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Treatment options, as opposed to hold and treat options, for wet weather overflows have heretofore been limited to those that could function in a very space-limited situation. System footprint therefore becomes a major consideration during technology selection phases of Long Term Control Planning (LTCP). Reviews of limited space alternatives for the treatment of the wet weather combined sewage included a technology called ballasted flocculation. Initial investigations indicated a construction footprint requirement that is 35 to 50 times smaller than one for conventional primary clarification.

Ballasted flocculation is a physical/chemical precipitation process that uses an iron or aluminum coagulant in conjunction with polymer to remove solids from suspension in the wastewater. In could easily be characterized as drinking water or industrial wastewater technology. The ballast in question is a microsand particle that greatly enhances floc settleability. This enhancement allows for much higher than normal hydraulic loading rates. Conventional primary clarifiers must be designed and operated for wet weather flow loadings of approximately 2500 gpd/sf (2000 to 3000); whereas the ballasted flocculation technology, manufactured under the trade name Actiflo by Kruger, Inc. of Cary, North Carolina (a Vivendi company), is designed to treat hydraulic loads of 80,000 to 90,000 gpd/sf. Because of the chemical precipitation function, discharges result in a far clearer effluent than conventional primary clarification. These high rates are the basis of the smaller footprint. The effluent clarity greatly improves disinfection capability.

Investigations into the technology included literature and pilot study results review, inspection of the technology in use in a surface water treatment application and inspection of the technology in use in a wastewater/CSO application. A pilot study was conducted in Ohio during the first two weeks of July 1999. Results of the study indicate good performance in solids removal (approximately 90%) and acceptable removals in CBOD5 (>45%). Phosphorus removal was excellent due to the use of the ferric chloride for coagulation (98%). The table below illustrates the average results of the pilot study under process “running” conditions. These performance levels, combined with the aesthetic appeal of the clear effluent, made ballasted flocculation an alternative worth cost-evaluating.

The paper will discuss the investigation into the ballasted flocculation technology and the results of the pilot studies performed at various WWTP's. One pilot study focused on the ability to eliminate what was termed a primary bypass, but equally important was the requirement to disinfect any secondary bypasses to NPDES permit levels. Treatment of these flows to any degree less than full secondary treatment would result in those flows being secondary bypasses.

Document Type: Research Article


Publication date: 2001-01-01

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