Water Treatment Desalination Systems Improve Efficiency and Maintenance Costs
When a Colombian mining company decided to replace their outdated desalination systems, they contracted ITT’s Water Equipment Technologies unit for reverse osmosis systems that provided improvements in energy and maintenance savings.
Puerto Bolívar is the most important coal terminal in Latin America and one of the largest in the world. It is located on the coast of La Guajira State, in the northeastern part of Colombia on the Caribbean Sea. The port serves the Cerrejón mine, the world’s largest export open pit coal mining operation with coal production capacity estimated at 22 million metric tons per year.
The geographically remote region around Puerto Bolívar is very arid, with desert-like conditions and little or no water available for the port’s operations. The requirements for water include potable water for the port’s workers and residents, as well as water for utilities and for industrial use. Because almost all of the coal from the Cerrejón mine is exported to Europe and the United States, an average of seven coal trains per day arrive from the mine (150 kilometer away) to load the ships destined for export markets. This loading operation requires a large amount of water to settle the carbon dust in the loading process—which can load the ships at a rate of 4,800 tons per hour.
In an effort to upgrade the production of water at the port, the company, Carbones Del Cerrejon, decided to replace the three existing desalination systems. The systems that needed replacement were approximately 20-years–old and employed a multi-flash evaporative process for desalinating sea water. In this process, the raw sea water is heated and through an evaporation process employing a vacuum, purified water is produced. The old systems used electricity to power the process, but were very inefficient—requiring about 48–50 kW of electricity for every gallon of water produced when the systems were new. As the systems aged, they became less efficient, costing even more in energy. In addition, the maintenance on the multi-flash evaporative systems was an ordeal. Maintenance workers had to use a great deal of chemicals to clean the system. As Mainor Vega, product manager for Latin America for the Water Equipment Technologies (WET) unit of ITT Sanitaire noted, “the amount of caustic used for cleaning was in the tons.”
After reviewing a number of competing bids and technologies, Carbones Del Cerrejon purchased three reverse osmosis (RO) desalination systems from WET. Each unit has the capacity of producing 550 m3 of purified water per day.
Desalination through RO works by cross filtration. Pumps apply enough pressure to overcome the osmotic pressure so that water molecules travel through a series of membranes. After the raw water has traveled through the process, there are two flows leaving the membrane; one is the permeate flow, or purified process water, and other is reject water with a high concentration of ions that is discharged back into the sea. A typical RO system has a recovery rate of 35–40 percent from the raw water.
Because of the arid region, raw water is drawn from an open intake seawater design—also used in the older system. In the RO system design, the raw water is pre–filtered to reduce suspended particles in order to protect the membranes in the RO system. Six tanks with multimedia sand filtration is used for this task. Each tank is loaded with seven different layers of sand as the filtration media. The particulate matter is reduced to a point where the 5 micron filter in the reverse osmosis unit is used for the final polishing prior to going into the membrane process.
Smaller footprint. The reverse osmosis systems from WET drastically reduced the footprint for the port’s desalination facilities. The multi-flash evaporative systems were approximately 20–30 ft. high, 40 ft. long and 20 ft. wide. The RO systems’ small size (approximately one-third the size of the old systems) allowed for all three systems to be located in a single, air-conditioned room that is protected from the elements. “Unlike the old systems that were exposed to sandstorms and carbon dust—which created maintenance headaches—the customer is now able to keep the systems in a cleaner environment,” Vega said.
Maintenance and energy use. Maintenance costs for the new RO systems are lower in a number of areas. The old multi-flash evaporative systems required about six separate pumps. The RO systems work with just two pumps: A flush pump that works only for 15–20 minutes every 48 hours, and a second, high pressure pump to power the reverse osmosis process.
Because of the pre-treatment scheme used for the RO system, requirements for system cleaning also were greatly reduced. The old system was cleaned two to three times per year, depending on the amount of efficiency-reducing fouling that occurred on the internal parts. The required maintenance cleaning on the RO systems is about once per year, with the amount of chemicals used reduced from “tons” to about 50 lbs. per system.
The energy needed to produce water also has dropped dramatically. The new RO systems use about 18–19 kW per 1,000 gal. of water produced as opposed to the 48–50 kW for the multi-flash evaporative systems. Vega estimated that the customer would save more than $200,000 per year per system in just energy costs alone. When combined with maintenance savings, the overall life cycle cost of the RO systems is very low in comparison to the systems they replaced.
Although, the purified water from the multi-flash evaporative systems has a lower salinity level than from the RO systems, the salinity level of the RO water is approximately 200 ppm—well under World Health Organization standards of 1,000 ppm. Therefore, WET’s seawater RO systems installed in Puerto Bolívar are producing purified water of excellent quality.
According to Vega, about one-third of the water output will be used for the work camp and utilities. The remaining two-thirds will be employed in settling the carbon dust from the coal transshipment operation.