Optimization of biological nutrient removal in a pilot-scale trickling filter-activated sludge process

A pilot-scale study was undertaken to further the development of a combined trickling filter-activated sludge process designed for biological phosphorus removal and nitrification-denitrification. The system is called the FGR-SGR (fixed growth reactorsuspended growth reactor) process. The innovative aspect of the FGR-SGR process is the incorporation of a fixed growth (trickling filter) component into the conventional suspended growth (activated sludge) biological nutrient removal treatment train. The objective of the study was to extend and optimize design and operational criteria for biological nutrient removal in the FGR-SGR process, including an assessment of optimum process reactor hydraulic retention times, internal recycle flow rates, and operating mixed liquor suspended solids (MISS) concentration. Two pilot-scale processes were operated in parallel, to compare controlled changes in design parameters. Both pilot plants consistently produced an effluent typically containing 10-15 mg/L suspended solids, less than 10 mg/L BOD₅,less than 0.01 mg N/L ammonia, and 2-3 mg N/L total kjeldahl nitrogen, regardless of design and operational changes. Nitrification in the FGRS accounted for greater than 85% of the process total nitrification, and the fixed growth nitrification was found to be first order with respect to ammonia concentration. A higher FGR irrigation (recycle) rate was associated with a significantly greater nitrification rate. On the other hand, phosphorus removal was highly dependent on design and operational changes to the process. Daily short-term increases in the FGR recycle rate to prevent excess solids buildup on the media greatly improved phosphorus removal. With daily pulse hydraulic loading, low effluent orthophosphate concentrations (less than 0.3 mg P/L) were observed when the ratio of the mass of volatile fatty acids (VFA) taken up in the anaerobic reactor to process influent total phosphorus (P) concentration was greater than 6 mg HAc/mg P; at lower ratios, effluent orthophosphate concentrations increased to greater than 1 mg P/L. The mass of volatile fatty acids taken up in the anaerobic reactor depended on the anaerobic actual hydraulic retention time, the steady-state process influent VFA concentration, and the steady-state mean MLSS concentration. Biological phosphorus removal was significantly better at an aeration basin mean MLSS concentration of approximately 3,000 mg/L, compared to one of 2,000 mg/L. Denitrification in the anoxic reactor was accompanied by bacterial uptake of orthophosphate. Phosphorus uptake in the anoxic reactor accounted for approximately 45% of overall process total phosphorus removal. Bench-scale batch tests showed that following the completion of denitrification, secondary phosphorus release occurred for the remainder of the anoxic phase. Allowing significant concentrations of VFA to reach the anoxic reactor induced phosphorus release during the first few minutes of denitrification, reducing net anoxic phosphorus uptake. Bench-scale batch tests designed to simulate the effects of manipulating internal recycle flow rates to dampen hydraulic shocks typically caused by the peak daily load in full-scale plants indicated that manipulation of the recycle flows has the potential to improve phosphorus removal in the process.Applied Science, Faculty ofCivil Engineering, Department ofGraduat