Another important aspect is the capital cost of desalination plant construction. An MSF or MED
plant with a capacity of 27,000 m
3
/day costs approximately $40 million. An RO plant with a
much higher capacity, approximately 100,000 m
3
/day, would cost around $50 million. So,
initially a higher cost would be required for an RO plant, but the end result would be a
significantly higher production rate
27
.
Environmental Impact. One other aspect in the implementation of a desalination process is its
impact on the surroundings. There are two main issues: the atmospheric emissions related to
mechanical or thermal energy input and the brine discharge
30
. As discussed above, RO has the
lowest energy requirements of the three main desalination processes, which means that a power
plant supporting an RO process would have lower atmospheric emissions than power plants
supporting MSF and MED
30
.
The overall impact of brine discharge is determined by its temperature, salinity, and chemical
content
30
. It is detrimental to the environment to introduce heated brine to a water source
because it negatively affects the water oxygen content, which, in turn, affects the water
ecosystem. Here, RO has an advantage because no additional heating is involved; the brine exits
the process at essentially the same temperature as the feed. However, MED and MSF employ
heating, and, therefore, the brine is at least 10°C warmer than the feed stream
30
.
Salt content also negatively affects the oxygen content of water
30
. Since RO processes typically
have a larger capacity and produce more desalinated water per day than the other processes, the
RO process results in the highest rate of brine discharge. However, all of the processes are
ultimately removing the same amount of salt and returning it to the original source, just at
different rates. So, the effects of all three processes are approximately the same
30
.
Finally, any chemical additives to the water should be considered in assessing environmental
impact. All three processes can employ biocides which are discharged
30
. RO processes can also
require anti-scaling additives to enhance membrane performance
30
. On the other hand, the
distillation processes use anti-foaming agents
30
. Therefore, none of the three methods are clear
cut winners when it comes to reduced chemical impact on the environment. RO, MED, and
MSF brine discharges all have high salinity and chemical additives, but when it comes to
discharge temperature, RO is the least destructive to the surroundings. This fact, coupled with
reduced emissions due to lower power consumption, makes RO a more environmentally friendly
option
30
.
Future Prospects for Membranes
With the proven success of membranes in the water treatment arena, membrane technology
continues to advance. Major problems still needing attention are membrane fouling and
membrane chemical stability. Reduced fouling would make membranes even more cost
effective by extending their operational lifetime and lowering their energy requirements. Work
in this area has focused on surface modification of membranes and increasing the pretreatment of
the feed water before it reaches the membranes. The chemical stability of membranes is also
being studied
35
. Improving the polyamide TFC membrane’s tolerance to chlorine would reduce
operation costs by eliminating pre-treatment dechlorination steps.
In addition to waste water treatment and desalination, new applications of membranes for water
purification are being pursued. One example is the purification of produced water, which is
13