Volume 43 Number 7
World Spotlight: RO Systems for Surface Water Treatment -- Judging the Progress Made in Brazil
Mucuri is situated in the midst of a jungle with beautiful beaches nearby. With more than 26,000 inhabitants, the area is rich in tourism and offers a delicate blend of both urban and rural landscape.
Currently, the Bahia Sul mill generates 555 tons per hour of steam -- or 280 cubic meters per hour (m3/hr) of demineralized water -- used in its production process. With this large demineralized water usage, it was imperative to find cost-effective ways to minimize water production costs, increase reliability of the demineralized water supply and increase water production capacity, if possible. An additional variable was that the make-up or raw water source is a river containing high levels of organic matter and total dissolved solids (TDS) during some parts of the year.
Make-up water at Bahia Sul is produced by taking water from the Mucuri River and passing it through a chlorine detention tank to prevent algae and bacteria growth. This also oxidizes and breaks up organic compounds found in the water. The chlorinated water is mixed with ferric sulfate and lime prior to entering a precipitator. The precipitator clarifies the water, which is then filtered through gravity sand filters. The clarified, filtered water is next passed through activated carbon filters that remove organic matter and traces of chlorine that may remain in the treated water (see Figure 1).
The four reverse osmosis (RO) units at Bahia Sul's station were installed in 1991. The setup can be seen in Figure 2. The RO system is rated at 80 m3/hr per train in order to supply two mixed bed demineralizer trains. In the past, membranes from more than one producer, including Filmtec, TriSep and Hydranautics, have been included in the mix used.
By late 1998, conditions of the installed membranes were such that a replacement was required. The plant was unreliable and the cleaning frequency was very high because of high levels of biofouling causing a high pressure drop on the system. Bahia Sul decided to try new fouling resistant (FR) membranes in one of the trains in conjunction with special operating and cleaning conditions. By March 1999, these membranes were installed on two trains. The reasons for choosing this approach were:
Technology & elements
The development of biofilms and the role they play in membrane processes may be the most misunderstood and underestimated factor in treatment of surface water and other industrial water systems by RO/nanofiltration (NF) technology. Simply stated, a biofilm consists of microbial cells (algae, fungal or bacterial) and the extracellular biopolymer -- slime -- they produce. Generally, bacterial biofilms are of most concern in RO industrial water systems because they're responsible for membrane fouling. This is due to the minimal amount of nutrients required for many species to proliferate at an alarming rate. Under ideal conditions a single bacteria, reproducing every 20 minutes, can produce progeny in excess of 4.71 x 1021 colony-forming units (cfu) within 24 hours.
Microorganisms can be found in both bulk water and on the surface of industrial water systems. Bacteria attach to surfaces by proteinaceous appendages, and once they have "glued" the cell to the surface, detachment of the organism is very difficult. One reason bacteria prefer to attach to surfaces is the organic molecules adsorbed there provide nutrients. Once "stuck," the organisms begin to produce slime. The amount of slime produced can exceed the mass of the bacterial cell by many times and tends to provide a more suitable protective environment for the organism's survival.
Once bacteria begin to colonize surfaces and produce biofilms, numerous symptoms begin to appear that result in membrane fouling or decreased water treatment system performance. To reduce that risk, developmental FR elements were made available to Bahia Sul for testing during the summer of 1999. One train, 65 m3/h in capacity, had spiral-wound elements replaced FR elements and the other train was installed at the same time with standard elements in order to do a side-by-side comparison study. These elements have now been in operation for more than a year along with the original standard ones.
Total system approach
As is common of most surface waters, the Mucuri River suffers a huge shift in organic matter (OM) levels and TDS throughout the year. For example, OM could change between 1-10 parts per million (ppm) and TDS between 250-700 ppm. It's very difficult to absorb these changes in pretreatment. That's why a fouling resistance membrane would be desired. It’s clear, however, that any membrane technology couldn't operate under these conditions for an extended period of time without significant performance decline. For close follow-up on the system, a special program was established to define specific operating conditions and cleaning procedures.
The following operating parameters were established:
1. Reject flow should be higher than 20 m3/h in order to achieve a turbulent flow on the brine channel to decrease the possibility of fouling (sweeping effect). This factor is related with the flux -- or liters of water per square meter of membrane per hour -- through the membrane surface.
Besides the above information, total organic carbon (TOC) is measured once a week and silt density index (SDI) daily.
Because of the low feed flow through the carbon filters and the lack of frequent sanitation, a microbiological study was done in May 1999 to review the performance of the carbon filter. Levels of bacteria, cfu per milliliter (cfu/mL), were measured before and after the carbon beds. No cfu/mL were found before the filters, and between 1×103 and 1×104 cfu/mL were present after the carbon beds. This is a common problem in carbon filters without an appropriate sanitation program (see Figure 3). The carbon bed will remove any residual chlorine in the top portion of the bed. The bottom portion of the carbon bed will contain a significant amount of organic species and will promote rapid bacteria growth. This ready source of bacteria will eventually result in biofouling of the RO membranes.
Because of the high biofouling potential, an 8-step cleaning methodology has been used that involves high and low pH cleaning, disinfection with DBNPA -- 2,2 dibromo 3 nitrilopropionamide -- and flushing with permeate water.
Criteria to stop the high pH cleaning was later implemented. This was based on the color of the cleaning solution after passing through the membranes. After passing a first load, it's common to obtain a color between 300–800 Platinum-Cobalt (Pt-Co) at 440 nanometers (nm). When the color drops below 50, the high pH cleaning is stopped.
Trains A and C have shown excellent, consistent performance since start-up in March 1999. The design throughput, pressure drop, normalized flow and water quality have been achieved with a product quality of < 6 microsiemens per centimeter (uS/cm) on the permeate. Figures 4-6 show salt rejection, differential pressure and normalized flow performance from start-up through 12 months of operation for Trains A and C.
After a year, pressure drops in the first stage changed from 1.0 bar to 1.8 bars in Train A (standard membrane) and from 1.0 bar to 1.5 bar in train C (FR membrane), which represents the longer life expected for FR membranes. This would take into consideration that Train C has been working to produce 20 percent more permeate water than Train A, as shown in Figure 7. Salt rejection and permeate flow are almost the same for both membranes.
After reviewing feed water composition, RO recovery and the Langelier saturation index (to predict CaCO3 stability), it was decided a reduction of anti-scalant consumption of 9,000 liter per year (US$36,000 a year) was feasible.
Since the RO system has been operating with new membranes and operating conditions, regeneration frequency of the demineralizers has been decreased substantially. They now need to be regenerated about every 12 days, compared to six days before. By extending time between regenerations, it's expected to eliminate use of 14,250 liters of hydrochloric acid (35 percent HCl = 16.7 tons) and 17,100 liters of sodium hydroxide (50 percent NaOH = 26 tons) annually.
The success of this project is due to the following factors: special operating conditions, cleaning and disinfection procedures, membrane technology, close system performance monitoring/analysis and people training/commitment. In other words, a total system approach has been taken in order to improve system reliability. As a result of the excellent performance obtained, Bahia Sul decided to replace one more train with the new membranes.
The author would like to thank Rodolfo Bayona Plata, Latin American technical service and development product steward based in Midland, Mich. He can be reached at (989) 636-7418 or email: email@example.com
1. FILMTEC Membranes, Technical manual, The Dow Chemical Company.
2. Zahid, Amjad, "Reverse Osmosis: Membrane Technology, Water Chemistry and Industrial Applications," Van Nostrand Reinhold, New York, 1992.
3. Coker, Steven, "Winning the Battle Against Biofilm Formation," Technical Literature Form No. 609-00261-199x, The Dow Chemical Company.
4. Technical Literature Form No. 609-24010-498QRP, FT30 Reverse Osmosis Membrane Biological Protection and Disinfection, The Dow Chemical Company.
About the author
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