giardia lamblia and Cryptosporidium cysts, algae, and some

    bacterial species. MF is not an absolute barrier to viruses; however, when used in combination with disinfection, MF appears

    to control these microorganisms in water.

▶ The primary impetus for the more widespread use of MF has been the increasingly stringent requirements for removing particles

    and micro-organisms from drinking water supplies. Additionally, there is a growing emphasis on limiting the concentrations and

     number of chemicals that are applied during water treatment. By physically removing the pathogens, membrane filtration can

     significantly reduce chemical addition, such as chlorination.

▶ Another application for the technology is for removal of natural or synthetic organic matter to reduce fouling potential. In its normal

    operation, MF removes little or no organic matter; however, when pretreatment is applied, increased removal of organic material,

     as well as a retardation of membrane fouling can be realized.

▶ Two other applications involve using MF asa pretreatment to RO or NF to reduce fouling potential. Both RO and NF have been

    tradition-ally employed to desalt or remove hardness from groundwater.

(Hollow-Fiber MF & UF Membrane) 

 PROCESS

 • MF membranes provide absolute removal of particulate contaminants from a feed stream by separation based on retention of

   contaminants on a membrane surface. It is the “loosest” of the membrane processes, and as a consequence of its large pore size,

   it is used primarily for removing particles and microbes and can be operated under ultra low pressure conditions.

 • In the simplest designs, the MF process involves prescreening raw water and pumping it under pressure onto a membrane.

   In comparison to conventional water clarification processes, where coagulants and other chemicals are added to the water

   before filtration, there are few pretreatment requirements for hollow-fiber systems when particles and microorganisms are the

   target contaminants.

(Individual Fiber Flow Pattern; MF/UF Membranes)

 • Prefilters are necessary to remove large particles that may plug the inlet to the fibers within the membrane module. More complex

   pretreatment strategies are sometimes employed either to reduce fouling or enhance the removal of viruses and dissolved organic

   matter. In such cases, pretreatment by adding coagulants or powdered activated carbon (PAC), has been employed. In some

   cases, the cake layer built up on the membrane during the water production cycle can remove some organic materials.

 • It may be necessary to adjust the feedwater pH by chemical dosing prior to membrane filtration in order to maintain the pH within

   the recommended operating range for the membrane material employed. It should be noted that pH adjustment is not required for

   scaling control, since MF membranes do not remove uncomplexed dissolved ions. o MF membranes, under the most conservative

   conditions, appear to act as an absolute barrier to selected bacteria and protozoan cysts and oocysts.

   Unlike UF however, MF does not remove appreciable densities of viruses. There-fore, it is necessary to complement MF with a

   post membrane disinfection process. Chemical disinfection may be employed by applying chlorine, chlorine dioxide, or

   chloramines; however, long contact times are required to inactivate viruses.

 UF(Ultrafiltration)

▶ UF involves the pressure-driven separation of materials from water using a membrane pore size of approximately 0.002 to 0.1

    microns, an MWCO of approximately 10,000 to 100,000 daltons, and an operating pressure of approximately 200 to 700 kPa

    (30 to 100 psi).

    UF will remove all microbiological species removed by MF (partial removal of bacteria), as well as some viruses (but not an

    absolute barrier to viruses)and humic materials. Disinfection can provide a second barrier to contamination and is therefore

    recommended.

(UF Membrane & Plant)

(Submerged Membrane System)

▶ The primary advantages of low-pressure UF membrane processes compared with conventional clarification and disinfection

   (post chlorination) processes are: o No need for chemicals (coagulants, flocculants, disinfectants, pH adjustment);

   • Size-exclusion filtration as opposed to media depth filtration;

   • Good and constant quality of the treated water in terms of particle and microbial removal;

   • Process and plant compactness;and o Simple automation.

▶ Fouling is the limiting phenomenon responsible for most difficulties encountered in membrane technology for water treatment.

    UF is certainly not exempt from this fouling control problem. Therefore, membrane productivity is still an important subject,

    which should be thoroughly researched in order to have a better understand-ing of this phenomenon and its mechanisms.

♦  PROCESS

   • UF is a pressure-driven process by which colloids, particulates, and high molecular masssoluble species are retained by a

      process of size exclusion, and, as such, provides means for concentrating, separating into parts, or filtering dissolved or

      suspended species.

      UF allows most ionic inorganic species to pass through the membrane and retains discrete particulate matter and nonionic and

      ionic organic species. 

   •  UF is a single process that removes many water-soluble organic materials, as well as microbiological contaminants.

      Since all UF membranes are capable of effectively straining protozoa, bacteria, and most viruses from water, the process

      offers a disinfected filtered product with little load on any post-treatment sterilization method, such as UV radiation, ozone

      treatment, or even chlorination.

   •  Unlike RO, the pretreatment requirement for UF is normally quite low. Fortunately, due to the chemical and hydrolytic stability

       of UF membrane materials, some of the pretreatments essential for RO membranes, such as adjustment of pH or chlorine

       concentration levels, do not apply.

     However, it may be necessary to adjust the pH to decrease the solubility of a solute in the feed so that it may be filtered out.

   •  UF is designed to remove suspended and dissolved macromolecular solids from fluids.

      The commercially available modules are there-fore designed to accept feed waters that carry high loads of solids. Because

       of the many uses for UF membranes, pilot studies are normally conducted to test how suitable a given stream is for direct UF.

   • Water containing dissolved or chelated iron and manganese ions needs to be treated by an adequate oxidation process

      in order to precipitate these ions prior to UF membrane filtration, as with all membrane processes. This is recommended to

      avoid precipitation of iron and manganese in the membrane, or even worse, on the permeate side of the membrane

     (membrane fouling during the backwash procedure).

   •  Preoxidation processes generally used include aeration, pH adjustment to a value greater than eight, or addition of strong

      oxidants, such as chlorine, chlorine dioxide, ozone, or potassium permanganate.

   •  Natural Organic Matter (NOM) is of great importance in potential fouling of the UF membrane and, consequently, in permeate flux

       that can be used under normal operating conditions. Thus, it is an interesting design option to use PAC or coagulants to pretreat

       the water to remove NOM and, consequently, decrease the surface of membrane needed.

      (Tech Brief, National Drinking Water Clearinghouse)

 MF/UF Fouling Mechanisms

♦ Particulates

  • Concentration polarization

  • Pore plugging

♦ Adsorption

  • Organics

♦ Inorganic

  • Scaling : CaCO3, CaSO4

  • Fe, Mn, etc

♦ Biofouling

  • Microbiology

  • Algae