Drip Drop Water Talk

Separating the sales pitch from science

Category: Membrane

Reverse Osmosis and Bacteria

Do reverse osmosis filters remove bacteria-

Q: Do reverse osmosis filters remove bacteria?  How?

A: Yes, bacteria will be removed via reverse osmosis due to size exclusion (i.e. the bacteria are larger than the pores in the reverse osmosis membrane so they can’t pass through).  However there can be defects in the membranes which allow the bacteria to pass through.  I’ve also seen it hypothesized that the bacteria will move around the o-ring which typically seals the reverse osmosis into place within its housing.  Secondary contamination, where bacteria will move up the pure water stream and attach to the membrane, is also a possibility.  This is why while bacteria can be removed via reverse osmosis, these systems are rarely certified to be microbiological purifiers.


Find this question and others I’ve answered on Quora.  

New Membrane Material Developed which may Compete with Current Desalination Membranes

MoS2 membrane

Move over graphene, there is a new membrane material in town!

Researchers at the University of Illinois at Urbana-Champaign have developed a new membrane material for water treatment.  This material is a nanometer-thick sheet of molybdenum disulfide (MoS2) which is riddled with tiny holes ranging from 1 to 10 nm in diameter called nanopores.  Today the market is full of reverse osmosis (RO) membranes which typically have a pore size of 0.1 nm but are thick due to their polymeric material.  This thickness has a direct relationship on the amount of energy it takes to push water through the membrane and current advances to increase the recovery rate of water through an RO membrane are limited to the orientation of membrane leaves (individual sheets of membrane through which water passes).  By creating a thinner membrane, less energy is required to filter water and lower operating costs are feasible.  Despite its thinness MoS2 is mechanically robust with an effective Young’s modulus of 270±100 GPa (about that of steel), which is not completely surprising considering that molybdenum is frequently used for making high strength steel alloys and superalloys.

In order to determine the effectiveness of the MoS2 membrane for water permeation, it was compared against conventional water treatment membranes: MFI-type zeolite, seawater RO, brackish water RO, nanofiltration, high-flux RO, and including graphene.  The permeation rate was found to be two to five orders of magnitude higher than conventional membranes and 70% greater than graphene.  This increased transport of water was attributed to the architecture of the pores within the membrane.  Molybdenum is located in the center of the membrane which attracts water, and the sulfur on the other side which pushes the water away.  Ion rejection rates of the MoS2 membrane were on par with that of seawater RO membranes and graphene.  Further testing is expected to look at fouling of the MoS2 membrane.

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