The WWETCO FlexFilter From WesTech Can Combine Tertiary And CSO/SSO Treatment

A high-rate compressible-media filter from WesTech can provide tertiary treatment, then easily switch to treat periodic wet-weather CSO and SSO flows.
The WWETCO FlexFilter From WesTech Can Combine Tertiary And CSO/SSO Treatment
A WWETCO FlexFilter system installed in the field.

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Clean-water agencies charged with treating heavy flows from storm events face large capital investments for processes they may use only a few times per year.

Now, WesTech Engineering offers a filtration technology that can treat flows with a wide range of TSS content. It is flexible enough to provide continuous tertiary treatment during dry-weather flow conditions, yet switch over easily to handle weather-related flows such as combined sewer overflows (CSOs) and sanitary sewer overflows (SSOs).

The WWETCO FlexFilter is a gravity-flow system that uses compressible media. The varied porosity of the filter bed allows the system to treat flows both high and low in solids. Its passive design requires no chemical additions and it functions with minimal operator attention.

WWETCO operates as a wholly owned subsidiary of WesTech. Jim Hanson, vice president and national sales manager for municipal products with WesTech and also president of WWETCO, talked about the technology in an interview with Treatment Plant Operator.

TPO: What need is this technology intended to fulfill?

Hanson: Municipalities are challenged with tighter regulations while their staffs often are decreasing through attrition. They are having to do more with less. This is a passive system that allows a plant to treat flows without requiring more operators or the addition of chemicals.

TPO: How does this filtration technology function in a combined role in tertiary treatment and in treating higher flows like CSOs and SSOs?

Hanson: Municipalities dealing with CSOs and SSOs face large capital investments and major construction to treat flows that only occur a few times a year. The beauty of the FlexFilter is that it can be used for tertiary treatment and then flip to the CSO or SSO mode seamlessly and automatically. One day it functions as a tertiary filter producing reuse-quality water. Then when you have a storm event, it simply switches over. It’s a way for plants to make use of that capital investment for wet-weather flows 365 days a year.

TPO: What other applications are appropriate for this technology?

Hanson: We can do primary treatment — the filter will handle a high solids loading and hold a large volume of solids before it requires a backwash. We can also handle stormwater treatment, raw water pretreatment for drinking water systems and industrial water pretreatment, either for incoming plant water or ahead of ultrafiltration or reverse osmosis membranes. In essence, anything a traditional gravity media filter or a sand media pressure filter can accomplish.

TPO: What does the compressible filter media consist of?

Hanson: The media consists of balls of a bicomponent fiber. There are 15,000 individual fibers stapled together to create each single ball.

TPO: How does the filtration process work?

Hanson: Our process uses no mechanical actuators or other moving parts. We rely on incoming hydraulic forces. The water coming in onto a flexible membrane actually squeezes the media, causing the compression. Once the water overflows into the media, the filtration process begins. There is no ramp-up period after bringing the system online. It treats the flow in a natural and passive way. You get the desired level of treatment right out of the gate until you shut it down.

TPO: How exactly does the compressible media capture the solids?

Hanson: As the bladder compresses the media, it creates a compression gradient from the bottom of the media to the top. The bottom is compressed the most, and the top of the media bed is actually under no compression. The larger particles are captured by the loosely compressed or uncompressed media, and the finer particles are captured down deeper where the media bed is at its highest compression. It’s the pressure gradient that allows the filter to capture a high volume of solids.

TPO: What happens during the backwash cycle?

Hanson: When the filter needs to be backwashed, it’s simply drained down. The flexible membrane relaxes, the media bed becomes decompressed, a low-pressure blower is activated along with a little backwash water, and the solids are carried off through an airlift pumping action.

TPO: Do you have an example of how this filter has performed in field conditions?

Hanson: The City of Springfield, Ohio, ran a pilot test from October 2010 to June 2011 in which they treated 16 wet-weather CSO events. The influent TSS in those events ranged from as high as 500 mg/L to as low as 150 mg/L. The filter produced an average effluent TSS of 22 mg/L. During that pilot study, they did short-duration and long-duration tests. They even ran the filter through the winter with the media effectively frozen at the start of the wet-weather event.

TPO: Based on the test results, did the city install the technology in full scale?

Hanson: It has been installed, and it started up in late 2014. This was an evaluated bid process in which the city evaluated three competing technologies. Three members of the city staff and three representatives from the city’s engineering consultants rated 14 weighted factors on a scale from one to five. Those that most differentiated our technology were the fact they would not have to increase plant staff, the simplicity of the process and the equipment’s operation, the fact that the process uses no chemicals, the system’s ability to passively ramp up and turn down, and the future capacity for tertiary filtration during dry weather, should they choose to operate the system for that purpose.  


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