Application and progress of membrane desalination technology?
The total global water reserves are 1.386 billion km3, sea water accounts for 96.5%, and the surface water and shallow groundwater available to humans are only 0.79%, and the distribution is extremely uneven with regional and seasonal changes. In order to obtain fresh water from the sea, membrane technology was put forward first in the early 1950s. By the 1970s, seawater desalination technology was commercialized in the world. After product upgrading and technological innovation, it has become the most economical Seawater desalination and high-salinity brackish water desalination technology.
1. Application overview
Seawater desalination refers to the desalination of 35000mg/L of seawater to drinking water below 500mg/L. At present, the world’s installed seawater desalination methods mainly include multi-stage flash evaporation (MSF), multi-effect evaporation (MED) and reverse osmosis (RO). Over the past half century, they have fed more than 100 million people in the world and promoted The economic and social development of arid desert regions and coastal areas of developed countries.
According to statistics from the International Desalination Association, as of the end of 1995, there were 11066 devices with a daily water output of more than 100 m 3 in the world, and 20.3 million m³ of fresh water was produced every day. As of the end of 1997, the total number of installations was 12,451, and the daily water production was 22.73 million m³, of which 13.37 million m 3 was desalinated seawater and 58.4 million m³ was desalinated brackish water, and the annual growth rate was 10-30%. As far as seawater desalination is concerned, the installed capacity is dominated by MSF, but RO has developed rapidly in recent years. Except for very large installations, MSF has had very few contracts since 1995. MSF dropped from 84.4% in 1989 to 76.04% in 1997. The RO increased from 5.6% in 1989 to 14% in 1997. MED accounted for 6.7% in 1989 and only 5.47% in 1997.
RO has an absolute advantage in desalination of coastal brackish water, accounting for 76.23%, and the investment and water production costs are lower. RO also accounts for about 65% and 94% of wastewater and water purification respectively. After 1995, the newly added installed capacity RO for seawater desalination and brackish water desalination was 90%. The largest RO desalination plant was built in Saudi Arabia, with a daily production of 128,000 fresh water per m³. The largest RO brackish water desalination plant was built in the United States, with a daily output of 270,000 m³.
At present, the annual sales of seawater desalination devices have reached more than 10 billion U.S. dollars, and they have continued to develop suppliers at an annual growth rate of about 20%. The application areas are mainly the Middle East, the Mediterranean region and the Caribbean, followed by Southeast Asia and North Africa.
2. Progress in reverse osmosis desalination technology
Reverse osmosis will become the main seawater desalination technology in the new century. Stable and reliable engineering and low water production costs are the main reasons for attracting users.
2.1 Improved membrane and module performance
Companies producing membrane modules in the world still attach great importance to the technological innovation of RO membranes and modules. The aim is to develop new membranes that are resistant to oxidation and bacterial erosion to improve the water production and desalination rate of membranes and modules. These efforts have made certain progress. Hyde Energy’s SW series reverse osmosis membrane is a membrane product specially developed for seawater desalination. Its average desalination rate has reached over 99.2%, and the average permeated water volume can reach 22.7M3/d.
2.2 Low project investment
The price of membrane modules in 1990, converted according to the consumer price index, was only 40% of that in 1973. After 1990 there was a significant decline. Based on the production of fresh water from standard seawater, the current project investment is MSF at 1800-2000 US $/m³ .d, low temperature MED at 1100-1600 US $/m³ .d, RO at 700-900 US $ m³ .d, and The RO seawater desalination plant has been constructed quickly, and the RO seawater desalination plant of 10 thousand m³ can be delivered to use within 7 months.
2.3 Energy consumption reduction
Using a work exchanger (WOrk Exchanrye Enerar Recovear), the pressure of the high-pressure concentrated water discharged from the RO module is recovered and transferred to the module feed water, and its conversion efficiency can be as high as 89-96%. Scott A. Shumway reported that a new type of energy recovery device has been successfully applied to 13600 m³/ d and 5000 m³/ d reverse osmosis seawater desalination units, with a process energy consumption of 2.6kwh / m³. Gord F. Leitner pointed out that with pretreatment energy consumption, the total energy consumption is 2.83kwh/m³. This is an outstanding progress in technology in recent years.
Between the desalination part of the reverse osmosis device and the stage or stage, an energy recovery turbine can be used to increase the water inlet pressure of the next stage or stage and increase the water production. The principle is shown in Figure 1. Steven J. Duranceau et al. reported that in 1996, Florida Water Service Company renovated the existing 15000 m³/d matrine water desalination on Marco Island. By using the energy recovery turbine between sections, the water output of the system increased by 3780 m³/ d, an increase of 25%.
The energy recovery turbine between stages is suitable for reverse osmosis desalination of brackish water with a salt content of 7500-10500mg/L, and the desalination energy consumption is reduced to 0.82kwh/m³.
In the reverse osmosis seawater desalination process, nanofiltration (NF) is used as the pretreatment, that is, the NF-RO system. NF removes part of the hardness and TDS, thereby increasing the RO operating pressure and water recovery rate, which can further reduce energy consumption by 25% , Water production cost can be reduced by 30%.
At present, various desalination methods cannot compete with RO in terms of energy consumption. RO is 4-5kwh/m³, ED is 14-16kwh/m³, MED is 9-10kwh/m³, MFS is 12-14kwh/m³.
3. Application of desalination and concentration in wastewater recycling
The deep desalination technology of seawater desalination into high salt water, in order to improve the recovery rate of raw water, there are also certain techniques for the concentration of the discharged concentrated water. This is very different from the technical characteristics of ultrafiltration and microfiltration to treat polluted or slightly polluted water. Wastewater treatment, especially inorganic wastewater treatment, has many common technologies, but special attention should be paid to the degree of applicability of the membrane to the material and liquid environment.
Reverse osmosis treatment of electroplating wastewater and radioactive wastewater is very mature. Since the 1970s, it has been used for nickel-plating rinsing wastewater treatment, and then used for chromium plating, copper plating, zinc plating, cadmium plating and other wastewater treatment. The American Chicago API Process Company uses B-9 aromatic polyamide hollow fiber membrane modules to treat Watt Ni rinsing water. The wastewater contains Ni 2+ 650mg/L, which is concentrated by RO 20 times to 13000mg/L. The separation rate of Ni 2+ is 92%. Beijing Broadcasting Equipment Factory uses cellulose acetate membrane to treat bright nickel and dark nickel rinsing wastewater, and the Ni 2+ in the wastewater is 1510-2400mg/L. The recovery rate of Ni 2+ in the system is> 99%.
As the power consumption of ED seawater desalination is three times that of RO, ED is less and less used in seawater desalination, and it still has a greater competitive advantage in the desalination of brackish water. Ion exchange membranes have strong acid, alkali, and oxidation resistance, and are widely used in the treatment of wastewater containing acid, alkali, and salt.