Membrane and Desalination Technologies - TCE Moodle Website

Potable Water Biotechnology, Membrane Filtration and Biofiltration 485
As the purification mechanism in a slow sand filtration is based on a balanced community in
the schmutzdecke, the filter should operate at a constant rate. When operation is stopped, the
microorganisms causing bacteriological degradation of trapped impurities lose their effectiveness.
To solve this problem, filter operation using declining rate filtration for additional water is
recommended (44).
3.2. Microfiltration
Microfiltration membrane is defined as a membrane separation process that has the ability
to remove suspended or colloidal particles via a sieving mechanism based on the membrane
pore size (0.1–0.2 mm), which is related to the sizes of particulate matter (2). However, the
microfiltration membranes with pore sizes of up to 10 mm are also available. The microfiltration
typically has a molecular weight cut off of greater than 100,000 Da with a relatively low
feedwater operating pressure of approximately 100–400 kPa (15–60 psi).
For drinking water applications, commercially available configurations for microfiltration
include spiral wound, tubular, and hollow capillary fiber. The hollow capillary fiber is the
most commonly used. In this configuration, the membranes are cast into small diameter tubes
or straws, which are bundled together longitudinally, potted in a resin on both ends, and
encased in a pressure vessel that is included as a part of the hollow-fiber module. The housing
of these bundles is usually made from PVC or stainless steel. The microfiltration membrane
may be constructed from a wide variety of materials including cellulose acetate (CA),
polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polypropylene (PP), polysulfone
(PS), polyethersulfone (PES), or other polymers. Types of material are selected based on
different properties with respect to surface charge, degree of hydrophobicity, pH and oxidant
tolerance, strength and flexibility (2).
In potable water treatment, microfiltration can remove various types of suspended particles
including sand, silt, and clays, Giardia lamblia and Cryptosporidium cysts, algae, and some
bacterial species. In addition, this technology is feasible in removal of natural or synthetic
organic matter when pretreatment is applied, as well as retardation of membrane fouling. As a
pretreatment step, microfiltration can be used to reduce fouling that might occur for the
following reverse osmosis and nanofiltration processes.
Generally, microfiltration provides the particulate removal from a feed stream by a
separation process based on retention of contaminants on a membrane surface. This system
can be operated under ultralow pressure conditions. Typically, the microfiltration process
involves prescreening raw water and pumping it under pressure onto a membrane. Pretreatment
process, such as filtration, is necessary to remove large particles that may block the inlet
to the fibers within the membrane module. To enhance the removal of viruses and DOM, the
complex pretreatments, such as addition of coagulants or adsorption by powdered activated
carbon can be employed. Potable water plants treating 1 MGD normally utilize membrane
units as their major component. Other units of the plant may include prescreens, feed pump,
cleaning tank, automatic gas backwash system, air compressor, membrane integrity monitor,
backwash water transfer tank, pressure break reservoir, air filter for the gas backwash,
controls for the programmable logic controller, and a coalescer.