Page 87 - Membranes for Industrial Wastewater Recovery and Re-Use

Membrane technology 67

Table 2.15 Common foulants and their pretreatment, reverse osmosis

Foulant Effect on performance Pretreatment
~
Suspended solids Decrease in permeability, < 2 mg IF1: 5 pm cartridge filter;
normally in first stage > 2 mg I-': multimedia deep-bed
filter
Cross-flow microfiltration
Organic and inorganic As above Ultrafiltration
colloidal matter, Precoat filtration
SDI 5 Multimedia deep bed filtration
In-line coagulation and filtration
Softening
Alkaline scales: CaC03 Decrease in salt rejection and LSI < 2: threshold inhibitor dosing:
slight decrease in permeability, LSI > 2: acid addition: LSI n 2: ion
normally in ha1 stage exchange or lime-soda softening
Active silica As above Lime-soda softening with MgO
Alkali dosing
Heat
Permanent hardness As above Scale inhibitor dosing, ion
exchange softening, lime-soda
softening
Sulphate scales, e.g. As above Threshold inhibitors
CaS04
Metal oxide (iron and As above Aeration and filtration, dissolved
manganese) air flotation (DAF), catalytic
oxidation (greensand filtration),
chemical oxidation and filtration
Free chlorine Permanent loss of rejection due Activated carbon, chemical
to degradation ofmembrane" reducing agent
Dissolved organics. e.g. Rapid decrease in permeability Adsorption (GAC), flotation
NOM, EPS, anionic
detergents
Biological matter Decrease in permeability Chlorination/dechlorination.
UV (+ microfiltration)
a Non-chlorine-resistant materials.

permeate water in the reverse direction through the membrane, and the
cleaning cycle in which the membrane is invariably chemically treated for a
more extended period. Whilst both backflushing and cleaning are routinely and
advantageously employed to recover some of the flux, nonetheless incur a cost
penalty through:

a the energy consumption,
a the loss of product water,
a the downtime, and
a the use of hazardous chemicals.

The energy consumption relates to the hydraulic resistance, as discussed in
Section 2.4.2. The loss of product water arises because of its use for both
backflushing and cleaning, and can be significant. For example, a 20 s backflush