Peak wet-weather flows have always been challenging to clean-water plants, and heavy rainfalls in the past few years have brought the problem into sharper focus.

The persistent question is how to handle peak flows while still producing effluent that meets permit limits and protects the receiving waterway.

Now Aqua-Aerobic Systems has introduced the AquaStorm pile cloth media filtration system. It is engineered for wet-weather applications and designed to provide high removal efficiency and to produce high-quality effluent, even under varying influent conditions.

The system uses a disc configuration with design features to deal with the specific challenges of peak flows. It uses the company’s OptiFiber PF-14 pile cloth media and operates with three zones of solids removal to treat peak flows without chemical addition. John Dyson, AquaStorm product manager, talked about the technology in an interview with Treatment Plant Operator.

TPO: What was the reason for bringing this product to market?

Dyson: Treatment plants have difficulty dealing with peak flows that exceed plant biological design capacity. This technology allows us to treat those peak flows without doing a full biological treatment process. It enables the plant to easily meet its permit limits while preventing the discharge of untreated peak flows, either at the plant or out in the collection network.

TPO: How and where can this system be deployed in the collection network?

Dyson: It could be at a pump station where the flows overwhelm the pumps or in places where combined sewer flows get so high so quickly that the network cannot physically move that water to the treatment plant. It provides 80% to 95% solids removal, after which the water can be effectively disinfected before discharge to a river or stream.

TPO: How would this technology fit into a typical clean-water plant process?

Dyson: One application is in a train parallel to the main treatment plant process. It allows the plant to operate up to its peak biological capacity where it can produce good effluent. The peak flow over that amount goes through the AquaStorm system to remove solids, BOD and any kind of particulate matter. Generally, because the influent in wet-weather conditions is quite diluted, we can produce very good effluent quality. Then we recombine the flow from our system with the effluent from the main plant and easily meet TSS, BOD and other permit limits. The other main option is to use the system in a dual treatment mode.

TPO: How does the AquaStorm process function in a dual treatment configuration?

Dyson: In dry weather, the system can work as a tertiary filter. When wet weather occurs, we can take the excess peak flow, route it around the biological process, recombine it with the effluent from secondary treatment and then filter all the water. In a way, it’s like getting tertiary filtration for free.

TPO: How does this technology differ from other cloth media filters?

Dyson: Because we’re dealing with primary influent, floatable material has to be removed. So the base design has been changed so we can remove scum and floatables periodically by hydraulic decanting. Primary influent also contains some large and heavy particulate matter. So we designed a collection system in the bottom of the unit that removes about 30% of the solids by a settling process.

TPO: What are the advantages of this particular system for treating wet-weather flows?

Dyson: First, the startup is very simple. Wet-weather flows often occur very quickly. Our system can sit dry, and when a peak flow comes, it goes into operation automatically. Second, we can achieve very high water quality without the addition of chemicals. If there is some dissolved component, such as phosphorus, that has to be removed, we could add a coagulant, but the chemical addition is simple because we just have to form a very tiny floc, which the pile cloth media filter then removes.

TPO: In simple terms, how does the process work?

Dyson: Influent enters the filtration tank, which contains the pile cloth media filter discs in a vertical arrangement. Larger and heavier solids are allowed to settle. Other solids collect on the surface of the cloth. When the water level rises to a high-level mark, backwash is initiated. The filter is never offline; we only backwash a portion of the discs at any given time. As the discs rotate, we use the water that’s being filtered for backwashing. It reverses back through at a very high velocity. That stands up the fibers and removes the solids from the cloth.

TPO: How are settled and floatable solids removed from the system?

Dyson: After six to 10 backwashes, we pump the settled solids from the collection system at the bottom of the unit. Every eight to 12 hours, we do a hydraulic decant of the scum and floatables. The water level is allowed to rise above the preset backwash level. As the water level increases, the scum is removed by flowing over a weir.

TPO: How does this system accommodate constrained spaces?

Dyson: It’s very compact. For perspective, based on engineering studies I have seen, it is generally about 10% smaller than a ballasted clarifier. It has about 15% of the footprint of a primary clarifier.

TPO: How well has this technology been accepted so far?

Dyson: We have one plant showing great results after almost two years of operation. Several other plants have systems under construction, and several more have entered into design. They vary from small facilities that might have to filter 1 or 2 mgd at peak flows to facilities that need to treat 150 to 250 mgd under peak-hour conditions. We see a lot of interest in the Eastern U.S. In the past couple of years, it has rained heavily from Oklahoma east. That means a lot of plants have been dealing with significant excess flow.