Page - 65 - in Clean Water Using Solar and Wind - Outside the Power Grid

Water treatment 65 5.2 MEMBRANE SEPARATION Membrane technology has a huge impact on water purification. Semi- permeable membranes are used to physically separate substances. Using pressure across the membrane can drive the process. Then the smallest molecules or particles in a given solution are pushed through the membrane while larger molecules or particles are kept back. Pressure- driven membrane separation can be divided into four different types: • Microfiltration (MF) screens particles from 0.1 to 0.5 microns (10−6 m); • Ultra-filtration (UF) screens particles from 0.005 to 0.05 microns; • Nanofiltration (NF) screens particles from 0.5 ⋅ 10−3 to 1 ⋅ 10−3 microns; • Reverse osmosis (RO) ranging molecular size down to about 1 angstrom (10−4 microns). At this size the “particles” are individual molecules. MF can remove suspended solids, high molecular weight species, bacteria, pathogens such as cryptosporidium and giardia in drinking water. Cryptosporidium is a parasite that commonly occurs in lakes and rivers, particularly when these water systems are contaminated with sewage or animal waste. The MF and UF techniques do not require any chemicals to inactivate the microbes. Water purification by UF can remove macromolecules, colloids, viruses, proteins and pectins. The UF does not remove all the natural minerals, such as calcium (Ca2+) or – more important – the salinity of seawater. NF can remove small molecules and polyvalent ions such as calcium (Ca2+) and magnesium (Mg2+), while RO is needed to remove soluble salts, smaller ions, colour and low molecular weight species. Another parameter that distinguishes the four types of membrane filtration from one another is the pressure under which they normally operate. The flux (the capacity of purified water, permeate, measured in litres per m2 of membrane per hour) depends on the feed pressure. MF and UF need relatively low pressures, while NF and RO require much more. Typically, NF would need 1–4 MPa (10–40 bar), while RO would require 1.5–8 MPa (15–80 bar). Above the optimum pressure clogging of “pores” occurs and the membrane is compacted. Downloaded from https://iwaponline.com/ebooks/book-pdf/520710/wio9781780409443.pdf by IWA Publishing user