Membrane filtration

▶ A membrane or, more properly, a semipermeable membrane, is a thin layer of material capable of separating substances when a

    driving force is applied across the membrane.

▶ Once considered a viable technology only for desalination, membrane processes are increasingly employed for removal of bacteria

    and other microorganisms, particulate material, and natural organic material, which can impart color, tastes, and odors to the water

    and react with disinfectants to form disinfection byproducts (DBP). As advancements are made in membrane production and

    module design, capital and operating costs continue to decline.

(Membrane & Pressure Vessel)

 Membrane filtration processes

▶ The pressure-driven membrane processes discussed in this fact sheet are microfiltration(MF), ultrafiltration (UF),

    nanofiltration (NF), Electrodialysis(ED or EDR), and reverse osmosis (RO).

 ♦ Membrane filtration: Alternative to Conventional filtration

    • Membrane filtration systems’ capital costs, on a basis of dollars per volume of installed treatment capacity, do not escalate

       rapidly as plant size decreases.

    • This factor makes membrane squite attractive for small systems. In addition, for ground water sources

       that do not need pretreatment, membrane technologies are relatively simple to install, and the systems require little more than

       a feed pump, a cleaning pump, the membrane modules, and some holding tanks.

    • According to a 1997 report by the National Research Council, most experts foresee that membrane filtration will be used with

       greater frequency in small systems as the complexity of conventional treatment processes for small systems increases.

 ♦ Comparing Membrane Filtration Systems
    • While all types of membranes work well under proper conditions, choosing the most appropriate membrane for a given application

      still remains crucial. In many cases, selection is complicated by the availability of new types of membranes, applications, or by

      site specific conditions. Bench and pilot tests are powerful tools for situations where process risks and uncertainties exist or the

      cost impacts from problems are potentially high.
    • Membrane classification standards vary considerably from one filter supplier to another. What one supplier sells as a UF product,

      another manufacturer calls a NF system. It is better to look directly at pore size, molecular weight cut off (MWCO), and applied

      pressure needed when comparing two membrane systems. MWCO, which can be regarded as a measure of membrane pore

      dimensions, is a specification used by membrane suppliers to describe a membrane’s retention capabilities.

 Membrane filtration Methods

▶ Two methods are often used in ratio membrane filtration operation; Direct Flow (Dead-end) filtration methods and Cross Flow

     filtration methods

♦ Direct Flow (Dead-end) filtration methods

    •

    • This method produces a right angle of flows to the membrane surface and filtrates the whole quantity which is the same as the

       former method of sand filtration. Regular cleansing on the membranes is essential.
    • Efficiency of energy by filtrating the whole supply water which requires small capacity of pumps.
    • Since it filters the whole supply water, the contamination of membrane is fast in progress, drawing the conclusion that its use is

      appropriate for intermittent operations
    • If cleansing is not done properly in time, recovery is usually incapable.

♦ Cross Flow filtration methods

    •

    • Cross flow means the parallel flow with membranes. When water is supplied, it filtrates suppressing colloids from piling up on

       the membrane surface.
    • Retaining high flux filtration is possible due to the suppression of accumulation of colloids.
    • Appropriate for continuous operations
    • Hhigh flux filtration requires bigger pumps which sometime result in inefficiency.

 Operation control system

▶ Static flux control method and static pressure control method are used

♦ Static flux control method
   • Maintains static amount of water in the operation. Filtration resistance increases as operating continued; this method uses static

     amount of pump or flow meters to retain a certain amount of water. 

(Static flow control method and static pressure control method)

▶Static pressure control method

   • Operation controling method by maintaining the static water supply pressure. Filtration resistance increases as operating

      continued; results in decreased amount of filtered water

 Major Design Items

▶ Main considerations when designing the facility are transmittal Flux, temperature, operation pressure, and retrieval rate.

    As it is closely related to the efficiency and functions of membranes, significant amount of examination is needed.

    Designing standards varies on the operation methods; when operating static pressure control system, flux lowest temperature

    is the standard; when operating static flux control system, membrane pressure on the lowest temperature is the standard.

(Inside & Out Side of the Membrane)

♦ Flux
  • Membrane filtration flux refers to filtered amounts of water per unit periods, unit flux per sizes
     - Flux(㎥/㎡.hr)= tranmittal amount(㎥/h)/square meters of membrane(㎡)
  • Dominant factors of membrane filtration flux are type of membranes, water temperature, quality of water; it is heavily affected by

     water temperature.

♦  Temperature
  • largely affects the flux as coefficient of viscosity changes according to the temperatures of water. When designing the process,

     sufficient consideration about temperatures and production capability at the lowest degree is required.

♦  Operation pressure
  • Operation pressure and transmittal flux have proportionate relationship theoretically, but in reality when pressure is higher, the

     increase rate of transmittal flux decreases. If operating pressure is higher, transmittal flux gets larger and the size of the facility

     gets smaller while designing for the process.

♦ Recovery rate
  • Recovery rate estimates the transmittal flux to supply water amount. It shows the sufficiency of dealing with filtration.
    - Recovery rate(%)=(transmittal flux/supply amount)×100 or (supply amount-accumulated amount)/supply amount×100
  • Recovery rate is deeply affected by membrane pollution degree which is estimated by quality, transmittal flux, and cleansing

     degree. Recovery rates are set as 35 ~ 40 percent for the case of salt-to-fresh water distillation, over 90 percent for normal

     membrane distillation.

(Different Membrane Integrity Testing)

  Classification by Driving Force

▶ Membrane Processes can be classified based on the driving forces yhat induce transport of materials across the membrane

 Silt Density Index(SDI) or Fouling-Index (FI).

▶ Silt is composed by suspended particulates of all types that accumulate on the membrane surface.

    Sources of silt are organic colloids, iron corrosion products,precipitated iron hydroxide, algae, and fine particular matter.

    Silt Density Index testing is a widely accepted method for estimating the rate at which colloidal and particle fouling will occur

    in water purification systems, especially using reverse osmosis (RO) or Nanofiltration membranes.
▶ SDI is a measurement of the fouling potential of suspended solids. It’s not measuring the quantity of particular matter, since the

    size, shape vary. Turbidity is a measurement of the amount of suspended solids. They are not the same and there is no direct

    correlation between them. In practical terms however, the membranes show very little fouling when the feed water has a turbidity

    of < 1 NTU. Correspondingly the membranes show very low fouling at a feed SDI of less than 5.
▶ The SDI test is used to predict and then prevent the particulate fouling on the membrane surface. Other names for it are the

    Fouling-Index (FI).
▶ It measures the time required to filter a fixed volume of water through a standard 0.45μm pore size microfiltration membrane with

    a constant given pressure of 30 psi (2,07 bar). The difference between the initial time and the time of a second measurement after

   normally 15 minutes (after silt-built up) represents the SDI value.

 Contamination of Membranes

▶ Contamination of membranes make the resistance of filtration increased, decreasing the transmission of Flux.

    Its main reason is the thickness of density caused by formation of cake on the surface of membranes.

▶ Factors of membrane contamination

  • Major factors include the quality of water, transmitted flux, speed of flows on the membrane surface, pressure of operation,

     temperatures, etc.

(Contamination of membranes; (A)Feed flow) 

 Cleaning Technologies

▶ There's no way that can perfectly get rid of the contaminants on membranes but generally it is recommended to choose proper

    treatments that varies to the elements in the water.

▶ Dealing with contaminated membranes is largely divided into two types of cleansing: mechanical,

    and chemical cleansing.

【Mechanical cleansing; flushing】

  • Mechanical cleansing is to flush down the contaminants on the surface of membranes with high

    amount of water in low pressure.

  • It is favorable to use electrically treated water supplied by RO filtration for cleansing.

【Chemical cleansing;using chemicals】

  • CIP(Clean in Place) is used when recovering the filtration capability with mechanical cleansing is insufficient.

     -  CIP is used as part of a recovery and restoration straegy; nominally undertaken from once per week to

         once in several months

  • It suspends the membrane filtration operation and removes the contaminants with chemicals.

  • It also sterilizes the whole facility when necessary.

  • Chemical cleansing is implemented by the following process with acid, or alkali cleansing.

    - chemical mixing→ chemical circulation(1hr)→ membrane deposition(2~15hr)→ chemical circulation(1hr)→ chemical drain

       → flushing→ cleansing