Cooling water supplied to reverse-osmosis (RO) and other demineralization systems from wells and rivers often must be pretreated to avoid fouling the RO membranes and compromising the performance of the water treatment system and downstream cooling operation. The standard practice is to use a multimedia depth filtration system, followed by disposable cartridge filters ahead of the RO system. However, conventional multimedia systems tend to be large and complex, and they require a substantial amount of backwash water to keep them clear of debris.
Using mechanical filters instead of multimedia systems can minimize space requirements, reduce piping and valving complexity, and increase efficiency, resulting in lower backwash water and maintenance requirements. While mechanical filtration has been used in a number of applications for more than 30 years, conventional screens were capable only of filtering out large particles (40 µm and above). Developments in the manufacturing technology of woven stainless steel screen over the past 10 years, however, have enabled mechanical filters to remove particles in the 1- to 5-µm range, making them suitable for use as prefilters to RO systems. Some mechanical filters incorporate an inline filter body that houses a self-cleaning screen to capture the suspended solids in the water stream.
Filter OperationSelf-cleaning mechanical filtration systems measure the differential pressure (DP) across the screen surface to determine when to initiate the cleaning cycle. As the water flows from the inside of the screen out, suspended material is collected on the inside screen surface. A typical installation will have a clean-condition pressure drop of 1 to 2 psi. As the silt builds on the screen, the differential pressure rises. When it reaches 7 psi, the cleaning cycle is initiated by a differential pressure switch.
The self-cleaning mechanism consists of a hollow shaft down the center of the filter body. This shaft has scanner nozzles that extend to approximately 0.125" from the screen surface. Some designs incorporate spring-loaded nozzles, which can enhance the system’s cleaning ability and reduce the amount of water used for backwash. In these devices, the shaft is sealed on one end and opens to an exhaust chamber at the top of the filter. When the exhaust valve is open, the differential between the supply pressure and the atmospheric pressure at the exhaust-chamber outlet creates a powerful vacuum effect at the end of each nozzle. The vacuum sucks the material from the screen surface through the shaft and out of the exhaust valve.
The start of the cleaning cycle causes the exhaust valve to open to the atmosphere and the electric motor to start. The motor simultaneously rotates and moves the shaft axially, so the nozzles cover the entire inner screen surface during each cleaning cycle. The cleaning cycle last 17 to 45 sec depending on the filter type.
Evaluation of Filter Efficiency. To evaluate the efficiency of this filter design for a given application, a scattered-light laser particle-counter can be used to establish the quantities and distribution of particles in the water. In this test, the water sample to be checked is dropped into a beaker of particle-free distilled water and ultrasonically cleaned for up to 30 sec. The beaker is then placed in the in-situ particle-counter, where a laser beam is passed through it to scan the volume for particles. Two different lenses are used to determine the size and number of particles in the 1- to 16-µm range and the 16- to 100-µm range. The size is measured and displayed on the computer screen and printout. This optical testing method appears to be precise. Testing of replicate samples has shown that the variation in particle counts is less than 5 percent.
Case in Point: Filtering a Well Water SourceA coal-fired utility plant near Las Vegas purchases water from a local water district that draws from a deep well. The water is supplied to an RO system. The original pretreatment for the system was a sand media filter; however, the plant experienced media carryover and high backwash water rates. Downstream of the sand filter, 1-µm cartridge filters were installed as part of the RO skid. The system was designed for 250 gal/min system flow.
The plant elected to eliminate the sand filter upstream of the cartridge filters to reduce the carryover problems, essentially using the raw well water for the RO feed. However, this setup required the 1-µm cartridge filters to be changed every two weeks, a significant maintenance expense.
The plant engineers reviewed the available filtration options and selected a self-cleaning screen filtration system from Amiad Filtration Systems Ltd., Oxnard, Calif., to pretreat the RO system. After installing a 6" self-cleaning mechanical filter from Amiad, the company operated the system for some time, then gathered water samples and analyzed them using a laser particle-counter. Results showed that the screen, which was rated by the filter manufacturer at 10 µm, removed 99 percent of the particles larger than 5 µm and 86 percent of the 1 µm and larger particles (figure 1). The total suspended solids (TSS) content of the raw well water was 10.46 ppm. The filter effluent TSS was determined to be 0.001 ppm. This system has been in operation for 10 years with no reported operating or maintenance issues other than normal wear and tear of the equipment.
Case in Point: Filtering a Reservoir Water SourceA coal-fired utility plant in eastern Wyoming draws water from a river-fed reservoir into a holding pond on the plant site. The original plant pretreatment system included a bank of carbon media filters followed by several 10-µm bag filters and 5-µm cartridge filters. The water then was fed into three RO machines. The plant was experiencing problems with plugging of the cartridge filters due to suspended solids material channeling through the media filter bed, as well as intermittent problems with the carbon media carrying over into the process stream.
A self-cleaning mechanical screen filter from Amiad was installed in June 1998. The system uses a single filter with 8" flanges to treat 800 gal/min. Particle size distribution analysis of the filter inlet and effluent stream has shown that the mechanical filter is removing more than 90 percent of the suspended material, reducing the TSS from 0.49 ppm to 0.04 ppm. Laser particle-counter analysis detected particles at 10 µm and below in all bins. However, the particle counts in the 5- to 10-µm bin were low - all less than 40 particle counts - and accounted for 0.03 ppm mass.
A primary factor in justifying the mechanical filter in this application was the assurance that if a major upset occurred in the carbon filters resulting in carbon carryover, the mechanical filter’s stainless steel screen offered 100 percent protection against any media reaching the RO membrane. With bags and cartridges, media can pass through torn bags or improperly seated cartridges
When designing a prefiltration system, it is imperative that a detailed particle size distribution (PSD) analysis be performed. Well water has a fairly consistent water quality with hard, suspended particles, and one PSD is generally adequate. Surface water supplies, depending on the type of water, regional location and level of suspended solids, often require a pilot test to be performed on-site.
In conclusion, mechanical filters offer several advantages over traditional multimedia systems.
- The capital cost of the equipment is 30 to 50 percent
- The water required for backwash is less than 1 percent on mechanical
filters compared to 5 to 7 percent for a media filter.
- Energy costs are lower for a mechanical system, with a 2 to 7 psi
operating range across the screen filter.
- Flocculation chemicals are not required.
- Mechanical systems require a smaller footprint and less complicated
valve arrangements, which is important in many retrofit applications.
- Mechanical screens have a service life of five to 10 years.
- Screens protect RO membranes, ensuring that suspended solids do not