Membrane and Desalination Technologies - TCE Moodle Website

Membrane Technologies for Oil–Water Separation 651
Specifically, nanofiltration is effective in removing divalent ions of Ca 2þ ,Mg 2þ , and SO4 2 ,
which are responsible for high water hardness, with average rejection efficiency of more than
95%. Nanofiltration may provide an efficient approach for water softening of selective water
streams that could be preferable to wider scale application of lime-softening technology. In
addition, nanofiltration is effective in the removal of total organic carbon. This should be
sufficient to remove acute toxicity associated with these materials in process-affected waters.
For the fouling consideration, the fouling in this membrane type is reversible. However, the
membrane permeate flux could be fully recovered (61).
3.4. Reverse Osmosis
Reverse osmosis is one type of membrane filtration that has been applied for treating a
wide variety of industrial effluents with the objective to separate oil from water. Sridhar et al.
(62) used the pilot-scale reverse osmosis to determine the feasibility of the process for
treating the wastewater from a vegetable oil industry. The system was found to be very
suitable for treating oil effluents having total dissolve solid (TDS) concentrations upto 52,215
mg/L. Reverse osmosis has been found to be a very promising separation process for
treatment of oil industry effluent and water recovery due to the high fluxes obtained alongside
with the significant rejection of TDS, COD, BOD, and color.
Application of reverse osmosis units in conditions when the feed may be contaminated
with crude oil and fuel oil spillages with feedwater comprising NaC1/water solutions of
2,000–35,000 mg/L concentration is also reported (63). In this work, hydrocarbons retained
in solution in water were not found to exhibit damaging effects on the performance of the
reverse osmosis membranes. However, it was reported that diesel contamination possibly
posed harmful effects with a capacity to reduce membrane fluxes to zero if present in large
concentrations for even short periods of time. Hexane, which is one of lighter crude oil
fraction can also cause serious deterioration of the performance of reverse osmosis membranes
when in contact, in pure or emulsified form. The damage is worse in more concentrated
hydrocarbon mixtures and at longer exposure times. The damaging effects of the
hydrocarbon contaminants are also dependent on types of membrane. Hodgkless et al. (63)
reported that the brackish water membrane suffers substantial reductions in flux and roughly
proportionate increases in salt passage. While the seawater membrane undergoes larger
deleterious effects on salt passage and much lower reductions in permeate flux, it appears
that the damage caused by exposure to hydrocarbons is difficult to reverse.
3.5. Integrated Membrane System
3.5.1. Combination of Ultrafiltration/Reverse Osmosis Processes
Generally, direct application of reverse osmosis for processing of oily wastewater would
require some physical and chemical pretreatment, depending on the used module configuration.
The simplest pretreatment would be for a tubular module, and can be done with an
“equalization tank,” from which the free-floating oil and the settleable solids are removed
(64). Application of ultrafiltration as a pretreatment stage before reverse osmosis should