Page 51 - Industrial Wastewater Treatment, Recycling and Reuse

34 Industrial Wastewater Treatment, Recycling, and Reuse

2. Applications for which alternative techniques are available, but mem-
branes offer a clear technical and commercial advantage. This is the case
in the production of ultrapure water and in the separation of certain food
products.
3. Applications where there is no reasonable alternative to membrane pro-
cesses. This is the case in certain drug delivery systems and artificial organs.
For membrane separations, established process applications include gaseous
separations such as O 2 /N 2 ,H 2 /CH 4 , Olefin/N 2 , and liquid separations such
as desalination and various applications in the beverage industry. In waste-
water treatment, MF, UF, and RO have established niche areas for them-
selves. Apart from these, numerous applications in combined processes such
as MBRs are being increasingly used.
Compared to conventional procedures, membrane processes are often
more energy efficient, simpler to operate, and yield higher-quality products.
The environmental impact of membrane processes is relatively low. No haz-
ardous chemicals are used in the processes that require a discharge step, and
there is no heat generation. However, membrane separations have their own
limitations that need to be carefully weighed before considering the appli-
cation. These include the following:
1. A major disadvantage of membranes, especially in water- and
wastewater-treatment processes, is that the long-term reliability has
not been completely proven.
2. Membrane processes sometimes require excessive pretreatment due to
their sensitivity to concentration polarization, chemical interaction with
water constituents, and fouling.
3. Membranes are mechanically not very robust and can easily be destroyed
by a malfunction in the operating procedure.
4. Another critical issue is the process cost. In general, membrane processes
are quite energy efficient. However, the energy consumption is only
part of the total process costs. Other factors determining the overall eco-
nomics of a process include the investment-related cost, which is deter-
mined by the cost of the membranes and other process equipment and
their useful life under operating conditions, and various pre- and post-
treatment procedures of the feed solutions and the products.
In wastewater treatment, mostly MF membranes are used for the removal of
large size species and RO membranes to obtain pure water. There are many
other variations possible in wastewater treatment such as membrane distil-
lation, which is commercially applied for removal of hydrochloric acid from
pickling wastewater streams. A large number of newer installations are in the