The Lawrence (Kan.) Wastewater Treatment Plant was recently expanded and improved to meet future growth needs, new regulatory requirements, and facility rehabilitation needs for the design year 2020.
The project expanded the main wastewater treatment
process and included an excess-flow facility to treat peak flows during storm events. The main process is designed to treat 95,000 m3/d (25 mgd); during wet weather events, peak flows in 2020 are predicted to reach 246,000 m3/d (65 mgd) so the excess-flow facility must handle 151,000 m3/d (40 mgd).
.....According to historical plant data, when peak flow is
more than twice the average flow, the amount of waste in the wastewater is significantly lower than usual. Based on pilot work with similar wastewater quality, the project team decided that ballasted flocculation and disinfection would meet excess-flow plant effluent requirements, which were imposed by the Kansas Department of Health and Environment. All effluent must meet the plant’s National Pollutant Discharge Elimination System (NPDES) permit.
.....The excess-flow facility consists of an ACTIFLO
ballasted flocculation system by Kruger Products
(Cary, N.C.). Kruger was responsible for the process and system design and supplied all the equipment. The system includes two 76,000-m3/d (20-mgd) treatment basins and is currently the largest ballasted flocculation facility for sanitary sewer overflow in the United States.

The Ballasted Flocculation System
The ballasted flocculation system is a chemical process for enhanced reduction of suspended solids and biochemical oxidation demand (BOD). Fundamentally, the process is very similar to conventional coagulation, flocculation, and sedimentation systems used in water treatment plants
(see figure, p. 33, and Table 1, p. 32).

As in conventional water treatment technology, a coagulant is used to destabilize suspended materials entering the process and a flocculation-aiding polymer is added to aggregate solids into larger masses. The resulting floc is removed by settling.
.....The ballasted flocculation system enhances this process by adding microsand (fine sand similar to silica powder) as seed for developing high-density floe, which is ballasted by the relatively high-density microsand and so more easily removed by settling. The benefit of the ballasted floc process is the ability to achieve good solids removal performance at a very high surface overflow rate. The process can be rapidly started and optimized even with variations in flow and water quality.
.....In addition to the ballasted flocculation basins, a flow
splitter—screening facility, a disinfection basin, and a
chemical storage and feed facility were constructed. The ballasted flocculation process is protected by fine screens, which remove trash, debris, and rags from the process stream. After screening, the flow passes through the ballasted flocculation basins.
....Each 76,000-m3/d 20-mgd) basin includes a coagulation chamber, an injection chamber, a flocculation (maturation) chamber, and a sedimentation chamber. The coagulant (ferric chloride) is added to wastewater flow at the excess-flow splitter-screening facility. A diffuser disperses ferric chloride at the entrance of the 105-cm (42-in.) excess-flow treatment basin influent pipes, ahead of the coagulation chamber. In the coagulation chamber, solids entering the ballasted flocculation process are destabilized. The coagulated water then enters the injection chamber, where It is mixed with polymer and microsand.
.....With most of the floe forming in the flocculation chamber, the ballasted floc settles in the sedimentation chamber, which is equipped with lamella plate settlers.