ADJUSTING FLOC CATIONS TO IMPROVE EFFLUENT QUALITY: THE CASE OF ALUMINUM ADDITION AT SIOUX CITY WASTEWATER TREATMENT FACILITY

The Sioux City Wastewater Treatment Facility experienced effluent quality problems. The problem was exacerbated by the introduction of a unique industrial stream to the plant influent with high fluctuations in the flow and makeup of the stream. Efforts to mitigate this problem with conventional methods of enhanced secondary clarification were unsuccessful. The research and development team of Veolia Water North America was working with Virginia Tech to investigate the role of trivalent cations, mainly aluminum and iron, on waste activated sludge floc properties including settling, digestion and dewatering. The project team working on mitigating the plant's problem hypothesized that the chronic poor effluent quality might be due to imbalance in the floc cation makeup.Research with VT showed that an optimum combined iron and aluminum ion floc content should be between 30-35 mg/g TS for producing good effluent quality with aluminum constituting about 10 mg/g TS. A multi month program was initiated to investigate the makeup of the mixed liquor floc content and provide a corrective action plan for the plant staff.Results showed that the ML floc aluminum content was well below the optimum range and in the order of ∼1.0 mg/g TS. The iron content was found to be within normal ranges. To adjust the floc aluminum content, the proper dose of alum was added at the primary effluent leading the aeration basin, thus allowing free aluminum to be available for floc building. The effluent quality in term of TSS was monitored and the secondary treatment parameters such as SVI and 30 min settlometer were measured.The addition of alum to improve the aluminum floc content resulted in improved effluent TSS. Moreover, the SVI and 30 min settlometer results showed more consistent values and the plant experienced system robustness during upset conditions as a result of wet weather flow. This new technology offers plant operators a new tool for predicting performance through monitoring their secondary treatment floc cations makeup. Keeping proper cations balance is important in obtaining and maintaining good effluent quality and enhanced plant performance.