HAZARDOUS WASTE TREATMENT - Environmentally Conscious Materials Handling - page 177

2.3.3 Catalytic Wet Air Oxidation (CWAO)
Wet oxidation can be defined as the oxidation of organic and inorganic substances
in an aqueous solution or suspension by means of an oxidant (usually oxygen or
air, but sometimes ozone and hydrogen peroxide) at elevated temperatures and
pressures. Typical conditions for wet oxidation range from 180 ◦ Cand2MPato
315 ◦ C and 15 MPa. Residence times may vary from 15 to 120 minutes. Insoluble
organic matter is converted to simpler organic compounds, which are oxidized
and eventually converted to carbon dioxide and water, without emissions of
NO x ,SO 2 , HCl, dioxins, furans, or ash. The last residual organic compounds
are carboxylic acids, especially acetic acid (Luck 1999). Catalytic wet oxida-
tion employs catalysts to reduce the severity of the already-mentioned reaction
conditions. Compared to conventional wet air oxidation, catalytic wet air oxi-
dation offers lower energy requirements and much higher oxidation efficiencies.
Current catalytic wet oxidation processes rely either on supported precious met-
als and/or base metal oxide catalysts, or on homogeneous catalysts such as Fe
or Cu (Pintar 2003). Matatov-Meytal and Sheintuch (1998) state that a catalyst
for aqueous phase oxidation should have high oxidation rates, should be nons-
elective, should be physically and chemically stable in hot acidic solutions and
mechanically strong and resistant to attrition, and should maintain a high activity
for a prolonged use at high temperatures.

2.4 Membrane Filtration — Reverse Osmosis

Membrane filtration involves separation of the contaminant (solute) from a liq-
uid phase (solvent), typically water. The objectives of the process are reduction
in volume of waste, recovery or purification of liquid waste, and concentra-
tion and/or recovery of the contaminant. Membranes used to retain material are
based on molecular size and shape. Most common membrane filtration processes
include reverse osmosis, electrodialysis, and ultrafiltration.
Osmosis is a process in which water moves under osmotic pressure to establish
equilibrium in the ionic strength of solutions across a semipermeable membrane.
A thin membrane separates waters with different salt concentrations. The water
moves from the more dilute side of the membrane to the more concentrated side.
In reverse osmosis, water is forced through the semipermeable membrane from
the concentrated side to the dilute side against natural osmotic pressure, as shown
in Figure 6.1. The driving force for mass transfer process is the pressure gradient.
The rate of flow through reverse osmosis membrane is directly proportional to
effective pressure.
The different kinds of membranes include cellulose acetate membranes, which
have high flow rate per unit area. They are used as spiral modules, which are
made of large membrane sheets. Another type is polyamide membranes, which
have a lower specific flow rate and are manufactured to form hollow fibers in
pressure vessel.