Case study: Mining Company Reclaims Wastewater in Mexico

• By Renato Ramos, Peter Aerts, Enrique Guzman, Juan Pacheco
• Jun 14, 2010

Increasing water stress and the resulting competition for high quality fresh water sometimes limits or restricts the use of conventional water sources for industrial cooling water and boiler water make-up. Met-Mex Peñoles, a Mexican mining and metallurgical company, filled that void by reclaiming tertiary treated municipal wastewater.

The company is one of the first adopters of wastewater reclamation for boiler water make-up when it first made a request in 1994 to Industrias Islas to design, build and start up a water reuse facility. This plant, which has been in operation for 12 years, was the first of four reverse osmosis (RO) plants and was designed to treat 1 million liters per day (MLD) (0.27 million gallons per day (mgd) of domestic wastewater. This wastewater is the tertiary treated effluent of a biological treatment process operated by Met-Mex Peñoles. The source of the wastewater is an urban wastewater treatment plant, whose effluent comes from Torreón City.

The RO plants allowed the company to continue operating and increase the capacity of its metallurgical operations. This site is located in Torreón, Coahuila, a desert area in Mexico where water reclamation is an appreciated, sustainable option. The company uses the treated wastewater as process water for cooling towers, boilers, and for the zinc electrolytic process.

The first RO plant was designed with a six-four array FILMTECTM BW30-8040 with four elements in every pressure vessel and operated at 50 percent recovery. The pretreatment consisted of dual media filters, activated carbon filters and a cartridge filter with particle retention up to 1 micron. The design capacity for this plant was 425 L/m2/d (10.4 gal/ft²/d) with a maximum allowable percent salt passage of 2 percent (83 mg/L in the product with 2044 mg/L in the feed). The design pressure was 8.8 bar (124 psig) and increased during operation to 13 bar (190 psig) due to fouling. High concentrations of calcium and silica in the water contributed to the fouling. Operators cleaned the plant four times a year as recommended by the manufacturer. This plant ceased operations in 2000, when additional RO plants came online.

The production of treated domestic wastewater increased over time and has doubled since 1999 from 7.8 to 15.6 MLD (2 to 4.1 mgd). Because of Peñoles' good experience with water reuse, it built more RO facilities ─ three more over 10 years (see table). These later plants have a three-stage design and all use FILMTEC BW30-400 membranes, operating at 70 to 75 percent recovery. Half of the tertiary wastewater (7.8 MLD) is fed into the RO modules to produce 5.2 – 7 MLD (1.4 – 1.85 mgd) of demineralized water for boiler water make-up for medium pressure boilers.

The fourth plant was designed to obtain a permeate flow with a chloride content below 30 parts per million (ppm) and a conductivity less than 70 micro Siemens (µSiemens). This process has a different setup and consists of two RO plants that treat domestic wastewater from the effluent of a biological process to obtain 2.2 MLD (0.58 mgd). The conductivity is typically ~ 1650 µSiemens in the feed and ~ 65 µSiemens in the permeate.

	Plant 1	Plant 2	Plant 3	Plant 4	BT Plant
Purpose	Reclamation of tertiary effluent for boiler water make-up			Low chlorine, low conduct.	Brine treatment
Design capacity (gal/ft2/d)	10.4	8.4	10.4	10.4	9.85
Operation time (yrs)	6	12	9	3	3
Number of elements	40	98	140	140	80
Plant design	6/4	4/2/1-7	7/5/2-5	7/4/3-5	8/6/4/2-4
Percent recovery	50	70-75	70-75	70-75	70-75
Number of skids	1	2	2	2	1
Element types FILMTEC	BW30-8040	BW30-400-FR	BW30-400-FR	BW30-400-FR	SW30-HRLE-400
Water source	Domestic treated wastewater	Domestic treated wastewater	Domestic treated wastewater	Domestic treated wastewater	RO concentrate
Pretreatment	Dual media filters + carbon filters + cartridge filter	Multimedia filters + carbon filters	Multimedia filters + carbon filters	Multimedia filters + carbon filters	Lime carbonate softener + sand filters +IEX softeners
Type of typical foulant	Colloidal, organic and mineral	Colloidal, organic and mineral	Colloidal, organic and mineral	Colloidal, organic and mineral	Mineral
Frequency of cleanings (per yr)	4	4	4	4	2
Post treatment	Ion exchange polishing

Additionally, one brine treatment RO plant started in 2006 to treat the concentrate from the second through fourth RO plants. The pretreatment for the RO concentrates of Plants 2, 3 and 4 are carried out by lime softening. The magnesium present in the treated concentrates helps to reduce the amount of silica in the water. Following the lime and carbonate softening, the water goes through two multimedia filters and two ionic exchange columns that are regenerated with a sodium hydroxide solution. In the last step, pretreated brine is forwarded to the RO plant. The brine feed has a conductivity of ~ 6000 µSiemens. This plant has four stages with an 8/6/4/2-4 array design using seawater RO elements, or FILMTEC SW30HRLE-400 membranes. The obtained permeate flow is 1.1 MLD (0.29 mgd) and a conductivity of ~ 135 µSiemens. Besides having a low chloride concentration (less than 30 ppm), the percent recovery of the treated effluent is optimized to between 70 and 75 percent.

The use of treated wastewater from municipal sewage treatment plants can be a valuable diversification strategy for cooling water and boiler water make-up for industrial purposes. This strategy has several benefits, including:

• Overall cost is lower because fresh water treatment costs are high.
• Treated effluent from highly urban areas, which is more stable in quality than river water, requires less maintenance and process control.
• Permits and administrative burdens are often less for reuse.
• Public acceptance for this type of reuse is high.

Met-Mex Peñoles has achieved long-term reliable and stable operation using best-in-class RO membranes to treat municipal wastewater. Without these water reclamation plants, the company would not have been able to operate at its current scale, due to water scarcity.