ACTIFLO® Ensures Compliance for Coal Yard Runoff

Contaminated storm water resulting from rainfall and snowmelt was previously collected in two large basins. As the basins approached capacity, the storm water was treated in situ and discharged to the river. The new NPDES permit for the discharge contained limitations for total suspended solids, iron and manganese that the existing treatment system could not consistently achieve, along with a compliance deadline. To further complicate matters, the amount of space available for the installation of additional treatment system components was limited.

The Solutions

ACTIFLO® high-rate clarification
ACTIFLO technology is a compact system for high-rate applications

Veolia's ACTIFLO® treatment process provided the solution to the facility's problem. This innovative technology utilizes sand-ballasted settling to treat water with changing flow rates within a small footprint, typically less than 10 percent of the area required for conventional settling technology. Veolia combined the high-rate settling process with pH adjustment and chemical pretreatment to precipitate the metals of concern and produce an effluent of consistent, compliant quality.

On-site investigations were first conducted to determine whether modifications or improved storm water management practices could be implemented to enable the facility to meet the new limitations.

Bench-scale treatability studies were then performed to assess the ability of Veolia's ACTIFLO® clarification process to achieve the required discharge quality.

In order to meet the compliance deadline, Veolia oversaw the installation and operation of a temporary treatment system using a mobile ACTIFLO®. Data generated from the temporary system were used to validate the process, establish design parameters and optimize operating conditions for the permanent treatment system.

Veolia prepared the environmental permit applications for installation as well as the design engineering and project management services for both the temporary and permanent treatment systems. Equipment procurement services were also provided for the permanent system.

Process Description

The key to the success of this technology is the use of microsand. While most conventional systems use polymer to aid in the settling process, ACTIFLO® combines tthe polymer addition with fine-grained sand that is recycled in the process.

In the treatment tank, the polymer, microsand and other small particles in the water conglomerate in a floc that settles very fast. The microsand provides both a "seed" for floc formation and a ballast to enhance settling. The settled sand and solids are pumped from the bottom of the tank to a hydrocyclone where the sand is separated from the solids and recycled.

The enhanced rate of settling allows for compact clarifier designs with high overflow rates and short detention times. These improved operating parameters result in system footprints that are from 5 to 20 times smaller than those of conventional clarification systems with similar capacity.

In industrial application, this process is able to remove suspended solids, oil, grease and heavy metals quickly and efficiently.

  1. Coagulation Tank: A coagulant such as an iron or aluminum salt is added to start floc formation.
  2. Injection Tank: The floc produced during the coagulation stage are ballasted by the dense microsand.
  3. Maturation Tank: Promotes denser floc formation using a mixer producing optimum velocity gradients, allowing larger floc to build for achieving high settling velocities.
  4. Settling Tank with Scraper and Lamella: Allows quick sludge settling and high water rise rate thanks to microsand ballasting of the floc.
  5. Effluent Discharge: Treated water exits the system.
  6. Microsand Recirculation: The underflow is pumped to the hydrocyclone where the sludge is separated from the microsand. The clean microsand is recovered and returned to the injection tank. The sludge is continuously removed for further processing.

The Results

The performance of the system for the constituents of concern is shown in the table below. The permanent treatment system has been installed and is currently providing consistent compliance with the facility's permit limitations. The values shown represent both the pilot and full-scale systems.


Effluent Range (mg/l)

 Average Monthly Limit (mg/l)

Total Suspended Solids (TSS)



Total Manganese 



Total Iron