The City of Orlando, FL, is very sensitive to pollution of any kind, but unsightly trash is especially disliked. The city is diligent in its continuous battle to keep the area clean, but until recently it has been almost impossible to keep trash and sediment out of area lakes. Two years ago, the city tested continuous deflection separation (CDS), a technology invented in 1992 by two Australians, Paul Blanche and Steve Compton. Orlando liked the results so well, it has already installed another unit and plans to install two more.
“The first time I saw a CDS unit, I thought it might help us solve some specific problems,” recalls Rick Howard, P.E., an engineer with City of Orlando. “We’ve tried different kinds of PCDs [pollution control devices] over the years, mainly chainlink fences at outfalls, but nothing worked well.
“We’ve always had a problem keeping our PCDs in place because they plug up, then the energy of the water blows out the fence panels and dumps all the debris into the receiving water. Before we installed the CDS unit, a lot of floatables got through our fences and into our lakes. I don’t think any floatables get by now. There’s no doubt about it, it works.
“The first installation in Orlando was in a 36-foot line, just upstream from an outfall,” Howard continues. “The unit captures 100% of the floating debris and a lot of other material, including suspended solids. We haven’t tried to measure the quantities because it is obvious that the unit is doing its job, but we clean the unit every five or six weeks with a vac truck and remove an amazing amount of trash and sediment. It’s certainly a lot easier to clean than the old fence traps, and it’s completely out of sight.”
Continuous Deflection Separation Devices
Continuous deflection separators are cylindrical devices constructed in any size necessary to treat stormwater from catchments as small as a single parking lot (1 cfs), or as large as an entire drainage basin (300 cfs). CDS technology uses the hydraulic energy of stormwater runoff entering the unit to gather and trap pollutants.
As water is diverted from a drainage pipe into the unit, it begins a circular motion that allows the water to pass through a perforated screen while forcing the pollutants to swirl toward the center of the cylinder. As the movement of the pollutants slows, most tend to settle into a central sump where they are no longer affected by the moving water above them. Floatable pollutants simply continue to swirl around the center of the cylinder until flow through the unit stops, or until the floatables are removed, but they cannot escape or be flushed back into the stormwater drain.
The units can even catch oil and grease. In one study, four different sorbents effectively removed used motor oil at the concentration typically found in stormwater runoff. The sorbents, added to a CDS unit separation chamber, captured 80–90% of the oil.
The swirling flow within the separation chamber behaves in the manner of a solid body in rotation. Therefore, objects in the flow that have a density greater than water will be forced outward and would be pressed against the perforated screen if it were not for the tangential flow around the chamber that continuously sweeps the screen and prevents blockage. The units are carefully designed to ensure that the tangential force at every point around the chamber screen is always greater than other forces acting upon pollutants which would otherwise tend to block the screen.
Orlando’s Second CDS Installation
Orlando’s most recent CDS installation is on a 60-in. storm sewer outfall and was prompted by the redevelopment of lakefront property. The outfall dumped floatables into the lake, in addition to a surprising amount of sand and other sediment that formed a sandbar in the lake, requiring frequent removal. Since the new development would severely limit access to the outfall, the city chose to install a CDS unit to capture the sediment and floatables at a more accessible location.
In addition to the normal unit, the city wanted to install a pump system to remove the screened water in the unit’s process chamber before the debris was vacuumed out. This would allow the vac truck to transport more debris and less water. Future plans call for the sump pump to be started remotely, so the water will be removed by the time the vac truck arrives. This will reduce cleaning time significantly.
John Faubert, job-site superintendent with Prime Construction Group Inc., explains that the installation was done in two steps because the work area was so limited and the excavation was 25 ft. deep. This also limited their exposure in case of bad weather. “We set the CDS unit first because it would be easier to match the poured-in-place diversion box to the unit than the other way around,” he recalls. The unit was precast in three sections: sump, separation chamber, and riser. As each piece was installed, it was backfilled. When the unit was in place, an excavation was made for the diversion box and pump vault.
The diversion box was inserted into the 60-in. stormwater drain to divert flow to and from the unit. It also contained a collapsible weir to allow overflow to bypass the unit. “The installation was straightforward,” reports Faubert. “That was the first unit we’ve installed, but it went in easily.”
The Future of Stormwater Regulations
“I would say that 80% of our installed units deal with stormwater,” states Robert Howard, P.E., manager with CDS Technologies Inc. in Morgan Hill, CA. “The remainder treats combined sewer overflows.
“The way stormwater regulations are going in the United States, it seems that responsibility will eventually be pushed back to the source of the pollution,” Howard adds. “That may be a commercial parking lot, an industrial plant, or a pig farm. As we become more serious about protecting our water quality, performance verification will also become more important. That will help us, because we can provide lab and field-performance data now, and it’s all from third-party testing.”
Howard points out that the Clean Water Act contains requirements for solids control, but the provision has not been enforced because of the lack of an affordable technology. That’s changing with the availability of CDS technology. “I think we’re seeing the rules begin to tighten now that the regulatory community is aware of this technology. For instance, we are seeing total maximum daily loading [TMDL] limits being set for trash and debris in southern California. That is the maximum amount of pollutants allowed to empty into a receiving water during any single day. Once set, the TMDL is expected to be reduced each year until nearly all of the trash and debris discharge is eliminated. Equipment such as CDS makes TMDL limits very achievable.
“Of course there are several ways to reduce the daily loading of storm sewers,” Howard continues. “You can educate the public so they will discard less litter. You can sweep streets, clean catch basins, and install many different kinds of trash traps, but all of those techniques are labor-intensive, and they certainly haven’t been able to solve the problem yet. That’s where I see CDS fitting in. It is an end-of-the line treatment that allows some latitude in the other programs that municipalities are expected to carry out. If a maintenance crew doesn’t make it out to clean a trash trap before it rains, all the trapped material gets flushed down the sewer, but the CDS unit will catch it. And once material is trapped in the unit, it cannot be flushed back into the storm sewer.
“Orlando has always emphasized water quality,” notes Rick Howard. “We have a long history at the forefront of stormwater treatment systems, and we have experimented with practically every system available. Those experiments have convinced me that almost every technology is best at something and that none is best at everything. That same experience has convinced me that nothing available today removes floating debris and suspended solids from stormwater as well as CDS.”
