The Role of Activated Sludge Solids in an Actiflo® System

This research was conducted using jar tests to investigate the impact of adding activated sludge solids to the Actiflo® process, a high-rate clarification (HRC) system, to increase BOD removal in HRC systems treating raw wastewater during periods of high wet-weather flows (WWF). A study of dosages and impacts of ferric sulfate, polymer, microsand and activated sludge on performance of the Actiflo® process was investigated. Experiments were designed to determine the dosages of activated sludge and other chemicals necessary to achieve an effluent BOD5 level of 30mg/L in this physical-chemical process with an influent BOD5 concentration of 100 mg/L. With a 490 mg/L dose of activated sludge and 15 minutes mixing time, the effluent BOD removal efficiency was enhanced by approximately 30% using lower chemical dosages in jar tests.

The Actiflo® process is a ballasted flocculation and high-rate settling process that utilizes microsand to increase floc settling rates (see Figure 1). Because the Actiflo® process is a physical-chemical process it is well adapted to rapid startup and automatic operation required for treating WWF, however it has limited capacity for removing soluble BOD. It was hypothesized that activated sludge addition would result in significantly higher levels of soluble BOD removal due to absorption into bacterial cells. (Nielsen et al., 2004) An additional hypothesis was that the biomass solids would also act as a kind of biosorbent/biocoagulant that enmeshes influent particles, possibly allowing for lower coagulant doses. (Desjardins et al., 2002; Huang and Li, 2000; Jacobsen and Hong, 2002; Sauvignet, 2003; Plum et al., 1998; Lmasuen et al., 2004; Sawey et al., 1999)

The jar tests were performed in a manner to simulate the addition of return activated sludge to the headworks of the Village Creek Wastewater Treatment Plant in Fort Worth, Texas. There is approximately 15 minutes of contact time in the pipe from the headworks to the Actiflo® process (see Figures 2 and 3). The Actiflo® system at the Village Creek WWTP was designed to meet a National Pollutant Discharge Elimination System (NPDES) permit that requires that the effluent BOD5 from the HRC be less than 45 mg/L at all times. (Sawey et al., 1999) A specific goal of this study was to determine whether activated sludge addition could lower the total effluent BOD5 and produce an effluent that meets a specific effluent BOD5 goal (25~35 mg/L BOD5) with a high degree of confidence for influent BOD5 values as high as 100 mg/L.

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