Page 80 - Membranes for Industrial Wastewater Recovery and Re-Use

60 Membranesfor Industrial Wastewater RecoverM and Re-use

the case of pressure-driven dense membrane processes (reverse osmosis and
nanofiltration), pretreatment is both critical and to some extent predictable from
the feedwater quality - specifically from derived indices pertaining to colloidal
particles and calcium carbonate scalant. Proscriptive methods for measuring
fouling propensity are available (ASTM D4189, D3 739, D4582, D4692,
D4993), and in the following sections pretreatment requirements for reverse
osmosis are discussed. A discussion of cleaning methods for fouled membranes
follows in Section 2.4.4.

Suspended solids
Suspended solids in the feed can accumulate at the membrane surface to an
extent dependent upon the degree of turbulence provided by the cross-flow.
Gross suspended solids are readily removed by pre-filtration using cartridge
filters (Section 2.1.4) which may or may not be preceded by depth (e.g. sand)
filtration. Colloidal matter, derived from aluminium silicate (clays), iron colloids,
organic materials, etc., is less readily removed. Colloids introduce problems in
membrane processes only when coagulation of the suspended colloidal particles
takes place in the membrane. Pretreatment is thus designed either to remove the
colloidal particles or else stabilise them to prevent their coagulation during
permeation.
The fouling nature of a water is best determined by the Fouling Index or Silt
Density Index (SDI), which measure the rate at which a membrane’s pores plug.
Standard test kits are available for determining SDI values. The test, a standard
empirical test described in the reference literature (ASTM D4189), involves
passing water at constant pressure (typically 2 bar) through a standard 0.45 pm-
rated filter. The time ti taken to collect a given volume of water, typically 100 ml,
from the clean filter is measured. Filtration is continued for a pre-set time (tt),
usually 15 minutes, and the time (tf) taken to collect a second sample of the same
volume is measured. The SDI is then given by:

loop - ti/tf)
SDI = (2.29)
tt

Suppliers of spiral wound RO and NJ? modules will normally specify an SDI
value below 5. Waters of higher SDI values must be pretreated either chemically
to stabilise the colloid, normally by addition of chemicals although occasionally
softening can be used to stabilise the colloid by removing colloid-destabilising
divalent species, or by some solid-liquid separation process to remove suspended
material. Softening is unlikely to be economically viable on this basis alone, but
in reducing the divalent ion level the scaling propensity of the water is also
reduced which then allows operation at a higher conversion. Physical
separation may demand pre-coagulation to increase the particle size and ensure
retention of the suspended solids by the filter. For some applications where the
feedwater contains natural organic matter the RO process may be preceded by
fine microfiltration or even ultrafiltration.