Phosphorous regulations are getting tougher for clean-water plants. One way to drive down effluent phosphorous levels is through tertiary filtration.

For that purpose, compressible media filters have been proven effective and can achieve high filtration efficiency in compact spaces. The technology also has applications for treating combined sewer overflows and for some industrial processes.

SUEZ now offers the FiltraFast compressible media filter for wastewater and process water treatment. It includes compressible media that the manufacturer says can handle up to 10 times the hydraulic loading rate of traditional media filters. It functions as a high-rate downflow gravity or pressure filter, using only hydraulic loading to create the media porosity required; no mechanical compressing devices are needed.

The backwash sequence is designed to enable maximum recovery, extend media life and limit energy consumption. The process requires minimal maintenance and has low operating costs. Emily Gilbert, applications engineer for separations with SUEZ, talked about the process in an interview with Treatment Plant Operator.

TPO: What was the rationale for bringing this filtration technology to market?

Gilbert: There is a huge market for tertiary filtration in the municipal market, as many municipalities are being asked to achieve 0.1 mg/L phosphorus, especially in the Chesapeake Bay region and along the Great Lakes. SUEZ had historically offered a traditional media filter that was difficult to differentiate from newer technologies, like disc filters and compressible media filters. So we investigated an alternative solution that we could offer.

TPO: What enables this filter to process high flows in a small footprint?

Gilbert: t’s the unique media we use. It looks like a comet. It has a central 1 cm, hard polymeric bead with strings coming out each end that extend the total length to about 3 inches. This media is very easy to fluidize and clean to remove solids captured while the filter is running at the high loading rates. The media is also easy to compress. By feeding water from the top of the basin, you compress the media from a 36-inch depth by almost 50 percent. This presses the filter fibers together to achieve approximately 5-micron filtration.

TPO: Does the media have a life limitation?

Gilbert: It lasts 7 to 10 years, which is quite similar to traditional filter media and other compressible filter media.

TPO: What distinguished this compressible media filter in terms of simplicity?

Gilbert: Our filter does not need a mechanical plate to compress the media. It compresses on its own simply by feeding water at the top of the basin. That means fewer mechanical parts to maintain, fewer components that can break, and less metal to buy on the front end.

TPO: Besides that, what would you say are the advantages of this technology?

Gilbert: It’s very cost-competitive with disc filters and has much lower operating costs than conventional media filters. That’s mainly because of the high loading rate. Because our filter can run at 30 to 40 gpm per square foot, we have smaller tanks that require less space, and that means lower cost. Energy usage is very similar to a disc filter.

TPO: How would you characterize this filter’s performance efficiency?

Gilbert: The recovery is incredibly high because we don’t backwash very frequently, and the backwashes are very intense when they do occur. Our recovery is typically greater than 96 percent, so less than 4 percent of the water coming to the filter is sent away as waste. We can achieve even higher recovery rates if we design a little more conservatively. In some applications there is a limit on the amount of waste that can be discharged from the site. In such cases, we can design at a slightly lower loading rate and can get up to 98 percent recovery. It’s a very flexible solution. 

TPO: In basic terms, how does the filtration process work?

Gilbert: It’s very similar to a conventional media filter. The flow enters the top of the tank through an influent pipe and is distributed across multiple independent filter cells. Most of our systems have at least two cells that are independent of each other, so when one goes into backwash, the other one is treating the full flow. The water flows down through the media to a plate underdrain. The filtrate exits through nozzles at the bottom of the filter and enters an effluent pipe.

TPO: What about the backwash cycle? How does that operate?

Gilbert: During backwash, we reverse the flow, pumping clean water through the bottom pipe to fluidize the media. We backwash with air and water at first and then with water alone. While the filter is backwashing, the solids are collected in a filter trough connected to a backwash waste line. After the backwash cycle, we allow some time for the media to settle down before bringing the system back into operation.

TPO: Is there a sweet spot for applications of this technology?

Gilbert: The inquiries we’re getting are for phosphorus removal down to 0.1 mg/L, and any filtration application where influent solids are less than 30 mg/L TSS, similar to other filtration technologies.

TPO: Can this technology be used for CSO treatment?

Gilbert: Yes. It would enable treatment of CSO flows in a very compact footprint. The media are stored wet in between storm events.

TPO: What is this product’s commercial application experience in the field?

Gilbert: Our first installation was delivered earlier this year to a soil remediation client in New York with a flow rate of 2,200 gpm (3.2 mgd).